Mutations in the Norrie disease gene

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Abstract
We report our experience to date in mutation identification in the Norrie disease (ND) gene. We carried out mutational analysis in 26 kindreds in an attempt to identify regions presumed critical to protein function and potentially correlated with generation of the disease phenotype. All coding exons, as well as noncoding regions of exons 1 and 2, 636 nucleotides in the noncoding region of exon 3, and 197 nucleotides of 5′ flanking sequence, were analyzed for single‐strand conformation polymorphisms (SSCP) by polymerase chain reaction (PCR) amplification of genomic DNA. DNA fragments that showed altered SSCP band mobilities were sequenced to locate the specific mutations. In addition to three previously described submicroscopic deletions encompassing the entire ND gene, we have now identified 6 intragenic deletions, 8 missense (seven point mutations, one 9‐bp deletion), 6 nonsense (three point mutations, three single bp deletions/frameshift) and one 10‐bp insertion, creating an expanded repeat in the 5′ noncoding region of exon 1. Thus, mutations have been identified in a total of 24 of 26 (92%) of the kindreds we have studied to date. With the exception of two different mutations, each found in two apparently unrelated kindreds, these mutations are unique and expand the genotype database. Localization of the majority of point mutations at or near cysteine residues, potentially critical in protein tertiary structure, supports a previous protein model for norrin as member of a cysline knot growth factor family (Meitinger et al., 1993). Genotype‐phenotype correlations were not evident with the limited clinical data available, except in the cases of larger submicroscopic deletions associated with a more severe neurologic syndrome. This lack of correlation suggests that complex epigenetic factors may play a significant role in the physiological and neurodevelopmental expression of the disease phenotype. Given the remarkable intra‐ and interfamilial variability in hearing and brain dysfunction in this disease, it may be expected that other phenotypic expressions of the disease gene, which do not match the “classical” Norrie phenotype, may be identified in the future.