MBNL Sequestration by Toxic RNAs and RNA Misprocessing in the Myotonic Dystrophy Brain

Abstract

SUMMARY For some neurological disorders, disease is primarily RNA-mediated due to expression of non-coding microsatellite expansion RNAs (RNAexp). Toxicity is thought to result from enhanced binding of proteins to these expansions and depletion from their normal cellular targets. However, experimental evidence for this sequestration model is lacking. Here, we use HITS-CLIP and pre-mRNA processing analysis of human control versus myotonic dystrophy (DM) brains to provide compelling evidence for this RNA toxicity model. MBNL2 binds directly to DM repeat expansions in the brain resulting in depletion from its normal RNA targets with downstream effects on alternative splicing and polyadenylation. Similar RNA processing defects were detected in Mbnl compound knockout mice, highlighted by dysregulation of Mapt splicing and fetal tau isoform expression in adults. These results demonstrate that MBNL proteins are directly sequestered by RNAexp in the DM brain and introduce a powerful experimental tool to evaluate RNA-mediated toxicity in other expansion diseases.

Distal Alternative Last Exons Localize mRNAs to Neural Projections

“…S4K). Furthermore, we observed a significant correlation of ALE ΔΔψ values between Mbnl1 KO and Mbnl2 KO cells, indicating that MBNL1 and MBNL2 may influence the localization of overlapping sets of mRNAs (and may partially compensate for each other’s absence) (Goodwin et al, 2015) (Fig. S4L).…”

The neurogenetics of alternative splicing

“…106). Although Mbnl2 -null brains did not show major changes in transcript levels, recent studies of Mbnl1/2 dKOs have highlighted MBNL activity in controlling alternative polyadenylation events 105,107,108 . Binding in 3′ UTRs is commonly observed for other splicing regulators and points to the need to distinguish phenotypes driven by splicing changes from those arising from altered mRNA stability, localization and/or translation (FIG.…”

“…MBNL proteins have been studied extensively in relation to the neuromuscular disorder myotonic dystrophy. In myotonic dystrophy, CTG or CCTG repeat expansions in expressed RNAs sequester MBNL proteins from their normal binding sites, altering MBNL-dependent splicing patterns 101–105 . Although all three MBNLs are expressed in the brain, only Mbnl2 -null mice exhibit obvious central nervous system phenotypes.…”

Roles for RNA-binding proteins in development and disease

“…Another clinical feature of DM1 is altered cognition. Mis-splicing of MBNL targets have been identified in brains of Mbnl2 knockout mice (Charizanis et al, 2012; Goodwin et al, 2015). The clinical features associated with expression of fetal isoforms and reversion of cytoplasmic functions to fetal patterns has demonstrated the necessity of the developmental transitions for normal function in adult tissues.…”