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The PAX6 gene is involved in ocular embryogenesis. This gene seems to be the master control gene for morphogenesis of the eye. Mutations in the PAX6 gene have been detected in various ocular anomalies suspected to have bilateral genetic backgrounds during development, including aniridia, Peters’ anomaly,1 and foveal hypoplasia.2
In 1994, a sporadic case of Peters’ anomaly and a small family with a range of anterior segment malformations, including Peters’ anomaly, were shown to have a mutation of the PAX6 gene.1 More recently, Azuma et al reported a subject with Peters’ anomaly having a missense mutation in the alternative splice region of the PAX6 gene in 1999.3 Here we report a novel PAX6 gene mutation in a patient with Peters’ anomaly.
The present study had the approval of Kyoto Prefectural University of Medicine ethics committee and was conducted in accordance with the World Medical Association Declaration of Helsinki. Genomic DNA samples were isolated from the whole blood of patients and their relatives after informed consent. Each exon of the PAX6 gene and its immediate flanking sequence were amplified by polymerase chain reaction (PCR). Purified amplified fragments were subjected to sequenced using an ABI Prism 3100 genetic analyser (Applied Biosystems, Foster City, CA, USA). To confirm the sequence of mutations, the SNaPshot method4 was performed.
Of the four patients studied, we detected a novel missense mutation in one patient. The patient, a 20 year old girl, had bilateral Peters’ anomaly showing corneal opacity with iridocorneal adhesion and nystagmus (fig 1). The fundus of both eyes could not be seen because of corneal opacity. No systemic associations with Peters’ anomaly were identified. Sequence analyses revealed a heterozygous mutation as A/G at the 38th position which resulted in Q13R substitution in the PAX6 gene. No mutation was found in her parents and elder brother, which is consistent with the fact that they show no abnormal findings on clinical examination (fig 2).
We have identified one missense mutation in the alternative splice region (exon5a) of the PAX6 gene in a subject with Peters’ anomaly. This missense mutation was the substitution from glutamine to arginine at the 13th codon, which is the second reported position in the exon5a.
The Pax family of developmentally regulated transcription factors share an amino terminal DNA binding motif known as the paired domain. The paired domain has a bipartite structure with a highly conserved N-terminal subdomain (NTS) and C-terminal subdomain, which bind distinctive consensus sequences, and the insertion of exon5a into the N-terminal subdomain abolishes the DNA binding activity of the NTS.5
Interestingly, in 1999, Azuma et al proved that the mutation in the NTS of the paired domain partially restored the DNA binding activity of the NTS, using functional analyses.3 In addition, because the amino acids glutamine and arginine belong to hydrophilic and basic amino acid respectively, they have different electronic charges. Therefore, this amino acid substitution may affect the structure of the PAX6 protein and then change DNA binding activity of the paired domain. In other words, such a single amino acid substitution is an important motif as the paired domain such as we have found may severely damage the normal protein function. In fact, except for aniridia, other missense mutations that result in a serious congenital eye disease such as Peters’ anomaly or foveal hypoplasia, also exist in the paired domain. The typical clinical presentation with a missense mutation in a highly conserved and functionally important region suggests there is a reasonable likelihood that this sequence variant is caused by the patient’s phenotype. Our report adds a novel and potentially structurally significant mutation in the PAX6 gene to the present spectrum of mutations.