Expansion of CUG RNA repeats causes stress and inhibition of translation in myotonic dystrophy 1 (DM1) cells

RBFOX1 cooperates with MBNL1 to control splicing in muscle, including events altered in myotonic dystrophy type 1

With the goal of identifying splicing alterations in myotonic dystrophy 1 (DM1) tissues that may yield insights into targets or mechanisms, we have surveyed mis-splicing events in three systems using a RT-PCR screening and validation platform. First, a transgenic mouse model expressing CUG-repeats identified splicing alterations shared with other mouse models of DM1. Second, using cell cultures from human embryonic muscle, we noted that DM1-associated splicing alterations were significantly enriched in cytoskeleton (e.g. SORBS1, TACC2, TTN, ACTN1 and DMD) and channel (e.g. KCND3 and TRPM4) genes. Third, of the splicing alterations occurring in adult DM1 tissues, one produced a dominant negative variant of the splicing regulator RBFOX1. Notably, half of the splicing events controlled by MBNL1 were co-regulated by RBFOX1, and several events in this category were mis-spliced in DM1 tissues. Our results suggest that reduced RBFOX1 activity in DM1 tissues may amplify several of the splicing alterations caused by the deficiency in MBNL1.

Myotonic dystrophies: An update on clinical aspects, genetic, pathology, and molecular pathomechanisms

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. However, it is now clear that additional pathogenic mechanism like changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. This review is an update on the recent advances in the understanding of the molecular mechanisms behind myotonic dystrophies. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

RNA-binding proteins in heart development

RNA-binding proteins (RBPs) are key players of posttranscriptional regulation occurring during normal tissue development. All tissues examined thus far have revealed the importance of RBPs in the regulation of complex networks involved in organ morphogenesis, maturation, and function. They are responsible for controlling tissue-specific gene expression by regulating alternative splicing, mRNA stability, translation, and poly-adenylation. The heart is the first organ form during embryonic development and is also the first to acquire functionality. Numerous remodeling processes take place during late cardiac development since fetal heart first adapts to birth and then undergoes a transition to adult functionality. This physiological remodeling involves transcriptional and posttranscriptional networks that are regulated by RBPs. Disruption of the normal regulatory networks has been shown to cause cardiomyopathy in humans and animal models. Here we review the complexity of late heart development and the current information regarding how RBPs control aspects of postnatal heart development. We also review how activities of RBPs are modulated adding complexity to the regulation of developmental networks.

High-content screening identifies small molecules that remove nuclear foci, affect MBNL distribution and CELF1 protein levels via a PKC-independent pathway in myotonic dystrophy cell lines

Myotonic dystrophy (DM) is a multi-system neuromuscular disorder for which there is no treatment. We have developed a medium throughput phenotypic assay, based on the identification of nuclear foci in DM patient cell lines using in situ hybridization and high-content imaging to screen for potentially useful therapeutic compounds. A series of further assays based on molecular features of DM have also been employed. Two compounds that reduce and/or remove nuclear foci have been identified, Ro 31-8220 and chromomycin A3. Ro 31-8220 is a PKC inhibitor, previously shown to affect the hyperphosphorylation of CELF1 and ameliorate the cardiac phenotype in a DM1 mouse model. We show that the same compound eliminates nuclear foci, reduces MBNL1 protein in the nucleus, affects ATP2A1 alternative splicing and reduces steady-state levels of CELF1 protein. We demonstrate that this effect is independent of PKC activity and conclude that this compound may be acting on alternative kinase targets within DM pathophysiology. Understanding the activity profile for this compound is key for the development of targeted therapeutics in the treatment of DM.

GSK3β is a new therapeutic target for myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1), an incurable, neuromuscular disease, is caused by the expansion of CTG repeats within the 3′ UTR of DMPK on chromosome 19q. In DM1 patients, mutant DMPK transcripts deregulate RNA metabolism by altering CUG RNA-binding proteins. Several approaches have been proposed for DM1 therapy focused on specific degradation of the mutant CUG repeats or on correction of RNA-binding proteins, affected by CUG repeats. One such protein is CUG RNA-binding protein (CUGBP1). The ability of CUGBP1 to increase or inhibit translation depends on phosphorylation at Ser302, which is mediated by cyclin D3-CDK4. The mutant CUG repeats increase the levels of CUGBP1 protein and inhibit Ser302 phosphorylation, leading to the accumulation of CUGBP1 isoforms that repress translation (i.e., CUGBP1^REP). Elevation of CUGBP1^REP in DM1 is caused by increased GSK3β kinase, which reduces the cyclin D3-CDK4 pathway and subsequent phosphorylation of CUGBP1 at Ser302. In this review, we discuss our recent discovery showing that correction of GSK3β activity in the DM1 mouse model (i.e., HSA^LR mice) reduces DM1 muscle pathology. These findings demonstrate that GSK3β is a novel therapeutic target for treating DM1.

STAU1 binding 3' UTR IRAlus complements nuclear retention to protect cells from PKR-mediated translational shutdown

For a number of human genes that encode transcripts containing inverted repeat Alu elements (IRAlus) within their 3' untranslated region (UTR), product mRNA is efficiently exported to the cytoplasm when the IRAlus, which mediate nuclear retention, are removed by alternative polyadenylation. Here we report a new mechanism that promotes gene expression by targeting mRNAs that maintain their 3' UTR IRAlus: Binding of the dsRNA-binding protein Staufen1 (STAU1) to 3' UTR IRAlus inhibits nuclear retention so as to augment the nuclear export of 3' UTR IRAlus-containing mRNAs (IRAlus mRNAs). Moreover, we found that 3' UTR IRAlus-bound STAU1 enhances 3' UTR IRAlus mRNA translation by precluding protein kinase R (PKR) binding, which obviates PKR activation, eukaryotic translation initiation factor 2α (eIF2α) phosphorylation, and repression of global cell translation. Thus, STAU1 binding to 3' UTR IRAlus functions along with 3' UTR IRAlus-mediated nuclear retention to suppress the shutdown of cellular translation triggered by PKR binding to endogenous cytoplasmic dsRNAs. We also show that a changing STAU1/PKR ratio contributes to myogenesis via effects on the 3' UTR IRAlus of mRNA encoding the microRNA-binding protein LIN28.

Molecular mechanisms of muscle atrophy in myotonic dystrophies

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) are multisystemic diseases that primarily affect skeletal muscle, causing myotonia, muscle atrophy, and muscle weakness. DM1 and DM2 pathologies are caused by expansion of CTG and CCTG repeats in non-coding regions of the genes encoding myotonic dystrophy protein kinase (DMPK) and zinc finger protein 9 (ZNF9) respectively. These expansions cause DM pathologies through accumulation of mutant RNAs that alter RNA metabolism in patients' tissues by targeting RNA-binding proteins such as CUG-binding protein 1 (CUGBP1) and Muscle blind-like protein 1 (MBNL1). Despite overwhelming evidence showing the critical role of RNA-binding proteins in DM1 and DM2 pathologies, the downstream pathways by which these RNA-binding proteins cause muscle wasting and muscle weakness are not well understood. This review discusses the molecular pathways by which DM1 and DM2 mutations might cause muscle atrophy and describes progress toward the development of therapeutic interventions for muscle wasting and weakness in DM1 and DM2. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Muscleblind-like1 undergoes ectopic relocation in the nuclei of skeletal muscles in myotonic dystrophy and sarcopenia

Muscleblind-like 1 (MBNL1) is an alternative splicing factor involved in postnatal development of skeletal muscles and heart in humans and mice, and its deregulation is known to be pivotal in the onset and development of myotonic dystrophy (DM). In fact, in DM patients this protein is ectopically sequestered into intranuclear foci, thus compromising the regulation of the alternative splicing of several genes. However, despite the numerous biochemical and molecular studies, scarce attention has been paid to the intranuclear location of MBNL1 outside the foci, although previous data demonstrated that in DM patients various splicing and cleavage factors undergo an abnormal intranuclear distribution suggestive of impaired RNA processing. Interestingly, these nuclear alterations strongly remind those observed in sarcopenia i.e., the loss of muscle mass and function which physiologically occurs during ageing. On this basis, in the present investigation the ultrastructural localization of MBNL1 was analyzed in the myonuclei of skeletal muscles from healthy and DM patients as well as from adult and old (sarcopenic) mice, in the attempt to elucidate possible changes in its distribution and amount. Our data demonstrate that in both dystrophic and sarcopenic muscles MBNL1 undergoes intranuclear relocation, accumulating in its usual functional sites but also ectopically moving to domains which are usually devoid of this protein in healthy adults. This accumulation/delocalization could contribute to hamper the functionality of the whole splicing machinery, leading to a lower nuclear metabolic activity and, consequently, to a less efficient protein synthesis. Moreover, the similar nuclear alterations found in DM and sarcopenia may account for the similar muscle tissue features (myofibre atrophy, fibre size variability and centrally located nuclei), and, in general, for the aging-reminiscent phenotype observed in DM patients.

Smaug/SAMD4A restores translational activity of CUGBP1 and suppresses CUG-induced myopathy

We report the identification and characterization of a previously unknown suppressor of myopathy caused by expansion of CUG repeats, the mutation that triggers Myotonic Dystrophy Type 1 (DM1). We screened a collection of genes encoding RNA-binding proteins as candidates to modify DM1 pathogenesis using a well established Drosophila model of the disease. The screen revealed smaug as a powerful modulator of CUG-induced toxicity. Increasing smaug levels prevents muscle wasting and restores muscle function, while reducing its function exacerbates CUG-induced phenotypes. Using human myoblasts, we show physical interactions between human Smaug (SMAUG1/SMAD4A) and CUGBP1. Increased levels of SMAUG1 correct the abnormally high nuclear accumulation of CUGBP1 in myoblasts from DM1 patients. In addition, augmenting SMAUG1 levels leads to a reduction of inactive CUGBP1-eIF2α translational complexes and to a correction of translation of MRG15, a downstream target of CUGBP1. Therefore, Smaug suppresses CUG-mediated muscle wasting at least in part via restoration of translational activity of CUGBP1.

Native tertiary structure and nucleoside modifications suppress tRNA's intrinsic ability to activate the innate immune sensor PKR

Interferon inducible protein kinase PKR is an essential component of innate immunity. It is activated by long stretches of dsRNA and provides the first line of host defense against pathogens by inhibiting translation initiation in the infected cell. Many cellular and viral transcripts contain nucleoside modifications and/or tertiary structure that could affect PKR activation. We have previously demonstrated that a 5'-end triphosphate-a signature of certain viral and bacterial transcripts-confers the ability of relatively unstructured model RNA transcripts to activate PKR to inhibit translation, and that this activation is abrogated by certain modifications present in cellular RNAs. In order to understand the biological implications of native RNA tertiary structure and nucleoside modifications on PKR activation, we study here the heavily modified cellular tRNAs and the unmodified or the lightly modified mitochondrial tRNAs (mt-tRNA). We find that both a T7 transcript of yeast tRNA(Phe) and natively extracted total bovine liver mt-tRNA activate PKR in vitro, whereas native E. coli, bovine liver, yeast, and wheat tRNA(Phe) do not, nor do a variety of base- or sugar-modified T7 transcripts. These results are further supported by activation of PKR by a natively folded T7 transcript of tRNA(Phe)in vivo supporting the importance of tRNA modification in suppressing PKR activation in cells. We also examine PKR activation by a T7 transcript of the A14G pathogenic mutant of mt-tRNA(Leu), which is known to dimerize, and find that the misfolded dimeric form activates PKR in vitro while the monomeric form does not. Overall, the in vitro and in vivo findings herein indicate that tRNAs have an intrinsic ability to activate PKR and that nucleoside modifications and native RNA tertiary folding may function, at least in part, to suppress such activation, thus serving to distinguish self and non-self tRNA in innate immunity.

CELFish ways to modulate mRNA decay

The CELF family of RNA-binding proteins regulates many steps of mRNA metabolism. Although their best characterized function is in pre-mRNA splice site choice, CELF family members are also powerful modulators of mRNA decay. In this review we focus on the different modes of regulation that CELF proteins employ to mediate mRNA decay by binding to GU-rich elements. After starting with an overview of the importance of CELF proteins during development and disease pathogenesis, we then review the mRNA networks and cellular pathways these proteins regulate and the mechanisms by which they influence mRNA decay. Finally, we discuss how CELF protein activity is modulated during development and in response to cellular signals. We conclude by highlighting the priorities for new experiments in this field. This article is part of a Special Issue entitled: RNA Decay mechanisms.

GSK3β mediates muscle pathology in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is a complex neuromuscular disease characterized by skeletal muscle wasting, weakness, and myotonia. DM1 is caused by the accumulation of CUG repeats, which alter the biological activities of RNA-binding proteins, including CUG-binding protein 1 (CUGBP1). CUGBP1 is an important skeletal muscle translational regulator that is activated by cyclin D3-dependent kinase 4 (CDK4). Here we show that mutant CUG repeats suppress Cdk4 signaling by increasing the stability and activity of glycogen synthase kinase 3β (GSK3β). Using a mouse model of DM1 (HSA(LR)), we found that CUG repeats in the 3' untranslated region (UTR) of human skeletal actin increase active GSK3β in skeletal muscle of mice, prior to the development of skeletal muscle weakness. Inhibition of GSK3β in both DM1 cell culture and mouse models corrected cyclin D3 levels and reduced muscle weakness and myotonia in DM1 mice. Our data predict that compounds normalizing GSK3β activity might be beneficial for improvement of muscle function in patients with DM1.

Competitive binding of CUGBP1 and HuR to occludin mRNA controls its translation and modulates epithelial barrier function

The present study shows that RNA-binding proteins CUGBP1 and HuR jointly regulate the translation of occludin and play a crucial role in the maintenance of tight junction integrity.

RNA-binding proteins CUG-binding protein 1 (CUGBP1) and HuR are highly expressed in epithelial tissues and modulate the stability and translation of target mRNAs. Here we present evidence that CUGBP1 and HuR jointly regulate the translation of occludin and play a crucial role in the maintenance of tight junction (TJ) integrity in the intestinal epithelial cell monolayer. CUGBP1 and HuR competed for association with the same occludin 3′-untranslated region element and regulated occludin translation competitively and in opposite directions. CUGBP1 overexpression decreased HuR binding to occludin mRNA, repressed occludin translation, and compromised the TJ barrier function, whereas HuR overexpression inhibited CUGBP1 association with occludin mRNA and promoted occludin translation, thereby enhancing the barrier integrity. Repression of occludin translation by CUGBP1 was due to the colocalization of CUGBP1 and tagged occludin RNA in processing bodies (P-bodies), and this colocalization was prevented by HuR overexpression. These findings indicate that CUGBP1 represses occludin translation by increasing occludin mRNA recruitment to P-bodies, whereas HuR promotes occludin translation by blocking occludin mRNA translocation to P-bodies via the displacement of CUGBP1.

The role of CUGBP1 in age-dependent changes of liver functions

Aging liver is characterized by alterations of liver biology and by a reduction of many functions which are important for the maintenance of body homeostasis. The main dysfunctions include appearance of enlarged hepatocytes, impaired liver regeneration after partial hepatectomy (PH), development of hepatic steatosis, reduction of secretion of proteins and alterations in the hepatic sinusoid. RNA binding proteins are involved in the regulation of gene expression in all tissues including regulation of biological processes in the liver. This review is focused on the role of a conserved, multi-functional RNA-binding protein, CUGBP1, in the development of aging phenotype in the liver. CUGBP1 has been identified as a protein which binds to RNA CUG repeats expanded in Myotonic Dystrophy type 1 (DM1). CUGBP1 is highly expressed in the liver and regulates translation of proteins which are critical for maintenance of liver functions. In livers of young mice, CUGBP1 forms complexes with eukaryotic translation initiation factor eIF2 and supports translation of C/EBPβ and HDAC1 proteins, which are involved in liver growth, differentiation and liver cancer. Aging changes several signaling pathways which lead to the elevation of the CUGBP1-eIF2α complex and to an increase of translation of C/EBPβ and HDAC1. These proteins form multi-protein complexes with additional transcription factors and with chromatin remodeling proteins causing epigenetic alterations of gene expression in livers of old mice. It appears that CUGBP1-mediated translational elevation of HDAC1 is one of the key events in the epigenetic changes in livers of old mice, leading to the development of age-associated dysfunctions of the liver. This review will also discuss a possible role of CUGBP1 in liver dysfunction in patients affected with DM1.

Skeletal muscle features in myotonic dystrophy and sarcopenia: do similar nuclear mechanisms lead to skeletal muscle wasting?

In the cell nucleus, the gene primary transcripts undergo molecular processing to generate mature RNAs, which are finally exported to the cytoplasm. These mRNA maturation events are chronologically and spatially ordered, and mostly occur on distinct ribonucleoprotein (RNP)-containing structures. Defects in the mRNA maturation pathways have been demonstrated in myotonic dystrophy type 1 (DM1) and type 2 (DM2) whose characteristic multisystemic features are caused by the expansion of two distinct nucleotide sequences: (CTG)n in the DMPK gene on chromosome 19q13 in DM1, and (CCTG)n in the ZNF9 gene on chromosome 3q21 in DM2. By combining biomolecular and cytochemical techniques, it has been shown that the basic mechanisms of DMs reside in the accumulation of CUG- or CCUG-containing transcripts in intranuclear foci where several RNA-binding proteins necessary for the physiological processing of pre-mRNA are sequestered. Moreover, a nucleoplasmic accumulation of splicing and cleavage factors has been found in DMs. This suggests that the dystrophic phenotype could depend on a general alteration of the pre-mRNA post-transcriptional pathway. Interestingly, the accumulation of pre-mRNA processing factors in the myonuclei of DM1 and DM2 patients is reminiscent of the nuclear alterations typical of sarcopenia, i.e., the loss of muscle mass and function which physiologically occurs during ageing. Consistently, in an in vitro study, we observed that satellite-cell-derived DM2 myoblasts show cell senescence alterations and impairment of the pre-mRNA maturation pathways earlier than the myoblasts from healthy patient. These results suggest possible common cellular mechanisms responsible for skeletal muscle wasting in sarcopenia and in myotonic dystrophy.

RNA Foci, CUGBP1, and ZNF9 are the primary targets of the mutant CUG and CCUG repeats expanded in myotonic dystrophies type 1 and type 2

Expansions of noncoding CUG and CCUG repeats in myotonic dystrophies type 1 (DM1) and DM2 cause complex molecular pathology, the features of which include accumulation of RNA aggregates and misregulation of the RNA-binding proteins muscleblind-like 1 (MBNL1) and CUG-binding protein 1 (CUGBP1). CCUG repeats also decrease amounts of the nucleic acid binding protein ZNF9. Using tetracycline (Tet)-regulated monoclonal cell models that express CUG and CCUG repeats, we found that low levels of long CUG and CCUG repeats result in nuclear and cytoplasmic RNA aggregation with a simultaneous increase of CUGBP1 and a reduction of ZNF9. Elevation of CUGBP1 and reduction of ZNF9 were also observed before strong aggregation of the mutant CUG/CCUG repeats. Degradation of CUG and CCUG repeats normalizes ZNF9 and CUGBP1 levels. Comparison of short and long CUG and CCUG RNAs showed that great expression of short repeats form foci and alter CUGBP1 and ZNF9; however, long CUG/CCUG repeats misregulate CUGBP1 and ZNF9 much faster than high levels of the short repeats. These data suggest that correction of DM1 and DM2 might be achieved by complete and efficient degradation of CUG and CCUG repeats or by a simultaneous disruption of CUG/CCUG foci and correction of CUGBP1 and ZNF9.

Massive expansion of SCA2 with autonomic dysfunction, retinitis pigmentosa, and infantile spasms

OBJECTIVE

To provide clinical data on a cohort of 6 patients with massive expansion (>200 CAG repeats) of spinocerebellar ataxia type 2 (SCA2) and investigate possible pathways of pathogenesis using bioinformatics analysis of ATXN2 networks.

METHODS

We present data on 6 patients with massive expansion of SCA2 who presented in infancy with variable combinations of hypotonia, global developmental delay, infantile spasms, and retinitis pigmentosa. ATXN2 is known to interact with a network of synaptic proteins. To investigate pathways of pathogenesis, we performed bioinformatics analysis on ATXN2 combined with known genes associated with infantile spasms, retinitis pigmentosa, and synaptic function.

RESULTS

All patients had a progressive encephalopathy with autonomic dysfunction, 4 had retinitis pigmentosa, and 3 had infantile spasms. The bioinformatics analysis led to several interesting findings. First, an interaction between ATXN2 and SYNJ1 may account for the development of retinitis pigmentosa. Second, dysfunction of postsynaptic vesicle endocytosis may be important in children with this progressive encephalopathy. Infantile spasms may be associated with interactions between ATXN2 and the postsynaptic structural proteins MAGI2 and SPTAN1.

CONCLUSIONS

Severe phenotype in children with massive expansion of SCA2 may be due to a functional deficit in protein networks in the postsynapse, specifically involving vesicle endocytosis.

Myotonic dystrophy, when simple repeats reveal complex pathogenic entities: new findings and future challenges

Expanded, non-coding RNAs can exhibit a deleterious gain-of-function causing human disease through abnormal interactions with RNA-binding proteins. Myotonic dystrophy (DM), the prototypical example of an RNA-dominant disorder, is mediated by trinucleotide repeat-containing transcripts that deregulate alternative splicing. Spliceopathy has therefore been a major focus of DM research. However, changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. The exciting finding of bidirectional transcription and non-conventional RNA translation of trinucleotide repeat sequences points to a new scenario, in which DM is not mediated by one single expanded RNA transcript, but involves multiple pathogenic elements and pathways. The study of the growing number of human diseases associated with toxic repeat-containing transcripts provides important insight into the understanding of the complex pathways of RNA toxicity. This review describes some of the recent advances in the understanding of the molecular mechanisms behind DM and other RNA-dominant disorders.

Myotonic dystrophy mouse models: towards rational therapy development

DNA repeat expansions can result in the production of toxic RNA. RNA toxicity has been best characterised in the context of myotonic dystrophy. Nearly 20 mouse models have contributed significant and complementary insights into specific aspects of this novel disease mechanism. These models provide a unique resource to test pharmacological, anti-sense, and gene-therapy therapeutic strategies that target specific events of the pathobiological cascade. Further proof-of-principle concept studies and preclinical experiments require critical and thorough analysis of the multiple myotonic dystrophy transgenic lines available. This review provides in-depth assessment of the molecular and phenotypic features of these models and their contribution towards the dissection of disease mechanisms, and compares them with the human condition. More importantly, it provides critical assessment of their suitability and limitations for preclinical testing of emerging therapeutic strategies.

RNA processing is altered in skeletal muscle nuclei of patients affected by myotonic dystrophy

Myotonic dystrophies (DMs) are characterised by highly variable clinical manifestations consisting of muscle weakness and atrophy, and a wide spectrum of extramuscular manifestations. In both DM1 and DM2 forms, expanded nucleotide sequences cause the accumulation of mutant transcripts in the nucleus, thus deregulating the function of some RNA-binding proteins and providing a plausible explanation for the multifactorial phenotype of DM patients. However, at the skeletal muscle level, no mechanistic explanation for the muscle wasting has so far been proposed. We therefore performed a study in situ by immunoelectron microscopy on biceps brachii biopsies from DM1, DM2 and healthy subjects, providing the first ultrastructural evidence on the distribution of some nuclear ribonucleoprotein (RNP)-containing structures and molecular factors involved in pre-mRNA transcription and maturation in dystrophic myonuclei. Our results demonstrated an accumulation of splicing and cleavage factors in myonuclei of both DM1 and DM2 patients, suggesting an impairment of post-transcriptional pre-mRNA pathways. The transcription of the expanded sequences in DM myonuclei would therefore hamper functionality of the whole splicing machinery, slowing down the intranuclear molecular trafficking; this would reduce the capability of myonuclei to respond to anabolic stimuli thus contributing to muscle wasting.