Muscleblind-like-1 (MBNL1) is a splicing regulatory factor controlling the fetal-to-adult alternative splicing transitions during vertebrate muscle development. Its capture by nuclear CUG expansions is one major cause for type 1 myotonic dystrophy (DM1). Alternative splicing produces MBNL1 isoforms that differ by the presence or absence of the exonic regions 3, 5, and 7. To understand better their respective roles and the consequences of the deregulation of their expression in DM1, here we studied the respective roles of MBNL1 alternative and constitutive exons. By combining genetics, molecular and cellular approaches, we found that (i) the exon 5 and 6 regions are both needed to control the nuclear localization of MBNL1; (ii) the exon 3 region strongly enhances the affinity of MBNL1 for its pre-mRNA target sites; (iii) the exon 3 and 6 regions are both required for the splicing regulatory activity, and this function is not enhanced by an exclusive nuclear localization of MBNL1; and finally (iv) the exon 7 region enhances MBNL1-MBNL1 dimerization properties. Consequently, the abnormally high inclusion of the exon 5 and 7 regions in DM1 is expected to enhance the potential of MBNL1 of being sequestered with nuclear CUG expansions, which provides new insight into DM1 pathophysiology.Splicing of pre-mRNA is a key post-transcriptional step in eukaryotic gene expression. A vast majority of vertebrate pre-mRNAs is alternatively spliced, allowing the production of several protein isoforms from transcripts of a given gene (1). The regulation of alternative splicing plays a major role in cell differentiation and in development and depends on the expression and activity of numerous splicing regulatory factors that are expressed differentially during development, according to the type of tissue. Defects in these alternative-splicing processes can contribute to pathogenesis, as demonstrated for a growing number of diseases, including neuromuscular diseases such as myotonic dystrophy type 1 (DM1) 9 (2, 3). DM1 is an autosomal disorder caused by an unstable CTG repeat expansion in the 3Ј-untranslated region (UTR) of the DMPK gene (4 -6). One of the main etiological hypotheses of DM1 is based on a toxic RNA gain of function, leading to the dysregulation of alternative splicing. Mutant transcripts bearing long-CUG repeats acquire unusual A-form doublestranded RNA structures (7), accumulate in the nucleus, and lead to small ribonucleoprotein inclusions, named foci (8) that sequester RNA-binding proteins such as Muscleblind-like 1 * This work was supported by the Association Française contre les Myopathies Grants 14269 and 15047, the Agence Nationale de Recherche NeuroSplicedeTau BLAN 1114 01, the EURASNET EU Contract FP6, life sciences, genomics and biotechnology for health, the Centre National de la Recherche Scientifique, the Institut National pour la Santé et la Recherche Mé dicale, the French Ministry for Youth, National Education and Research, and the Lorraine Region. The abbreviations used are: DM1, myotonic dystrophy type...

BackgroundCurrent therapies have succeeded in controlling AIDS pandemic. However, there is a continuing need for new drugs, in particular those acting through new and as yet unexplored mechanisms of action to achieve HIV infection cure. We took advantage of the unique feature of proviral genome to require both activation and inhibition of splicing of viral transcripts to develop molecules capable of achieving long lasting effect on viral replication in humanized mouse models through inhibition of Rev-mediated viral RNA biogenesis.ResultsCurrent HIV therapies reduce viral load during treatment but titers rebound after treatment is discontinued. We devised a new drug that has a long lasting effect after viral load reduction. We demonstrate here that ABX464 compromises HIV replication of clinical isolates of different subtypes without selecting for drug resistance in PBMCs or macrophages. ABX464 alone, also efficiently compromised viral proliferation in two humanized mouse models infected with HIV that require a combination of 3TC, Raltegravir and Tenofovir (HAART) to achieve viral inhibition in current protocols. Crucially, while viral load increased dramatically just one week after stopping HAART treatment, only slight rebound was observed following treatment cessation with ABX464 and the magnitude of the rebound was maintained below to that of HAART for two months after stopping the treatment. Using a system to visualize single HIV RNA molecules in living cells, we show that ABX464 inhibits viral replication by preventing Rev-mediated export of unspliced HIV-1 transcripts to the cytoplasm and by interacting with the Cap Binding Complex (CBC). Deep sequencing of viral RNA from treated cells established that retained viral RNA is massively spliced but importantly, normal cellular splicing is unaffected by the drug. Consistently ABX464 is non-toxic in humans and therefore represents a promising complement to current HIV therapies.ConclusionsABX464 represents a novel class of anti-HIV molecules with unique properties. ABX464 has a long lasting effect in humanized mice and neutralizes the expression of HIV-1 proviral genome of infected immune cells including reservoirs and it is therefore a promising drug toward a functional cure of HIV.Electronic supplementary materialThe online version of this article (doi:10.1186/s12977-015-0159-3) contains supplementary material, which is available to authorized users.

ABX464 is a first-in-class, clinical-stage, small molecule for oral administration that has shown strong anti-inflammatory effects in the DSS-model for inflammatory bowel disease (IBD) and also prevents replication of the HIV virus. ABX464 which binds to cap binding complex (CBC) has demonstrated safety and efficacy in a phase 2a proof-of-concept clinical trial in patients with Ulcerative colitis. Previously, with limited technologies, it was not possible to quantify the effect of ABX464 on viral and cellular RNA biogenesis. Here, using RNA CaptureSeq and deep sequencing, we report that ABX464 enhances the splicing of HIV RNA in infected PBMCs from six healthy individuals and also the expression and splicing of a single long noncoding RNA to generate the anti-inflammatory miR-124 both ex vivo and in HIV patients. While ABX464 has no effect on pre-mRNA splicing of cellular genes, depletion of CBC complex by RNAi leads to accumulation of intron retention transcripts. These results imply that ABX464 did not inhibit the function of CBC in splicing but rather strengthens it under pathological condition like inflammation and HIV infection. The specific dual ability of ABX464 to generate both anti-inflammatory miR-124 and spliced viral RNA may have applicability for the treatment of both inflammatory diseases and HIV infection.