Résumé
Purpose : Choroideremia (CHM) is an X-linked recessive chorioretinal dystrophy, which exhibits high inter- and intra-familial variability. It is caused by mutations involving the CHM gene located at Xq21.2, but no genotype-phenotype correlation has been identified. Over 30% of patients harbour nonsense mutations (nm), and nonsense suppression therapy may offer a potential treatment. However, CHM mRNA transcripts can be degraded by nonsense-mediated decay (NMD), a cell's natural surveillance mechanism, thereby reducing the substrate for drug activity. NMD is variable even amongst patients with the same mutation and in different tissues of the same individual. In this study, we investigate levels of mRNA and related NMD activity, as this may provide a prognostic indicator of response to treatment.Methods : We took whole blood from 9 affected male nmCHM patients and 6 age- and gender-matched controls. Using RT-qPCR, we assayed levels of CHM and UPF1 (a key modulator of NMD) mRNA expression. We compared mRNA levels in fibroblasts and patient derived iPSC-RPE to look for any change between tissue types of the same patient, and also compared phenotype.Results : We found that nmCHM patients have a 2.8-fold lower level of CHM mRNA than controls, and this varied widely amongst patients, with 40% variation between those with the same UGA mutation (c.715C>T; p.R239*). These results indicate that although NMD machinery is at work; the efficiency is highly variable and not wholly dependent on mutation position. There was no statistically significant correlation between CHM mRNA expression and genotype, phenotype or UPF1 transcript levels. CHM transcript levels were similar in both cell types, however UPF1 expression varied considerably amongst different tissues.Conclusions : Baseline CHM mRNA levels may provide a prognostic indicator for response to nonsense suppression therapy. It may guide the choice of small molecule drugs that either have dual NMD inhibition and nonsense suppression activity, such as amlexonox, or the use of NMD inhibitors as a useful adjunct to enhance treatment efficacy where indicated.