Genetic characteristics of 234 Italian patients with macular and cone/cone-rod dystrophy

Abstract

Two-hundred and thirty-four Italian patients with a clinical diagnosis of macular, cone and cone-rod dystrophies (MD, CD, and CRD) were examined using next-generation sequencing (NGS) and gene sequencing panels targeting a specific set of genes, Sanger sequencing and-when necessary-multiplex ligation-dependent probe amplification (MLPA) to diagnose the molecular cause of the aforementioned diseases. When possible, segregation analysis was performed in order to confirm unsolved cases. Each patient's retinal phenotypic characteristics were determined using focal and full-field ERGs, perimetry, spectral domain optical coherence tomography and fundus autofluorescence. We identified 236 potentially causative variants in 136 patients representing the 58.1% of the total cohort, 43 of which were unpublished. After stratifying the patients according to their clinical suspicion, the diagnostic yield was 62.5% and 53.8% for patients with MD and for those with CD/CRD, respectively. The mode of inheritance of all cases confirmed by genetic analysis was 70% autosomal recessive, 26% dominant, and 4% X-linked. The main cause (59%) of both MD and CD/CRD cases was the presence of variants in the ABCA4 gene, followed by variants in PRPH2 (9%) and BEST1 (6%). A careful morpho-functional evaluation of the phenotype, together with genetic counselling, resulted in an acceptable diagnostic yield in a large cohort of Italian patients. Our study emphasizes the role of targeted NGS to diagnose MDs, CDs, and CRDs, as well as the clinical usefulness of segregation analysis for patients with unsolved diagnosis.

Figure 1

(A) Percentage of inheritance patterns based on molecular diagnosis. Data derived from 136 genetically diagnosed patients out of 234 tested individuals. (B) Comparison between the inheritance patterns deduced from the pedigrees and those found by the genetic test.

Figure 2

(A) Prevalence of gene variants in 80 MD patients. (B) Functional classification of variants identified in MD patients. (C) Mean age of onset for each gene was calculated on 131 genetically solved subjects (probands and affected relatives). Bars indicate ± SD.

Figure 3

(A) Prevalence of gene variants in 56 CD/CRD patients. (B) Functional classification of variants identified in CD/CRD patients. (C) Mean age of onset for each gene was calculated on 58 genetically solved subjects (probands and affected relatives). Bars indicate ± SD.

Figure 4

Overview of the 43 unpublished variants found in 44 unrelated patients: (A) distribution in genes; (B) classification of pathogenicity; (C) distribution by clinical phenotype (MD and CD/CRD) and by inheritance pattern (autosomal dominant, autosomal recessive and X-linked).

Figure 5

Median age of onset of MD and CD/CRD by inheritance after molecular testing, calculated on 131 genetically solved subjects (both probands and affected relatives). AD-MD 36.5 (IQR, 44.75–11) years; AD-CD/CRD 25 (IQR, 40–6.25) years; AR-MD 16 (IQR, 23.5–11.5) years; AR-CD/CRD 13 (IQR, 25–6.5) years; XL-CD/CRD 19 (IQR, 24–16) years.