Missense mutation at the C-terminus of PAX6 negatively modulates homeodomain function

PAX6 is essential for ocular morphogenesis. Mutations in the PAX6 gene prod uce various phenotypes, including aniridia, Peters' anomaly, foveal hypopla sia, autosomal dominant keratitis and congenital cataracts. PAX6 functions as a transcription factor and has two DNA binding domains (a paired domain and a homeodomain) which are joined by a linker, and a transactivation doma in enriched in proline, serine and threonine (PST) at the C-terminus, The m echanism of PAX6 function is not clearly understood, and few target genes i n vertebrates have been identified, We examined disease-causing missense mu tations in the PST domain to understand how they affect the function of PAX 6, Upon examining the DNA samples of aniridia patients, we identified three missense mutations in the PST domain: P375Q (a novel mutation) and the pre viously reported Q422R and X423L mutations. On the basis of functional anal ysis, the P375Q mutant appears to have a normal transactivation activity bu t lower DNA binding through the paired domain than the wild-type. The Q422R mutation resulted in the loss of DNA binding ability of the PAX6 homeodoma in, Substitution analyses of the C-terminal amino acid (codon 422) indicate d that an amino acid at codon 422 is required for DNA binding of the homeod omain of intact PAX6 and that the polarity and charge of the side-chain of the terminal amino acid influence this binding.