Albinism, Ocular, Type I

A number sign (#) is used with this entry because ocular albinism type I is caused by mutation in the GPR143 gene (300808) on chromosome Xp22.

See also isolated X-linked congenital nystagmus-6 (NYS6; 300814), which is an allelic disorder.

Description

Ocular albinism type I (OA1) is the most common form of ocular albinism. Clinical presentation of OA1 in Caucasians is characterized by nystagmus, impaired visual acuity, iris hypopigmentation with translucency, albinotic fundus, macular hypoplasia, and normally pigmented skin and hair. Carrier females usually have punctate iris translucency and a mottled pattern of fundus pigmentation. In contrast to Caucasian patients, black or Japanese patients with OA1 often have brown irides with little or no translucency and varying degrees of fundus hypopigmentation, the so-called 'nonalbinotic fundus' (summary by Xiao and Zhang, 2009).

Clinical Features

In affected men the pupillary reflex is characteristic of albinism. The fundus is depigmented and the choroidal vessels stand out strikingly. Nystagmus, head nodding, and impaired vision also occur. Pigmentation is normal elsewhere than in the eye. In carrier females the fundus, especially in the periphery, shows a mosaic of pigmentation, as first recognized by Vogt (1942). Lyon (1962) pointed out that the fundus finding in heterozygous females supports her theory of X chromosome inactivation. Nystagmus is an associated feature. In fact, the ocular albinism has been commented on only obliquely or not at all in some reports of X-linked nystagmus in families that almost certainly had ocular albinism. The family studied by Waardenburg and Van den Bosch (1956) was earlier reported by Engelhard (1915) as a family with hereditary nystagmus. One family studied by Fialkow et al. (1967) had been reported by Lein et al. (1956) as sex-linked nystagmus.

Fundus drawings of heterozygous carriers were provided by Francois and Deweer (1953), and by others. (See frontispiece, McKusick, 1964.) Theoretically one should be able to count the number of pigmented spots and arrive at an estimate of the number of anlage cells present at the time of lyonization. Unfortunately, most of the available drawings are probably too crude to be relied on for this use. Furthermore, the drawings suggest appreciable variation in the number and size of pigmented areas, a finding to be expected from the considerations of the Lyon hypothesis. By electron microscopy, O'Donnell et al. (1976) showed that the skin as well as the eyes shows macromelanosomes in affected males and carrier females. Creel et al. (1978) demonstrated abnormal optic projections similar to those in total albinism. Hence, the abnormal decussation is a consequence of the lack of ocular pigment and not specific for any particular defect.

Schnur et al. (1994) studied 119 individuals from 11 families with OA1 with respect to their clinical phenotypes and their linkage genotypes. One of the families was a 4-generation Australian family in which 2 affected males and an obligatory carrier lacked the cutaneous melanin macroglobules (MMGs) considered typical of OA1; ocular features, on the other hand, were identical to those of Nettleship-Falls OA1. Furthermore, in this family, there was no evidence of linkage heterogeneity when compared with 6 families with biopsy-proven MMGs in at least 1 affected male.

Rosenberg and Schwartz (1998) determined phenotypic characteristics of 25 male patients from families in Denmark with identified mutations in the GPR143 gene. All patients had congenital nystagmus, and all but 1 had significant iris translucency. Only 1 patient had high myopia. Most of the remaining 24 patients (48 eyes) showed various degrees of hypermetropia.

Using MRI, Schmitz et al. (2003) found that the size and configuration of the optic chiasm in humans with albinism are distinctly different from the chiasms of normal control subjects. These chiasmal changes reflect the atypical crossing of the optic fibers, irrespective of the causative gene mutation. Eight patients had tyrosinase gene-related OCA1 (203100), 4 patients had P gene (611409)-related OCA2 (203200), and 1 had ocular albinism; the albinism-causing mutation had not been identified in 4 other patients.

Clinical Variability

Preising et al. (2001) reported a 3-generation family in which 3 affected males had variable features of ocular albinism due to a splice site mutation in the GPR143 gene (300808.0010). The male proband was diagnosed with OA1 at age 3 months with typical clinical features, including congenital nystagmus, iris translucency, macular hypoplasia, fundus hypopigmentation, and normal pigmentation of skin and hair. Examination at age 4 years showed increased pigmentation of the iris and fundus and improved visual acuity. A 51-year-old maternal uncle also had congenital nystagmus, clear macular hypoplasia and stromal focal hypopigmentation of the iris, but no iris translucency or fundus hypopigmentation. Macromelanosomes were present on skin biopsy. A 79-year-old maternal relative had congenital nystagmus and high myopia with macular change, but no iris translucency. Two carrier females had mosaic pattern of hypopigmented retinal epithelium, consistent with a carrier status of ocular albinism. Preising et al. (2001) suggested that this mutation results in a hypomorphic allele that causes impaired membrane fusion of melanosomes and the plasma membrane. They proposed a model of OA1 in this family that allowed increase of pigmentation with age. Thus, postnatal normalization of the extracellular dopamine levels due to delayed distribution and membrane budding or fusion of melanosomes in melanocytes could result in increasing pigmentation and a seemingly variable phenotype.

Xiao and Zhang (2009) studied a Chinese patient with ocular albinism, who had nystagmus since early childhood, without photophobia or night blindness. He was diagnosed with high myopia and amblyopia at 3 years of age. Ocular examination at age 8 years revealed high myopia and bilateral pendular nystagmus, and there was hyperpigmentation with tiny pigmentary nodules in the pupillary portion of the irides. The peripheral iris was brown without translucency. Fundus changes resembled those seen in high myopia; however, mild variegated pigmentary changes were observed in the midperiphery. Macular hypoplasia was confirmed by optical coherence tomography. The patient's mother had normal visual acuity without nystagmus or photophobia. She had iris hyperpigmentation in the pupillary portion, like her son, but had mild partial hypopigmentation in her peripheral iris. Mild hyperpigmentation was notable in her posterior fundus, and obvious mottled pigmentary deposits were present in the midperipheral retina. The patient's father had a normal iris and fundus.

Cytogenetics

Meindl et al. (1993) described the clinical features of a patient with a large Xp22.3 deletion who had ocular albinism in addition to short stature, chondrodysplasia punctata, mental retardation, ichthyosis, and Kallmann syndrome (KAL; 308700)--a total of 6 X-linked disorders. The deletion involved at least 10 Mb of DNA. Both the mother and the sister of the patient were carriers of the deletion that showed a number of traits seen in Turner syndrome. The diagnosis of ocular albinism was confirmed in the patient and his mother, who showed iris translucency, patches and streaks of hypopigmentation in the fundus, and macromelanosomes in epidermal melanocytes.

Mapping

Fialkow et al. (1967) estimated that the recombination fraction for ocular albinism (OA) and the Xg blood group (314700) is about 0.17. This was confirmed by Pearce et al. (1968) in an English kindred. From a Newfoundland kindred, Pearce et al. (1971) presented data that reduce the estimate of the interval between Xg and ocular albinism from 17 to 15. Confirmation of the linkage with Xg and demonstration of stripe-like areas of retinal hypopigmentation in carriers were also provided. Kidd et al. (1985) found RFLP markers tightly linked to OA.

Schnur et al. (1989) described the combination of ocular albinism and X-linked ichthyosis in 3 cytogenetically normal half brothers. A fourth half brother died at age 8 months of complications related to perinatal hypoxia and prematurity. Each of the 4 boys had a different father. No additional clinical features that have been described with other deletion syndromes in this area were present. The STS locus was entirely deleted on Southern blots in the affected males, but the MIC2X (313470) and several anonymous DNA loci were not deleted. The mother had patchy fundal hypopigmentation consistent with random X inactivation in an OA1 carrier. Flow cytometry analysis of cultured lymphoblasts detected a deletion of about 3.5 million bp or about 2% of the X chromosome. The observations with markers suggested that OA1 is located in the Xp22.3 region.

Bergen et al. (1991) presented information from a multipoint linkage analysis involving several DNA markers in the distal portion of Xp. Bergen et al. (1992) reported carrier detection in this disorder by use of markers that flank the OA1 locus. By multipoint linkage analysis in 16 British families, Charles et al. (1992) placed the OA1 locus between DXS85 proximally and DXS237 distally. They found that OA1 lies close to DXS143, but in the absence of recombinants the order of the loci could not be determined.

From genetic linkage studies in the large Newfoundland family reported previously by Pearce et al. (1971), Charles et al. (1993) found evidence that the order is Xpter--XG--DXS237--DXS143--OA1--DXS85. Bergen et al. (1993) found a similar order: Xpter--STS--DXS237--KAL--(OA1, DXS143)--DXS85--DXS16--Xcen.

By comparative deletion mapping, Meindl et al. (1993) located the OA1 gene proximal to DXS143 and distal to DXS85. Using a dinucleotide repeat polymorphism at the Kallmann locus to study 17 OA1 families, Zhang et al. (1993) found close linkage between KAL and OA1 with a maximum lod score of 30.14 at a recombination fraction of 0.06. There was looser linkage to the Xg blood group. Both KAL and XG are distal to OA1. By deletion analysis, Bouloux et al. (1993) concluded that the OA1 locus is located proximal to the STS locus (300747).

By combined multipoint analysis (LINKMAP) in 11 families with OA1 and analysis of individual recombination events, Schnur et al. (1994) confirmed that the major locus for OA1 resides within the DXS85-DXS143 interval.

Bassi et al. (1995) mapped the OA1 gene (GPR143) to chromosome Xp22.3-p22.2, approximately 20 kb on the telomeric side of the APXL gene (SHROOM2; 300103). APXL spans 80 of the 110 kb of the OA1 critical region.

Molecular Genetics

Bassi et al. (1995) identified 5 patients with OA1 who were carrying mutations within the GPR143 gene. Five intragenic deletions and a 2-bp insertion resulting in a premature stop codon (300808.0001) were identified by DNA analysis of patients with OA1. Some of these deletions were not overlapping, making it highly unlikely that the mutation involved in OA1 is located in an intron of the gene. Fine molecular characterization of the gene in one of these patients demonstrated that the deletion removed part of a coding exon. The APXL gene was completely deleted in 1 patient with isolated OA1. However, an extensive search for point mutations was performed in the 4,848-bp coding region of APXL from 57 patients and no functionally relevant mutation was identified.

Schiaffino et al. (1995) screened the entire OA1 coding region and 5-prime and 3-prime sequences for mutations and detected mutations in only one-third (21 of 60) of their patients with OA, including 2 frameshifts (e.g., 300808.0002) and a splice site mutation leading to truncated OA1 proteins, a deletion of a threonine codon at position 290, and 4 missense mutations (e.g., 300808.0008), 2 of which involve amino acids located within putative transmembrane domains.

Schnur et al. (1998) reported results of deletion and mutation screening of the full-length OA1 gene in 29 unrelated North American and Australian OA probands, including 5 with additional, nonocular phenotypic abnormalities (Schnur et al., 1994). They detected 13 intragenic gene deletions, including 3 of exon 1, 2 of exon 2, 2 of exon 4, and 6 others, which span exons 2 to 8. They also identified 8 novel missense mutations, which clustered within exons 1, 2, 3, and 6 in conserved and/or putative transmembrane domains of the protein. There was also a splice acceptor site mutation, a nonsense mutation, a single base deletion, and a previously reported 17-bp exon 1 deletion. All patients with nonocular phenotypic abnormalities had detectable mutations. All told, 26 (approximately 90%) of 29 probands had detectable alterations in the OA1 gene, thus confirming that OA1 is the major locus for X-linked OA.

In Denmark, Rosenberg and Schwartz (1998) performed a retrospective survey of 112 patients with ocular albinism identified in a national register, including 60 male patients with proven or presumed X-linked ocular albinism. Based on the birth year cohorts 1960 to 1989, a point prevalence for OA1 at birth of 1 in 60,000 live born was calculated. They identified 14 OA1 families in the Danish population and obtained DNA from affected persons in 9 families. Mutation analysis demonstrated 7 presumed pathogenic mutations in the 9 families: 5 single nucleotide substitutions predicting a change of conserved amino acids, including G35D (300808.0008) and W133R (300808.0006), when compared with the mouse OA1 homolog, 1 deletion leading to the skipping of exon 2, and 1 example of a single nucleotide substitution expected to affect the 5-prime splice site of intron 2 (300808.0007). Subsequent genealogic investigations in the 3 families harboring the same mutation, W133R, disclosed that 2 of the 3 belonged to the same family. Clinical examination failed to identify any phenotype-genotype pattern except for the finding of a milder phenotype lacking iris translucency in the patient with the 5-prime splice site mutation of intron 2.

Oetting (2002) found that a total of 25 missense, 2 nonsense, 9 frameshift, and 5 splicing mutations in the OA1 gene had been reported in association with type I ocular albinism. There were also reports of several deletions of some or all exons of the OA1 gene with deletions of exon 2 resulting from unequal crossing-over, due to flanking Alu repeats. Oetting (2002) referred to an albinism database website.

In a Chinese patient with ocular albinism and his carrier mother, who both demonstrated an unusual phenotype of iris hyperpigmentation without translucency, with apparent mosaic pigmentation of the fundus. Xiao and Zhang (2009) identified an intragenic deletion in the GPR143 gene (300808.0013).

Pathogenesis

D'Addio et al. (2000) characterized 19 independent missense mutations with respect to processing and subcellular distribution on expression in COS-7 cells. Eleven of the 19 OA1 mutants (approximately 60%) were retained in the endoplasmic reticulum, showing defective intracellular transport and glycosylation, consistent with protein misfolding. The remaining 8 OA1 mutants (approximately 40%) displayed sorting and processing behaviors indistinguishable from those of the wildtype protein. Most of the latter mutations clustered within the second and third cytosolic loops, 2 regions that in canonical GPCRs are known to be critical for their downstream signaling, including G protein coupling and effector activation.

Population Genetics

Bassi et al. (2001) found a rather striking difference in the frequency of large deletions in the OA1 gene as the cause of ocular albinism type 1 in patients from Europe and North America: large deletions accounted for only 8% (3 of 36) of mutations identified in European OA1 patients; large deletions were found in 57% (8 of 14) of North American OA1 patients. The explanation for this distribution was unclear. The authors stated that their findings have major relevance for the molecular diagnosis of OA1 and need to be considered in any mutation testing program for this disorder.

Animal Model

Incerti et al. (2000) generated and characterized Oa1-deficient mice by gene targeting. The knockout males were viable, fertile, and phenotypically indistinguishable from the wildtype littermates. Ophthalmologic examination showed hypopigmentation of the ocular fundus in mutant animals compared with wildtype. Analysis of the retinofugal pathway revealed a reduction in the size of the uncrossed pathway, demonstrating a misrouting of the optic fibers at the chiasm, as observed in OA1 patients. Microscopic examination of the RPE showed the presence of giant melanosomes comparable with those described in OA1 patients. Ultrastructural analysis of the RPE cells suggested that the giant melanosomes may form by abnormal growth of single melanosomes rather than by the fusion of several organelles.

Palmisano et al. (2008) found reduced melanosome number and abnormal melanosome distribution toward the apical pole of Oa1 -/- RPE at embryonic stages that preceded the formation of macromelanosomes. In cultured -/- skin melanocytes, melanosomes were depleted from the perinuclear area and accumulated toward the cell periphery. In Oa1 -/- melanocytes, melanosomes interacted normally with the microtubule cytoskeleton and recruited factors required for actin-mediated melanosome capture; however, Oa1 -/- melanosomes appeared unable to release from the peripheral actin filaments. Palmisano et al. (2008) concluded that OA1 plays a regulatory role in distributing melanosomes between microtubule- and actin-based cytoskeletal elements.