RP2 and RPGR mutations and clinical correlations in patients with X-linked retinitis pigmentosa

Abstract

We determined the mutation spectrum of the RP2 and RPGR genes in patients with X-linked retinitis pigmentosa (XLRP) and searched for correlations between categories of mutation and severity of disease. We screened 187 unrelated male patients for mutations, including 135 with a prior clinical diagnosis of XLRP, 11 with probable XLRP, 30 isolate cases suspected of having XLRP, and 11 with cone-rod degeneration. Mutation screening was performed by single-strand conformation analysis and by sequencing of all RP2 exons and RPGR exons 1-14, ORF15, and 15a. The refractive error, visual acuity, final dark-adapted threshold, visual field area, and 30-Hz cone electroretinogram (ERG) amplitude were measured in each patient. Among the 187 patients, we found 10 mutations in RP2, 2 of which are novel, and 80 mutations in RPGR, 41 of which are novel; 66% of the RPGR mutations were within ORF15. Among the 135 with a prior clinical diagnosis of XLRP, mutations in the RP2 and RPGR genes were found in 9 of 135 (6.7%) and 98 of 135 (72.6%), respectively, for a total of 79% of patients. Patients with RP2 mutations had, on average, lower visual acuity but similar visual field area, final dark-adapted threshold, and 30-Hz ERG amplitude compared with those with RPGR mutations. Among patients with RPGR mutations, those with ORF15 mutations had, on average, a significantly larger visual field area and a borderline larger ERG amplitude than did patients with RPGR mutations in exons 1-14. Among patients with ORF15 mutations, regression analyses showed that the final dark-adapted threshold became lower (i.e., closer to normal) and that the 30-Hz ERG amplitude increased as the length of the wild-type ORF15 amino acid sequence increased. Furthermore, as the length of the abnormal amino acid sequence following ORF15 frameshift mutations increased, the severity of disease increased.

Figure 1

Location of ORF15 mutations. Mutations that have been previously published by other groups are depicted on the left. Mutations reported in the present study are depicted on the right. The numbers along the ORF15 bar represent the amino acid numbers. Arrows (←) indicate mutations not reported by other groups, asterisks (*) indicate mutations causing cone-rod degeneration, the number sign (#) indicates a case with probable X-linked cone dystrophy (Vervoort et al. 2000), and the tilde (∼) indicates that ocular measurements were not provided by the authors (Pusch et al. 2002).

Figure 2

Plots of ocular function by age for XLRP patients with RP2 mutations (blackened diamonds) or RPGR mutations (unblackened circles). The regression lines were fitted by least-squares analysis to the RP2 data (solid lines) or RPGR data (stippled lines).

Figure 3

Plots of ocular function by age for patients with XLRP with RPGR mutations in exons 1–14 (blackened diamonds) or in ORF15 (unblackened circles). The regression lines were fitted by least-squares analysis to the exon 1–14 data (solid lines) or ORF15 data (stippled lines).

Figure 4

Plots of the dark-adapted threshold elevation (upper panels) and 30-Hz ERG amplitude (lower panels) versus the altered ORF15 codon (left panels) and the number of mutant residues (right panels). By multiple regression, each side panel controls for the relationship in the other side panel, as well as for age and (for the lower panels) for refractive error. Both X and Y coordinates have been adjusted statistically to remove the effects of these covariates on the measures of ocular function.

Figure 5

The predicted effect of ORF15 mutations on translated proteins. The unblackened bars represent a normal amino acid sequence, and the striped and blackened bars represent an aberrant amino acid sequence due to a frameshift mutation of type −1 (striped) or −2 (blackened). The Xs within the bar designate the repetitive domain. The numbers above the top bar are amino acid numbers for ORF15.