Mutations of the PAX6
gene are known to cause many cases of inherited and sporadic aniridia. The PAX6
gene is clearly vital for the normal development of the eye [28
]. The phenotype presumably results from heterozygous insufficiency such that if only one copy of the gene is active insufficient protein is produced to support normal development of the eye. The protein products of the mutant genes could theoretically have DNA binding activity and compete with the wild type protein. However, the fact that aniridia caused by intragenic mutation is phenotypically indistinguishable from aniridia caused by deletion of entire PAX6
gene makes it likely that the mutant alleles are in fact null alleles [4
The pathological mutations will be found throughout the gene including the promoter and the other regulatory regions. In these patients all the mutations if translated, would result in truncated protein products. The translated proteins would be unable to carry out normal PAX6 function because functional domains are deleted. The two DNA binding domains are important for the normal activity of the protein [7
]. Proband 27–1 with c.1201delA also has another change, c.1239A>G downstream of the deletion, however its impact on transcriptional product is unknown. The downstream mutation may be less important because it is likely that the other mutation (c.1201delA) is the one responsible for aniridia. Though the two nonsense mutations C40X and W156X were previously reported in other population, we report for the first time in aniridic patients in south Indian population and thus it could be a recurrent mutation.
The change in the splice acceptor region of intron 9 (IVS9-12C>T) is a neutral polymorphism that has been observed before and the C>T substitution in the splice acceptor consensus sequence is not predicted to have any effect on function [29
]. In addition, other studies with this change in patients with aniridia have a clear pathological mutation elsewhere in the gene [19
]. It is likely that the patient 18–1 might have a pathological mutation elsewhere in the PAX6
gene, perhaps in one of the cis
Analysis of PAX6
mutants showed that the pathogenicity effects appear to be loss-of-function mutations, which leads to aniridia (Fig. ) and other variable phenotypes [25
]. The mutations showed varying phenotypic severity of the disease. The proband with c.715ins5 mutation showed the presence of sclerocornea with nystagmus in both eyes. A similar phenotype (Foveal hypoplasia) was observed in probands with c.1201delA and c.482C>A. Proband 10–1 with c.901delA revealed ptosis, microcornea with dislocated cataractous lens. Marfan syndrome and ectopia lentis was observed in both eyes of the proband 16–1 with c.830G>A. Even though Marfan syndrome is rarely associated with aniridia [3
] we observed the syndrome in the proband. The peripheral corneal pannus with bullous keratopathy and corneal ectasia was observed in IVS9-12C>T. It is worthwhile to note that all the mutations observed in this study are associated with common phenotypes.
Our data provide evidence that some of the PAX6
gene mutations identified in aniridia patients lead to disruption of PAX6
gene expression due to the premature termination and most of which are assumed to cause loss of activity of one allele [30
]. The Nonsense mutations would be predicted to result in truncated proteins due to a mechanism called 'nonsense-mediated decay' which degrades mRNAs containing premature stop codons (nonsense mutations). Therefore, PAX6
mRNAs that contain nonsense mutations would be degraded and would not be translated [31
]. Identifying new mutations contributes valuable information for carrier detection for specific effect mutation and genetic counseling.