Identification of IDH3G, encoding the gamma subunit of mitochondrial isocitrate dehydrogenase, as a novel candidate gene for X-linked retinitis pigmentosa.

Objective: Retinitis pigmentosa (RP) is a genetically heterogeneous group of retinal degenerative disorders characterized by the loss of rod and cone photoreceptors, leading to visual impairment and blindness. To date, to our knowledge, X-linked RP has been associated with variants in 3 genes (RPGR, RP2, and OFD1), whereas genetic defects at 3 loci (RP6, RP24, and RP34) are yet unidentified. The aim of this study was to identify a novel candidate gene underlying X-linked RP.

Methods: Participants were identified from cohorts of genetically unsolved male individuals affected by RP, who underwent genome sequencing, exome sequencing, or candidate gene screening via direct Sanger sequencing at 3 referral centers. Specifically, 2 probands were identified at the National Reference Centre for Rare Retinal Diseases (Paris, France), 2 at the Massachusetts Eye and Ear Hospital (Boston, MA), and 1 at the National Reference Centre for Inherited Sensory Diseases (Montpellier, France). The pathogenicity of the identified variants was assessed using bioinformatic predictions, protein expression analyses, and mitochondrial function assays.

Results: We identified 4 rare single-nucleotide variants in IDH3G (HGNC:5386), located at the RP34 locus on the X chromosome, and a complete gene deletion, in 5 unrelated male individuals affected with nonsyndromic RP. The variants segregated with the phenotype in all available family members. In all cases, the disease severity was intermediate. None had high myopia. IDH3G encodes the γ subunit of mitochondrial isocitrate dehydrogenase (IDH3), an enzyme involved in the citric acid cycle, which is expressed in the inner segments of photoreceptors. Variants in IDH3A and IDH3B, encoding the other subunits of IDH3, have already been associated with nonsyndromic autosomal recessive RP. Bioinformatic predictions and functional assays support a pathogenic role for the variants identified in this study, possibly through partial loss of enzymatic activity and mitochondrial function.

Conclusions: Our findings suggest that variants in IDH3G are a novel cause of X-linked RP.