Pathogenicity of a novel missense variant associated with choroideremia and its impact on gene replacement therapy

Choroideremia (CHM) is an inherited retinal dystrophy characterised by progressive degeneration of photoreceptors, retinal pigment epithelium (RPE) and underlying choroid. It is caused by loss-of-function mutations in CHM, which has an X-linked inheritance, and is thus an ideal candidate for gene replacement strategies. CHM encodes REP1, which plays a key role in the prenylation of Rab GTPases. We recently showed that an induced pluripotent stem cell (iPSc)-derived RPE model for CHM is fully functional and reproduces the underlying prenylation defect. This criterion can thus be used for testing the pathogenic nature of novel variants. Until recently, missense variants were not associated with CHM. Currently, at least nine such variants have been reported but only two have been shown to be pathogenic. We report here the characterisation of the third pathogenic missense CHM variant, p.Leu457Pro. Clinically, the associated phenotype is indistinguishable from that of loss-of-function mutations. By contrast, this missense variant results in wild type CHM expression levels and detectable levels of mutant protein. The prenylation status of patient-specific fibroblasts and iPSc-derived RPE is within the range observed for loss-of-function mutations, consistent with the clinical phenotype. Lastly, considering the current climate of CHM gene therapy, we assayed whether the presence of mutant REP1 could interfere with a gene replacement strategy by testing the prenylation status of patient-specific iPSc-derived RPE following AAV-mediated gene transfer. Our results show that correction of the functional defect is possible and highlight the predictive value of these models for therapy screening prior to inclusion in clinical trials.