Residual Cystine Transport Activity for Specific Infantile and Juvenile CTNS Mutations in a PTEC-Based Addback Model

Cystinosis is a rare, autosomal recessive, lysosomal storage disease caused by mutations in the gene , leading to cystine accumulation in the lysosomes. While cysteamine lowers the cystine levels, it does not cure the disease, suggesting that CTNS exerts additional functions besides cystine transport. This study investigated the impact of infantile and juvenile mutations with discrepant genotype/phenotype correlations on CTNS expression, and subcellular localisation and function in clinically relevant cystinosis cell models to better understand the link between genotype and CTNS function. Using CTNS-depleted proximal tubule epithelial cells and patient-derived fibroblasts, we expressed a selection of CTNS under various promoters. -driven expression led to substantial overexpression, resulting in CTNS protein levels that localised to the lysosomal compartment. All CTNS tested also reversed cystine accumulation, indicating that CTNS still exert transport activity, possibly due to the overexpression conditions. Surprisingly, even CTNS expression driven by the less potent and promoters reversed the cystine accumulation, contrary to the CTNS missense mutant. Taken together, our findings shed new light on mutations, highlighting the need for robust assessment methodologies in clinically relevant cellular models and thus paving the way for better stratification of cystinosis patients, and advocating for the development of more personalized therapy.