PORTEFEUILLE

There is currently no cure for myotonic dystrophy type 1 (DM1). The disease affects about 1 in 8000 individuals world-wide and patients develop problems with muscle relaxation (myotonia), muscle weakness and wasting, as well as cardiac abnormalities. Patients also develop cataracts and diabetes in addition to experiencing decreased motivation and daytime sleepiness.

DM1 is results from the expansion of a trinucleotide repeat in an unstranslated region of the mRNA coding for the myotonic dystrophy protein kinase (DMPK). Importantly, the longer the repeat tract, the more severe the symptoms are. It has therefore been proposed that reducing – referred to as contracting – the repeat tract down to a normal length would effectively remove the underlying cause of the disease and provide a therapeutic avenue. The goal of this project is to test this idea and provide the proof-of-concept that this is a viable option to treat patients.

Most treatments currently being developed, predominantly based on pharmaceutical molecules, aim to reduce the symptoms of the disease and are meant to be administered throughout a patient’s life, which brings up issues of long term toxicity. An alternative could include treatments designed to tackle the cause rather than the symptoms of DM1.

The correlation between repeat length and the severity of the symptoms was noticed over 20 years ago, yet repeat contraction has never been tested for its potential beneficial effects. This is mainly due to the lack of approaches that can specifically induce contractions. This is a limitation because if expansions are also caused by the treatment, then the disease symptoms would worsen. Here are testing a new customizable system, CRISPR-Cas9, for the ability to induce contractions.

DM1 is part of a family of neurodegenerative, neuromuscular, and neurological diseases caused by the expansion of trinucleotide repeats. While these diseases are phenotypically distinct, they all share a common cause: the expanded repeat. We therefore anticipate the approach tested here for DM1 will also be applicable to a wide variety of devastating diseases.

We now have shown that using a variant of the CRISPR-Cas9 enzyme (Cas9 D10A) that makes nicks on the DNA rather than cutting both strands, we can specifically induce contractions in human cell lines. This effect is specific for the expanded allele, leaving the essential normal allele that patients also carry intact. Furthermore, we were unable to detect off-target mutations using this approach, which could target all 14 different diseases caused by the expansion of CAG/CTG repeats. We are now turning our attention to the question of whether repeat contraction can reverse the cellular phenotypes in patient-derived cells and in murine models for multiple expanded repeat diseases.

Cinzia Cinesi, Lorène Aeschbach, Bin Yang, and Vincent Dion. Contracting CAG/CTG repeats using the CRISPR-Cas9 nickase. Nat. Commun. 7, 13272 doi: 10.1038/ncomms13272 (2016).