The human transcriptome is composed of a vast RNA population that undergoes further diversification by splicing. Detecting specific splice sites in this large sequence pool is the responsibility of the major and minor spliceosomes in collaboration with numerous splicing factors. This complexity makes splicing susceptible to sequence polymorphisms and deleterious mutations. Indeed, RNA mis-splicing underlies a growing number of human diseases with substantial societal consequences. Here, we provide an overview of RNA splicing mechanisms followed by a discussion of disease-associated errors, with an emphasis on recently described mutations that have provided new insights into splicing regulation. We also discuss emerging strategies for splicing-modulating therapy.

SUMMARY The RNA-mediated disease model for myotonic dystrophy (DM) proposes that microsatellite C(C)TG expansions express toxic RNAs which disrupt splicing regulation by altering MBNL1 and CELF1 activities. While this model explains DM manifestations in muscle, less is known about the effects of C(C)UG expression on the brain. Here, we report that Mbnl2 knockout mice develop several DM-associated CNS features including abnormal REM sleep propensity and deficits in spatial memory. Mbnl2 is prominently expressed in the hippocampus and Mbnl2 knockouts show a decrease in NMDAR synaptic transmission and impaired hippocampal synaptic plasticity. While Mbnl2 loss did not significantly alter target transcript levels in the hippocampus, mis-regulated splicing of hundreds of exons was detected using splicing microarrays, RNA-seq and HITS-CLIP. Importantly, the majority of the Mbnl2-regulated exons examined were similarly mis-regulated in DM. We propose that major pathological features of the DM brain result from disruption of the MBNL2-mediated developmental splicing program.

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.