Characterisation of DMPK and MBNL1 expression in cell models of Myotonic Dystrophy: A platform for drug screening

Myotonic dystrophy type I (DM1) is caused by CTG repeat expansions in the DMPK gene leading to mRNA toxicity and sequestration of the splicing regulator MBNL1, affecting many tissues. We have developed an in vitro screening platform based on ddPCR and in-cell western to quantify these mRNAs and proteins and characterised more than 20 cell models to define DM1 biomarkers that could be useful for drug screening. DMPK protein levels were reduced in DM1-immortalised myoblasts and myotubes, but not in fibroblasts, while MBNL1 protein was consistently lower in all DM1 myogenic cultures, whether primary or immortalised. Myogenic differentiation of cultures led to an increase in DMPK mRNA expression, which was translated into increased MBNL1 sequestration in foci. We further corroborated the platforms ability to assess therapeutic outcomes, evaluating the effect of a DMPK gapmer ASO and one siRNA: while the gapmer increased MBNL1 protein levels, the siRNA had no significant effect on MBNL1 release. Our platform and the in-depth characterisation of some of the most used models would be of use to the DM1 research community. Significance statementMyotonic dystrophy type I (DM1) is a multisystemic disease with a complex pathogenesis and multiple outcome measures for drug assessment in vitro. In the last years, the increasing number of new potential therapies targeting DM1 in clinical trials has increased the need for robust and rapid evaluation of preclinical candidates, as well as in-depth knowledge of the cell models used. Here, we present a new cell-based platform that enables robust quantification of DMPK and MBNL1 in cell culture for cell model characterisation and drug screening. Indeed, we highlight the differences observed in DMPK and MBNL1 protein quantification in primary fibroblasts and myotubes, immortalised fibroblasts, myoblasts and myotubes. We then, we performed a proof-of-concept drug evaluation of potential therapeutic strategies targeting DMPK, showing the most suitable for targeting the DMPK expanded transcript.