Dominant RDH12-retinitis pigmentosa impairs photoreceptor development and cone function in retinal organoids

Retinal dehydrogenase 12 (RDH12) is a photoreceptor NADPH-dependent retinal reductase enzyme, which converts all-trans-retinal to all-trans-retinol. In rare cases, heterozygous variants in RDH12 are associated with a mild, late-onset autosomal dominant (AD) retinitis pigmentosa (RP). There are no dominant RDH12 animal models, hence, we generated retinal organoids from human induced pluripotent stem cells (hiPSC) derived from dermal fibroblasts from a RDH12-AD patient with the heterozygous pathogenic variant c.759del p.(Phe254Leufs*24) and an unrelated unaffected individual (WT). Retinal organoids were characterised at different stages of differentiation; at 18-weeks there were no differences in rod and cone photoreceptors with correct localisation of RDH12 in the photoreceptor inner segments in both WT and RDH12-AD. Transmission electron microscopy on week 37 retinal organoids showed a normal outer limiting membrane and mitochondria, but photoreceptors were 1.8 times less abundant and shorter in RDH12-AD mutant compared to WT. Transcriptomic analyses on week 44 retinal organoids revealed visual cone function, retinol biosynthesis and the vitamin A pathway are highly affected in RDH12-AD retinal organoids. Our study is the first to describe a human RDH12 retinal organoid model, and we found subtle differences between the WT and mutant at later stages of photoreceptor differentiation. This is in keeping with the milder disease course seen in humans, but it provides insights into the aetiology and possible targets for future therapeutic development.