Variegated yet non-random rod and cone photoreceptor disease patterns in RPGR-ORF15-associated retinal degeneration

Abstract

Mutations in the ORF15 exon of the RPGR gene cause a common form of X-linked retinitis pigmentosa, which often results in severe loss of vision. In dogs and mice, gene augmentation therapy has been shown to arrest the progressive degeneration of rod and cone photoreceptors. However, the distribution of potentially treatable photoreceptors across the human retinas and the rate of degeneration are not known. Here, we have defined structural and functional features of the disease in 70 individuals with ORF15 mutations. We also correlated the features observed in patients with those of three Rpgr-mutant (Rpgr-ko, Rd9, and Rpgr-cko) mice. In patients, there was pronounced macular disease. Across the retina, rod and cone dysfunction showed a range of patterns and a spectrum of severity between individuals, but a high symmetry was observed between eyes of each individual. Genotype was not related to disease expression. In the Rpgr-ko mice, there were intra-retinal differences in rhodopsin and cone opsin trafficking. In Rd9 and Rpgr-cko mice, retinal degeneration showed inter-ocular symmetry. Longitudinal results in patients revealed localized rod and cone dysfunction with progression rates of 0.8 to 1.3 log per decade in sensitivity loss. Relatively retained rod and cone photoreceptors in mid- and far-peripheral temporal-inferior and nasal-inferior visual field regions should be good targets for future localized gene therapies in patients.

Figure 1.

Spectrum of spatial patterns of disease in RPGR-ORF15 evaluated with melanin autofluorescence (left panels) and OCT (right panels) across the fovea, macula and superior and inferior para-macular regions. Specific patients exemplify the altitudinal pattern with central maculopathy (A–C), anisotropic pattern with parafoveal defects (D–F), isotropic patterns with or without parafoveal defects (G–H), and end stage disease with indeterminate pattern (I). Arrowheads indicate the transition between diseased (darker) and healthier (brighter) retina near the boundaries of the macula region while arrows mark diseased regions near the fovea. On OCT scans, ONL layer is painted blue and IS/OS line is painted yellow for visibility. All eyes are shown as equivalent right eyes and images are individually contrast stretched for visibility of features. P, patient number, F, family number from Supplementary Material, Table S1. Age in years.

Figure 2.

Photoreceptor degeneration across the central retina in RPGR-ORF15. (A) Central ONL thickness along the vertical meridian in RPGR-ORF15 is divided into three groups (Left: subjects with detectable IS/OS line outside the fovea; centre: subjects with detectable IS/OS line in the fovea only; right: subjects with no detectable IS/OS line). Blue lines indicate regions with no detectable IS/OS line and orange lines denote regions with detectable IS/OS line. Grey area indicates 95% confidence interval of ONL thickness from normal data. S, superior; I, inferior retina. (B-D) ONL thicknesses at fovea (B), 20° superior (C) and 20° inferior (D) retina show a wide spectrum among the patient population. Abscissae represent patient IDs which are ordered in terms of predicted truncation location; truncations closer to the 3’ end are to the right. Orange bars show subjects with detectable IS/OS line, blue bars show patients with no detectable IS/OS line. X denotes undetectable ONL.

Figure 3.

Lack of relationship between severity of rod- or cone-mediated vision loss and age and RPGR-ORF15 genotype. (A–I) Extents of kinetic visual fields (V-4e) demonstrating examples of severity in younger and older patients with early (A–F) or late truncations (G–I). Note that patients shown in A–D are from the same family. (J) Retina wide severity of cone (upper) and rod (lower) photoreceptor mediated function plotted against age. Symbols define genotype categorized as early or late truncations and predicted aberrant protein tails of none, short or long. At any given age group, a range of severity is observed independent of genotype. CSL, cone sensitivity loss; RSL, rod sensitivity loss.

Figure 4.

Retina-wide maps of retained rod and cone function show complex spatial patterns in RPGR-ORF15-RD. (A–F) Topographic representation of rod- and cone-mediated sensitivity losses (RSL and CSL, smaller greyscale maps) and classification of whether rod (R, blue), cone (C, red) or both photoreceptors (R/C, white) are retained across the full extent of the retina (larger colour maps) in representative patients showing substantially different patterns and severities. Black squares represent no detectable vision. Half-filled colour squares represent loci where one of the photoreceptor function was not measurable but one was measurable. Loci corresponding to the fovea and locus of the optic nerve (white squares with cross) are not included in the analyses. S, superior; I, inferior; N, nasal; T, temporal visual field. (G) Percentage of the retina-wide loci showing R-, C-, R/C-retention or no vision in all 70 patients. Abscissa represents patient IDs which are ordered in terms of predicted truncation location; truncations closer to the 3’ end are to the right. (H) Spatial distribution of retinal loci showing R-, C- and R/C-retention across all patients. Pseudocolour coding describes the percentage of patients at a given retinal locus that show a specific type of dysfunction.

Figure 5.

Long-term progression of rod and cone dysfunction. (A) Rod- and cone-mediated sensitivity losses (RSL and CSL, smaller greyscale maps) and classification of whether rod, cone or both photoreceptors are retained (same as Fig. 4) in a representative patient recorded at ages 10 and 18 show substantial and non-homogeneous progression. (B) Calculated progression rates in dB per year across the retina tiled into seven neighbouring regions. Progression rates for rods and cones are specified for those loci that were R/C-retained at first visit (R and centre C, respectively), and for those loci that were C-retained at first visit (right C). Progression rates for loci with R-retention at first visit were not calculated since rod function at most of these points was undetectable at the second visit. (C) Rod and cone function progression rates across all patients with long-term followup as a function of retinal region described in Panel B. Rod progression is specified at loci with R- and R/C-retention at first visit, and cone progression at loci with C- and R/C-retention at the first visit.