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In vivo RNA structure analysis has become a powerful tool in molecular biology, largely due to the coupling of an increasingly diverse set of chemical approaches with high-throughput sequencing. This has resulted in a transition from single target to transcriptome-wide approaches. However, these methods require sequencing depths that preclude studying low abundance targets, which are not sufficiently captured in transcriptome-wide approaches. Here we present a ligation-free method to enrich for low abundance RNA sequences, which improves the diversity of molecules analyzed and results in improved analysis. In addition, this method is compatible with any choice of chemical adduct or read-out approach. We utilized this approach to study an autoregulated event in the pre-mRNA of the splicing factor, muscleblind-like splicing regulator 1 (MBNL1).

The spinocerebellar ataxias (SCAs) are a group of dominantly inherited neurodegenerative diseases, several of which are caused by CAG expansion mutations (SCAs 1, 2, 3, 6, 7 and 12) and more broadly belong to the large family of over 40 microsatellite expansion diseases. While dysregulation of alternative splicing is a well defined driver of disease pathogenesis across several microsatellite diseases, the contribution of alternative splicing in CAG expansion SCAs is poorly understood. Furthermore, despite extensive studies on differential gene expression, there remains a gap in our understanding of presymptomatic transcriptomic drivers of disease. We sought to address these knowledge gaps through a comprehensive study of 29 publicly available RNA-sequencing datasets. We identified that dysregulation of alternative splicing is widespread across CAG expansion mouse models of SCAs 1, 3 and 7. These changes were detected presymptomatically, persisted throughout disease progression, were repeat length-dependent, and were present in brain regions implicated in SCA pathogenesis including the cerebellum, pons and medulla. Across disease progression, changes in alternative splicing occurred in genes that function in pathways and processes known to be impaired in SCAs, such as ion channels, synaptic signalling, transcriptional regulation and the cytoskeleton. We validated several key alternative splicing events with known functional consequences, including Trpc3 exon 9 and Kcnma1 exon 23b, in the Atxn1154Q/2Q mouse model. Finally, we demonstrated that alternative splicing dysregulation is responsive to therapeutic intervention in CAG expansion SCAs with Atxn1 targeting antisense oligonucleotide rescuing key splicing events. Taken together, these data demonstrate that widespread presymptomatic dysregulation of alternative splicing in CAG expansion SCAs may contribute to disease onset, early neuronal dysfunction and may represent novel biomarkers across this devastating group of neurodegenerative disorders. [ABSTRACT FROM AUTHOR]

Myotonic dystrophy (DM) is a genetic disorder caused by the expression (as RNA) of expanded CTG or CCTG repeats. The alternative splicing factor MBNL1 is sequestered to the expanded RNA repeats, resulting in missplicing of a subset of pre-mRNAs linked to symptoms found in DM patients. Current data suggest that if MBNL1 is released from sequestration, disease symptoms may be alleviated. We identified the small molecules pentamidine and neomycin B as compounds that disrupt MBNL1 binding to CUG repeats in vitro. We show in cell culture that pentamidine was able to reverse the missplicing of 2 pre-mRNAs affected in DM, whereas neomycin B had no effect. Pentamidine also significantly reduced the formation of ribonuclear foci in tissue culture cells, releasing MBNL1 from the foci in the treated cells. Furthermore, pentamidine partially rescued splicing defects of 2 pre-mRNAs in mice expressing expanded CUG repeats. alternative splicing | triplet repeats | MBNL1 | muscleblind-like 1 | RNA repeats www.pnas.org/cgi/doi/10.1073/pnas.0903234106

Myotonic dystrophy (DM) type 1 is associated with an expansion of (>50) CTG repeats within the 3' untranslated region (UTR) of the dystrophin myotonin protein kinase gene (dmpk). In the corresponding mRNA transcript, the CUG repeats form an extended stem-loop structure. The double-stranded RNA of the stem sequesters RNA binding proteins away from their normal cellular targets resulting in aberrant transcription, alternative splicing patterns, or both, thereby leading to DM. To better understand the structural basis of DM type 1, we determined to 1.58-[Angstrom] resolution the x-ray crystal structure of an 18-bp RNA containing six CUG repeats. The CUG repeats form antiparallel double-stranded helices that stack end-on-end in the crystal to form infinite, pseudocontinuous helices similar to the long CUG stem loops formed by the expanded CUG repeats in DM type 1. The CUG helix is very similar in structure to A-form RNA with the exception of the unique U-U mismatches. This structure provides a high-resolution view of a toxic, trinucleotide repeat RNA. toxic RNA | U-U mismatches | x-ray crystallography

Author(s): James E. Galagan (corresponding author) [1]; Sarah E. Calvo [1]; Katherine A. Borkovich [2]; Eric U. Selker [3]; Nick D. Read [4]; David Jaffe [1]; William FitzHugh [5]; Li-Jun [...]

A branchpoint sequence binding protein (BBP) has been identified in yeast. Cooperative interaction between the mammalian splicing factor U2AF65 and mBBP/SF1 is helpful to branchpoint region recognition. U2AF65 binds to the polypyrimidine (PY) tract, which is adjacent. The cooperative RNA binding aids in initial recognition of the branchpoint sequence (BPS) in pre-mRNA splicing. A branchpoint sequence-polypyrimidine tract-containing RNA is involved. The third RNA-binding domain (RBD) of U2AF65 is required for the two proteins to interact, with or without RNA.

Ultra-violet (UV) cross-linking techniques were utilized to characterize the direct association between the branchpoint bridging protein (BBP) yeast splicing factor and the pre-messenger RNA (mRNA). Ribonuclease T1 digestion and specific immunoprecipitation indicated the ability of BBP to form UV cross-links to the pre-mRNA near the pre-mRNA branchpoint sequence during commitment-complex assembly. Furthermore, recombinant murine BBP recognized the UACUAAC sequence within the commitment complex.