Muscleblind‐like 2 (Mbnl2) ‐deficient mice as a model for myotonic dystrophy

Abstract

Myotonic dystrophy (DM), the most common adult-onset muscular dystrophy, is caused by CTG or CCTG microsatellite repeat expansions. Expanded DM mRNA microsatellite repeats are thought to accumulate in the nucleus, sequester Muscleblind proteins, and interfere with alternative mRNA splicing. Muscleblind2 (Mbnl2) is a member of the family of Muscleblind RNA binding proteins (that also include Mbnl1 and Mbnl3) that are known to bind CTG/CCTG RNA repeats. Recently, it was demonstrated that Mbnl1-deficient mice have characteristic features of human DM, including myotonia and defective chloride channel expression. Here, we demonstrate that Mbnl2-deficient mice also develop myotonia and have skeletal muscle pathology consistent with human DM. We also find defective expression and mRNA splicing of the chloride channel (Clcn1) in skeletal muscle that likely contributes to the myotonia phenotype. Our results support the hypothesis that Muscleblind proteins and specifically MBNL2 contribute to the pathogenesis of human DM.

Myotonic Dystrophy Type 1: Focus on the RNA Pathology and Therapy

“…In another Drosophila model of DM1 that shows degenerative phenotypes in muscle and eye tissue as well as key histopathological features of the DM1, including accumulation of the expanded transcripts in nuclear foci and their co-localization with MBNL1 protein, reduced levels of MBNL1 aggravate the muscle and eye phenotypes of DM1 flies whereas MBNL1 overexpression suppresses the degenerative phenotypes (de Haro et al, 2006). Mbnl2-deficient mice developed myotonia, skeletal muscle pathology consistent with human DM1 and defective CLCN1 mRNA splicing in skeletal muscle, supporting the hypothesis that MBNL proteins and specifically MBNL2 contribute to the pathogenesis of human DM1 (Hao et al, 2008). These results are consistent with the notion that Mbnl1 deficiency alone is not sufficient to fully replicate the human DM1 phenotype ).…”

Huntington disease skeletal muscle is hyperexcitable owing to chloride and potassium channel dysfunction

“…Consistent with this, we found reduced levels of normal mature Clcn1 and Kcnj2 mRNA. A disruption of mRNA splicing has been shown to cause a decrease in ClC-1 expression in myotonic dystrophy type 1, another trinucleotide repeat disorder (27)(28)(29)(30)(31)(32)(33)(34)(35)(36). We measured elevated levels of aberrant Clcn1 mRNA containing exon 7a in HD muscle, which indicates a similar disruption in Clcn1 pre-mRNA splicing.…”

“…The loss of chloride channels in myotonic dystrophy is thought to be due to an accumulation of RNA with CUG or CCUG repeats in the nucleus that disrupt the function of RNA binding proteins, such as muscleblind-like 1 and 2; consequently, aberrantly spliced Clcn1 mRNA that contains exon 7a is degraded via nonsense-mediated decay (27)(28)(29)(30)(31)(32)(33)(34)(35)(36). We tested for this mechanism in HD interosseous muscle (Fig.…”

“…To quantify total Clcn1 and Kcnj2 mRNA levels, real-time quantitative RT-PCR was performed with a BioRad DNA engine Opticon 2. Clcn1 was quantified using primers that spanned either exons 10-14 or exons 16-19 as previously described (31). Kcnj2 was quantified using a Taqman assay (Mm00434616.m1, Applied Biosystems).…”

Muscleblind-Like Proteins

“…Lin et al 31 found that a mouse with Ͼ90% reduction in MBNL2 had no effect on muscle histology, myotonia or splicing of three RNAs that are affected by mbnl1 knockout (Serca1, ZASP and titin); however, this study was done in adult muscle, which may contain relatively little MBNL2 compared with MBNL1. More recently, Hao et al 53 described a different mbnl2 knockout mouse that did display myotonia and chloride channel mRNA splicing defects, though much less severe than the mbnl1 knockout. This appears to suggest that MBNL1 and MBNL2 proteins have similar functions, but is difficult to reconcile with the lack of DM phenotype in heterozygotes of the MBNL1-deficient mice.…”