There has been a remarkable increase in knowledge on genetic disorders of the eye in the last 40 years, and genetics has become increasingly important in ophthalmic practice. In the 1980s, RB1 was discovered as the first tumor suppressor gene, after which the mechanism of the onset of retinoblastoma and heredity was elucidated. Subsequent advances in molecular genetics have led to the identification and understanding of the pathophysiology of RB1 dysfunctional variants (gene mutations) and RB1 genetic testing of cells in the blood of patients with retinoblastoma can now provide information not only for the patient but also for family care and genetic counseling. Moreover, over the past 10 years, there have been major paradigm shift changes in the analyses of Mendelian disorders. More specifically, a widely used analysis method based on the principle of mass parallel sequencing, the so-called next-generation sequencing (NGS), has become wide spread. This technique has greatly improved the efficiency of genetic analyses of patients with genetic disorders and their families. Our group has performed genetic analyses focusing on hereditary corneal dystrophy, optic nerve disease, and inherited retinal disorders (IRD). For corneal dystrophy, the frequent pathological variants have been well characterized and conventional Sanger sequencing is often efficient in drawing genetic conclusions. However, for IRD, genetic heterogeneity hampers identification of responsible gene for disease in many patients. Moreover, over half of our patients have retinitis pigmentosa, which can sometimes be difficult to differentiate from advanced macular dystrophy or cone-rod dystrophy. Due to this genetic heterogeneity, NGS has been widely used for genetic analyses of IRD and has led to remarkable achievements in understanding the disease. In many cases, either targeted sequencing where only a large number of known (or potential) causative genes are analyzed simultaneously or exome analyses where all exons (protein coding region) are analyzed in principle is performed; however, whole-genome analyses where all genes are analyzed may also be performed. We introduced targeted sequencing to analyze 130 genes and have been able to identify the cause with reasonable efficiency (approximately 40%) compared with reports from other institutions. Technical and genetic challenges need to be addressed to reduce the proportion of unresolved cases. Challenges such as functional evaluations of variants of unknown significance (VUS) seen at considerable high frequencies and the detection of gene defects and copy number variations (CNV) need to be addressed. Various computer analysis programs have been used to analyze the effects of genetic changes, but the importance of wet analysis using cells with genetic variants has again become apparent. Recent advances in cell reprogramming technology could enable ophthalmologists to establish in vitro models of diseases by generating retinal organoids using induced pluripotent stem cells (iPSC). At present, iPSCs are established from monocytes of patients with retinitis pigmentosa, cone dystrophy, and Stargardt disease for which causal variants have been identified. Despite the various challenges presented by this field, we hope to establish experimental systems using retinal organoids to aid in the search for new therapeutic agents.
Nippon Ganka Gakkai Zasshi (J Jpn Ophthalmol Soc) 125: 210-229, 2021.