Disrupted prenatal RNA processing and myogenesis in congenital myotonic dystrophy

Abstract

Myotonic dystrophy type 1 (DM1) is a CTG microsatellite expansion (CTG exp ) disorder caused by expression of CUG exp RNAs. These mutant RNAs alter the activities of RNA processing factors, including MBNL proteins, leading to re-expression of 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. We identify prominent alternative splicing and polyadenylation abnormalities in infant CDM muscle, and, although most are also misregulated in adult-onset DM1, dysregulation is significantly more severe in CDM. Furthermore, analysis of alternative splicing during human myogenesis reveals that CDM-relevant exons undergo prenatal RNA isoform transitions and are predicted to be disrupted by CUG exp -associated mechanisms in utero. To test this possibility and the contribution of MBNLs to CDM pathogenesis, 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 that RNA misprocessing is a major pathogenic factor in CDM and provides novel mouse models to further examine roles for cotranscriptional/post-transcriptional gene regulation during development.

Congenital myotonic dystrophy—an RNA-mediated disease across a developmental continuum

“…Thomas et al (2017) addressed this question by showing that disrupted RNA processing also underlies CDM pathology, with key differences from adult-onset DM1 that may explain the severity and the early onset of CDM. Thomas et al (2017) first quantified splicing dysregulation in CDM by comparing the skeletal muscle transcriptomes of CDM-affected infants with disease control (spinal muscular atrophy type I) and healthy control transcriptomes. They identified hundreds of potentially disease-relevant isoforms that were differentially spliced in CDM muscle relative to controls as well as dysregulated alternative polyadenylation.…”

“…In contrast, CDM exhibits early, with in utero onset of disease. Thomas et al (2017) therefore tested whether RNA misprocessing similarly occurred in utero in CDM-affected individuals. The investigators first assessed whether splicing transitions are widespread during skeletal muscle development in utero.…”

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Thomas and colleagues (pp. 1122-1133)

demonstrate severe dysregulation of developmentally regulated alternative splicing and polyadenylation in congenital myotonic dystrophy (CDM).

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“…CDM tissue had the lowest inferred functional MBNL concentration and more extreme differential splicing relative to adult-onset DM1 samples. These results indicate that CDM is a spliceopathy like adult-onset DM1, albeit with more severe splicing dysregulation that is presumably due to the presence of long CTG repeats in the prenatal genome.Having established the severity of RNA misprocessing in postnatal CDM muscle, Thomas et al (2017) next tackled the timing of misregulation. In adult-onset DM1, pathogenic RNA misprocessing occurs postnatally.…”

“…These analyses suggested that CDM is characterized by dysregulated prenatal splicing and reversion of developmental splicing transitions that are important for muscle development. Thomas et al (2017) next sought to establish murine models of CDM. First, they showed that conditional knockout of Mbnl1 and Mbnl2 recapitulated much of the spliceopathy and several phenotypes of CDM but resulted in more modest differential gene expression than they observed in CDM.…”