Abstract

Volume.127 Number.3

Toward Genomic Medicine: Research and Clinical Application in Ocular Genetics
Yoshihiro Hotta
Department of Ophthalmology, Hamamatsu University School of Medicine

Genetics has made great progress in the last three decades. I have consistently studied genes for intractable ocular diseases and congenital anomalies, from the days of viewing genes via Southern blotting to the current era of next-generation sequencing. I would like to discuss my experiences and challenges, focusing on what my collaborators and I have learned.
I. The dawn of molecular genetics
In the early days of molecular genetics, we discovered abnormalities in the ornithine aminotransferase (OAT) gene in patients with gyrate chorioretinal atrophy and demonstrated a codominant mode of inheritance. In the 1990s, with the availability of polymerase chain reaction, we found that detection of the mutation at nucleotide position (nt) 11778 in mitochondrial DNA was useful for the genetic diagnosis of Leber hereditary optic neuropathy (LHON). Furthermore, we observed a high frequency of this mutation in Japanese patient with LHON. We also discovered the first Japanese patient with retinitis pigmentosa (RP) caused by a rhodopsin gene mutation. While studying at Juntendo University and then at Nagoya University, I have reported many findings concerning the clinical characteristics and genotype-phenotype relationship in cases of corneal dystrophy and retinal dystrophy, respectively.
II. Molecular genetics of retinitis pigmentosa and Leber congenital amaurosis (LCA)
At the Hamamatsu University School of Medicine, I set up a laboratory for genetic analysis and collaborated with domestic and overseas institutions. We determined that all exon sequences of the eyes shut homolog (EYS) gene via Sanger sequencing. EYS gene mutations are the most frequent cause of RP in the Japanese populations. Today, we can analyze the entire gene of a patient comprehensively using a next-generation sequencer. The spectrum of genes responsible for adult RP and pediatric LCA is much different.
III. Molecular genetics of complex genes and complex genetic variants
Surprisingly, genetic ocular diseases are each caused by only one mutation. Currently, it is common to examine variants using whole-exome sequencing (WES) and whole-genome sequencing (WGS) and to confirm them by Sanger sequencing. However, there are challenges associated with complex genes, such as the OPN1LW/OPN1MW gene cluster in blue cone monochromacy, the IKBKG/NEMO in incontinentia pigmenti, and an imbalanced translocation involving the FOXC1 region in childhood glaucoma. Moreover, it is challenging to accurately analyze the ORF15 in the RPGR, or the Alu insertion of the RP1. There are rare cases of structural gene abnormalities that cannot be identified without WGS and complex gene abnormalities such as uniparental disomy.
IV. Two mutations influenced an ocular finding and a single mutation caused two ocular diseases
The GNAT1 mutation impairs rod function and the ABCA4 mutation impairs cone function. We reported siblings whose two mutations were thought to affect retinal function. We also reported a case of an adult with comorbid RP and LHON that harbored COQ2 gene mutations. In the future, we will be able to elucidate the role of introns, the regulation of gene expression, and the interaction of genes, as well as to perform gene analysis at the single-cell level. A more accurate understanding of the influence of a genetic variant on diseases may reveal differences in the clinical characteristics of patients with the same variant.
V. A message to fellow ophthalmologists
Genetic ocular diseases are often severe and difficult to treat, rendering a clinician as myself helpless. As the era of gene therapy has begun, I would like to point out; 1) the need for appropriate genetic counseling, 2) the merits such as more precise treatment and care based on the causative variant, and 3) the demerits, such as the impact on close relatives and incidental finding or secondary findings. If we can understand the diversity of patients in terms of their genetic diagnoses and the response to the environment and drugs through more accurate parameters, it will be possible to determine the therapeutic effect on genetic ocular diseases in a short period of time and establish evidence-based therapy. I hope that the next generation will overcome intractable genetic diseases.
Nippon Ganka Gakkai Zasshi (J Jpn Ophthalmol Soc) 127: 297-328, 2023.

Key words
Hereditary eye disease, Intractable ocular disease, Congenital anomaly, Gyrate chorioretinal atrophy, Leber hereditary optic neuropathy (LHON), Corneal dystrophy, Retinal dystrophy, Retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), Blue cone monochromacy, Incontinentia pigmenti, Childhood glaucoma, Uniparental disomy, Genetic counseling, Gene therapy
Reprint requests to
Yoshihiro Hotta, M. D. Department of Ophthalmology, Hamamatsu University School of Medicine. 1-20-1 Handayama, Higashi-ku, Hamamatsu-shi 431-3192, Japan