Characterization 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 characterized >20 cell models to define DM1 biomarkers that could be useful for drug screening. DMPK protein levels were reduced in DM1-immortalized myoblasts and myotubes, but not in fibroblasts, while MBNL1 protein was consistently lower in all DM1 myogenic cultures, whether primary or immortalized. 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 platform's 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 characterization of some of the most used models would be of use to the DM1 research community.