Introduction
Retinal dystrophies caused by RPGRIP1 gene mutations present significant diagnostic challenges due to variable clinical manifestations. This nationwide Japanese cohort study investigates how specific genetic variants correlate with two distinct phenotypes: Leber congenital amaurosis (LCA) and achromatopsia (ACHM). Both conditions cause severe visual impairment but follow different clinical trajectories. Understanding these genotype-phenotype relationships is crucial for accurate diagnosis and personalized treatment approaches.
Study Design and Methodology
Researchers conducted a retrospective multicenter analysis across Japanese university hospitals, enrolling 34 patients from 26 families with confirmed biallelic pathogenic RPGRIP1 variants. Comprehensive genetic testing included polymerase chain reaction assays targeting the exon 18 deletion variant, whole-exome sequencing, and whole-genome sequencing. Ophthalmological assessments measured best-corrected visual acuity (BCVA), visual field performance, full-field electroretinography (ERG) responses, and multimodal retinal imaging including OCT and fundus photography to evaluate structural changes.
Key Genetic Findings
Fourteen distinct RPGRIP1 variants were identified across the cohort, with the exon 18 deletion variant (exon 18-DEL) dominating at 63.2% of alleles. This variant appeared in both LCA and ACHM cases, suggesting it represents a Japanese founder mutation. Notable genotype-phenotype patterns emerged: ACHM cases predominantly carried exon 18 variants, while LCA patients exhibited more diverse mutation combinations across the gene. This distribution indicates variant location significantly influences disease expression.
Clinical Phenotype Comparisons
Visual acuity outcomes differed substantially between groups. LCA patients demonstrated significantly worse BCVA from infancy, while ACHM patients maintained better visual function initially. Interestingly, both groups showed similar rates of vision decline over time. Retinal imaging revealed striking differences: LCA patients maintained relatively preserved macular structure even in advanced age, whereas ACHM demonstrated progressive outer retinal degeneration that worsened with age. ERG results showed near-complete loss of rod and cone function in LCA infants, contrasting with ACHM’s initially preserved rod function that gradually deteriorated.
Imaging and Functional Assessments
Multimodal imaging provided crucial insights into disease progression patterns. Optical coherence tomography clearly demonstrated earlier and more severe photoreceptor layer disruption in ACHM compared to LCA. Fundus autofluorescence revealed abnormal patterns in both groups, but with distinct topographic distributions. Electroretinography proved essential for differential diagnosis, showing non-recordable responses in LCA versus measurable but progressively declining signals in ACHM. These functional tests complement genetic findings for accurate phenotyping.
Significance and Clinical Implications
This research establishes clear genotype-phenotype correlations in RPGRIP1 disorders and identifies exon 18-DEL as a prevalent founder mutation in Japan. These findings have important clinical applications: they enable more precise prognostic counseling based on variant type and location, guide targeted genetic testing strategies, and inform patient selection for emerging gene therapies. The relatively preserved macular structure in LCA suggests this group may benefit from future intervention approaches targeting central retinal preservation.
Therapeutic Perspectives
Several therapeutic avenues appear promising based on this research. Gene replacement therapy could potentially address the underlying genetic defect, particularly for the prevalent exon 18-DEL variant. Neuroprotective approaches might help slow retinal degeneration in ACHM patients with progressive disease. Ongoing clinical trials investigating antisense oligonucleotides and optogenetic therapies could offer additional treatment options. These findings provide critical baseline data for future interventional studies in this population.
Conclusions
This landmark study establishes that RPGRIP1 mutations cause two distinct clinical entities through different molecular mechanisms. The exon 18 deletion serves as the predominant founder mutation in the Japanese population. The research significantly expands our understanding of RPGRIP1-associated retinal diseases, providing essential insights for diagnosis, prognosis, and therapeutic development. These findings will help clinicians deliver personalized counseling and management while accelerating the development of precision treatments for inherited retinal disorders.
