Opitz Gbbb Syndrome, Type I

A number sign (#) is used with this entry because of evidence that the X-linked form of Opitz GBBB syndrome (GBBB1) is caused by mutation in the MID1 gene (300552) on Xp22.

Description

The Opitz GBBB syndrome is a congenital midline malformation syndrome characterized by hypertelorism, hypospadias, cleft lip/palate, laryngotracheoesophageal abnormalities, imperforate anus, developmental delay, and cardiac defects (So et al., 2005).

This disorder was first reported as 2 separate entities, BBB syndrome and G syndrome; subsequent reports of families in which the BBB and G syndromes segregated within a single kindred suggested that they represent a single entity.

Genetic Heterogeneity

See also GBBB2 (145410), caused by mutation in the SPECC1L gene (614140) on chromosome 22q11.

Clinical Features

Robin et al. (1996) compared the phenotypic features of the X-linked and autosomal (145410) forms of the Opitz syndrome. They found that anteverted nares and posterior pharyngeal cleft were seen only in the X-linked form. However, all other manifestations of the syndrome, such as hypertelorism, swallowing difficulties, hypospadias, and developmental delay, were seen in both forms. For a discussion of clinical features in early reports of this syndrome, see 145410.

Monozygotic twinning, which is unusually frequent in these families, may be a manifestation of the defect; unusually severe cases with early lethality occur as twins (Opitz, 1996).

De Silva et al. (1998) reported a family with X-linked Opitz syndrome in which the diagnosis was established initially in the 64-year-old grandmother after she developed recurrent aspiration pneumonia due to esophageal reflux and pharyngeal dysmotility.

Jacobson et al. (1998) reviewed the spectrum of congenital heart disease and genitourinary anomalies seen in Opitz syndrome. They reported an infant with this disorder and a combination of complex congenital heart disease (double outlet right ventricle with pulmonary atresia, malalignment ventriculoseptal defect, right-sided aortic arch with left ductus arteriosus) and bladder exstrophy. Neither of these defects had previously been reported in association with Opitz syndrome. Since both were midline defects, they further characterized Opitz syndrome as an impairment in midline development.

Brooks et al. (1998) described pituitary macroadenoma and cranial osteoma in a 43-year-old woman, a mother of a boy with typical Opitz syndrome and herself a manifesting heterozygote. She had telecanthus, anteverted nares, and a history of frequent respiratory and urinary tract infections. Her son had hypertelorism, bilaterally cleft lip and palate, hypospadias, and dysphagia with multiple episodes of aspiration pneumonia. Linkage analysis had demonstrated X-linked inheritance in this family (Robin et al., 1995). Brooks et al. (1998) concluded that cranial osteomas are not associated with growth hormone hypersecretion and that therefore cranial osteomas and perhaps pituitary tumors should be included in the nosography of the Opitz GBBB syndrome.

Reviewing all MID1-mutated Opitz syndrome patients thus far reported, De Falco et al. (2003) confirmed that hypertelorism and hypospadias were the most frequent manifestations, being present in almost all individuals. Laryngotracheoesophageal defects were also common anomalies, being manifested by all MID1-mutated male patients. Congenital heart and anal abnormalities were less frequent than reported in the literature. De Falco et al. (2003) included limb defects in the clinical synopsis of Opitz syndrome, because they found MID1-mutated patients showing syndactyly.

Pinson et al. (2004) found vermis hypoplasia or agenesis in 4 of 9 patients with MID1 mutations, including 1 patient with no developmental delay, and suggested that this is an important clinical feature that should be routinely sought even in patients without mental retardation.

Funke et al. (2006) presented a case of Opitz syndrome complicated by congenital chylothorax (603523), a collection of lymph in the pleural space. The mother had marked hypertelorism. The boy also had bilateral cleft lip and palate, hypertelorism, hypospadias, right cryptorchidism, left hydrocele, widow's peak, prominent forehead, low-set and posteriorly angulated ears, and flat, broad nasal bridge. No mutation was detected in MID1. The patient was treated with octreotide, a somatostatin analog, which had proven to be effective for treatment of congenital cases of chylothorax (Goto et al., 2003; Young et al., 2004), intractable post-operative chylothorax (Clarke et al., 2005) and even intestinal lymphangiectasia in Noonan syndrome (163950) (Strehl et al., 2003).

Pathogenesis

Trockenbacher et al. (2001) showed that mutation in the MID1 gene leads to a marked accumulation of the catalytic subunit of protein phosphatase 2A (PP2CA; 176915), a central cellular regulator. This accumulation is caused by an impairment of the E3 ubiquitin ligase activity of the MID1 protein that normally targets PP2CA for degradation through binding to its alpha-4 regulatory subunit, as demonstrated in an embryonic fibroblast line derived from a fetus with Opitz syndrome. Elevated PP2A catalytic subunit caused hypophosphorylation of microtubule-associated proteins, a pathologic mechanism that is consistent with the Opitz syndrome phenotype. The alpha-4 regulatory subunit of PP2A is also known as immunoglobulin-binding protein 1 (IGBP1; 300139). Trockenbacher et al. (2001) pointed out that the IGBP1 gene maps to the same linkage interval in Xq13 as does the FG syndrome (305450). Similar to Opitz syndrome, FG syndrome is characterized by mental retardation combined with imperforate anus, congenital heart defects, and characteristic facies. Trockenbacher et al. (2001) speculated that IGBP1 has a role in the pathogenesis of FG syndrome.

Mapping

Verloes et al. (1995) reported a large pedigree in which Opitz GBBB syndrome cosegregated with a pericentric inversion of the X chromosome: inv(X)(p22.3q26). This suggested the existence of a true X-linked form of GBBB that does not appear phenotypically different from its autosomal counterpart.

Robin et al. (1995) demonstrated that the Opitz BBBG syndrome is a heterogeneous disorder, with X-linked and autosomal (22q-linked; 145410) forms, here designated type I and type II, respectively. In a study of multiple families, they found 3 in which there was linkage to DXS987 in Xp22, with a lod score of 3.53 at zero recombination; and 5 in which there was linkage to D22S345 from chromosome 22q11.2, with a lod score of 3.53 at zero recombination. No phenotypic differences between the 2 linkage types were discerned. In both there are craniofacial anomalies, hypospadias, swallowing difficulties, and developmental delay. The original G family (Opitz et al., 1969) was shown by linkage to have the X-linked form. Robin et al. (1995) pictured an affected brother and sister from family 5 which also showed X linkage; both had widely spaced eyes and the boy had repaired cleft lip and tracheotomy necessitated by a laryngotracheal cleft.

Although the Opitz syndrome maps to Xp22 in approximately the same area as craniofrontonasal dysplasia (304110), Muenke (1996) concluded that they represent separate loci.

Diagnosis

Prenatal Diagnosis

Hogdall et al. (1989) made the diagnosis of the BBB syndrome at 19 weeks' gestation by ultrasonographic demonstration of hypertelorism and hypospadias. The pedigree showed affected individuals in 3 generations in a pattern consistent with X-linked inheritance with minor expression in the heterozygous females.

Molecular Genetics

Quaderi et al. (1997) identified mutations in the MID1 gene in 3 Opitz syndrome families: a 3-bp deletion involving a methionine codon (300552.0001), a 24-bp duplication causing addition of 8 amino acids (300552.0002), and a 1-bp insertion resulting in a frameshift and loss of 101 amino acid residues (300552.0003). All these mutations were in the C-terminal region of the MID1 gene.

Among 15 patients with Opitz syndrome, Cox et al. (2000) identified 7 novel mutations in the MID1 gene, 2 of which disrupt the N terminus of the protein. The most severe of these, glu115 to ter (E115X; 300552.0005), is predicted to truncate the protein before the B-box motifs. Another mutation, leu626 to pro (L626P; 300552.0004), represented the most C-terminal alteration reported to date. Green fluorescent protein (GFP) fusion constructs of 2 N-terminal mutants showed no evidence of cytoplasmic aggregation, suggesting that this feature is not pathognomonic for X-linked Opitz syndrome.

Pinson et al. (2004) identified 1 previously reported and 5 novel mutations in the MID1 gene in 14 patients with Opitz syndrome.

Among 63 male individuals referred to De Falco et al. (2003) as instances of sporadic or familial X-linked Opitz syndrome, they found novel mutations of the MID1 gene in 11. The mutations were scattered throughout the gene, although more were represented in the 3-prime region. The low frequency of mutations in MID1 and the high variability of the phenotype suggested the involvement of other genes in the causation of OS.

Genotype/Phenotype Correlations

Among 70 patients with clinically diagnosed Opitz syndrome, So et al. (2005) compared the phenotypes of patients with and without a MID1 mutation to determine if there were distinct clinical patterns in these groups. They identified 10 novel mutations, of which 5 were detected in familial cases, 2 in sporadic cases, and 3 in families in which it was not clear whether the disorder was familial or sporadic. X-linked Opitz syndrome patients with MID1 mutations were less severely affected than patients with MID1 mutations reported in previous studies, particularly in terms of functionally significant neurologic, laryngotracheoesophageal (LTE), anal, and cardiac abnormalities. Minor anomalies were more common in patients with MID1 mutations compared to those without mutations. Female MID1 mutation carriers had milder phenotypes compared to male MID1 mutation carriers, with the most common manifestation being hypertelorism in both sexes. The authors found that most of the anomalies they observed did not correlate with MID1 mutation type, with the possible exception of LTE malformations. So et al. (2005) demonstrated the wide spectrum of severity and manifestations of Opitz syndrome, and they emphasized that X-linked Opitz syndrome patients with MID1 mutations may be less severely affected than previously reported.

History

There had long been disagreement as to whether the disorder is an X-linked recessive or an autosomal dominant with male preference. Beginning with the third edition (1971) and continuing through the seventh edition (1986) of Mendelian Inheritance in Man, the disorder was listed in the X-linked catalog. Although the original pedigrees were consistent with either X-linked or autosomal dominant inheritance, male-to-male transmission in subsequent reports suggested that the disorder is inherited as an autosomal dominant. Opitz (1987) gave a follow-up on his original family and presented information on a large number of unpublished cases. In conclusion, he wrote as follows: 'We would petition again that the G syndrome be moved from its present entry in the X-linked catalog (in Mendelian Inheritance in Man) into the autosomal dominant section since there is now good evidence of male-to-male transmission with female involvement almost as common (but generally less severe) than male involvement in newly referred proposita.' As it turned out, there are in fact both X-linked and autosomal dominant (145410) forms of this disorder.