Genetic Analysis in a Swiss Cohort of Bilateral Congenital Cataract

Abstract

Importance: Identification of geographic population-based differences in genotype and phenotype heterogeneity are important for targeted and patient-specific diagnosis and treatment, counseling, and screening strategies.

Objective: To report disease-causing variants and their detailed phenotype in patients with bilateral congenital cataract from a single center in Switzerland and thereby draw a genetic map and perform a genotype-phenotype comparison of this cohort.

Design, setting, and participants: This clinical and molecular-genetic cohort study took place through the collaboration of the Department of Ophthalmology at the University Hospital Zurich and the Institute of Medical Molecular Genetics, University of Zurich, Schlieren, Switzerland. Thirty-seven patients from 25 families with different types of bilateral congenital cataract were included. All participating family members received a comprehensive eye examination. Whole exome sequencing was performed in the index patients, followed by a filtering process to detect possible disease-associated variants in genes previously described in association with congenital cataract. Probable disease-causing variants were confirmed by Sanger sequencing in available family members. All data were collected from January 2018 to June 2020, and the molecular-genetic analyses were performed from January 2019 to July 2020.

Main outcomes and measures: Identification of the underlying genetic causes of bilateral congenital cataract, including novel disease-causing variants and phenotype correlation.

Results: Among the 37 patients (18 [49%] male and 19 [51%] female; mean [SD] age, 17.3 [15.9] years) from 25 families, pathogenic variants were detected in 20 families (80% detection rate), which included 13 novel variants in the following genes: BCOR, COL4A1, CRYBA2, CRYBB2, CRYGC, CRYGS, GJA3, MAF, NHS, and WFS1. Putative disease-causing variants were identified in 14 of 20 families (70%) as isolated cases and in 6 of 20 families (30%) with syndromic cases. A recessive variant in the CRYBB2 gene in a consanguineous family with 2 affected siblings showing a nuclear and sutural cataract was reported in contrast to previously published reports. In addition, the effect on splicing in a minigene assay of a novel splice site variant in the NHS gene (c.[719-2A>G]) supported the pathogenicity of this variant.

Conclusions and relevance: This study emphasizes the importance of genetic testing of congenital cataracts. Known dominant genes need to be considered for recessive inheritance patterns. Syndromic types of cataract may be underdiagnosed in patients with mild systemic features.

Conflict of interest statement

Conflict of Interest Disclosures: None reported.

Figures

Figure 1.. Spectrum of Pathogenic Variants

Relative proportion of 14 different genes in which disease-causing variants were identified using next-generation sequencing in our cohort. aIndicates syndromic pediatric cataract.

Figure 2.. Phenotype Spectrum

A, Slitlamp photography demonstrating the cataract phenotype with pathogenic variants in the CRYBB2 gene in patient III:1 of family 2 (left eye: nuclear, partially sutural cataract). B, A pathogenic variant in the CRYGS gene is identified in patient III:2 of family 6 with a lamellar cataract and a distinct shape resembling a lip (left eye). C, The different cataract phenotype with a pathogenic variant in the GJA8 gene is shown in patient II:1 of family 9 (right eye with a pulverulent cataract restricted to the nucleus). Detailed phenotype images are available in eFigure 3 in the Supplement.

Figure 3.. Functional Analysis by Minigene Assay of a Novel NHS (Nance-Horan Syndrome) Splice Site Variant

The NHS variant c.[719-2A>G] detected in family 20 shows a different splice product in a minigene assay. A, Schematic diagram of the transcript derived from the minigene plasmid construct. Major splice products obtained by polymerase chain reaction (PCR) (primers indicated) and analyzed by Sanger sequencing (eFigure 2 in the Supplement) are shown for the reference sequence and the NHS variant c.[719-2A>G]. B, Gel electrophoresis of amplified complementary DNA derived from minigene construct transfected HEK293T cells using primers indicated in part A. Two independent clone constructs were used for each separate transfection. C, Reverse transcriptase (RT)–PCR on RNA from whole blood obtained from family 20 (patient I:2) carrying the NHS variant c.[719-2A>G] in heterozygous state and a control sample (CS). PCR was performed using primers in exon 2 and 3 of the NHS gene. Expected PCR product size of the reference transcript is 122 base pairs (bp). D, Alamut Visual splicing window view of NHS variant c.[719-2A>G]. Different indicated estimation algorithms show a cryptic splicing acceptor site 83 nucleotides (83nt) upstream of NHS exon 3 (ex3). The red box indicates premature termination codon (PTC) of the NHS variant splice product if the reading frame from NHS exon 2 is continued. NTC indicates no template control; part, partial; and RHO, rhodopsin.