Identification of two single nucleotide polymorphisms in exon 8 of PAX2.

We estimate the allele frequencies of two single nucleotide polymorphisms (1410 C --> T) and (1521 A --> C) in the coding region of PAX2. The coding region single nucleotide polymorphisms (cSNPs) were identified by sequencing of amplimers of PAX2 exon 8 exhibiting variant migration patterns in the course of genomic DNA mutation screening from patients with renal-coloboma syndrome. Allele frequencies of the two polymorphisms were 0.94 for 1410C and 0.72 for 1521A. Cosegregation analyses of both alleles suggest that they are each in Hardy-Weinberg equilibrium and jointly in linkage equilibrium and may represent ancient polymorphisms. Characterization of PAX2 exon 8 cSNPs will serve as useful tools for mapping at the PAX2 locus.

Homonucleotide expansion and contraction mutations of PAX2 and inclusion of Chiari 1 malformation as part of renal-coloboma syndrome.

Renal-Coloboma syndrome, an autosomal dominant disorder characterized by colobomatous eye defects, vesicoureteral reflux, and abnormal kidneys, results from mutations in PAX2. The purpose of this study was to identify mutations in PAX2 and understand the associated patient phenotypes. We report a severely affected girl and a mildly affected mother and daughter, all of whom have PAX2 homoguanine tract (7 G) missense mutations. The mother and daughter have optic nerve colobomas and the daughter has vesicoureteral reflux. The severely affected girl developed renal failure and has bilateral colobomatous eye defects. Additionally, this girl developed hydrocephalus associated with platybasia and a Chiari 1 malformation. We examined genomic DNA from these individuals by SSCP and sequencing. The mother and daughter had a novel mutation: a contraction in a string of 7 G's to 6 G's in one allele of PAX2, leading to a premature stop codon two amino acids downstream. The severely affected girl had an expansion to 8 G's, leading to a premature stop codon 27 amino acids downstream. The 8 G expansion has been found in other patients without brain anomalies and has occurred spontaneously in a mouse model, PAX2(1Neu). We expand the known phenotype associated with mutations in PAX2 to include brain malformations. The homoguanine tract in PAX2 is a hot spot for spontaneous expansion or contraction mutations and demonstrates the importance of homonucleotide tract mutations in human malformation syndromes.

Further delineation of renal-coloboma syndrome in patients with extreme variability of phenotype and identical PAX2 mutations.

Renal-coloboma syndrome is a recently described autosomal dominant syndrome of abnormal optic nerve and renal development. Two families have been reported with renal-coloboma syndrome and mutations of the PAX2 gene. The PAX2 gene, which encodes a DNA-binding protein, is expressed in the developing ear, CNS, eye, and urogenital tract. Ocular and/or renal abnormalities have been consistently noted in the five reports of patients with renal-coloboma syndrome, to date, but PAX2 expression patterns suggest that auditory and CNS abnormalities may be additional features of renal-coloboma syndrome. To determine whether additional clinical features are associated with PAX2 mutations, we have used PCR-SSCP to identify PAX2 gene mutations in patients. We report here four patients with mutations in exon 2, one of whom has severe ocular and renal disease, microcephaly, and retardation, and another who has ocular and renal disease with high-frequency hearing loss. Unexpectedly, extreme variability in clinical presentation was observed between a mother, her son, and an unrelated patient, all of whom had the same PAX2 mutation as previously described in two siblings with renal-coloboma syndrome. These results suggest that a sequence of seven Gs in PAX2 exon 2 may be particularly prone to mutation.