X-linked cone-rod dystrophy (locus COD1): identification of mutations in RPGR exon ORF15

Abstract

X-linked cone-rod dystrophy (COD1) is a retinal disease that primarily affects the cone photoreceptors; the disease was originally mapped to a limited region of Xp11.4. We evaluated the three families from our original study with new markers and clinically reassessed all key recombinants; we determined that the critical intervals in families 2 and 3 overlapped the RP3 locus and that a status change (from affected to probably unaffected) of a key recombinant individual in family 1 also reassigned the disease locus to include RP3 as well. Mutation analysis of the entire RPGR coding region identified two different 2-nucleotide (nt) deletions in ORF15, in family 2 (delAG) and in families 1 and 3 (delGG), both of which result in a frameshift leading to altered amino acid structure and early termination. In addition, an independent individual with X-linked cone-rod dystrophy demonstrated a 1-nt insertion (insA) in ORF15. The presence of three distinct mutations associated with the same disease phenotype provides strong evidence that mutations in RPGR exon ORF15 are responsible for COD1. Genetic heterogeneity was observed in three other families, including the identification of an in-frame 12-nt deletion polymorphism in ORF15 that did not segregate with the disease in one of these families.

Figure 1

ORF15 mutations and RPGR exon 9 sequence alteration, with the corresponding predicted translation products. The locations for sequence alterations are indicated by arrows. The partial sequences of ORF15 (a and b) and RPGR exon 9 (c) are shown on the right side of each chromatogram, with the resulting changes in the open reading frame underlined. a, The 2-nt deletions identified in the original three families, which cause a frameshift resulting in very similar truncated protein products (46 or 44 novel amino acids, and premature truncation resulting in the loss of 76 amino acids). b, The 1-nt insertion identified in another individual with X-linked cone-rod dystrophy, which results in three novel amino acids and in premature truncation resulting in the loss of 43 amino acids. c, A nonconservative amino acid change (Asn345Asp) in RPGR exon 9 (A1092G), which segregated with the mutation in families 1 and 3 and suggests a shared disease haplotype.