Next-generation sequencing identifies unexpected genotype-phenotype correlations in patients with retinitis pigmentosa

Abstract

Retinitis pigmentosa (RP) is an inherited degenerative disease causing severe retinal dystrophy and visual impairment mainly with onset in infancy or adolescence. Targeted next-generation sequencing (NGS) has become an efficient tool to encounter the enormous genetic heterogeneity of diverse retinal dystrophies, including RP. To identify disease-causing mutations in unselected, consecutive RP patients, we conducted Sanger sequencing of genes commonly involved in the suspected genetic RP subtype, followed by targeted large-panel NGS if no mutation was identified, or NGS as primary analysis. A high (70%) detection rate of disease-causing mutations was achieved in a large cohort of 116 unrelated patients. About half (48%) of the solved RP cases were explained by mutations in four genes: RPGR, EYS, PRPF31 and USH2A. Overall, 110 different mutations distributed across 30 different genes were detected, and 46 of these mutations were novel. A molecular diagnosis was achieved in the majority (82–100%) of patients if the family history was suggestive for a particular mode of inheritance, but only in 60% in cases of sporadic RP. The diagnostic potential of extensive molecular analysis in a routine setting is also illustrated by the identification of unexpected genotype-phenotype correlations for RP patients with mutations in CRX, CEP290, RPGRIP1, MFSD8. Furthermore, we identified numerous mutations in autosomal dominant (PRPF31, PRPH2, CRX) and X-linked (RPGR) RP genes in patients with sporadic RP. Variants in RP2 and RPGR were also found in female RP patients with apparently sporadic or dominant disease. In summary, this study demonstrates that massively parallel sequencing of all known retinal dystrophy genes is a valuable diagnostic approach for RP patients.

Genetic testing in patients with retinitis pigmentosa: Features of unsolved cases

“…Such pedigree characteristics were present in 49% (38/77) and 20% (7/35) of genetically solved and genetically unsolved patients, respectively (Figure D). As previously described, the likelihood to obtain a molecular diagnosis was higher in case of a discernible inheritance pattern (depending on the mode of inheritance: 83%‐100%) than in patients with sporadic RP (58%) …”

“…Re‐evaluating patients with atypical findings and identified mutations revealed potential explanations: Patients with an early‐onset central retinal atrophy exhibited RDH12 and CRB1 related retinopathy, both of which are known to be associated with such a phenotype. The patient with an absence of bone spicule pigmentation was relatively young and carried disease‐causing mutations in CDHR1 , which is known to cause a delayed‐onset retinal degeneration; thus, the patient will likely develop bone spicule pigmentation later in life . Asymmetric disease manifestation was observed in two female patients with sporadic RP in whom only one eye was suggestive for their X‐linked carrier status, likely due to skewed X‐inactivation in the more severely affected eye .…”

“…Prior to genetic testing, the diagnosis of RP was made based on compatible structural retinal changes and a reported history of nyctalopia and/or visual field defects. Four patients of the original cohort (n = 116) were excluded because their clinical classification was reversed after genetic testing . Disease‐causing mutations were identified in 77 (69%) of the 112 patients (see Ref.…”

“…Disease‐causing mutations were identified in 77 (69%) of the 112 patients (see Ref. for a detailed description including methods of molecular screening). For the purpose of this study, patients with two variants in a gene associated with autosomal recessive RP were assumed to be “genetically solved” even if phase information was not available.…”

“…These characteristic findings were used to distinguish patients with atypical morphological fundus features. Indicators for phenocopies: post‐inflammatory conditions and neoplastic or non‐neoplastic autoimmune retinopathy may mimic an RP phenotype. Thus, a comprehensive general medical history and a precise history of visual symptoms were obtained and analysed. Age of first symptoms: even though late‐onset disease may occur in patients with RP, nyctalopia as an early symptom is typically present within the first decades of life. Evidence for a particular inheritance pattern: a family history indicating a particular mode of inheritance (autosomal recessive, autosomal dominant or X‐linked; defined as previously described) would be less likely in diseases mimicking RP.…”