Domain disruption and mutation of the bZIP transcription factor, MAF, associated with cataract, ocular anterior segment dysgenesis and coloboma

Human congenital cataract and ocular anterior segment dysgenesis both demonstrate extensive genetic and phenotypic heterogeneity. We identified a family where ocular developmental abnormalities (cataract, anterior segment dysgenesis and microphthalmia) co-segregated with a translocation, t(5;16)(p15.3;q23.2), in both balanced and unbalanced forms. We hypothesized that this altered the expression of a gene of developmental significance in the human lens and ocular anterior segment. Cloning the 16q23.2 breakpoint demonstrated that it transected the genomic-control domain of MAF, a basic region leucine zipper (bZIP) transcription factor, first identified as an oncogene, which is expressed in vertebrate lens development and regulates the expression of the eye lens crystallins. The homozygous null mutant Maf mouse embryo demonstrates defective lens formation and microphthalmia. Through mutation screening of a panel of patients with hereditary congenital cataract we identified a mutation in MAF in a three-generation family with cataract, microcornea and iris coloboma. The mutation results in the substitution of an evolutionarily highly conserved arginine with a proline at residue 288 (R288P) in the basic region of the DNA-binding domain of MAF. Our findings further implicate MAF/Maf in mammalian lens development and highlight the role of the lens in anterior segment development. The 16q23.2 breakpoint transects the common fragile site, FRA16D, providing a molecular demonstration of a germline break in a common fragile site.