RPE65-Associated Retinopathies in the Italian Population: A Longitudinal Natural History Study

Abstract

Purpose: To investigate the course of inherited retinal degenerations (IRD) due to mutations in the RPE65 gene.

Methods: This longitudinal multicentric retrospective chart-review study was designed to collect best corrected visual acuity (BCVA), Goldman visual field, optical coherence tomography (OCT), and electroretinography (ERG) measurements. The data, including imaging, were collected using an electronic clinical research form and were reviewed at a single center to improve consistency.

Results: From an overall cohort of 60 Italian patients with RPE65-associated IRD, 43 patients (mean age, 27.8 ± 19.7 years) were included and showed a mean BCVA of 2.0 ± 1.0 logMAR. Time-to-event analysis revealed a median age of 33.8 years and 41.4 years to reach low vision and blindness based on BCVA, respectively. ERG (available for 34 patients) showed undetectable responses in most patients (26; 76.5%). OCT (available for 31 patients) revealed epiretinal membranes in five patients (16.1%). Central foveal thickness significantly decreased with age at a mean annual rate of -0.6%/y (P = 0.044). We identified 43 different variants in the RPE65 gene in the entire cohort. Nine variants were novel. Finally, to assess genotype-phenotype correlations, patients were stratified according to the number of RPE65 loss-of-function (LoF) alleles. Patients without LoF variants showed significantly (P < 0.05) better BCVA compared to patients with one or two LoF alleles.

Conclusions: We described the natural course of RPE65-associated IRD in an Italian cohort showing for the first time a specific genotype-phenotype association. Our findings can contribute to a better management of RPE65-associated IRD patients.

Figure 1.

(A) Pie chart of BCVA measurements of the best-seeing eye of the 43 RPE65 patients at the study baseline. (B) Comparison of BCVA measurement between both eyes measured in the 43 RPE65 patients at the study baseline.

Figure 2.

Examples of qualitative analysis of OCT scans in selected patients: (A) an initial thinning of the outer nuclear layer (ONL), an intact external limiting membrane (ELM) and initial parafoveal defects of the ellipsoid zone (EZ); (B) thinning of the ONL, preservation of the ELM and disruption of the EZ, whereas the RPE layer is still preserved; (C) final stage of retinal degeneration with complete atrophy of the ELM, EZ and RPE layers; the outer plexiform layer can instead be observed as a rarefied hyperreflective band lying above the ONL.

Figure 3.

Genetic variants detected in RPE65 in the study cohort. (A) Schematic drawing of the RPE65 exons showing the position of the variants identified and their frequency among index cases. The nucleotide change and variant position is reported on the top. The colored symbol shape indicates the mutation type (specified in the legend box). (B) Pie charts showing the relevant representation of the different mutation types among the 43 different variants identified (left panel) and among the 120 alleles of the entire cohort (right panel). (C) Pie chart showing the classification of the identified sequence variants according to the ACMG guidelines using the Varsome automated variant classification.

Figure 4.

Comparison of BCVA measurements (best-seeing eye) at the study baseline in the 43 RPE65 patients stratified according to the genotype. Patients harboring biallelic LoF variants (Group C, red circles) had a significantly worse BCVA compared to the other groups (Group B, compound heterozygotes for an LoF and a missense variant, yellow circles; Group A, biallelic missense variants, green circles).

Figure 5.

Time-to-event analysis for low vision, i.e., 20/400 ≤ BCVA in the better-seeing eye < 20/60, (A) and blindness, i.e., BCVA in the better-seeing eye < 20/400, (B) based on longitudinal observations (n = 35; mean follow-up period: 5.0 ± 4.5 years; median, 4.6 years; range, 1–18 years) in the study sample stratified according to genotype.