Mutations in the human PAX6 gene produce various phenotypes, including
aniridia, Peters' anomaly, autosomal dominant keratitis and familial
foveal dysplasia, The various phenotypes may arise from different muta
tions in the same gene, To test this theory, we performed a functional
analysis of two missense mutations in the paired domain: the R26G mut
ation, previously reported in a case of Peters' anomaly, and an unrepo
rted I87R mutation, which we identified in a patient with aniridia. Wh
ile both the R26 and the I87 positions are conserved in the paired box
es of all known PAX genes, X-ray crystallography has shown that only R
26 makes contact with DNA, We showed that the R26G mutant failed to bi
nd a subset of paired domain binding sites but, surprisingly, bound ot
her sites and successfully transactivated promoters containing those s
ites, In contrast, the I87R mutant had lost the ability to bind DNA at
all tested sites and failed to transactivate promoters, Our data supp
ort the haploid-insufficiency hypothesis of aniridia, and the hypothes
is that R26G is a hypomorphic allele.