Compound loss of muscleblind-like function in myotonic dystrophy

Myotonic dystrophy (DM) is a multi-systemic disease that severely impacts cardiac and skeletal muscle functions as well as the central nervous system. DM is unusual because it is RNA-mediated disease due to the expression of C(C)UG expansion RNAs that inhibit the activities of the muscleblind-like (MBNL) proteins. In mice, studies using Mbnl1 and Mbnl2 single knockouts have revealed that Mbnl1 plays a predominant role in skeletal and heart muscle alternative splicing regulation while Mbnl2 performs an analogous splicing function in the brain. However, Mbnl single knockout models fail to recapitulate the full-range of adult-onset DM muscle symptoms. Here, we report that Mbnl1; Mbnl2 double knockouts are embryonic lethal while Mbnl1-/-; Mbnl2+/- mice, which express no Mbnl1 and reduced levels of Mbnl2, are viable but develop cardinal features of adult-onset DM cardiac and skeletal muscle disease including reduced lifespan, heart conduction block, severe myotonia and progressive skeletal muscle weakness. Mbnl2 protein levels are elevated in both Mbnl1-/- and Mbnl1-/-; Mbnl2+/- knockouts where Mbnl2 targets Mbnl1-regulated exons. These findings support the MBNL loss-of-function model for DM and provide novel Mbnl compound knockout models to investigate the molecular pathways disrupted by RNA-mediated disease. Mbnl2 protein-RNA interactions were assessed in 4-month-old WT and Mbnl1-/- quadriceps muscles in triplicates by HITS-CLIP.