A new nonsense mutant mouse model for cystinosis
Presented by Fatima Tokhmasfshan, Research Institute of McGill University Health Centre, Montreal, Canada
Authors*: F. Tokhmafshan, L.L. Chu, M.M. Akpa, P.R. Goodyer
Lay Summary: We have previously shown that aminolgycosides can ameliorate the problem with gene expression as a result of nonsense mutations (i.e. premature STOP codons). To test the efficacy of aminoglycosides in treating proximal tubulopathy as a result of nonsense mutations in the CTNS gene, we have created a new mouse model for cystinosis that harbours the most common human nonsense mutation: W138X. We will study the disease course in these mice and inject them with a new class of less toxic and more efficient aminoglycoside, and evaluate whether early injection of the drug can protect against proximal tubulopathy.
Oral cysteamine therapy slows organ deterioration in cystinosis but doesn’t obviate eventual need for renal transplantation and has never been shown to prevent proximal tubulopathy (Fanconi Syndrome). We have hypothesized that novel translational readthrough drugs may benefit about 15% of cystinosis patients worldwide who carry one or more CTNS nonsense mutations. The most common of these is the CTNSW138X which was introduced from Ireland into the French-Canadian population and accounts for 40-50% of cystinosis alleles in Quebec. The W138X responds well to the novel aminoglycoside, ELX-02, in vitro, and preliminary data from a Phase II clinical trial shows that the drug reduced leukocyte cystine in young adults with biallelic CTNSW138X/W138X mutations.
To test the effect of early treatment with translational readthrough drugs, we used the CRISPR-Cas9 system to create a knock-in mutant mouse model. Three founders were shown to have biallelic CtnsW138X/W138X mutations in the germline, the mice were free of other unintended Ctns mutations.
Measuring kidney cystine levels in these mice at three-month of age showed that the three mutant mouse lines had levels that were elevated 6X, 3X and 2X above wildtype littermates. We are now conducting a longitudinal urinalysis study to characterize the onset of Fanconi syndrome in our mouse model, assaying low molecular-weight proteinuria, amino aciduria and glucosuria in mice 3-9 months old. Our results thus far show that the mutant mice indeed present with proteinuria starting at 3 months of age.
These data will identify an age at which to introduce readthrough drug therapy (prior to onset of Fanconi Syndrome) so that we can ascertain whether early therapy can prevent proximal tubulopathy. This could help guide a new treatment course for individuals with cystinosis due to nonsense mutations.
* Authors’ affiliation
Research Institute of McGill University Health Centre, Montreal, Canada