Commitment to Myogenic Differentiation Significantly Aggravates the RNA Phenotype in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a severe neuromuscular disorder classified as a spliceopathy, caused by a (CTG)n repeat expansion in the 3' UTR of the DMPK gene. The expansion in DMPK transcripts sequesters key splicing regulators of the MBNL family, leading to dysregulated alternative splicing. DM1 presents heterogeneous symptoms, with prevalent muscle weakness and myotonia, highlighting the need to understand its impact on the myogenesis process in more detail. This study aims to understand the impact of myotonic dystrophy type 1 (DM1) on myogenesis by investigating RNA expression during the differentiation of DM1 and isogenic CRISPR/Cas9-corrected DM∆ myoblast cell lines into myotubes.

RNA samples were collected at various stages of myogenesis from DM1 and control DM∆ myoblast cell lines. Gene expression patterns and alternative splicing signatures were analysed using high-coverage sequencing.

Proliferating myoblasts exhibited a mild phenotype, with only a few differentially expressed genes and aberrant splicing events. However, upon commitment to fusion in differentiating cultures, there was a marked increase in differentially expressed genes between DM1 and corrected cells, particularly those related to muscle function and ion transport. Notably, aberrant alternative splicing, enriched for MBNL1 binding motifs, aggravated during differentiation, affecting genes associated with muscle organization, contraction and cell junctions.

These findings highlight that the disturbance of myogenesis becomes particularly evident upon commitment to differentiation, emphasising the critical role of differentiation- and MBNL1-dependent splicing throughout myogenesis.