| Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disease. There are two types of DM, DM1 and DM2, which are caused by unstable microsatellite repeat expansions in the non-coding regions of DMPK and ZNF9, respectively. Despite different mutations, DM1 and DM2 exhibit very similar symptoms including myotonia, ocular cataracts, heart conduction defects and neuropsychiatric manifestations. Recent evidence has implicated the aberrant regulation of pre-mRNA splicing as the underlying molecular defect in DM. However, correlation between the mutation and splicing defect has not been elucidated.;To explain how mutations in the non-coding regions of two different genes cause similar diseases, we have proposed a RNA-mediated pathogenesis model. According to this model, mutant DMPK and ZNF9 transcripts form stable hairpin structure which recruit, and then sequester, a novel family of nuclear double-stranded (ds) RNA-binding proteins called muscleblind. Consequently, muscleblind proteins are depleted, which results in muscleblind loss-of-function and DM.;This study focused on testing the RNA-mediated pathogenesis model. The experimental strategy was to generate mouse muscleblind knockout mice and determine if they recapitulate aspects of the DM disease phenotype. In mammals, muscleblind belongs to a three gene family consisting of Mbnl1, Mbnl2 and Mbnl3. To select the optimal gene for knockout studies, the adult and embryonic expression profiles of these three genes were analyzed. Based on Mbnl1 gene expression profile, Mbnl1 was targeted. Multiple Mbnl1 isoforms are generated by alternative splicing, and only some of these isoforms bind to the mutant transcript. Therefore, isoform-specific Mbnl1 knockout mice were generated. These Mbnl1DeltaE3/DeltaE3 mice develop a DM-like phenotype including skeletal muscle myotonia, dust-like subcapsular ocular cataracts and cardiac defects. These defects result from a defect in the alternative pre-mRNA splicing of several genes including Clcn1, Tnnt2, and Tnnt3. Thus, the Mbnl1DeltaE/DeltaE3 mouse model provides substantial support for the RNA-mediated pathogenesis model and suggest that DM disease is caused by inappropriate retention of fetal exons in the adult. |
