TRUNCATION MUTATIONS IN THE TRANSACTIVATION REGION OF PAX6 RESULT IN DOMINANT-NEGATIVE MUTANTS

PAX6 is a transcription factor with two DNA-binding domains (paired bo x and homeobox) and a proline-serine-threonine (PST)-rich transactivat ion domain. PAX6 regulates eye development in animals ranging from jel lyfish to Drosophila to humans. Heterozygous mutations in the human PA X6 gene result in various phenotypes, including aniridia, Peter's anom aly, autosomal dominant keratitis, and familial foveal dysplasia, It i s believed that the mutated allele of PAX6 produces an inactive protei n and aniridia is caused due to genetic haploinsufficiency, However, s everal truncation mutations have been found to occur in the C-terminal half of PAX6 in patients with Aniridia resulting in mutant proteins t hat retain the DNA-binding domains but have lost most of the transacti vation domain, It is not clear whether such mutants really behave as l oss-of-function mutants as predicted by haploinsufficiency. Contrary t o this theory, our data showed that these mutants are dominant-negativ e in transient transfection assays when they are coexpressed with wild -type PAX6. We found that the dominant-negative effects result from th e enhanced DNA binding ability of these mutants. Kinetic studies of bi nding and dissociation revealed that various truncation mutants have 3 -5-fold higher affinity to various DNA-binding sites when compared wit h the wild-type PAX6, These results provide a new insight into the rol e of mutant PAX6 in causing aniridia.