Inherited retinal disorders: a genotype-phenotype correlation in an Indian cohort and the importance of genetic testing and genetic counselling

PURPOSE

Recent advances in sequencing technologies have enabled radical and rapid progress in the genetic diagnosis of inherited retinal disorders (IRDs). Although the list of gene variations continues to grow, it lacks the genetic etiology of ethnic groups like South Asians. Differences in racial backgrounds and consanguinity add to genetic heterogeneity and phenotypic overlaps.

METHODS

This retrospective study includes documented data from the Gen-Eye clinic from years 2014 to 2019. Medical records and pedigrees of 591 IRD patients of Indian origin and genetic reports of 117 probands were reviewed. Genotype-phenotype correlations were performed to classify as correlating, non-correlating and unsolved cases.

RESULTS

Among the 591 patients, we observed a higher prevalence of clinically diagnosed retinitis pigmentosa (38.9%) followed by unspecified diagnoses (28.5%). Consanguinity was reported to be high (55.6%) in this cohort. Among the variants identified in 117 probands, 36.4% of variants were pathogenic, 19.2% were likely pathogenic, and 44.4% were of uncertain significance. Among the pathogenic and likely pathogenic variants, autosomal recessive inheritance showed higher prevalence. About 35% (41/117) of cases showed genotype-phenotype correlation. Within the correlating cases, retinitis pigmentosa and Stargardt disease were predominant. Novel variants identified in RP, Stargardt, and LCA are reported here.

CONCLUSION

This first-of-a-kind report on an Indian cohort contributes to existing knowledge and expansion of variant databases, presenting relevant and plausible novel variants. Phenotypic overlap and variability lead to a differential diagnosis and hence a clear genotype-phenotype correlation helps in precise clinical confirmation. The study also emphasizes the importance of genetic counselling and testing for personalized vision care in a tertiary eye hospital.

The Natural History of Leber's Hereditary Optic Neuropathy in an Irish Population and Assessment for Prognostic Biomarkers

In this study we have assessed the clinical and genetic characteristics of an Irish Leber's hereditary optic neuropathy (LHON) cohort and assessed for useful biomarkers of visual prognosis. We carried out a retrospective review of clinical data of patients with genetically confirmed LHON presenting to an Irish tertiary referral ophthalmic hospital. LHON diagnosis was made on classic clinical signs with genetic confirmation. Alternate diagnoses were excluded with serological investigations and neuro-imaging. Serial logarithm of the minimum angle of resolution (logMAR) visual acuity (VA) was stratified into 'on-chart' for logMAR 1.0 or better and 'off-chart' if worse than logMAR 1.0. Serial optical coherence tomography scans of the retinal nerve fibre layer (RNFL) and ganglion cell complex (GCC) monitored structure. Idebenone-treated and untreated patients were contrasted. Statistical analyses were performed to assess correlations of presenting characteristics with final VA. Forty-four patients from 34 pedigrees were recruited, of which 87% were male and 75% harboured the 11778 mutation. Legal blindness status was reached in 56.8% of patients by final review (mean 74 months). Preservation of initial nasal RNFL was the best predictor of on-chart final VA. Females had worse final VA than males and patients presenting at < 20 years of age had superior final VA. Idebenone therapy (50% of cohort) yielded no statistically significant benefit to final VA, although study design precludes definitive comment on efficacy. The reported cases represent the calculated majority of LHON pedigrees in Ireland. Visual outcomes were universally poor; however, VA may not be the most appropriate outcome measure and certain patient-reported outcome measures may be of more use when assessing future LHON interventions.

Target 5000: a standardized all-Ireland pathway for the diagnosis and management of inherited retinal degenerations

Introduction

Inherited retinal degenerations (IRD) are rare genetic disorders with > 300 known genetic loci, manifesting variably progressive visual dysfunction. IRDs were historically underserved due to lack of effective interventions. Many novel therapies will require accurate diagnosis (phenotype and genotype), thus an efficient and effective pathway for assessment and management is required.

Methods

Using surveys of existing practice patterns and advice from international experts, an all-Ireland IRD service (Target 5000) was designed. Detailed phenotyping was followed by next generation genetic sequencing in both a research and accredited laboratory. Unresolved pedigrees underwent further studies (whole gene/whole exome/whole genome sequencing). Novel variants were interrogated for pathogenicity (cascade screening, in silico analysis, functional studies). A multidisciplinary team (MDT; ophthalmologists, physicians, geneticists, genetic counsellors) reconciled phenotype with genotype. A bespoke care plan was created for each patient comprising supports, existing interventions, and novel therapies/clinical trials.

Results and discussion

Prior to Target 5000, a significant cohort of patients were not engaged with healthcare/support services due to lack of effective interventions. Pathogenic or likely pathogenic variants in IRD-associated genes were detected in 62.3%, with 11.6% having variants of unknown significance. The genotyping arm of Target 5000 allowed a 42.73% cost saving over independent testing, plus the value of MDT expertise/processing. Partial funding has transferred from charitable sources to government resources.

Conclusion

Target 5000 demonstrates efficacious and efficient clinical/genetic diagnosis, while discovering novel IRD-implicated genes/variants and investigating mechanisms of disease and avenues of intervention. This model could be used to develop similar IRD programmes in small/medium-sized nations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01841-1.

Application of CRISPR Tools for Variant Interpretation and Disease Modeling in Inherited Retinal Dystrophies

Inherited retinal dystrophies are an assorted group of rare diseases that collectively account for the major cause of visual impairment of genetic origin worldwide. Besides clinically, these vision loss disorders present a high genetic and allelic heterogeneity. To date, over 250 genes have been associated to retinal dystrophies with reported causative variants of every nature (nonsense, missense, frameshift, splice-site, large rearrangements, and so forth). Except for a fistful of mutations, most of them are private and affect one or few families, making it a challenge to ratify the newly identified candidate genes or the pathogenicity of dubious variants in disease-associated loci. A recurrent option involves altering the gene in in vitro or in vivo systems to contrast the resulting phenotype and molecular imprint. To validate specific mutations, the process must rely on simulating the precise genetic change, which, until recently, proved to be a difficult endeavor. The rise of the CRISPR/Cas9 technology and its adaptation for genetic engineering now offers a resourceful suite of tools to alleviate the process of functional studies. Here we review the implementation of these RNA-programmable Cas9 nucleases in culture-based and animal models to elucidate the role of novel genes and variants in retinal dystrophies.

Advances in the genetics of eye diseases

PURPOSE OF REVIEW

An update on heritable eye disease will allow informed patient counseling and improved patient care.

RECENT FINDINGS

New loci and genes have been associated with identifiable heritable ocular traits. Molecular genetic analysis is available for many of these genes either as part of research or for clinical testing. The advent of gene array technologies has enabled screening of samples for known mutations in genes linked to various disorders. Exomic sequencing has proven to be particularly successful in research protocols in identifying the genetic causation of rare genetic traits by pooling patient resources and discovering new genes. Further, genetic analysis has led improvement in patient care and counselling, as exemplified by the continued advances in our treatment of retinoblastoma.

SUMMARY

Patients and families are commonly eager to participate in either research or clinical testing to improve their understanding of the cause and heritability of an ocular condition. Many patients hope that testing will then lead to appropriate treatments or cures. The success of gene therapy in the RPE65 form of Leber congenital amaurosis has provided a brilliant example of this hope; that a similar trial may become available to other patients and families burdened by genetic disease.