Identification of an Intronic Single-Point Mutation inRP2as the Cause of Semidominant X-linked Retinitis Pigmentosa

PURPOSE: A large family with 11 males and 2 females with X-linked retinitis pigmentosa (XLRP) was analyzed in search of pathologic mutations. METHODS: Of the two major XLRP genes, RPGR was analyzed by SNP cosegregation and RP2 was directly screened for mutations. The pathogenicity of a new variant was assessed in silico, in vivo, and in vitro. RESULTS: The results of cosegregation analysis with SNPs closely located to RPGR excluded this gene as the cause of the disease in this family. Sequencing of RP2 showed a putative pathogenic variant in intron 3 at the conserved polypyrimidine tract (c.1073-9T>A). This substitution cosegregated with the disease and was not found in 220 control chromosomes. In silico analyses using online resources indicated a decreased score of intron 3 acceptor splice site for the mutated sequence. Real-time RT-PCR analysis of the RP2 splicing pattern in blood samples of patients and carrier females showed skipping of exon 4, causing a frame shift that introduced a premature stop codon. Further verification of the pathogenicity of this point mutation was obtained by expression of a minigene RP2 construct in cultured cells. CONCLUSIONS: A transversion (T>A) at position -9 in intron 3 of RP2 causes XLRP by altering the splicing pattern and highlights the pathogenicity of intronic variants. The single point RP2 mutation leads to a wide range of phenotypic traits in carrier females, from completely normal to severe retinal degeneration, thus supporting that RP2 is also a candidate for semidominance in XLRP.