Mutational risk in highly repetitive exon ORF15 of the RPGR multidisease gene is not associated with haplotype background

Exon ORF15 is an alternative exon in the retinitis pigmentosa GTPase regulator (RPGR) gene containing a highly repetitive, purine-rich internal region. It constitutes a mutational hot spot giving rise to a group of heterogeneous X-linked retinal disorders. We sought to determine whether non-pathogenic substitutions and sequence length variations in the repetitive sequence have an influence on the risk of pathogenic exon ORF15 mutations. The type and distribution of exon ORF15 polymorphisms were assessed by genotyping 107 healthy German males using standard procedures. Polymorphisms were grouped into haplotypes and their frequencies determined in normal controls and previously analyzed patients with X-linked retinitis pigmentosa (XLRP). In the control group we identified 6 complex variants of the most common, ancestral exon ORF15 allele corresponding to the GenBank reference sequence (accession no. AF286472). Exon ORF15 mutations in XLRP patients were associated with the ancestral allele in 75% of affected cases. Four of the most recent founder haplotypes termed H3, H4, H6 and H7 were not identified in the patient samples. Our analysis and review of polymorphism data from the published literature suggests the presence of common exon ORF15 haplotypes in the European population. While the mutational risk in the RPGR gene appears not to be altered by the haplotype background, exon ORF15 haplotype analysis may be useful for tracing the evolutionary history of RP3-associated diseases.