Cystinosis is a rare, recessive disorder caused by mutations of the CTNS gene, which encodes a lysosomal cystine channel. The disease is characterized by intralysosomal cystine accumulation and progressive organ dysfunction in all tissues of the body. The kidney is the first organ affected, with children presenting with renal Fanconi syndrome at 4-6 months of age and progressing to end-stage renal failure by 10-11 years. In Quebec, about 50% of cystinosis patients harbor the W138X nonsense mutation, causing a STOP codon in exon 7. Nonsense mutations account for about 20% of cystinotic mutations worldwide.

Aminoglycosides, such as geneticin (G418) and gentamicin, are known to have nonsense suppressor activity, which permits translational read-through of premature STOP codons and reduced nonsense-mediated decay (NMD). We transfected CTNS^Del/Del human fibroblasts with a pCMV-driven CTNS^W138X-DSRed reporter construct and observed DSRed signal after treatment with G418 for 24 hours. In a CTNS^W138X/W138X fibroblast line, endogenous CTNS protein was detectable by Western immunoblotting and intracellular cystine levels were reduced by 60% after 24-hour exposure to G418. Interestingly, baseline CTNS transcript levels in CTNS^W138X/W138X fibroblasts were 20-45% of normal fibroblasts, indicating nonsense- mediated mRNA decay. In contrast, CTNS transcript levels were nearly normalized after treatment with G418 for 24 hours.

These observations indicate that geneticin-like compounds may offer an effective therapeutic strategy for cystinosis caused by CTNS nonsense mutations.