Retinoblastoma

A number sign (#) is used with this entry because hereditary retinoblastoma is caused by a heterozygous germline mutation on one allele and a somatic mutation on the other allele of the RB1 gene (614041) on chromosome 13q14.

See also the chromosome 13q14 deletion syndrome (613884) in which retinoblastoma is a feature.

Retinoblastoma (RB) is an embryonic malignant neoplasm of retinal origin. It almost always presents in early childhood and is often bilateral. Spontaneous regression ('cure') occurs in some cases. The retinoblastoma gene (RB1) was the first tumor suppressor gene cloned. It is a negative regulator of the cell cycle through its ability to bind the transcription factor E2F (189971) and repress transcription of genes required for S phase (Hanahan and Weinberg, 2000).

Connolly et al. (1983) reported a 4-generation family with 3 patterns of expression of the retinoblastoma gene: frank retinoblastoma, unilateral or bilateral; retinoma; and no visible retinal pathology except for 'normal degeneration' with age. ('Paving stone degeneration' of the type observed in 2 of 3 RB carriers, aged 49 and 59, is said by Duane (1980) to occur in about 20% of the adult population.)

In a review, Balmer et al. (2006) noted that the most common presenting signs of retinoblastoma are leukocoria (a late sign) and strabismus (an early sign), but that many other ocular or general signs have been observed. Although the malignant tumor is curable with early treatment, there remains in the heritable form a major risk of second nonocular primary tumors.

Retinoma

Gallie and Phillips (1982) described benign lesions in the retina in retinoblastoma patients. The distinctive characteristics of these lesions, referred to by the authors as retinomas, included a translucent, grayish retinal mass protruding into the vitreous, 'cottage-cheese' calcification in 75%, and retinal pigment epithelial migration and proliferation in 60%. They suggested that retinomas represent not the heterozygous state postulated by the Knudson 2-stage model of carcinogenesis but rather the homozygous state occurring in differentiated cell(s). Gallie et al. (1982) suggested that retinomas represent either spontaneous regression of a retinoblastoma or a benign manifestation of the RB gene.

Trilateral Retinoblastoma

Brownstein et al. (1984) described 3 children with bilateral retinoblastoma and a morphologically similar neoplasm in the region of the pineal. They referred to this as trilateral retinoblastoma. The pineal gland has sometimes been referred to as 'the third eye.' Lueder et al. (1991) described a fourth case of pinealoma associated with bilateral retinoblastoma. The patient was one of 56 with heritable RB. Amoaku et al. (1996) reported 5 patients with trilateral retinoblastoma (including 2 previously reported), diagnosed among 146 consecutive patients with retinoblastoma in the West Midlands Health Authority Region in England between 1957 and 1994. This represented an incidence of 3%. There were 4 patients with pineoblastoma, only 1 of whom had a positive family history. The mean age at diagnosis of RB in the entire series was 6 months, whereas the patients with pineoblastoma were diagnosed at a mean age of 2 years. The tumors were not evident on the initial computed tomographic (CT) scans. One child presented with a calcified suprasellar mass 13 months before bilateral sporadic retinoblastoma was identified. Death occurred within 1 month of diagnosis of the intracranial tumor in 3 patients who received no treatment. In the other 2 patients, who were treated, death occurred at 15 months and 2 years, respectively, after diagnosis of intracranial tumor.

Kivela (1999) performed a metaanalysis of trilateral retinoblastoma by reviewing the literature systematically and contacting authors to obtain missing information. Data from 106 children were used to determine frequency and Kaplan-Meier survival curves. No sex predilection was found. Median age at diagnosis of retinoblastoma was 5 months (range, 0 to 29 months); age at diagnosis was younger among 47 children (47%) with familial retinoblastoma compared with age at diagnosis among 52 children (53%) with sporadic retinoblastoma (2 vs 6.5 months; P less than 0.0001). Trilateral retinoblastoma usually affected the second or third generation with retinoblastoma. Median time from retinoblastoma to trilateral retinoblastoma was 21 months (range, 6 months before to 141 months after); time to trilateral retinoblastoma was longer for 78 (77%) pineal tumors compared with 23 (23%) suprasellar tumors (32 vs 6.5 months; P less than 0.0001). The size and prognosis of pineal and suprasellar tumors were similar. Screening by neuroimaging improved outcome. The cure rate was improved when the tumors were at or below 15 mm at the time of detection.

Karatza et al. (2006) reported a pineal cyst simulating pineoblastoma in 11 children with retinoblastoma (2 familial and 9 sporadic).

Second Primary Tumors

The risk of osteogenic sarcoma is increased 500-fold in bilateral retinoblastoma patients, the bone malignancy being at sites removed from those exposed to radiation treatment of the eye tumor (Abramson et al., 1976). Francois (1977) concluded that there is a special predisposition to osteogenic sarcoma, both radiogenic and nonradiogenic, in retinoblastoma patients and possibly in their relatives. Matsunaga (1980) estimated that the relative risk of development of nonradiogenic osteosarcoma in persons with the retinoblastoma gene is 230. That osteosarcoma is a direct effect of the genomic change that underlies retinoblastoma is indicated by the cases of osteosarcoma without retinoblastoma but with genomic changes like those of retinoblastoma.

Chauveinc et al. (2001) reviewed retinoblastoma survivors who subsequently developed osteosarcoma. They found that osteosarcomas occurred 1.2 years earlier inside than outside the radiation field in patients who had undergone external beam irradiation. Also, the latency between radiotherapy and osteosarcoma was 1.3 years shorter inside than outside the radiation field. Bimodal distribution of latency periods was observed for osteosarcomas arising inside but not outside the radiation field: 40% occurred after a short latency, while the latency for the remaining 60% was comparable to that of osteosarcoma arising outside the radiation field. The authors suggested that different mechanisms may be involved in the radiocarcinogenesis. They hypothesized that a radiation-induced mutation of the second RB1 allele may be the cause of osteosarcomas occurring after a short delay, while other genes may be responsible for osteosarcomas occurring after a longer delay.

To understand why the RB protein is specifically targeted in osteosarcoma, Thomas et al. (2001) studied its function in osteogenesis. Loss of RB but not p107 (116957) or p130 (180203) blocked late osteoblast differentiation. RB physically interacted with the osteoblast transcription factor, CBFA1 (600211), and associated with osteoblast-specific promoters in vivo in a CBFA1-dependent fashion. Association of RB with CBFA1 and promoter sequences resulted in synergistic transactivation of an osteoblast-specific reporter. This transactivation function was lost in tumor-derived RB mutants, underscoring a potential role in tumor suppression. Thus, RB functions as a direct transcriptional coactivator promoting osteoblast differentiation, which may contribute to the targeting of RB in osteosarcoma.

Friend et al. (1987) found that the same DNA segment that they had isolated from 13q14 and showed to have attributes of the retinoblastoma gene is additionally the target of somatic mutations in mesenchymal tumors among patients having no apparent predisposition to retinoblastoma. Almost two-thirds of the secondary tumors arising in patients with retinoblastoma are mesenchymal in origin. Over 60% of the mesenchymal tumors are osteosarcomas; the soft tissue sarcomas include fibrosarcoma, leiomyosarcoma, liposarcoma, and others. Friend et al. (1987) specifically demonstrated homozygous deletions of the RB1 locus in sporadic cases of leiomyosarcoma, malignant fibrous histiocytoma, and undifferentiated sarcoma in the absence of any history of retinoblastoma. Friend et al. (1988) reviewed the subject of tumor-suppressing genes in retinoblastoma and other disorders.

Henson et al. (1994) found loss of heterozygosity for the RB1 protein in 16 of 54 informative high-grade astrocytomas but not in 12 low-grade gliomas. Deletion mapping with ranking markers revealed that the retinoblastoma locus was preferentially targeted by the deletions. SSCP analysis and direct DNA sequencing demonstrated mutations in the remaining retinoblastoma allele. This evidence suggested to the authors that whereas mutation of the p53 tumor suppressor gene (191170) is an early event in the formation of many astrocytomas, mutation of the retinoblastoma gene is associated with progression of the astrocytoma into high-grade astrocytoma or glioblastoma multiforme. Previous work had demonstrated frequent loss of 9p, 10, 13q, 17p, 19q, and 22q in astrocytomas. Mutations in the RB1 gene were described.

Cance et al. (1990) found that leiomyosarcomas and other soft tissue sarcomas in which expression of the RB gene product was decreased were more aggressive than tumors in which this protein was expressed by nearly all cells.

Moll et al. (2001) evaluated the influence of age at external beam irradiation (EBRT) on the occurrence of second primary tumors (SPTs) inside and outside the irradiation field in 263 patients with hereditary retinoblastoma. They calculated cumulative incidences of SPT in 3 subgroups: irradiation before 12 months of age (early EBRT), irradiation after 12 months of age (late EBRT), and no irradiation. They found that hereditary RB conferred an increased risk of the development of SPT, especially in patients treated with EBRT before 12 months of age. However, they concluded that the presence of similar numbers of SPTs inside and outside the irradiation field suggested that irradiation was not the cause. The authors concluded that their study did not show an age effect on radiation-related risk, but rather that early EBRT is probably a marker for other risk factors of SPT.

Kivela et al. (2001) analyzed the association between retinoblastoma and sebaceous carcinoma (SC) of the eyelid to improve surveillance of RB survivors. They studied 11 children with hereditary RB who subsequently developed eyelid SC. Nine of the children developed SC within the field of radiation. All 9 had received a median of 46 Gy (range, 21-89) of EBRT at a median age of 16 months (range, 0.5-15 years of age). Their median age at SC diagnosis was 14 years (range, 8-30 years) and median interval from EBRT to SC diagnosis was 11 years (range, 5-26 years). The 2 children who had never received EBRT developed eyelid SC at ages 32 and 54 years. In this series, the cumulative probability of a 5-year survival of eyelid SC was 87%. The authors concluded that SC of the eyelid may occur in patients with hereditary RB regardless of primary treatment, especially within the EBRT field 5 to 15 years after radiotherapy.

Brantley et al. (2002) examined expression of p53 and Rb tumor suppressor pathways in uveal melanomas following plaque radiotherapy. They found that plaque radiotherapy damaged DNA, inhibited cell division, and promoted cell death. They stated that these changes might be due, at least in part, to induction of p53, which activates genes involved both in cell cycle arrest and apoptosis. Their results also showed that plaque radiotherapy can cause alterations in the expression of Rb, but the authors noted that the significance of the latter finding would require further study.

Gombos et al. (2007) identified 15 retinoblastoma patients with secondary acute myelogenous leukemia (sAML; see 601626), 13 of whom developed sAML in childhood. Mean latent period from RB to AML diagnosis was 9.8 years (median, 42 months). Nine cases were of the M2 or M5 French-American-British subtypes. Twelve patients (79%) had received chemotherapy with a topoisomerase II inhibitor, and 8 (43%) had received chemotherapy with an epipodophyllotoxin. Ten children died of their leukemia. Gombos et al. (2007) questioned whether chemotherapy was a risk factor for the development of sAML in this series of patients.

Bonaiti-Pellie et al. (1975) found an increased frequency of malformations, especially cleft palate, in association with retinoblastoma and proposed that this argued for germinal mutation rather than somatic mutation.

Gibbons et al. (1995) presented a patient in whom sporadic unilateral retinoblastoma occurred at an unusually early age (4 months) in the presence of Fanconi anemia (227650). In a second child, they found retinoblastoma in association with Bloom syndrome (210900), another chromosome breakage syndrome.

Shields et al. (2000) analyzed patient management and prognosis after vitrectomy in eyes with unsuspected retinoblastoma. Retinoblastoma may present with atypical features such as vitreous hemorrhage or signs of vitreous inflammation, particularly in older children. Vitrectomy should be avoided in these cases until the possibility of underlying retinoblastoma has been excluded. The authors concluded that if vitrectomy has been performed in an eye with unsuspected retinoblastoma, enucleation combined with adjuvant chemotherapy, radiotherapy, or both, should be done without delay to prevent systemic tumor dissemination.

Honavar et al. (2001) reviewed 45 consecutive patients who underwent an intraocular surgery after treatment for retinoblastoma: 34 (76%) underwent a single procedure (cataract surgery, scleral buckling procedure, or pars plana vitrectomy) and 11 (24%) underwent a combination of 2 or more procedures. Sixteen patients (36%) achieved final visual acuity better than 20/200 (legal blindness). Unfavorable outcomes included recurrence of retinoblastoma in 14 patients (31%), enucleation in 16 (36%), and systemic metastasis in 3 (7%). Five patients (20%) who underwent cataract surgery, 5 (63%) who underwent scleral buckling, and 9 (75%) who underwent pars plana vitrectomy had an unfavorable outcome. Median interval between completion of treatment for retinoblastoma and intraocular surgery was 26 months in patients with a favorable outcome versus 6 months in those with an unfavorable outcome. The authors concluded that cataract surgery was safe and effective in most cases. However, they cautioned that scleral buckling and pars plana vitrectomy might be associated with a much higher risk of recurrence of retinoblastoma, enucleation, or systemic metastasis.

Honavar et al. (2002) reviewed their experience with 80 consecutive patients with unilateral sporadic retinoblastoma who had been treated by primary enucleation and had high-risk characteristics for metastasis on histopathology, e.g., anterior chamber seeding, iris infiltration, ciliary body infiltration, massive choroidal infiltration, invasion of optic nerve lamina cribrosa, retrolaminar optic nerve invasion, invasion of the optic nerve transection, scleral infiltration, or extrascleral extension. A single high-risk characteristic was present in 62.5%, while 37.5% had more than 2 high-risk characteristics. Postenucleation adjuvant therapy (chemotherapy with or without orbital external beam radiotherapy) was administered to 58%. Adjuvant therapy was not administered to 42% of patients. Metastasis occurred in 13% of patients at a median of 9 months (range 6-57 months). Only 4% (2/46) of patients who had received adjuvant therapy developed metastasis, versus 24% (8/34) of patients who had not received adjuvant therapy. This difference was statistically significant (P = .02). The authors reported no serious systemic complications of adjuvant therapy. Thus, they concluded that postenucleation adjuvant therapy was safe and effective in significantly reducing the occurrence of metastasis in patients with retinoblastoma manifesting high-risk histopathologic characteristics.

In a retrospective study that included 100 enucleation specimens belonging to 96 patients with retinoblastoma, Abramson et al. (2003) analyzed the differences between the length of the optic nerve measured by the ophthalmologist in the operating room after enucleation and the length measured by the pathologist after fixation. They found a significant degree of shrinkage of the optic nerve after fixation prior to pathologic analysis. They cautioned that this must be taken into account when comparing different series and making recommendations for chemoprophylaxis based solely on histopathologic examination.

Camassei et al. (2003) found that FAS (600212) activation increased with increased retinoblastoma aggressiveness and postulated that FAS inhibition could represent an alternative treatment strategy in advanced and resistant retinoblastomas.

Poulaki et al. (2005) found that human retinoblastoma cell lines were resistant to death receptor (see DR5; 603612)-mediated apoptosis because of a deficiency of caspase-8 (CASP8; 601763) expression secondary to epigenetic gene silencing by overmethylation. Treatment with a demethylating agent restored CASP8 expression and sensitivity to apoptosis. Poulaki et al. (2005) suggested that a combination of demethylating agents with DR-activating modalities, such as TNF-related apoptosis-inducing ligand receptor (see 603163) monoclonal antibodies, might benefit patients with retinoblastoma.

Siffroi-Fernandez et al. (2005) examined fibroblast growth factor (FGF) high and low affinity receptor (FGFR) expression, activation of FGFR1 (136350) by acidic FGF (FGF1; 131220), and proliferative effects on Y79 retinoblastoma cells. They found that Y79 retinoblastoma expresses protein and mRNA of all 4 FGFRs. FGFR1 was differentially phosphorylated by FGF1. Proliferation of Y79 cells induced by FGF1 was entirely mediated by FGFR1. FGF1-induced proliferation was dependent on the presence and sulfation of heparan sulfate proteoglycan (HSPG; 142460). Siffroi-Fernandez et al. (2005) concluded that their study demonstrated a role for the FGF1/FGFR1 pathway in retinoblastoma proliferation and might contribute to developing therapeutic strategies to limit retinoblastoma growth.

De Jong et al. (2006) documented the growth, clinical course, and histopathology of retinoblastomas in the well-functioning fellow eye of a 27-year-old man whose left eye was enucleated at age 2 years for retinoblastomas. Despite irradiation, transscleral cryocoagulation, argon laser photocoagulation of tumors and their feeder vessels, and combination chemotherapy, the remaining eye required enucleation due to tumor recurrences and seeding, pseudohypopyon, and elevated intraocular pressure. The authors thought there was sufficient evidence indicating that this was not a recurrence of a spontaneously regressed retinoblastoma, and that the patient's rare mutation in the RB1 gene (614041.0026) could explain the atypical course. The case report also showed that the germinative lens epithelial layer in adults can still function despite high doses of ionizing radiation and chemotherapy.

Smith and Sorsby (1958) concluded that bilateral cases of retinoblastoma are most often familial. In their opinion, estimates of mutation rate of 2.3 x 10(-5) as given by Falls and Neel (1951) were too high. Many unilateral cases may be sporadic with a low risk (empirically, about 4%) to subsequent children or to offspring of the proband.

Knudson (1971) proposed that a 2-mutation model best fits the data. In this view, a fraction of cases are nonhereditary and result from 2 somatic mutational events in one cell. The remainder are hereditary cases, occurring in persons susceptible by reason of having inherited one of the mutational events. (See review of Knudson (1986) on the 2-mutation model and other aspects of the genetics of human cancer.)

Matsunaga (1982) suggested that the almost synchronous appearance of bilateral retinoblastoma argues against the 2-mutation model, which assumes that in the gene carriers the eyes acquire tumors independently.

Age-specific incidence rates for 96 New Zealand patients with sporadic retinoblastoma peaked earlier for bilateral cases than for unilateral ones (Fitzgerald et al., 1983). The cumulative log survival until diagnosis for bilateral and unilateral patients followed linear and quadratic curves, respectively, thus supporting the 2-hit hypothesis. A germ cell mutation rate of 9.3 x 10(-6) to 10.9 x 10(-6) was estimated. Interestingly, retinoblastoma, which behaves as a dominant in pedigrees, results from a gene that is expressed only in the homozygote, i.e., is recessive. Since retinoblastoma results from the homozygous or hemizygous state of a gene at 13q14, it should perhaps be stated that the susceptibility is dominant. The total experience with retinoblastoma has been that 5 to 10% of cases have been inherited; 20 to 30% have been new germinal mutations; and 60 to 70% have been sporadic, i.e., somatic mutations.

Dryja et al. (1989) and Zhu et al. (1989) found that in bilateral retinoblastoma there is a preferential retention of the paternal chromosome in the process of loss of heterozygosity (LOH). This may indicate that either (1) mutation of RB1 is more common during spermatogenesis than oogenesis as a result of differences between male and female meiosis, DNA methylation or environmental exposure; or (2) the paternal chromosome in the early embryo is more at risk for mutation, or deficient in DNA repair. Dryja et al. (1989) analyzed the parental origin of mutations at the retinoblastoma locus using RFLPs known to map at chromosomal band 13q14. Ten of 10 new germline mutations were derived from the father's chromosome 13, while 4 of 7 somatic mutations occurred in the maternal retinoblastoma allele. The authors suggested that new germline mutations at the retinoblastoma locus arise more frequently during spermatogenesis than during oogenesis, but that genomic imprinting does not play an important role in the development of somatic mutations leading to this malignancy.

Matsunaga et al. (1990) found no parental age effect in 225 sporadic cases of bilateral retinoblastoma and in 10 sporadic cases of chromosome deletion or translocation involving 13q14 that was identified as of paternal origin. Parental exposure to ionizing radiation or chemical mutagens, the effect of which accumulates with age, does not seem to play a major role in the production of germinal mutations at the RB1 locus. Since analysis of month of birth of 753 children with sporadic unilateral retinoblastoma showed no significant deviation from controls or a cyclic trend, nonheritable retinoblastoma is not likely to be associated with viruses whose activity varies markedly with season. DerKinderen et al. (1990) found that fathers 50 years of age or older had a relative risk of 5.0 to have a child with sporadic hereditary retinoblastoma compared with fathers in the population in general and that mothers 35 years of age or older had a relative risk of 1.7 compared with mothers in the population in general. Sporadic retinoblastoma was defined as occurrence without a family history of the disease. These cases were further classified as sporadic hereditary retinoblastoma when the patient was bilaterally affected or when a patient with unilateral retinoblastoma without family history later had the birth of a child with retinoblastoma; or as sporadic nonhereditary retinoblastoma when there was no family history, only unilateral retinoblastoma, and no occurrence of a relative in whom retinoblastoma was subsequently diagnosed. In connection with retinoblastoma, there is no parent of origin effect in the inherited disorder to suggest imprinting; however, there is evidence of imprinting in relation to osteosarcoma (Hall, 1993). The gene is more often transmitted from the father in the case of RB-related osteosarcoma. New mutations in the retinoblastoma gene most often occur in the father. Munier et al. (1992) found evidence of segregation distortion in a study of 8 kindreds with hereditary retinoblastoma by concomitant ophthalmologic examination and determination of 7 intragenic RFLPs. There was preferential transmission of the mutant allele from fathers; there was no difference from a 1:1 segregation ratio among the children of female carriers.

Epidemiologic studies indicated a preferential paternal transmission of mutant retinoblastoma alleles to offspring (Munier et al. (1992)), suggesting the occurrence of meiotic drive. To investigate this mechanism, Girardet et al. (2000) analyzed sperm samples from 6 individuals from 5 unrelated families affected with hereditary retinoblastoma. Single-sperm typing techniques were performed for each sample by study of 2 informative short tandem repeats located either in or close to the RB1 gene. The segregation probability of mutant RB1 alleles in sperm samples was assessed by use of the SPERMSEG program, which includes experimental parameters, recombination fractions between the markers, and segregation parameters. A total of 2,952 single sperm from the 6 donors were analyzed. They detected a significant segregation distortion in the data as a whole (P = 0.0099) and a significant heterogeneity in the segregation rate across donors (0.0092). Further analysis showed that this result could be explained by segregation distortion in favor of the normal allele in 1 donor only and that it does not provide evidence of the significant segregation distortion in the other donors. The segregation distortion favoring the mutant RB1 allele does not seem to occur during spermatogenesis, and, thus, meiotic drive may result either from various mechanisms, including a fertilization advantage or a better mobility in sperm bearing a mutant RB1 gene, or from the existence of a defectively imprinted gene located on the human X chromosome.

Naumova and Sapienza (1994) presented epidemiologic and genetic analyses of sporadic and familial retinoblastoma suggesting the existence of an X-linked gene (308290) involved in the genesis of a significant fraction of new bilateral cases of the disease. From the finding of both sex-ratio distortion in favor of males and transmission-ratio distortion in favor of affecteds among the offspring of males with bilateral sporadic disease, they proposed the existence of a defective imprinting gene on the X chromosome. Sporadic cases of Wilms tumor and embryonal rhabdomyosarcoma exhibit preferential loss of maternal alleles at loci linked to the putative tumor suppressor genes on 11p; sporadic cases of osteosarcoma and bilateral retinoblastoma similarly show preferential loss of maternal alleles at loci linked to the tumor suppressor gene on 13q. These observations may be explained by preferential germline mutation of the father's tumor suppressor gene or by genome imprinting. Naumova et al. (1994) examined 74 cases of sporadic retinoblastoma for tumors in which at least 2 genetic events had occurred: loss of heterozygosity for 13q markers and formation of an isochromosome 6p. They found 16 cases containing both events. In 13 of 16 such tumors, the chromosome 13q that was lost and the chromosome 6p that was duplicated were derived from the same parent. Because the formation of an isochromosome 6p is thought to be a somatic event and perhaps related to tumor progression (Horsthemke et al., 1989), the germline mutation model does not predict a relationship between the 2 events. The genome imprinting model, on the other hand, assumes that the original cell that gave rise to the tumor bore a genome imprint. Because the imprinting process affects loci on many different chromosomes, genetic events involving 2 unlinked loci would be expected to be related with respect to parent of origin, if both loci are imprinted. Toguchida et al. (1989) examined 13 sporadic osteosarcomas and in 12 found evidence indicating that the initial mutation was in the paternal gene. The finding suggests the involvement of germinal imprinting in producing differential susceptibility of the 2 genes to mutation. There was a growing body of data indicating a difference in behavior of maternally and paternally derived autosomal genes. Germinal imprinting may be mediated by some epigenetic process such as de novo DNA methylation and be carried over to postzygotic stages.

Sippel et al. (1998) evaluated 156 families with retinoblastoma in which the initial oncogenic mutation in the RB1 gene had been identified. In 15 of the families (approximately 10%) they were able to document mosaicism for the initial mutation in the retinoblastoma gene, either in the proband or in one of the proband's parents. Sippel et al. (1998) stated that the true incidence of mosaicism in this group was probably higher than 10%; in some additional families mosaicism was likely but could not be proven, because somatic or germline DNA from key family members was unavailable. In one mosaic father the mutation was detected in both sperm and leukocyte DNA; in a second, the mutation was detected only in sperm. The possibility of mosaicism should always be considered during genetic counseling of newly identified families with retinoblastoma and other disorders in which a high proportion of cases represent new mutations. Genetic tests of germline DNA can provide valuable information that is not available through analysis of somatic (leukocyte) DNA.

Based on genomewide methylation analysis of a patient with multiple imprinting defects, Kanber et al. (2009) identified a differentially methylated CpG island in intron 2 of the RB1 gene. The CpG island is part of a 5-prime truncated, processed pseudogene derived from the KIAA0649 gene (614056) on chromosome 9 and corresponds to 2 small CpG islands in the open reading frame of the ancestral gene. It is methylated on the maternal chromosome 13 and acts as a weak promoter for an alternative RB1 transcript on the paternal chromosome 13. In 4 other KIAA0649 pseudogene copies, which are located on chromosome 22, the 2 CpG islands have deteriorated and the CpG dinucleotides are fully methylated. By analyzing allelic RB1 transcript levels in blood cells, as well as in hypermethylated and 5-aza-2-prime-deoxycytidine-treated lymphoblastoid cells, Kanber et al. (2009) found that differential methylation of the CpG island (CpG 85) skews RB1 gene expression in favor of the maternal allele. Thus, Kanber et al. (2009) concluded that RB1 is imprinted in the same direction as CDKN1C (600856), which operates upstream of RB1. The imprinting of 2 components of the same pathway indicates that there has been strong evolutionary selection for maternal inhibition of cell proliferation.

Penetrance

Macklin (1959) demonstrated irregularities in the inheritance, suggesting incomplete penetrance. In 10.5% of cases, affected persons were identified in collateral lines. Examples included (1) a bilateral case, his unilaterally affected brother and a bilaterally affected daughter of the latter person; (2) 6 bilaterally affected offspring of a woman who had 1 microphthalmic eye but refused examination; (3) several instances of 2 or more affected sibs with normal parents. Incomplete penetrance was reported by Connolly et al. (1983) and Onadim et al. (1992) among others. A striking difference in penetrance between 2 generations of the family described by Connolly et al. (1983) suggested the segregation of an additional epistatic, host-resistance gene. Bundey and Morten (1981) reported a rather similar pattern of intergenerational difference in penetrance. Scheffer et al. (1989) identified nonpenetrance of the RB gene by the use of linkage markers in family studies.

Dryja et al. (1993) examined the molecular basis of incomplete penetrance in some retinoblastoma families. In 1 family, a germline deletion was shared by affected and unaffected obligate carriers. The deletion encompassed exon 4 of the RB gene and corresponded to a mutant protein without residues 127-166. In another pedigree, the phenomenon was only 'pseudo-low penetrance' because the appearance of incomplete penetrance was created by the fact that 2 distant relatives had independently derived mutations. Munier et al. (1993) reported 2 families with 'pseudo-low penetrance' resulting from familial aggregation of sporadic cases caused by independently derived mutations.

Bia and Cowell (1995) noted that rare families show evidence of incomplete penetrance where individuals transmit the mutant gene without being affected themselves. Formal proof of incomplete penetrance requires identification of the predisposing mutation. They reported an extraordinary family in which different mutations were identified in first cousins with bilateral disease. One cousin carried a C-to-T transition in exon 8, which changed CGA (arg) at codon 123 to TGA (stop). This mutation was present also in his affected mother. The other cousin carried an 8-bp deletion in exon 20 that resulted in the generation of a downstream stop codon. This mutation was not present in his mother who was a half sister of the affected mother of his affected cousin. Thus, this was an example not of reduced penetrance but of independent constitutional germline mutations.

Sakai et al. (1991) observed a germline mutation within the promoter region of RB in 2 different families showing incomplete penetrance. In a study of 5 RB families with low penetrance, Otterson et al. (1997) identified 3 separate germline RB mutations which showed different degrees of partial functional inactivation of the RB protein. Apparently, there are 2 categories of mutant low-penetrant RB alleles, those affecting the promoter region and those resulting in mutant proteins that retain partial activity.

Familial retinoblastoma with incomplete penetrance is characterized by the absence of clinical disease in obligate carriers or the presence of children with unifocal tumors that are more characteristic of sporadic retinoblastoma. In an effort to quantitate these clinical observations, Lohmann et al. (1994) proposed a disease-eye ratio (DER) that scored for each family the ratio of the sum of the number of eyes with retinal tumors over the number of obligate carriers. Kindreds with classic familial retinoblastoma characteristically showed a DER score that approached 2.0, whereas families with incomplete penetrance had DER scores less than 1.5.

Otterson et al. (1999) studied the RB-pocket-binding properties of 3 independent, mutant RB alleles that were present in the germline of 12 kindreds with the phenotype of incomplete penetrance of familial retinoblastoma. Each arose from alterations of single codons within the RB pocket domain, designated del480 (614041.0023), 661W (614041.0019), or 712R (614041.0024). The 3 mutants lacked pocket protein-binding activity in vitro but retained the wildtype ability to undergo cyclin-mediated phosphorylation in vivo. Each of the low-penetrant RB mutants exhibited marked enhancement of pocket protein binding when the cells were grown at reduced temperature. In contrast, in this temperature range no change in binding activity was seen with wildtype RB, a 706F mutant, or an adjacent, in vitro-generated point mutation (707W). Otterson et al. (1999) demonstrated that many families with incomplete penetrance of familial retinoblastoma carry unstable, mutant RB alleles with temperature-sensitive pocket protein-binding activity.

Harbour (2001) stated that recent advances in understanding of the structure and function of the RB protein provided insights into the molecular basis of low-penetrance retinoblastoma. Low-penetrance retinoblastoma mutations either cause a reduction in the amount of normal RB that is produced (class 1 mutations) or result in a partially functional mutant RB (class 2 mutations).

In a survey of germline RB1 gene mutations in Spanish retinoblastoma patients, Alonso et al. (2001) found splicing mutations associated with a low-penetrance phenotype. Most of the mutations affecting splice junctions corresponded to retinoblastoma cases of either sporadic or hereditary nature with delayed onset (32 months on average). In contrast, most of the nonsense and frameshift mutations were associated with an early age at diagnosis (8.7 months on average).

In 2 unrelated families with incompletely penetrant retinoblastoma, Klutz et al. (2002) identified a splice-site mutation (IVS6+1G-T; 614041.0025) in the RB1 gene. Analysis of RNA from white blood cells showed that this mutation causes skipping of exon 6. Although this deletion results in a frameshift, most carriers of the mutation did not develop retinoblastoma. The relative abundance of the resultant nonsense mRNA varied between members of the same family and was either similar to or considerably lower than the transcript level of the normal allele. Moreover, variation of relative transcript levels was associated with both the sex of the parent that transmitted the mutant allele and phenotypic expression: all 8 carriers with similar abundance of nonsense and normal transcript had received the mutant allele from their mothers, and only 1 of them had developed retinoblastoma; by contrast, all 8 carriers with reduced abundance of the nonsense transcript had received the mutant allele from their fathers, and all but 2 of them had retinoblastoma. After treatment with cycloheximide, the relative abundance of transcripts from paternally inherited mutant alleles was partly restored, thus indicating that posttranscriptional mechanisms, rather than transcriptional silencing, were responsible for low levels of mutant mRNA. The data suggested that a specific RB1 mutation can be associated with differential penetrance, on the basis of the sex of the transmitting parent.

Fitzek et al. (2002) observed an unexpected hypersensitivity in ionizing radiation in skin fibroblasts derived from unaffected parents of children with hereditary retinoblastoma. In at least 4 of these 5 families, there was no family history of retinoblastoma, indicating a new germline mutation. Fitzek et al. (2002) hypothesized that the increased parental cell sensitivity to radiation may reflect the presence of an as yet unrecognized genetic abnormality occurring in one or both parents of children with retinoblastoma. Chuang et al. (2006) used DNA microarray technology to determine whether differences in gene expression profiles occurred in the unaffected parents of patients with hereditary retinoblastoma compared to 'normal' individuals. Microarray analyses were validated by quantitative reverse transcription-PCR measurements. A distinct difference was observed in the patterns of gene expression between unaffected retinoblastoma parents and normal controls. The differences between the 2 groups were identified when as few as 9 genes were analyzed.

Macklin (1960) stated that in the U.S. the frequency of retinoblastoma is about 1 in 23,000 live births. Jensen and Miller (1971) found that at ages 2 to 3 years a peak of mortality occurred which was 2.5 times greater in blacks than in whites. Whether this reflects a truly high frequency in blacks or some other factor such as higher mortality from delayed diagnosis is not clear.

Pendergrass and Davis (1980) found an incidence of 3.58 cases among each million children under age 15 years. Over 90% were diagnosed before age 5 years. No difference was found between whites and blacks, but other non-whites had rates more than 4 times greater than those of whites. Bilateral disease occurred in 20%. No nonhereditary retinoblastomas (which represent 55-65% of all retinoblastoma cases) are bilateral. Bilateral and unilateral hereditary retinoblastoma represent, respectively, about 25-30% and 10-15% of all cases.

The earliest example of a cytogenetic change in a solid tumor was the description of partial deletion of a D group chromosome in retinoblastoma by Stallard (1962) and Lele et al. (1963).

In 12 reported patients with a deletion of the long arm of a D chromosome, 7 had retinoblastoma, which in 3 instances was bilateral (Taylor, 1970; Gey, 1970). Cytogenetic evidence suggested that a locus for retinoblastoma is on the long arm of chromosome 13. In the patients of Orye et al. (1971) and a patient of Wilson et al. (1969), in which a 14q- karyotype was found, no clinical features of the type usually associated with 13q- were present. Orye et al. (1971) found deletion of a distal part of the long arm of one chromosome 13 in a case of bilateral retinoblastoma. The broadest of the three Giemsa bands normally present on the long arm was missing. Grace et al. (1971) described a patient with typical 13q- syndrome plus retinoblastoma (613884). The karyotype contained a ring D chromosome. Wilson et al. (1973) restudied their case of bilateral retinoblastoma with new banding techniques and concluded that it, like all the other deleted D-chromosome cases, was an instance of 13q-. Orye et al. (1974) suggested that deletion of 13q21 is mainly responsible for retinoblastoma. With the advent of banding techniques, the chromosome involved was identified as chromosome 13 and the critical segment common to all deletions as band 13q14 (Francke, 1976).

Deletion of 13q22 was found by Riccardi et al. (1979), who reviewed published cases of retinoblastoma with abnormality of chromosome 13. They noted that, contrariwise, duplication of this segment has only mildly deleterious consequences. In 1 of 8 patients with retinoblastoma, Davison et al. (1979) found a reciprocal translocation of chromosomes 1 and 13. The breakpoint in chromosome 13 was at band q12, suggesting that the retinoblastoma locus is more proximal than thought from other data. Sparkes et al. (1979, 1980) found that retinoblastoma and esterase D (ESD; 133280) map to the same band, 13q14. They observed that quantitative and qualitative expression of esterase D in 5 persons with partial deletions or duplications of chromosome 13 supported localization of the gene to 13q14. The same band had been found deleted in cases of retinoblastoma. Rivera et al. (1981) concluded that the retinoblastoma and esterase D loci are in the proximal half of the 13q14 band. Benedict et al. (1983) studied a patient who had ESD activity 50% of normal but no deletion of 13q14 at the 550-band level. However, in 2 stem lines identified in a retinoblastoma from this patient, they found a missing chromosome 13 and no detectable ESD activity was found in the tumor. Therefore, in the tumor, the patient had total loss of genetic information at the location of the retinoblastoma gene. Thus, homozygosity appears to underlie this tumor.

Knight et al. (1980) studied linkage of familial retinoblastoma with fluorescent markers of chromosome 13. Instances of discordant segregation were attributed to crossing-over. Nichols et al. (1980) studied a patient with a 13q;Xp translocation and retinoblastoma. The 13q14 band was translocated intact to the X chromosome rather than being the breakpoint of the translocation. Genetic inactivation of the derivative X chromosome shown by late labeling and other findings with resulting functional monosomy of the 13q14 band was considered likely. About 20 cases of abnormality involving the 13q14 band, as an aberration in all somatic cells, had been described by 1981 (Balaban-Malenbaum et al., 1981).

Mosaicism for a 13q- cell line was found in 2 of 3 patients in a series of 42 that had abnormal karyotypes (Motegi, 1981). All 3 had bilateral sporadic retinoblastoma and the 2 mosaic cases had an apparently normal phenotype except for the eye tumors.

Comparison of the structural changes in the tumor cells with those in fibroblasts of some patients supported Knudson's 2-hit hypothesis. Using 2 probes, Horsthemke et al. (1987) found deletions in nearly 20% of patients with bilateral or multifocal unilateral retinoblastoma. One probe also detected a RFLP useful as a genetic marker in some families. Three of 8 deletions did not include the esterase D locus and were undetectable by conventional cytogenetic analysis. Sequence analysis of the RB cDNA clones demonstrated a long open reading frame encoding a hypothetical protein with features suggestive of a DNA-binding function.

Ejima et al. (1988) showed that cytogenetically visible germline mutations resulting in retinoblastoma are usually in the paternally derived gene. Such a bias would not be expected for sporadic (nonfamilial) tumors, where both mutations occur in somatic tissue, but there had been some indication of a bias toward initial somatic mutation in the paternally derived gene on chromosome 11 in sporadic Wilms tumor (Shroeder et al., 1987). Lemieux et al. (1989) used a method of minute band analysis to locate the RB locus in subband 13q14.11. The method involved high resolution G banding by BrdU using antibodies and gold. The banding was visualized by electron microscopy. Lemieux et al. (1989) suggested that the gain in resolution afforded by immunochemical banding, coupled with electron microscopy, would prove highly useful in detecting small chromosome anomalies in clinical syndromes and neoplasms. Greger et al. (1990) studied a family in which the father and 2 of his children had bilateral retinoblastoma. The father was found to be a mosaic for 2 different deletions, one of which showed a complex rearrangement. The 2 deletions shared 1 breakpoint but extended in opposite directions.

Both retinoblastoma and Wilms tumor are rare childhood tumors associated with loss or inactivation of RB1, located on 13q14, and WT1 (607102), located on 11p13, respectively. Punnett et al. (2003) reported a unique family in which an insertional translocation of a chromosomal segment that included band 13q14 inserted into 11p13, causing childhood Wilms tumor in the father, and whose child developed bilateral retinoblastoma. Thus, the insertional translocation caused both tumors. The estimated risk for an offspring of this father to develop Wilms tumor was up to 50%, to develop retinoblastoma 25%, to have neither tumor 25%, and to have both tumors 0%.

Sparkes et al. (1983) showed that the locus for the nondeletion form of retinoblastoma is closely linked to the ESD locus. Tight linkage to ESD was established by Connolly et al. (1983). Dryja et al. (1983) found quantitatively normal esterase D in all of 51 patients with retinoblastoma and no known chromosomal abnormality. Cowell et al. (1986) surveyed 200 retinoblastoma patients (75% bilateral and 25% familial) for dosage evidence of deletion of the esterase D locus. Nine patients with about half-normal levels were found; 5 had not been previously recognized as carriers of deletion.

Duncan et al. (1987) found that a cDNA probe hybridized most strongly to 13q14.2 and 13q14.3. They restudied an individual in whom a deletion was said to have separated the closely linked ESD and RB1 loci, placing ESD proximal to RB1. Quantitative in situ hybridization studies of this deletion showed, however, that ESD was missing from the deleted chromosome 13 and duplicated on a normal homolog. This led them to conclude that the deletion in this individual cannot be used to determine the orientation or sublocalization of ESD and RB1 within the 13q14 region.

Cowell et al. (1988) found a 50% level of esterase D activity and a small deletion in region 13q14 in a patient with retinoblastoma. The mother showed the same 50% of normal esterase D level and the same deletion, but had no retinal abnormality. Cowell et al. (1988) stated that this was the first instance of direct transmission of the deletion and also the first instance in which the deletion did not predispose to tumor formation.

Higgins et al. (1989) used field inversion gel electrophoresis (FIGE) to construct a restriction map of approximately 1,000 kb of DNA surrounding the RB1 locus and to detect the translocation breakpoints in 3 retinoblastoma patients. The large size of the presumed RB1 gene, approximately 200 kb, and its multiple dispersed exons, complicate molecular screening for prenatal and presymptomatic diagnosis and for carrier detection. By FIGE, Higgins et al. (1989) could detect and map the translocation breakpoints of all 3 retinoblastoma patients within the putative RB1 gene, thus substantiating the authenticity of this candidate sequence and demonstrating the utility of FIGE in detecting chromosomal rearrangements affecting this locus.

In the course of constructing a long-range restriction map around the RB gene by means of PFGE analysis, Blanquet et al. (1991) found evidence of possible genomic imprinting: the NruI restriction pattern differed according to the parental origin of the deletion or other rearrangement.

Diagnosis and Counseling

Ophthalmoscopic examination typically shows a white 'cat's eye' reflex and a retinal tumor in one or both eyes, usually by age 3 years.

Sparkes et al. (1979, 1980) suggested that if linkage between retinoblastoma and esterase D were found, this would provide a means of genetic counseling and early diagnosis, including prenatal diagnosis. Turleau et al. (1983) added a fourth family to those with retinoblastoma due to deletion of the critical portion of 13q in the offspring of a parent with a balanced insertional translocation (Riccardi et al., 1979; Rivera et al., 1981; Strong et al., 1981). They pointed out that without karyotyping the recurrence risk after the birth of 1 case from normal parents might be thought to be virtually nil whereas in fact it is 25% if one of the normal parents is a carrier of an insertional translocation. The same risk, 25%, applies to occurrence of a trisomic offspring.

Cavenee et al. (1985) showed that the chromosome 13 remaining in tumors from 2 hereditary retinoblastoma cases was derived from the affected parents. The ability to identify which chromosome of an affected parent carries the mutation predisposing to retinoblastoma would have obvious usefulness in genetic counseling. Cavenee et al. (1986) demonstrated the usefulness of multiple RFLP and isozymic markers flanking the RB1 locus in prenatal and postnatal prediction of susceptibility to retinoblastoma.

Wiggs et al. (1988) used cloned fragments of a 'retinoblastoma gene' that detected RFLPs and tested the usefulness of these RFLPs in predicting cancer in 20 families with hereditary retinoblastoma. Predictions were possible in 19 of the 20 families; in 18 of the 19, the marker RFLPs showed a consistent association with the mutation predisposing to retinoblastoma. In the 19th kindred, there was a lack of cosegregation, which, however, may have been due to inaccuracy of the clinical diagnosis of the retinal lesion in the key member of the kindred. Greger et al. (1988) demonstrated the usefulness of linkage analysis with DNA markers in genetic counseling of families with hereditary retinoblastoma.

Maat-Kievit et al. (1993) detected an enormous retinoblastoma at 21 weeks of gestation by means of ultrasound.

Since about 75% of cases of retinoblastoma due to constitutional mutations represent new mutations, Janson and Nordenskjold (1994) recognized a need for methods to identify carriers of such germline mutations so that informed genetic counseling can be offered. Using pulsed field gel electrophoresis in a screening of 20 unrelated cases with bilateral retinoblastoma, 1 constitutional mutation was detected and found to be caused by a balanced translocation t(4;13), with the breakpoint within intron 17 of the retinoblastoma gene.

Timely molecular diagnosis of RB1 mutations has many benefits: it enables earlier treatment, lower risk, and better health outcomes for patients with retinoblastoma; empowers families to make informed family-planning decisions; and costs less than conventional surveillance. However, complexity hinders its clinical implementation. Most RB1 mutations are unique and distributed throughout the RB1 gene, with no real hotspots. Richter et al. (2003) devised a sensitive and efficient strategy to identify RB1 mutations that combined quantitative multiplex PCR, double-exon sequencing, and promoter-targeted methylation-sensitive PCR. Optimization of test order by stochastic dynamic programming and the development of allele-specific PCR for 4 recurrent point mutations decreased the estimated turnaround time to less than 3 weeks and decreased direct costs by one-third. Using this multistep method, Richter et al. (2003) detected 89% of mutations (199 of 224) in bilaterally affected probands and both mutant alleles in 84% (112 of 134) of tumors from unilaterally affected probands. By revealing those family members who did not carry the mutation found in the related proband, molecular analysis enabled 97 at-risk children from 20 representative families to avoid 313 surveillance examinations under anesthetic and 852 clinic visits. The average savings in direct costs from clinical examinations avoided by children in these families substantially exceeded the cost of molecular testing. Moreover, health care savings would continue to accrue, as children in succeeding generations would avoid unnecessary repeated anesthetics and examinations.

Rushlow et al. (2009) found that the RB1 gene mutation detection rate in 1,020 retinoblastoma families was increased by the use of highly sensitive allele-specific PCR (AS-PCR) to detect low-level mosaicism for 11 recurrent RB1 nonsense mutations. Mosaicism was evident in 23 (5.5%) of 421 bilaterally affected probands and in 22 (3.8%) of 572 unilaterally affected probands, as well as in 1 unaffected mother of a unilateral proband. Noting that half of the mosaic mutations were detectable only by AS-PCR, Rushlow et al. (2009) suggested that significant numbers of low-level mosaics with other classes of RB1 mutations might remain unidentified by current technologies. In addition, since only 1 (0.7%) of 142 unaffected parents showed somatic mosaicism for the proband's mutation, in contrast to an overall 4.5% somatic mosaicism rate for retinoblastoma patients, Rushlow et al. (2009) suggested that mosaicism for an RB1 mutation is highly likely to manifest as retinoblastoma.

Screening

Noorani et al. (1996) compared the direct health care costs of molecular and conventional screening of relatives of individuals affected with retinoblastoma. With variables set at the most likely values (baseline), the expected cost (in 1994 Canadian dollars) of conventional screening was $31,430 for a prototype family consisting of 7 at-risk relatives. The cost included 3 clinic examinations and 8 examinations under anesthetic over the first 3 years of life for each relative. Using baseline variables, the molecular strategy consisted of the screening of a prototype family of 1 proband and 7 at-risk relatives at a cost of $8,674, including identification of the RB1 mutation in the proband, subsequent testing of the relevant relatives for that mutation, and clinical follow-up similar to the conventional strategy for relatives with the mutation. Sensitivity analysis over the range of values for each variable revealed a significant saving of health care dollars by the molecular route, indicating the benefit of redirecting economic resources to molecular diagnosis in retinoblastoma.

Zeschnigk et al. (1999) reported a PCR-based assay for the detection of methylation at the RB1 promoter. The assay gave results which were concordant with those achieved by Southern blot analysis in 40 samples.

Tsai et al. (2004) reported the usefulness of protein truncation testing (PTT) for rapid detection and sequencing of germline mutations in the RB1 gene. Nineteen (70%) of 27 probands tested positive for germline mutations by PTT. In 1 kindred, the proband had negative PTT results but an additional affected relative had positive PTT results. Using a multitiered approach to genetic testing, 23 (85%) of the 27 kindreds had mutations identified, and those detected by PTT received a positive result in as few as 7 days. In control subjects, PTT produced no false-positive results. The authors concluded that when used as an initial screen, PTT can increase the yield of additional testing modalities, providing a timely and cost-effective approach for the diagnosis of heritable germline mutations in patients with retinoblastoma.

Fung et al. (1987) used a cDNA probe to determine the lesion in retinoblastomas. In 16 of 40 retinoblastomas studied with a cDNA probe by Fung et al. (1987), a structural change in the RB gene was identifiable, including, in some cases, homozygous internal deletions with corresponding truncated transcripts. An osteosarcoma also had a homozygous internal deletion with a truncated transcript. Possible hotspots for deletion were identified within the RB genomic locus. Among those tumors with no identifiable structural change, there was either absence of an RB transcript or abnormal expression of the RB transcript.

Bookstein et al. (1988) identified at least 20 exons in genomic clones of the RB gene and provisionally numbered them. With a unique sequence probe from intron 1, they detected heterozygous deletions in genomic DNA from 3 retinoblastoma cell lines and genomic rearrangements in fibroblasts from 2 hereditary retinoblastoma patients, indicating that intron 1 includes a frequent site for mutations conferring predisposition to retinoblastoma. Demonstration of a DNA deletion of exons 2-6 from 1 RB allele, as well as the demonstration of other deletions, explains the origin of shortened RB mRNA transcripts.

The retinoblastoma candidate gene, 4.7R, does not show gross deletion or rearrangement in most retinoblastomas. Dunn et al. (1988) searched for more subtle mutations using the ribonuclease protection method for analysis of 4.7R mRNA from retinoblastomas. In the ribonuclease (RNase) protection assay, RNases A and T1 cleave single-stranded RNA at basepair mismatches in RNA:RNA or RNA:DNA hybrids. The test identifies only about 50% of single basepair mutations. Dunn et al. (1988) found that 5 of 11 RB tumors, which exhibited normal 4.7R DNA and normal-sized RNA transcripts, showed abnormal ribonuclease cleavage patterns. Three of the 5 mutations affected the same region of the mRNA, consistent with an effect on splicing involving an as yet unidentified 5-prime exon.

Canning and Dryja (1989) found deletions in the retinoblastoma gene in 12 of 49 tumors from patients with retinoblastoma or osteosarcoma. Mapping of the deletion breakpoints revealed coincidence of no 2 breakpoints. Thus they could not support the conclusion of others regarding the existence of a 'hotspot' for deletion breakpoints in this gene. In 4 tumors, they sequenced 200 basepairs surrounding each deletion breakpoint. Three deletions had termini within pairs of short, direct repeats ranging in size from 4 to 7 basepairs. They interpreted this as indicating that 'slipped mispairing' may predominate in the generation of deletions at the RB locus. Short, direct repeats were incriminated in 15 of 20 deletions in the beta-globin locus. In other loci, Alu sequences at breakpoints are frequently found, e.g., in the LDL receptor gene (606945), the ADA gene (608958) and the beta-hexosaminidase gene (606869). Efstratiadis et al. (1980) argued that the repeats found at the breakpoints of deletions of the beta-globin locus are not long enough to mediate unequal crossing over via homologous recombination, thus requiring the alternative model, 'slipped mispairing' during DNA replication.

Dunn et al. (1989) extended the characterization of mutations in RB1 using RNase protection of RB1 transcripts to locate probable mutations, followed by polymerase chain reaction (PCR) to amplify and sequence the mutant allele. Mutations were identified in 15 of 21 RB tumors; in 8 tumors, the precise error in nucleotide sequence was characterized. Each of 4 germline mutations involved a small deletion or duplication while 3 somatic mutations were point mutations leading to splice alterations and loss of an exon from the mature RB1 mRNA. By PCR techniques, Yandell et al. (1989) demonstrated single nucleotide changes in tumors from 7 patients with simplex retinoblastoma (with no family history of the disease). In 4 patients, the mutation involved only the tumor cells, and in 3 it involved normal somatic cells as well as tumor cells but was not found in either parent. Thus, these 3 represent new germinal mutations. All 3 were C-to-T transitions in the coding strand in the retinoblastoma gene. Two of the 3 occurred at CpG pairs. Since new germinal mutations of the retinoblastoma gene are more likely to occur on the paternal allele (Dryja et al., 1989), the overrepresentation of C-to-T transitions is probably the result of processes that occur during male gametogenesis. Direct analysis of disease-causing mutations are particularly applicable to diseases characterized by a high proportion of new mutations. This approach had been used successfully with Lesch-Nyhan syndrome (300322) and Duchenne muscular dystrophy (310200), which are associated with high frequencies of new point mutations and new deletions, respectively. Analysis of genomic DNA obviates the difficulty of having an RNA transcript or protein gene product for analysis and allows the detection of mutations that may occur at splice sites or other sequences that are excluded from the RNA transcript. Of the 10 mutations (7 retinoblastoma tumors plus 3 others), 5 occurred in the exon 21-24 region which represents only 15% of the coding sequence of the retinoblastoma gene. It has been found that aberrant proteins from which amino acids coded by these exons have been deleted lack binding activity for the adenovirus E1A dominant transforming protein.

Blanquet et al. (1995) performed a mutation survey of the RB1 gene in 232 patients with hereditary or nonhereditary retinoblastoma. They systematically explored all 27 exons and flanking sequences, as well as the promoter. All types of point mutations were represented and found to be unequally distributed along the RB1 gene sequence. In the population studied, exons 3, 8, 18, and 19 were preferentially altered. Correlations between the phenotypic expression and molecular alterations were difficult to discern, at least for the missense and in-frame mutations. However, Blanquet et al. (1995) observed that some patients carrying mutations in exon 19 also developed nonocular tumors such as pineoblastoma, fibrosarcoma, or osteosarcoma. Germline mutations were detected in 36% of 25 familial cases, in 20.5% of bilateral sporadic or unilateral multifocal cases, and in 7.1% of unilateral sporadic cases. Because of the low level of detection of germline mutations in hereditary cases, they reasoned that other mechanisms of inactivation of RB1 must be involved.

Lohmann et al. (1996) studied 119 patients with hereditary retinoblastoma for germline RB1 mutations. Southern blot hybridization and PCR fragment-length analysis revealed mutations in 48 patients. In the remaining 71 patients, they detected mutations in 51 (72%) by applying heteroduplex analysis, nonisotopic SSCP, and direct sequencing. Rare sequence variants were also found in 4 patients. No region of the RB1 gene was preferentially involved in single base substitutions. Recurrent transitions were observed at most of the 14 CGA codons within the RB1 gene. No mutation was observed in exons 25-27, although this region contains 2 CGA codons. This suggested to the authors that mutations within the 3-prime terminal region of the RB1 gene may not be oncogenic. For the entire series of 119 patients, mutations were identified in 99 (83%). The spectrum comprised 15% large deletions, 26% small length alterations, and 42% base substitutions.

Lohmann et al. (1997) investigated the frequency and nature of constitutional RB1-gene mutations in patients with isolated unilateral retinoblastoma. A total of 45 mutations were detected in tumors from 36 patients. Thirty-nine of the mutations--including 34 small mutations, 2 large structural alterations, and hypermethylation in 3 tumors--were not detected in the corresponding peripheral blood DNA. In 6 (17%) of the 36 patients, a mutation was detected in constitutional DNA, and 1 of these mutations was known to be associated with reduced expressivity. The presence of a constitutional mutation was not associated with an early age at treatment. In 1 patient, somatic mosaicism was demonstrated by molecular analysis of DNA and RNA from peripheral blood. In 2 patients without a detectable mutation in peripheral blood, mosaicism was suggested because 1 of the patients showed multifocal tumors and the other later developed bilateral retinoblastoma.

Hagstrom and Dryja (1999) investigated loss of heterozygosity in a set of matched retinoblastoma and leukocyte DNA samples from 158 patients informative for DNA polymorphisms. Loss of heterozygosity at the RB locus was observed in 101 cases, comprising 7 cases with a somatic deletion causing hemizygosity and 94 with homozygosity (isodisomy). Homozygosity was approximately equally frequent in tumors from male and female patients, among patients with a germline versus somatic initial mutation, and among patients in whom the initial mutation occurred on the maternal versus paternal allele. A set of 75 tumors exhibiting homozygosity was investigated with markers distributed in the interval 13cen-q14. Forty-one tumors developed homozygosity at all informative marker loci, suggesting that homozygosity occurred through chromosomal nondisjunction. The remaining cases exhibited mitotic recombination. There was no statistically significant bias in apparent nondisjunction versus mitotic recombination among male versus female patients or among patients with germline versus somatic initial mutations. Hagstrom and Dryja (1999) compared the positions of somatic recombination events in the analyzed interval with a previously reported meiotic recombination map. Although mitotic crossovers occurred throughout the assayed interval, they were more likely to occur proximally than a comparable number of meiotic crossovers. They observed 4 triple-crossover cases, suggesting negative interference for mitotic recombination, the opposite of what is usually observed for meiotic recombination.

Bremner et al. (1997) studied a 4-kb deletion spanning exons 24 and 25 of the RB1 gene and associated with low penetrance, since only 39% of eyes at risk in this family developed retinoblastoma. This was said to have been the largest deletion observed in a low penetrance family. Unlike the usual RB mutations, which cause retinoblastoma in 95% of at-risk eyes and yield no detectable protein, the del24-25 allele transcribed a message splicing exon 23 to exon 26, resulting in a detectable protein that lacks 58 amino acids from the C-terminal domain, proving that this domain is essential for suppression of retinoblastoma. Two functions were partially impaired by del24-25, namely, nuclear localization and repression of E2F, consistent with the idea that low penetrance mutations generate 'weak alleles' by reducing but not eliminating essential activities. However, del24-25 ablated interaction of the RB protein with MDM2.

Sampieri et al. (2006) identified mutations in the RB1 gene in 13 (37%) of 35 unrelated Italian patients with retinoblastoma. Mutations were identified in 6 of 9 familial cases and 7 of 26 sporadic cases. Eleven of the 13 mutations were novel.

Gratias et al. (2007) analyzed 22 short tandem repeat loci on chromosome 16q in 58 patients with known RB1 mutations and detected loss of heterozygosity in 18 (31%) of 58 tumors, with a 5.7-Mb minimum deleted region in the telomeric part of 16q24 with a centromeric boundary at Mb 82.7 in exon 10 of the CDH13 gene (601364). There was no loss of expression of CDH13 or 2 other candidate suppressors at 16q24, CBFA2T3 (603870) and WFDC1 (605322), in retina compared to retinoblastoma tissue. Gratias et al. (2007) noted that almost all retinoblastomas with chromosome 16q24 loss showed diffuse intraocular seeding, suggesting that genetic alterations in the minimal deleted region are associated with impaired cell-to-cell adhesion.

Zhang et al. (2012) showed that the retinoblastoma genome is stable, but that multiple cancer pathways can be epigenetically deregulated. To identify the mutations that cooperate with RB1 loss in retinoblastoma, Zhang et al. (2012) performed whole-genome sequencing of retinoblastomas. The overall mutational rate was very low; RB1 was the only known cancer gene mutated. Zhang et al. (2012) then evaluated the role of RB1 in genome stability and considered nongenetic mechanisms of cancer pathway deregulation. For example, the protooncogene SYK (600085) is upregulated in retinoblastoma and is required for tumor cell survival. Targeting SYK with a small molecule inhibitor induced retinoblastoma tumor cell death in vitro and in vivo. Thus, Zhang et al. (2012) concluded that retinoblastomas may develop quickly as a result of the epigenetic deregulation of key cancer pathways as a direct or indirect result of RB1 loss.

Dommering et al. (2014) studied 529 patients with retinoblastoma from 433 unrelated families in the Dutch National Retinoblastoma Register. Mutations in RB1 were detected in 92% of bilateral and/or familial patients and in 10% of nonfamilial unilateral cases. Overall, germline mutations were detected in 187 (43%) of 433 RB families; the mutations were 37% nonsense, 21% splicing, 20% frameshift, 9% large indel, 5% missense, and 1% promoter, with 7% chromosomal deletions. Ten percent of patients were mosaic for the RB1 mutation. Six 3-generation families with incompletely penetrant RB1 mutations were identified. Dommering et al. (2014) noted that the frequency of type of RB1 mutation in this unbiased national cohort was the same as the mutation spectrum described worldwide.

Lohmann et al. (1996) found no correlation between the location of frameshift or nonsense mutations and phenotypic features of retinoblastoma, including age at diagnosis, the number of tumor foci, and the manifestations of nonocular tumors.

Taylor et al. (2007) studied 165 RB1 mutation carriers from 50 unrelated pedigrees with a family history of retinoblastoma. Twenty-five (50%) families had nonsense or frameshift mutations and showed high or complete disease penetrance. Two families with nonsense mutations in exon 1 showed slightly reduced penetrance, suggesting transcriptional modifiers or resistance to nonsense-mediated decay. Aberrant splicing mutations were identified in 13 (26%) families and associated with incomplete penetrance and variable expressivity. Eight (16%) families had large gene rearrangements associated with high penetrance. Promoter and missense mutations were associated with low penetrance. Six fully penetrant mutation carriers developed a secondary sarcoma at a median age of 15 years, regardless of mutation type. Retinomas were observed in patients with truncating mutations or large gene rearrangements.

Murphree and Benedict (1984) suggested that the RB1 gene is a model for a class of recessive human cancer genes that have a 'suppressor' or 'regulatory' function. The primary mechanism in the development of retinoblastoma is loss or inactivation of both alleles of this gene. This mechanism contrasts with that of putative human oncogenes which are thought to induce cancers following activation or alteration. The high incidence of second primary tumors among patients who inherit one retinoblastoma gene suggests that this cancer gene plays a key role in the etiology of several other primary malignancies. In some retinoblastomas, extra nonrandom copies of specific chromosomal regions occur, suggesting that an 'expressor' gene (possibly an oncogene) may be involved.

On morphologic criteria, retinoblastoma had long been considered a malignancy of the photoreceptor cell lineage. Bogenmann et al. (1988) showed that this tumor grown in vitro expresses highly specific photoreceptor cell genes: transducin alpha subunit, which is specific to the cone cell, and transcripts for red or green cone cell photopigment. Bogenmann et al. (1988) found no marker genes specific to rod cells.

Weichselbaum et al. (1988) presented evidence suggesting that in soft tissue sarcomas as well as in osteosarcomas, transcriptional inactivation or posttranscriptional downregulation of the RB gene may be important in etiology. They used hybridization with a cDNA probe for RB mRNA to analyze 3 soft tissue sarcomas and 4 osteosarcomas in patients without retinoblastoma. Most of the tumors studied did not express detectable levels of RB mRNA, whereas normal cells and epithelial tumor cells did. One osteosarcoma expressed a 2.4-kb transcript in addition to a normal 4.7-kb species.

Kimchi et al. (1988) demonstrated that retinoblastoma cells lack receptors for TGF-beta. Whereas in control cells, in this case retinal cells from fetuses, treatment with TGF-beta inhibited cell division, treatment of retinoblastoma cells with the same substance had no effect on DNA synthesis, cell division, or cell morphology.

Huang et al. (1988) demonstrated suppression of the neoplastic phenotype in cultured retinoblastoma or osteosarcoma cells by introduction of a cloned RB gene via retrovirus-mediated gene transfer. This is compelling evidence that the inactivation of the endogenous RB gene in these cells was a crucial step in tumorigenesis.

Benedict et al. (1990) suggested that retinoblastoma may be unique in that functional loss of both RB alleles suffices for malignant development. In osteosarcoma, additional change of the p53 locus may be necessary, and in small-cell lung carcinoma, changes in both p53 and the short arm of chromosome 3 may be necessary.

The results of many experiments indicate that the normal function of the RB gene product is a negative regulator of cellular proliferation which is achieved by sequestering a variety of nuclear proteins involved in cellular growth. DNA tumor virus oncoproteins transform cells, at least in part, by releasing these cellular proteins from inactive complex. Two of the cellular proteins that are bound, E2F (189971) and the MYC protein (NMYC; 164840), are transcription factors. Bandara et al. (1991) reported a naturally occurring loss-of-function RB allele encoding a protein that fails to complex with DRTF1 (189902). They also showed that cyclin A (CCNA; 123835) also complexes with DRTF1 and facilitates the efficient assembly of the RB protein into the complex.

Goodrich et al. (1991) found that injection into cells of either full-length or a truncated form of the RB protein containing the T antigen-binding region inhibited progression from G1 into the S phase. Coinjection of anti-RB antibodies antagonized this effect. The results indicated that RB regulates cell proliferation by restricting cell cycle progression at a specific point in G1 and established a biologic assay for RB activity. Coinjection of RB with a T-antigen peptide or injection into cells expressing T antigen was accompanied by no inhibition of progression into S phase. This was interpreted as indicating that the transforming proteins of some DNA tumor viruses, including SV40 T antigen and adenovirus E1A, may promote cell growth, at least in part, by binding and inactivating RB.

Sakai et al. (1991) studied the methylation pattern at the 5-prime end of the RB gene, including its promoter region and exon 1, in DNA purified from 56 primary retinoblastomas. The purpose was to investigate the possibility that hypermethylation of the promoter region might be responsible for loss of function of the gene giving rise to tumor. In 5 of the tumors, they found evidence for hypermethylation; all tumors were from unilateral, 'simplex' patients. No methylation abnormality was detected in DNA purified from the leukocytes of these patients. In 1 of the tumors, the hypermethylation was confined to 1 allele. No mutations to account for the allele-specific hypermethylation was found in a 1,306-bp sequence including the hypermethylated region. If hypermethylation indeed is the cause of the tumor, therapeutic agents that interfere with methylation of DNA might be effective.

A significant proportion of disease-causing mutations in the RB1 gene result in the premature termination of protein synthesis, and most of these mutations occur as C-to-T transitions at CpG dinucleotides. Mancini et al. (1997) presented evidence confirming the view that such recurring CpG mutations are the result of the deamination of 5-methylcytosine within these CpGs. They used the sodium-bisulfite conversion method to detect cytosine methylation in representative exons of RB1. They analyzed DNA from a variety of tissues and specifically targeted CGA codons in RB1, where recurrent termination mutations had been reported. They found that DNA methylation within RB1 exons 8, 14, 25, and 27 appeared to be restricted to CpGs, including 6 CGA codons. Other codons containing methylated cytosines had not been reported to be mutated. Therefore, disease-causing mutations at CpGs in RB1 appear to be determined by several factors, including the constitutive presence of DNA methylation at cytosines within CpGs, the specific codon within which the methylated cytosine is located, and the particular region of the gene within which that codon resides.

Gallie (1997) commented that the 'biology of the protein product of RB1 has ignited the field of cell cycle regulation, so that much more literature now refers to the retinoblastoma protein than to the disease.' The main point of her editorial, however, was to emphasize the need for a new kind of partnership between research, innovative entrepreneurial activities, and clinical management to get the most out of the investment that has been made in identifying human disease genes such as RB1.

Nevins (2001) reviewed the role of the Rb/E2F pathway in cell proliferation, cell fate determination, and cancer.

Van Aken et al. (2002) studied the cadherin-catenin complex in retinoblastoma and normal retina tissues. In both cases, they found that N-cadherin (114020) was associated with alpha- and beta-catenin (116805; 116806) but not with E- or P-cadherin. Moreover, retinoblastoma cells, in contrast with normal retina, expressed an N-cadherin/catenin complex that was irregularly distributed and weakly linked to the cytoskeleton. In retinoblastoma, this complex acted as an invasion promoter.

Mohan et al. (2007) found an increase in N-cadherin and alpha-catenin expression and loss of E-cadherin and CD9 (143030) expression in invasive retinoblastoma and suggested that these antigens may contribute to RB tumor invasiveness.

Tucker and Friedman (2002) reviewed the mechanisms through which hereditary tumors may arise and concluded that loss of both alleles of a particular tumor suppressor gene is a frequent, but not invariably necessary or sufficient, event. They presented 4 models, with possible examples, of how various tumors arise in patients with inherited tumor predisposition syndromes such as hereditary retinoblastoma, tuberous sclerosis complex (see 191100), or neurofibromatosis (see 162200). They noted that even tumors of 1 particular type may develop by more than 1 mechanism. The multistep model is similar to that proposed by Vogelstein and Kinzler (1993) for colorectal cancer. In the recruitment model, a second hit occurs at the disease locus, resulting in complete loss of function. These cells recruit other surrounding stromal cells that still retain 1 functional copy of the disease locus into the region, which promotes tumor progression. In the LOH model, all cells possess 1 mutation in the inherited disease-causing gene. One cell requires another mutation at an additional locus; this mutation could allow for tumor formation or a second somatic 'hit' at the disease locus may result in a tumor. In the haploinsufficiency model, cells having only 1 functional copy of the disease gene are more sensitive to proliferating stimuli, allowing a tumor to form.

Xu et al. (2014) showed that postmitotic human cone precursors are uniquely sensitive to RB depletion. RB knockdown induced cone precursor proliferation in prospectively isolated populations and in intact retina. Proliferation followed the induction of E2F-regulated genes, and depended on factors having strong expression in maturing cone precursors and crucial roles in retinoblastoma cell proliferation, including MYCN (164840) and MDM2 (164785). Proliferation of RB-depleted cones and retinoblastoma cells also depended on the RB-related protein p107 (RBL1; 116957), SKP2 (601436), and a p27 (CDKN1B; 600778) downregulation associated with cone precursor maturation. Moreover, RB-depleted cone precursors formed tumors in orthotopic xenografts with histologic features and protein expression typical of human retinoblastoma. Xu et al. (2014) concluded that these findings provide a compelling molecular rationale for a cone precursor origin of retinoblastoma.

Windle et al. (1990) created transgenic mice by microinjecting fertilized ova with a chimeric gene containing the protein coding region of the SV40 T antigen (Tag) driven by the promoter of the luteinizing hormone beta-subunit gene. One of the male founders developed bilateral retinoblastomas at about age 5 months. The phenotype was heritable with complete penetrance in transgenic offspring in whom the tumors were first observed at about 2 months. Windle et al. (1990) demonstrated specific association between p105(Rb) and T antigen in mouse retinoblastoma tumor cells. Thus, evidence is provided for oncogenesis due to the ocular-specific expression of an Rb-binding oncoprotein that can functionally inactivate the Rb protein.

Marino et al. (2000) generated a mouse model for medulloblastoma (155255) by Cre-LoxP-mediated inactivation of Rb and p53 tumor suppressor genes in the cerebellar external granular layer (EGL) cells. Recombination mediated by Gfap (137780) promoter-driven Cre was found both in astrocytes and in immature precursor cells of the EGL in the developing cerebellum. Gfap-Cre-mediated inactivation of Rb in a p53-null background produced mice that developed highly aggressive embryonal tumors of the cerebellum with typical features of medulloblastoma. These tumors were identified as early as 7 weeks of age on the outer surface of the molecular layer, corresponding to the location of the EGL cells during development. Marino et al. (2000) concluded that loss of function of Rb is essential for medulloblastoma development in the mouse and stated that their results strongly support the hypothesis that medulloblastomas arise from multipotent precursor cells located in the EGL.

Lee et al. (1992) generated mice deficient for Rb by targeted disruption. The mice were nonviable and showed defects in neurogenesis and hematopoiesis (Lee et al., 1992; Jacks et al., 1992; Clarke et al., 1992). Inactivation of Rb in mice results in unscheduled cell proliferation, apoptosis, and widespread developmental defects, leading to embryonic death by day 14.5. Wu et al. (2003) showed that loss of Rb leads to excessive proliferation of trophoblast cells and a severe disruption of the normal labyrinth architecture in the placenta. This is accompanied by a decrease in vascularization and a reduction in placental transport function. Wu et al. (2003) used complementary techniques--tetraploid aggregation and conditional knockout strategies--to demonstrated that Rb-deficient embryos supplied with a wildtype placenta can be carried to term, but die soon after birth. Most of the neurologic and erythroid abnormalities thought to be responsible for the embryonic lethality of Rb-null animals were virtually absent in rescued Rb-null pups. Wu et al. (2003) concluded that these findings identified and defined a key function of RB in extraembryonic cell lineages that is required for embryonic development and viability, and provided a mechanism for the cell-autonomous versus non-cell-autonomous roles of RB in development.

In a transgenic retinoblastoma mouse model (LH-beta-Tag mouse), Dawson et al. (2003) found that low doses of 1-alpha-hydroxyvitamin D2 (0.1-0.3 micrograms 5 times weekly for 5 weeks) inhibited retinoblastoma with no significant increase in mortality.

To model sporadic cancers associated with inactivation of the RB tumor suppressor gene in humans, Sage et al. (2003) produced a conditional allele of the mouse Rb gene. Sage et al. (2003) demonstrated that acute loss of Rb in primary quiescent cells is sufficient for cell cycle entry and has phenotypic consequences different from germline loss of Rb function. This difference is explained in part by functional compensation by the Rb-related gene p107 (116957). Sage et al. (2003) also showed that acute loss of Rb in senescent cells leads to reversal of the cellular senescence program.

Adipocyte precursor cells give rise to 2 major cell populations with different physiologic roles: white and brown adipocytes. Hansen et al. (2004) demonstrated that the retinoblastoma protein regulates white versus brown adipocyte differentiation. Functional inactivation of the retinoblastoma protein in wildtype mouse embryo fibroblasts and white preadipocytes by expression of simian virus-40 large T antigen resulted in the expression of the brown fat-specific uncoupling protein-1 (UCP1; 113730) in the adipose state. Rb-null mouse embryo fibroblasts and stem cells, but not the corresponding wildtype cells, differentiated into adipocytes with a gene expression pattern and mitochondria content resembling brown adipose tissue. From these and other observations, Hansen et al. (2004) proposed that the retinoblastoma protein acts as a molecular switch determining white versus brown adipogenesis, suggesting a previously uncharacterized function of this key cell cycle regulator in adipocyte lineage commitment and differentiation.

By targeted disruption, Sage et al. (2006) deleted the Rb gene primarily in the inner ear. During early postnatal development, Rb -/- hair cells continued to divide and transduced mechanical stimuli. However, adult Rb -/- mice exhibited profound hearing loss due to progressive degeneration of the organ of Corti. Many Rb -/- vestibular hair cells survived and continued to divide in adult mice, and vestibular hair cells were functional.

Day et al. (2002) developed Rb -/- prostate epithelial (PrE) cells from Rb -/- mouse embryos. Rb -/- PrE cells showed serum independence in culture and immortality in vivo. Cell cycle analysis revealed elevated S-phase DNA content accompanied by increased expression of cyclin E1 and proliferating cell nuclear antigen (PCNA; 176740). Rb -/- PrE cultures also exhibited a diminished ability to growth arrest under high-density culture conditions.

MacPherson et al. (2003) generated mouse embryos with conditional Rb deletion in the central nervous system (CNS), peripheral nervous system (PNS), and lens, while maintaining normal erythropoiesis. In contrast to the massive CNS apoptosis in Rb-null embryos at embryonic day 13.5, conditional mutants did not have elevated apoptosis in the CNS, although there was significant apoptosis in the PNS and lens. Rb -/- cells in the CNS, PNS, and lens underwent inappropriate S-phase entry at embryonic day 13.5. By day 18.5, conditional mutants had increased brain size and weight and defects in skeletal muscle development. MacPherson et al. (2003) hypothesized that hypoxia is a necessary cofactor in the death of CNS neurons in the developing Rb mutant embryo.

Haigis et al. (2006) found that Rb was expressed in all epithelial cells of mouse colon, whereas p107 was expressed predominantly in the lower half of the crypt, and p130 was expressed in the upper portion of the crypt and in the epithelium lining the lumen. Similarly, undifferentiated cells in the mouse small intestinal crypt expressed Rb and p107, whereas differentiated cells in the villi expressed Rb and p130. Conditional deletion of Rb or p130 increased p107 levels, and Rb/p130 double mutants had even higher levels of p107. Although mutating any of these 3 genes singly had little or no effect, loss of Rb and p107 or p130 together produced chronic hyperplasia and dysplasia of the small intestinal and colonic epithelium. In Rb/p130 double mutants, this hyperplasia was associated with defects in terminal differentiation of specific cell types and was dependent on the increased proliferation seen in the epithelium of mutant animals.

In the transgenic retinoblastoma mouse model LH-beta-Tag, Jockovich et al. (2007) detected increased cell proliferation and angiogenesis in the retinal inner nuclear layer before morphologic neoplastic changes were evident. As tumor size increased, angiogenesis diminished concomitantly with the appearance of new vessels. Treatment with CA4P and anecortave acetate resulted in significant reductions in total vessel density. However, neither drug reduced the amount of alpha-smooth muscle actin-positive, mature vessels. Jockovich et al. (2007) concluded that these findings suggest a high potential value in targeting the process of angiogenesis in the treatment of children with retinoblastoma.

Using optical coherence tomography (OCT) imaging in the T-antigen retinoblastoma (TAg-RB) mouse model, Wenzel et al. (2015) characterized TAg-positive cells as early as 2 weeks, corresponding to the earliest stages at which tumors are histologically evident, and well before thay are evident with funduscopy. Wenzel et al. (2015) concluded that OCT is a noninvasive imaging modality for tracking early TAg-RB tumor growth in vivo.

Fearon (1997) reviewed more than 20 hereditary cancer syndromes that had been defined and attributed to specific germline mutations in inherited cancer genes. All but 4 of these behaved as autosomal dominant disorders. Retinoblastoma was one of the first to be defined at the molecular level.