Disrupted prenatal RNA processing and myogenesis in congenital myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a CTG microsatellite expansion (CTGexp) disorder caused by expression of CUGexp RNAs. These mutant RNAs alter the activities of RNA processing factors, including MBNL proteins, leading to reversion to specific fetal isoforms in adult tissues and DM1 pathology. While this pathogenesis model accounts for adult-onset disease, the molecular basis of congenital DM (CDM) is unknown. Here, we test the hypothesis that disruption of developmentally regulated RNA alternative processing pathways contributes to CDM disease. Analysis of alternative splicing (AS) transitions during human myogenesis reveals hundreds of AS events that undergo prenatal isoform transitions and both AS and alternative polyadenylation abnormalities are prominently detectable in infant CDM muscle biopsies. While the majority of RNA targets are also mis-regulated in adult-onset DM1, splicing dysregulation is significantly more severe in CDM. Since many of these mis-processing events are MBNL-regulated, we generated mouse Mbnl double (Mbnl1; Mbnl2) and triple (Mbnl1; Mbnl2; Mbnl3) muscle-specific knockout models that recapitulate the congenital myopathy, gene expression and spliceopathy defects characteristic of CDM. This study demonstrates RNA mis-processing is a major pathogenic factor in CDM and provides novel mouse models to further examine roles for co/post-transcriptional gene regulation during tissue development. Overall design: Disease-control (SMA type 1) and CDM skeletal muscle (biceps brachii) was used for RNA-seq and polyA-seq and transcriptome analysis (gene expression, alternative splicing, and alternative cleavage and polyadenylation) was performed to identify RNA mis-processing associated with disease. Newborn (P0) quadriceps muscle from wild-type, double (Mbnl1; Mbnl2) knockout (DKO), and TKO (Mbnl1; Mbnl2; Mbnl3) knockout (TKO) muscle and wild-type and Mbnl3 knockout (3KO) myoblasts were also used for transcriptome analysis to test the contribution of MBNL loss-of-function to disease features.