Aberrant regulation of insulin receptor alternative splicing is associated with insulin resistance in myotonic dystrophy

Altered mRNA splicing of dystrophin in type 1 myotonic dystrophy

Myotonic dystrophy type1 (DM1) is a multisystemic disorder caused by a CTG repeat expansion in the DMPK gene. Aberrant mRNA splicing of several genes has been reported to contribute to some of the symptoms, including myotonia and insulin resistance, but the cause of muscle wasting is unknown. Dystrophin is a cytoskeletal protein that is required for structural stability and signaling at the sarcolemma and has several spliced isoforms. We investigated the alternative splicing of dystrophin in skeletal and cardiac muscle of DM1 patients by using reverse transcriptase-polymerase chain reaction and found that dystrophin isoforms lacking exon 71 or 78, which is suggested to encode an important region for protein binding and hydrophobicity, were significantly increased. We suggest that the aberrantly spliced dystrophin is responsible for the muscle wasting in DM1.

The impact of alternative splicing in vivo: mouse models show the way

Alternative splicing is widely believed to have a major impact on almost all biological processes since it increases proteome complexity and thereby controls protein function. Recently, gene targeting in mice has been used to create in vivo models to study the regulation and consequences of alternative splicing. The evidence accumulated so far argues for a nonredundant, highly specific role of individual splicing factors in mammalian development, and furthermore, demonstrates the importance of distinct protein isoforms in vivo. In this review, we will compare phenotypes of mouse models for alternative splicing to crystallize common themes and to put them into perspective with the available in vitro data.

A correlation with exon expression approach to identify cis-regulatory elements for tissue-specific alternative splicing

Correlation of motif occurrences with gene expression intensity is an effective strategy for elucidating transcriptional cis-regulatory logic. Here we demonstrate that this approach can also identify cis-regulatory elements for alternative pre-mRNA splicing. Using data from a human exon microarray, we identified 56 cassette exons that exhibited higher transcript-normalized expression in muscle than in other normal adult tissues. Intron sequences flanking these exons were then analyzed to identify candidate regulatory motifs for muscle-specific alternative splicing. Correlation of motif parameters with gene-normalized exon expression levels was examined using linear regression and linear splines on RNA words and degenerate weight matrices, respectively. Our unbiased analysis uncovered multiple candidate regulatory motifs for muscle-specific splicing, many of which are phylogenetically conserved among vertebrate genomes. The most prominent downstream motifs were binding sites for Fox1- and CELF-related splicing factors, and a branchpoint-like element acuaac; pyrimidine-rich elements resembling PTB-binding sites were most significant in upstream introns. Intriguingly, our systematic study indicates a paucity of novel muscle-specific elements that are dominant in short proximal intronic regions. We propose that Fox and CELF proteins play major roles in enforcing the muscle-specific alternative splicing program, facilitating expression of unique isoforms of cytoskeletal proteins critical to muscle cell function.

Huntington's disease--like 2 is associated with CUG repeat-containing RNA foci

OBJECTIVE

Huntington's disease-like 2 (HDL2) is caused by a CAG/CTG expansion mutation on chromosome 16q24.3. The repeat falls, in the CTG orientation, within a variably spliced exon of junctophilin-3 (JPH3). The existence of a JPH3 splice variant with the CTG repeat in 3' untranslated region suggested that transcripts containing an expanded CUG repeat could play a role in the pathogenesis of HDL2, similar to the proposed pathogenic role of expanded CUG repeats in myotonic dystrophy type 1 (DM1). The goal of this study, therefore, was to test the plausibility of an RNA gain-of-function component in the pathogenesis of HDL2.

METHODS

The presence and composition of RNA foci in frontal cortex from HDL2, Huntington's disease, DM1, and control brains were investigated by in situ hybridization and immunohistochemistry. An untranslatable JPH3 transcript containing either a normal or an expanded CUG repeat was engineered and expressed in human embryonic kidney 293 and HT22 cells to further test the toxic RNA hypothesis. The formation of RNA foci and the extent of cell death were quantified.

RESULTS

RNA foci resembling DM1 foci were detected in neurons in HDL2 cortex and other brain regions. Similar to DM1, the foci colocalize with muscleblind-like protein 1, and nuclear muscleblind-like protein 1 in HDL2 cortical neurons is decreased relative to controls. In cell experiments, expression of a JPH3 transcript with an expanded CUG repeat resulted in the formation of RNA foci that colocalized with muscleblind-like protein 1 and in cell toxicity.

INTERPRETATION

These results imply that RNA toxicity may contribute to the pathogenesis of HDL2.

Regulation of the promoter of CUG triplet repeat binding protein, Cugbp1, during myogenesis

CUG triplet repeat binding protein, CUGBP1, plays a critical role in the development of skeletal muscle pathology in patients with Myotonic Dystrophy 1 (DM1). In this paper, we have characterized transcriptional regulation of mouse Cugbp1 gene during normal myogenesis. There are several Cugbp1 mRNA species with variable 5' ends. We found that these mRNA species have different patterns of expression during myogenesis. Isoforms 1 and 2 are mainly expressed in myotubes, while expression of isoform 3 is increased during transition of myoblasts to myotubes and during transition of myotubes to myofibers. We have cloned a short region of the Cugbp1 promoter, which is responsible for the regulation of the isoform 3, and have identified within this region three different transcription start sites. This promoter region exhibits high activity in myoblasts and the activity of this region is significantly increased in myotubes. The Cugbp1 promoter contains three E-box elements. A mutation of one of the E-boxes, E(3), significantly reduces activity of the Cugbp1 promoter. Gelshift and ChIP assays showed that E(3)-box is occupied by E12, CBP and p300 proteins in myoblasts, while in differentiated myotubes this element is occupied by myogenin, E12 and p300. The binding of myogenin to the Cugbp1 promoter correlates with activation of the promoter during differentiation. Our data show that myogenin is a key regulator of the Cugbp1 promoter since overexpression of myogenin increases the activity of the Cugbp1 promoter; while the inhibition of myogenin reduces activity of the Cugbp1 promoter. These data show that transcription of Cugbp1 gene in muscle is regulated by myogenin and E proteins and suggest that the co-operation of several transcription factors is important for the activation of the Cugbp1 promoter.

Regulating insulin signaling and beta-cell function through IRS proteins

Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity onset diabetes of youth, MODY), autoimmune-mediated beta-cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2). Type 2 diabetes is the most prevalent form that usually occurs at middle age; it afflicts more than 30 million people over the age of 65, but is appearing with greater frequency in children and adolescents. Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders--including dyslipidemia, hypertension, cardiovascular disease, and female infertility. Understanding the molecular basis of insulin resistance can prevent these disorders and their inevitable progression to type 2 diabetes.

Alternative isoform discrimination by the next generation of expression profiling microarrays

Microarray expression profiling has revolutionised the way that many therapeutic targets have been identified over the past 10 years. High-density microarrays have allowed scientists to simultaneously scrutinise the expression of all genes encoded on a given genome. Although the data collected from classically designed microarrays greatly enriched the pool of information available to help guide the selection and design of new therapeutic strategies, they were unable to tell the complete story. The major limitation with most array designs is that they can only produce a global expression value for all transcripts produced from a specific locus and cannot monitor each individual alternative isoform produced from the interrogated locus. Recently, new array designs have been described, and become commercially available, that can efficiently monitor individual alternatively spliced isoforms produced from a single locus, allowing the research community to get a more accurate picture of the biological landscape of the expressed transcripts.

Expression of MBNL and CELF mRNA transcripts in muscles with myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is an autosomal dominant disorder that causes muscle wasting, myotonia, cardiac conduction abnormalities, and other multi-systemic symptoms. Current evidence supports a pathogenic mechanism involving aberrantly expanded CTG repeats in the 3'-untranslated region of the DM protein kinase (DMPK) gene. The repeats are thought to recruit various RNA-binding proteins such as muscleblind-like (MBNL) proteins into foci in the nuclei of DM cells, resulting in loss of function. However, aberrant regulation of transcription or subsequent RNA processing of MBNL-family mRNAs might also be part of the pathogenic mechanism of DM. We used real-time RT-PCR analysis to examine the possibility that MBNL mRNA expression is altered in DM1 patients. We also examined mRNA expression for members of the CUG-BP and ETR-3-like factor (CELF) family of RNA-binding proteins given that CELF proteins regulate alternative splicing and are also implicated in DM. We found that DM1 muscles displayed aberrant regulation of alternative splicing as reported previously; however, the levels of MBNL and CELF mRNA expression did not show any significant changes. Our results suggest that the expression and stability of the mRNA for these RNA-binding proteins are unaffected in DM1.

Abnormalities of the nucleus and nuclear inclusions in neurodegenerative disease: a work in progress

Neurodegenerative diseases are characterized pathologically by the abnormal accumulation of pathogenic protein species within the cell. Several neurodegenerative diseases feature intranuclear protein aggregation in the form of intranuclear inclusion bodies. Studies of these intranuclear inclusions are providing important clues regarding the cellular pathophysiology of these diseases, as exemplified by recent progress in defining the genetic basis of a subset of frontotemporal dementia cases. The precise role of intranuclear inclusion bodies in disease pathogenesis is currently a focus of debate. The present review provides an overview of the diverse family of neurodegenerative diseases in which nuclear inclusions form part of the neuropathological spectrum. In addition, current pathogenetic concepts relevant to these diseases will be reviewed and arguments for and against a protective role for intranuclear inclusions will be presented. The relationship of pathological intranuclear inclusions to functional intranuclear bodies will also be discussed. Finally, by analogy with pathological intranuclear inclusions, I will speculate on the possibility that intranuclear protein aggregation may represent a constitutive cellular protective mechanism occurring in neurons under physiological conditions.

Myotonic dystrophy: Emerging mechanisms for DM1 and DM2

Myotonic dystrophy (DM) is a complex multisystemic disorder linked to two different genetic loci. Myotonic dystrophy type 1 (DM1) is caused by an expansion of a CTG repeat located in the 3' untranslated region (UTR) of DMPK (myotonic dystrophy protein kinase) on chromosome 19q13.3. Myotonic dystrophy type 2 (DM2) is caused by an unstable CCTG repeat in intron 1 of ZNF9 (zinc finger protein 9) on chromosome 3q21. Therefore, both DM1 and DM2 are caused by a repeat expansion in a region transcribed into RNA but not translated into protein. The discovery that these two distinct mutations cause largely similar clinical syndromes put emphasis on the molecular properties they have in common, namely, RNA transcripts containing expanded, non-translated repeats. The mutant RNA transcripts of DM1 and DM2 aberrantly affect the splicing of the same target RNAs, such as chloride channel 1 (ClC-1) and insulin receptor (INSR), resulting in their shared myotonia and insulin resistance. Whether the entire disease pathology of DM1 and DM2 is caused by interference in RNA processing remains to be seen. This review focuses on the molecular significance of the similarities and differences between DM1 and DM2 in understanding the disease pathology of myotonic dystrophy.

A variably spliced region in the type 1 ryanodine receptor may participate in an inter-domain interaction

The aim of the present study was to examine residues that are variably spliced in the juvenile and adult isoforms of the skeletal-muscle RyR1 (type 1 ryanodine receptor). The juvenile ASI(-) splice variant is less active than the adult ASI(+) variant and is overexpressed in patients with DM (myotonic dystrophy) [Kimura, Nakamori, Lueck, Pouliquin, Aoike, Fujimura, Dirksen, Takahashi, Dulhunty and Sakoda (2005) Hum. Mol. Genet. 14, 2189-2200]. In the present study, we explore the ASI region using synthetic peptides corresponding to rabbit RyR1 residues Thr3471-Gly3500 either containing [PASI(+)] or lacking [PASI(-)] the ASI residues. Both peptides increased [3H]ryanodine binding to rabbit RyR1s, increased Ca2+ release from sarcoplasmic reti-culum vesicles and increased single RyR1 channel activity. The peptide PASI(-) was more active in each case than PASI(+). [3H]Ryanodine binding to recombinant ASI(+)RyR1 or ASI(-)-RyR1 was enhanced more by PASI(-) than PASI(+), with the greatest increase seen when PASI(-) was added to ASI(-)RyR1. The activation of the RyR channels is consistent with the hypo-thesis that the peptides interrupt an inhibitory inter-domain inter-action and that PASI(-) is more effective at interrupting this interaction than PASI(+). We therefore suggest that the ASI(-) sequence interacts more tightly than the ASI(+) sequence with its binding partner, so that the ASI(-)RyR1 is more strongly inhibited (less active) than the ASI(+)RyR1. Thus the affinity of the binding partners in this inter-domain interaction may deter-mine the activities of the mature and juvenile isoforms of RyR1 and the stronger inhibition in the juvenile isoform may contribute to the myopathy in DM.

Spliceosome assembly and composition

Cells control alternative splicing by modulating assembly of the pre-mRNA splicing machinery at competing splice sites. Therefore, a working knowledge of spliceosome assembly is essential for understanding how alternative splice site choices are achieved. In this chapter, we review spliceosome assembly with particular emphasis on the known steps and factors subject to regulation during alternative splice site selection in mammalian cells. We also review recent advances regarding similarities and differences between the in vivo and in vitro assembly pathways, as well as proofreading mechanisms contributing to the fidelity of splice site selection.

Alternative splicing in disease

Alternative splicing is a major source of diversity in the human proteome. The regulation of alternative splicingmodulates the composition of this diversity to fulfill the physiological requirements of a cell. When control of alternative splicing is disrupted, the result can be a failure to meet cellular and tissue requirements resulting in dysfunction and disease. There are several well-characterized examples in which disruption of alternative splicing is a cause of disease. Investigations into how the mis-regulation of alternative splicing causes disease complements investigations of normal regulatory processes and enhances our understanding of regulatory mechanisms in general Ultimately, an understanding of how alternative splicing is altered in disease will facilitate strategies directed at reversing or circumventing mis-regulated splicing events.

Gene expression analysis in myotonic dystrophy: indications for a common molecular pathogenic pathway in DM1 and DM2

An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins. An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins.

CUG-BP1/CELF1 requires UGU-rich sequences for high-affinity binding

CUG-BP1 [CUG-binding protein 1 also called CELF (CUG-BP1 and ETR3 like factors) 1] is a human RNA-binding protein that has been implicated in the control of splicing and mRNA translation. The Xenopus homologue [EDEN-BP (embryo deadenylation element-binding protein)] is required for rapid deadenylation of certain maternal mRNAs just after fertilization. A variety of sequence elements have been described as target sites for these two proteins but their binding specificity is still controversial. Using a SELEX (systematic evolution of ligand by exponential enrichment) procedure and recombinant CUG-BP1 we selected two families of aptamers. Surface plasmon resonance and electrophoretic mobility-shift assays showed that these two families differed in their ability to bind CUG-BP1. Furthermore, the selected high-affinity aptamers form two complexes with CUG-BP1 in electrophoretic mobility assays whereas those that bind with low affinity only form one complex. The validity of the distinction between the two families of aptamers was confirmed by a functional in vivo deadenylation assay. Only those aptamers that bound CUG-BP1 with high affinity conferred deadenylation on a reporter mRNA. These high-affinity RNAs are characterized by a richness in UGU motifs. Using these binding site characteristics we identified the Xenopus maternal mRNA encoding the MAPK (mitogen-activated protein kinase) phosphatase (XCl100alpha) as a substrate for EDEN-BP. In conclusion, high-affinity CUG-BP1 binding sites are sequence elements at least 30 nucleotides in length that are enriched in combinations of U and G nucleotides and contain at least 4 UGU trinucleotide motifs. Such sequence elements are functionally competent to target an RNA for deadenylation in vivo.

RNA-dominant diseases

Several examples have come to light in which mutations in non-protein-coding regions give rise to a deleterious gain-of-function by non-coding RNA. Expression of the toxic RNA is associated with formation of nuclear inclusions and late-onset degenerative changes in brain, heart or skeletal muscle. In the best studied example, myotonic dystrophy, it appears that the main pathogenic effect of the toxic RNA is to sequester binding proteins and compromise the regulation of alternative splicing. This review describes some of the recent advances in understanding the pathophysiology of RNA-dominant diseases.

Kinetics of FcRn-mediated recycling of IgG and albumin in human: pathophysiology and therapeutic implications using a simplified mechanism-based model

The nonclassical MHC class-I molecule, FcRn, salvages both IgG and albumin from degradation. Here we introduce a mechanism-based kinetic model for human to quantify FcRn-mediated recycling of both ligands based on saturable kinetics and data from the literature using easily measurable plasma concentrations rather than unmeasurable endosomal concentrations. The FcRn-mediated fractional recycling rates of IgG and albumin were 142% and 44% of their fractional catabolic rates, respectively. Clearly, FcRn-mediated recycling is a major contributor to the high endogenous concentrations of these two important plasma proteins. While familial hypercatabolic hypoproteinemia is caused by complete FcRn deficiency, the hypercatabolic IgG deficiency of myotonic dystrophy could be explained, based on the kinetic analyses, by a normal number of FcRn with lowered affinity for IgG but normal affinity for albumin. A simulation study demonstrates that the plasma concentrations of IgG and albumin could be dynamically controlled by both FcRn-related and -unrelated parameters.

Interaction of muscleblind, CUG-BP1 and hnRNP H proteins in DM1-associated aberrant IR splicing

In myotonic dystrophy (DM1), both inactivation of muscleblind proteins and increased levels of CUG-BP1 are reported. These events have been shown to contribute independently to aberrant splicing of a subset RNAs. We demonstrate that steady-state levels of the splice regulator, hnRNP H, are elevated in DM1 myoblasts and that increased hnRNP H levels in normal myoblasts results in the inhibition of insulin receptor (IR) exon 11 splicing in a manner similar to that observed in DM1. In normal myoblasts, overexpression of either hnRNP H or CUG-BP1 results in the formation of an RNA-dependent suppressor complex consisting of both hnRNP H and CUG-BP1, which is required to maximally inhibit IR exon 11 inclusion. Elevated levels of MBNL1 show RNA-independent interaction with hnRNP H and dampen the inhibitory activity of increased hnRNP H levels on IR splicing in normal myoblasts. In DM1 myoblasts, overexpression of MBNL1 in conjunction with si-RNA mediated depletion of hnRNP H contributes to partial rescue of the IR splicing defect. These data demonstrate that coordinated physical and functional interactions between hnRNP H, CUG-BP1 and MBNL1 dictate IR splicing in normal and DM1 myoblasts.

Flies deficient in Muscleblind protein model features of myotonic dystrophy with altered splice forms of Z-band associated transcripts

Myotonic dystrophy (DM) is a dominantly inherited neuromuscular disorder characterised by muscle weakness and wasting. There are two forms of DM; both of which are caused by the expansion of repeated DNA sequences. DM1 is associated with a CTG repeat located in the 3' untranslated region of a gene, DMPK and DM2 with a tetranucleotide repeat expansion, CCTG, located in the first intron of a different gene, ZNF9. Recent data suggest a dominant RNA gain-of-function mechanism underlying DM, as transcripts containing either CUG or CCUG repeat expansions accumulate as foci in the nuclei of DM1 and DM2 cells respectively, where they exert a toxic effect, sequestering specific RNA binding proteins such as Muscleblind, which leads to splicing defects and the disruption of normal cellular functions. Z-band disruption is a well-known histological feature of DM1 muscle, which has also been reported in Muscleblind deficient flies. In order to determine whether there is a common molecular basis for this abnormality we have examined the alternative splicing pattern of transcripts that encode proteins associated with the Z-band in both organisms. Our results demonstrate that the missplicing of ZASP/LDB3 leads to the expression of an isoform in DM1 patient muscle, which is not present in normal controls, nor in other myopathies. Furthermore the Drosophila homologue, CG30084, is also misspliced, in Muscleblind deficient flies. Another Z-band transcript, alpha actinin, is misspliced in mbl mutant flies, but not in DM1 patient samples. These results point to similarities but subtle differences in the molecular breakdown of Z-band structures in flies and DM patients and emphasise the relevance of Muscleblind proteins in DM pathophysiology.

Novel regulators of RyR Ca2+ release channels: insight into molecular changes in genetically-linked myopathies

There are many mutations in the ryanodine receptor (RyR) Ca2+ release channel that are implicated in skeletal muscle disorders and cardiac arrhythmias. More than 80 mutations in the skeletal RyR1 have been identified and linked to malignant hyperthermia, central core disease or multi-minicore disease, while more than 40 mutations in the cardiac RyR2 lead to ventricular arrhythmias and sudden cardiac death in patients with structurally normal hearts. These RyR mutations cause diverse changes in RyR activity which either excessively activate or block the channel in a manner that disrupts Ca2+ signalling in the muscle fibres. In a different myopathy, myotonic dystrophy (DM), a juvenile isoform of the skeletal RyR is preferentially expressed in adults. There are two regions of RyR1 that are variably spiced and developmentally regulated (ASI and ASII). The juvenile isoform (ASI(-)) is less active than the adult isoform (ASI(+)) and its over-expression in adults with DM may contribute to functional changes. Finally, mutations in an important regulator of the RyR, the Ca2+ binding protein calsequestrin (CSQ), have been linked to a disruption of Ca2+ homeostasis in cardiac myocytes that results in arrhythmias. We discuss evidence supporting the hypothesis that mutations in each of these situations alter protein/protein interactions within the RyR complex or between the RyR and its associated proteins. The disruption of these protein-protein interactions can lead either to excess Ca2+ release or reduced Ca2+ release and thus to abnormal Ca2+ homeostasis. Much of the evidence for disruption of protein-protein interactions has been provided by the actions of a group of novel RyR regulators, domain peptides with sequences that correspond to sequences within the RyR and which compete with the endogenous residues for their interaction sites.

Reversal of RNA missplicing and myotonia after muscleblind overexpression in a mouse poly(CUG) model for myotonic dystrophy

RNA-mediated pathogenesis is a recently developed disease model that proposes that certain types of mutant genes produce toxic transcripts that inhibit the activities of specific proteins. This pathogenesis model was proposed first for the neuromuscular disease myotonic dystrophy (DM), which is associated with the expansion of structurally related (CTG)(n) and (CCTG)(n) microsatellites in two unrelated genes. At the RNA level, these expansions form stable hairpins that alter the pre-mRNA splicing activities of two antagonistic factor families, the MBNL and CELF proteins. It is unclear which altered activity is primarily responsible for disease pathogenesis and whether other factors and biochemical pathways are involved. Here, we show that overexpression of Mbnl1 in vivo mediated by transduction of skeletal muscle with a recombinant adeno-associated viral vector rescues disease-associated muscle hyperexcitability, or myotonia, in the HSA(LR) poly(CUG) mouse model for DM. Myotonia reversal occurs concurrently with restoration of the normal adult-splicing patterns of four pre-mRNAs that are misspliced during postnatal development in DM muscle. Our results support the hypothesis that the loss of MBNL1 activity is a primary pathogenic event in the development of RNA missplicing and myotonia in DM and provide a rationale for therapeutic strategies designed either to overexpress MBNL1 or inhibit MBNL1 interactions with CUG and CCUG repeat expansions.

Reversible model of RNA toxicity and cardiac conduction defects in myotonic dystrophy

Myotonic dystrophy (DM1), the most common muscular dystrophy in adults, is caused by an expanded (CTG)n tract in the 3' UTR of the gene encoding myotonic dystrophy protein kinase (DMPK), which results in nuclear entrapment of the 'toxic' mutant RNA and interacting RNA-binding proteins (such as MBNL1) in ribonuclear inclusions. It is unclear if therapy aimed at eliminating the toxin would be beneficial. To address this, we generated transgenic mice expressing the DMPK 3' UTR as part of an inducible RNA transcript encoding green fluorescent protein (GFP). We were surprised to find that mice overexpressing a normal DMPK 3' UTR mRNA reproduced cardinal features of myotonic dystrophy, including myotonia, cardiac conduction abnormalities, histopathology and RNA splicing defects in the absence of detectable nuclear inclusions. However, we observed increased levels of CUG-binding protein (CUG-BP1) in skeletal muscle, as seen in individuals with DM1. Notably, these effects were reversible in both mature skeletal and cardiac muscles by silencing transgene expression. These results represent the first in vivo proof of principle for a therapeutic strategy for treatment of myotonic dystrophy by ablating or silencing expression of the toxic RNA molecules.

RNA-MEDIATED NEUROMUSCULAR DISORDERS

Myotonic dystrophy type 1 (DM1) is caused by a CTG expansion mutation located in the 3' untranslated portion of the dystrophica myotonin protein kinase gene. The identification and characterization of RNA-binding proteins that interact with expanded CUG repeats and the discovery that a similar transcribed but untranslated CCTG expansion in an intron causes myotonic dystrophy type 2 (DM2) have uncovered a new type of mechanism in which microsatellite expansion mutations cause disease through an RNA gain-of-function mechanism. This review discusses RNA pathogenesis in DM1 and DM2 and evidence that similar mechanisms may play a role in a growing number of dominant noncoding expansion disorders, including fragile X tremor ataxia syndrome (FXTAS), spinocerebellar ataxia type 8 (SCA8), SCA10, SCA12, and Huntington's disease-like 2 (HDL2).

Alternative pre-mRNA splicing in the human system: unexpected role of repetitive sequences as regulatory elements

Alternative splicing is a process by which multiple messenger RNAs (mRNAs) are generated from a single pre-mRNA, resulting in functionally distinct protein products. This is accomplished by the differential recognition of splice sites in the pre-mRNA, often regulated in a tissue- or development-specific manner. Alternative splicing constitutes not only an important mechanism in controlling gene expression in humans, but also an essential source for increasing proteome diversity. In this review we summarize the underlying mechanistic principles, focussing on the cis-acting regulatory elements. In particular, the role of short sequence repeats, which are often polymorphic, in splicing regulation is discussed.

Increased CUG triplet repeat-binding protein-1 predisposes to impaired adipogenesis with aging

Preadipocyte differentiation capacity declines between middle and old age. Expression of the adipogenic transcription factors, CCAAT/enhancer-binding protein (C/EBP) alpha and peroxisome proliferator-activated receptor gamma (PPARgamma), is lower in differentiating preadipocytes from old than young animals, although no age-related changes occur in C/EBPbeta mRNA, which is upstream of C/EBPalpha and PPARgamma. C/EBPbeta-liver-enriched inhibitory protein (C/EBPbeta-LIP), a truncated C/EBPbeta isoform that is a dominant inhibitor of differentiation, increases with aging in rat fat tissue and preadipocytes. CUG triplet repeat-binding protein-1 (CUGBP1) binds to C/EBPbeta mRNA, increasing C/EBPbeta-LIP translation. Abundance and nucleotide binding activity of CUGBP1 increased with aging in preadipocytes. CUGBP1 overexpression in preadipocytes from young animals increased C/EBPbeta-LIP and impaired adipogenesis. Decreasing CUGBP1 in preadipocytes from old rats by RNA interference reduced C/EBPbeta-LIP abundance and promoted adipogenesis. Tumor necrosis factor-alpha, levels of which are elevated in fat tissue with aging, increased CUGBP1 protein, CUGBP1 binding activity, and C/EBPbeta-LIP in preadipocytes from young rats. Thus, CUGBP1 contributes to regulation of adipogenesis in primary preadipocytes and is responsive to tumor necrosis factor-alpha. With aging, preadipocyte CUGBP1 abundance and activity increases, resulting in enhanced translation of the C/EBPbeta-LIP isoform, thereby blocking effects of adipogenic transcription factors, predisposing preadipocytes from old animals to resist adipogenesis. Altered translational processing, possibly related to changes in cytokine milieu and activation of stress responses, may contribute to changes in progenitor differentiation and tissue function with aging.

MBNL1 and CUGBP1 modify expanded CUG-induced toxicity in a Drosophila model of myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by a CTG expansion in the 3' UTR of the dystrophia myotonica protein kinase (DMPK) gene. It has been hypothesized that the pathogenesis in DM1 is triggered by a toxic gain of function of the expanded DMPK RNA. This expanded RNA is retained in nuclear foci where it sequesters and induces alterations in the levels of RNA-binding proteins (RNA-BP). To model DM1 and study the implication of RNA-BP in CUG-induced toxicity, we have generated a Drosophila DM1 model expressing a non-coding mRNA containing 480 interrupted CUG repeats; i.e. [(CUG)20CUCGA]24. This (iCUG)480 transcript accumulates in nuclear foci and its expression leads to muscle wasting and degeneration in Drosophila. We also report that altering the levels of two RNA-BP known to be involved in DM1 pathogenesis, MBNL1 and CUGBP1, modify the (iCUG)480 degenerative phenotypes. Expanded CUG-induced toxicity in Drosophila is suppressed when MBNL1 expression levels are increased, and enhanced when MBNL1 levels are reduced. In addition, (iCUG)480 also causes a decrease in the levels of soluble MBNL1 that is sequestered in the CUG-containing nuclear foci. In contrast, increasing the levels of CUGBP1 worsens (iCUG)480-induced degeneration even though CUGBP1 distribution is not altered by the expression of the expanded triplet repeat. Our data supports a mechanism for DM1 pathogenesis in which decreased levels of MBNL and increased levels of CUGBP mediate the RNA-induced toxicity observed in DM1. Perhaps more importantly, they also provide proof of the principle that CUG-induced muscle toxicity can be suppressed.

Failure of MBNL1-dependent post-natal splicing transitions in myotonic dystrophy

In myotonic dystrophy (DM), expression of RNA containing expanded CUG or CCUG repeats leads to misregulated alternative splicing of pre-mRNA. The repeat-bearing transcripts accumulate in nuclear foci, together with proteins in the muscleblind family, MBNL1 and MBNL2. In transgenic mice that express expanded CUG repeats, we show that the splicing defect selectively targets a group of exons that share a common temporal pattern of developmental regulation. These exons undergo a synchronized splicing switch between post-natal day 2 and 20 in wild-type mice. During this post-natal interval, MBNL1 protein translocates from a predominantly cytoplasmic to nuclear distribution. In the absence of MBNL1, these physiological splicing transitions do not occur. The splicing defect induced by expanded CUG repeats in mature muscle fibers is closely reproduced by deficiency of MBNL1 but not by deficiency of MBNL2. A parallel situation exists in human DM type 1 and type 2. MBNL1 is depleted from the muscle nucleoplasm because of sequestration in nuclear foci, and the associated splicing defects are remarkably similar to those observed in MBNL1 knockout mice. These results indicate that MBNL1 participates in the post-natal remodeling of skeletal muscle by controlling a key set of developmentally regulated splicing switches. Sequestration of MBNL1, and failure to maintain these splicing transitions, has a pivotal role in the pathogenesis of muscle disease in DM.

Mammalian CELF/Bruno-like RNA-binding proteins: molecular characteristics and biological functions

In mammals, the CELF/Bruno-like family of RNA-binding proteins contains six members. The founder members of the family are the CUG-BP1 (CELF1) and ETR-3 (CELF2) proteins. Four other members have been identified mainly by sequence similarity. The founder members were cloned or identified in a number of laboratories which has lead to a profusion of names and two separate naming systems. In addition, different members of the CELF/Bruno-like protein family have been shown to be implicated in two major post-transcriptional regulatory processes, namely the alternative splicing and the control of translation and stability of target mRNAs. Several studies have indicated a certain functional redundancy between the CELF proteins in fulfilling these functions. The multiplicity of gene names and the eventual functional redundancy is a source of potential confusion in published work. We present here a synthetic picture of the present situation and, where possible, models are proposed that can account for the data obtained in the various laboratories with different biological models. Furthermore, we have highlighted some important questions that still need to be resolved.

Myotonic dystrophy protein kinase monoclonal antibody generation from a coiled-coil template

Myotonic dystrophy protein kinase (DMPK) was the initial representative of a ubiquitous protein kinase family that regulates cell size and shape. DMPK is highly expressed in heart and skeletal muscle and transgenic over-expression induces cardiac hypertrophy. The characterization of DMPK has been limited by the paucity of immunological reagents with high affinity and well-defined specificity. Amino acid sequence data was used to predict the surface exposure of the coil-coiled domain of DMPK. These exposed amino acids were substituted into an extremely stable coiled-coil template to produce a peptide antigen. Sera from mice immunized with the peptide conjugated to keyhole limpet hemocyanin were screened against recombinant DMPK using Western blots. Murine spleens expressing DMPK antibodies were used to produce hybridoma cell lines. Hybridoma supernatants were further screened against recombinant DMPK and four clonal hybridoma cell lines expressing DMPK antibodies were generated. These four monoclonal antibodies recognized recombinant DMPK in Western blots of COS-1 cell lysates expressing high levels of recombinant DMPK and immunoprecipitated recombinant DMPK from COS-1 cell lysates. The identity of the immunoprecipitated DMPK was confirmed by MALDI-TOF mass spectrometry and peptide mass fingerprinting. DMPK was the only protein detected in the immunoprecipitates, indicating the high specificity of the antibodies. Western blots immunostained with two of the monoclonal antibodies specifically recognized the two isoforms of endogenous DMPK, DMPK-1 and DMPK-2, that are expressed at low levels in the human heart. The recognition of low amounts of DMPK-1 and DMPK-2 indicates the high affinity of these antibodies. A human heart lysate was subjected to ammonium sulfate precipitation and column chromatography to produce a fraction that was enriched in DMPK. One of the monoclonal antibodies immunoprecipitated endogenous DMPK from this fraction. This antibody was used for immuno-localization studies of an adenoviral DMPK construct, expressed in adult mouse cardiac myocytes. This construct was localized to the intercalated disc, the site of endogenous DMPK, indicating that this antibody is applicable to immuno-localization studies. This study demonstrates the utility of the described procedure for generation of specific monoclonal antibodies with high affinity for epitopes in coiled-coiled domains of mammalian proteins expressed at low levels.

Nuclear targeting of the CaMKII anchoring protein αKAP is regulated by alternative splicing and protein kinases

alphaKAP is an anchoring protein for the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and is encoded within the same gene as the CaMKIIalpha isoform. alphaKAP co-assembles with CaMKII and targets such heteromers to the membrane of the sarcoplasmic reticulum, where CaMKII can regulate Ca(2+) homeostasis. CaMKII has also nuclear functions in skeletal muscle, however, the nuclear targeting mechanism has been elusive. We show here that developmentally regulated splicing of exon Ealpha(B) generates a functional nuclear localization signal (NLS) in alphaKAP(B), the dominant alphaKAP variant in mature muscle. The alphaKAP(A) variant lacks the NLS and dominates in developing muscle before and around birth. Both alphaKAP variants localize to membranes, but a small fraction of alphaKAP(B) is additionally found in the nucleus. Indeed, alpha-karyopherins that mediate nuclear import bound to alphaKAP(B) but not alphaKAP(A) in vitro. When the N-terminal membrane anchor of alphaKAP was deleted, localization of alphaKAP(B) but not alphaKAP(A) became predominantly nuclear. Co-expression of constitutively active CaMKI and IV, which do not bind to alphaKAP, interfered with nuclear localization of alphaKAP(B). CaMKIIalpha was found essentially exclusively in the cytoplasm when expressed in cell lines but was targeted to the nucleus when co-expressed with the nuclear form of alphaKAP(B). Thus, nuclear targeting of cytoplasmic CaMKII isoforms by alphaKAP may be regulated by developmentally controlled alternative splicing and by protein kinases.

The Muscleblind family of proteins: an emerging class of regulators of developmentally programmed alternative splicing

Alternative splicing is widely used to generate protein diversity and to control gene expression in many biological processes, including cell fate determination and apoptosis. In this review, we focus on the Muscleblind family of tissue-specific alternative splicing regulators. Muscleblind proteins bind pre-mRNA through an evolutionarily conserved tandem CCCH zinc finger domain. Human Muscleblind homologs MBNL1, MBNL2 and MBNL3 promote inclusion or exclusion of specific exons on different pre-mRNAs by antagonizing the activity of CUG-BP and ETR-3-like factors (CELF proteins) bound to distinct intronic sites. The relative activities of Muscleblind and CELF proteins control a key developmental switch. Defined transcripts follow an embryonic splice pattern when CELF activity predominates, whereas they follow an adult pattern when Muscleblind activity prevails. Human MBNL proteins show functional specializations. While MBNL1 seems to promote muscle differentiation, MBNL3 appears to function in an opposing manner inhibiting expression of muscle differentiation markers. MBNL2, on the other hand, participates in a new RNA-dependent protein localization mechanism involving recruitment of integrin alpha3 protein to focal adhesions. Both muscleblind mutant Drosophila embryos and Mbnl1 knockout mice show muscle abnormalities and altered splicing of specific transcripts. In addition to regulating terminal muscle differentiation through alternative splicing control, results by several groups suggest that Muscleblind participates in the differentiation of photoreceptors, neurons, adipocytes and blood cell types. Misregulation of MBNL activity can lead to human pathologies. Through mechanisms not completely identified yet, expression of transcripts containing large non-coding CUG or CCUG repeat expansions mimics muscleblind loss-of-function phenotypes. Archetypical within this class of disorders are myotonic dystrophies. Our understanding of the biology of Muscleblind proteins has increased dramatically over the last few years, but several key issues remain unsolved. Defining the mechanism of the activity of Muscleblind proteins, their splicing partners, and the functional relevance of its several protein isoforms are just a few examples.

Effect of the [CCTG]n repeat expansion on ZNF9 expression in myotonic dystrophy type II (DM2)

Myotonic dystrophy is caused by two different mutations: a (CTG)n expansion in 3' UTR region of the DMPK gene (DM1) and a (CCTG)n expansion in intron 1 of the ZNF9 gene (DM2). The most accredited mechanism for DM pathogenesis is an RNA gain-of-function. Other findings suggest a contributory role of DMPK-insufficiency in DM1. To address the issue of ZNF9 role in DM2, we have analyzed the effects of (CCTG)n expansion on ZNF9 expression in lymphoblastoid cell lines (n=4) from DM2 patients. We did not observe any significant alteration in ZNF9 mRNA and protein levels, as shown by QRT-PCR and Western blot analyses. Additional RT-PCR experiments demonstrated that ZNF9 pre-mRNA splicing pattern, which includes two isoforms, is unmodified in DM2 cells. Our results indicate that the (CCTG)n expansion in the ZNF9 intron does not appear to have a direct consequence on the expression of the gene itself.

ETR-3 represses Tau exons 2/3 inclusion, a splicing event abnormally enhanced in myotonic dystrophy type I

Altered splicing of transcripts, including the insulin receptor (IR) and the cardiac troponin (cTNT), is a key feature of myotonic dystrophy type I (DM1). CELF and MBNL splicing factor members regulate the splicing of those transcripts. We have previously described an alteration of Tau exon 2 splicing in DM1 brain, resulting in the favored exclusion of exon 2. However, the factors required for alternative splicing of Tau exon 2 remain undetermined. Here we report a decreased expression of CELF family member and MBNL transcripts in DM1 brains as assessed by RT-PCR. By using cellular models with a control- or DM1-like splicing pattern of Tau transcripts, we demonstrate that ETR-3 promotes selectively the exclusion of Tau exon 2. These results together with the analysis of Tau exon 6 and IR exon 11 splicing in brain, muscle, and cell models suggest that DM1 splicing alteration of several transcripts involves various factors.

Misregulation of alternative splicing causes pathogenesis in myotonic dystrophy

Myotonic dystrophy (DM), the most common form of adult onset muscular dystrophy, affects skeletal muscle, heart, and the central nervous system (CNS). Mortality results primarily from muscle wasting and cardiac arrhythmias. There are two forms of the disease: DM1 and DM2. DM1, which constitutes 98% of cases, is caused by a CTG expansion in the 3' untranslated region (UTR) of the DMPK gene. DM2 is caused by a CCTG expansion in the first intron of the ZNF9 gene. RNA containing CUG- or CCUG-expanded repeats are transcribed but are retained in the nucleus in foci. Disease pathogenesis results primarily from a gain of function of the expanded RNAs, which alter developmentally regulated alternative splicing as well as pathways of muscle differentiation. The toxic RNA has been implicated in sequestration of splicing regulators and transcription factors thereby causing specific symptoms of the disease. Here we review the proposed mechanisms for the toxic effects of the expanded repeats and discuss the molecular mechanisms of splicing misregulation and disease pathogenesis.

Myotonic dystrophy expanded CUG repeats disturb the expression and phosphorylation of τ in PC12 cells

Mental retardation is a main feature of the congenital form of myotonic dystrophy (DM1), however, the molecular mechanisms underlying the central nervous system symptoms of DM1 are poorly understood. We have established a PC12 cell line-based model expressing the DM1 expanded CUG repeats (CTG90 cells) to analyze the effects of this mutation on neuronal functions. Previously, we have reported that CTG90 cells displayed impaired NGF-induced neuronal differentiation. Because disruption of normal expression of the microtubule associated protein tau and neuronal aggregates of hyperphosphorylated tau have been associated with DM1, this study analyzes the behavior of tau in the CTG90 cells. Several alterations of tau were observed in the PC12 cells that express expanded CUG repeats, including a subtle but reproducible reduction in the expression of the tau mRNA splicing isoform containing exon 10, decreased expression of tau and hyperphosphorylation of both tau and high molecular weight tau as well as abnormal nuclear localization of tau phosphorylated at Ser396/404. Interestingly, phosphorylation regulates negatively the activity of tau as microtubule-associated protein. In addition, impaired activity of the Akt/GSK3beta pathway, which phosphorylates tau, was also identified in the CTG90 cells. Besides tau phosphorylation, the Akt/GSK3beta signaling pathway regulates other key processes of PC12 cells, such as apoptosis and neuronal differentiation. Our results indicate that defective neuronal differentiation exhibited by the PC12 cells expressing expanded CUG repeats could be the result of combinatory effects derived from the altered behavior of tau and the impaired activation of the Akt/GSK3beta signaling pathway.

Brain-specific change in alternative splicing of Tau exon 6 in myotonic dystrophy type 1

Alternative splicing is altered in myotonic dystrophy of type 1 (DM1), a syndrome caused by an increase of CTG triplet repeats in the 3' untranslated region of the myotonic dystrophy protein kinase gene. Previously, we reported the preferential skipping of Tau exon 2 in DM1 brains. In this study, we analyze the alternative splicing of Tau exon 6 which can be inserted in three different forms (c, p and d) depending on the 3' splice site used. In fact, inclusion of exon 6c decreases in DM1 brains compared to control brains whereas inclusion of 6d increases. Alteration of exon 6 splicing was not observed in DM1 muscle although this exon was inserted in RNAs from normal muscle and DM1 splicing alterations were first described in this organ. In contrast, alteration of exon 2 of Tau mRNA was observed in both muscle and brain. However, co-transfections of a minigene containing exon 6 with CELF or MBNL1 cDNAs, two splicing factor families suspected to be involved in DM1, showed that they influence exon 6 splicing. Altogether, these results show the importance of determining all the exons and organs targeted by mis-splicing to determine the dysregulation mechanisms of mis-splicing in DM1.

Perinucleolar compartment and transformation

The perinucleolar compartment (PNC) is a sub-nuclear structure that preferentially localizes to the nucleolar periphery. The PNC is found predominantly in transformed cells both in vitro and in vivo. PNC prevalence (the percentage of cells containing at least one PNC) positively correlates with the progression of breast cancer and patient survival. PNCs are highly enriched with newly synthesized RNA polymerase III transcripts and RNA-binding proteins. The structural integrity of the PNC is dependent upon the transcription of these RNAs and a critical level of the polypyrimidine tract binding (PTB) protein, as assayed by the localization of other PNC-associated proteins. These observations suggest a model in which the PNC is a dynamic, functional organelle that forms under specific physiological conditions favoring cellular transformation and might be involved in the metabolism of RNA polymerase III transcripts.

Dynamic balance between activation and repression regulates pre-mRNA alternative splicing during heart development

Cardiac troponin T (cTNT) exon 5 splicing is developmentally regulated such that it is included in embryonic but not adult heart. CUG-BP and ETR-3-like factor (CELF) proteins promote exon inclusion, whereas polypyrimidine tract binding protein (PTB) and muscleblind-like (MBNL) proteins repress inclusion. In this study, we addressed what happens to these regulatory proteins during heart development to shift the regulatory balance of cTNT alternative splicing. Using dominant-negative proteins, we found that both CELF and PTB activities are required for appropriate splicing in cardiomyocytes. Two CELF proteins, CUG-BP and ETR-3, are nuclear and cytoplasmic in embryonic heart but are down-regulated in adult heart concomitant with loss of exon inclusion. In contrast, PTB and MBNL1 are expressed throughout heart development. The patterns of cTNT splicing and expression of its regulatory factors are conserved between mouse and chicken. Thus, alternative splicing is determined by a balance between positive and negative regulation, and modulation of expression levels of auxiliary splicing regulators may drive developmental splicing changes. ETR-3 and CUG-BP proteins are also down-regulated in other tissues during development, suggesting that CELF proteins play a broad role in developmental splicing regulation.

Diseases of unstable repeat expansion: mechanisms and common principles

The list of developmental and degenerative diseases that are caused by expansion of unstable repeats continues to grow, and is now approaching 20 disorders. The pathogenic mechanisms that underlie these disorders involve either loss of protein function or gain of function at the protein or RNA level. Common themes have emerged within and between these different classes of disease; for example, among disorders that are caused by gain-of-function mechanisms, altered protein conformations are central to pathogenesis, leading to changes in protein activity or abundance. In all these diseases, the context of the expanded repeat and the abundance, subcellular localization and interactions of the proteins and RNAs that are affected have key roles in disease-specific phenotypes.

Cardiac tissue-specific repression of CELF activity disrupts alternative splicing and causes cardiomyopathy

Members of the CELF family of RNA binding proteins have been implicated in alternative splicing regulation in developing heart. Transgenic mice that express a nuclear dominant-negative CELF protein specifically in the heart (MHC-CELFDelta) develop cardiac hypertrophy and dilated cardiomyopathy with defects in alternative splicing beginning as early as 3 weeks after birth. MHC-CELFDelta mice exhibit extensive cardiac fibrosis, severe cardiac dysfunction, and premature death. Interestingly, the penetrance of the phenotype is greater in females than in males despite similar levels of dominant-negative expression, suggesting that there is sex-specific modulation of splicing activity. The cardiac defects in MHC-CELFdelta mice are directly attributable to reduced levels of CELF activity, as crossing these mice with mice overexpressing CUG-BP1, a wild-type CELF protein, rescues defects in alternative splicing, the severity and incidence of cardiac hypertrophy, and survival. We conclude that CELF protein activity is required for normal alternative splicing in the heart in vivo and that normal CELF-mediated alternative splicing regulation is in turn required for normal cardiac function.

Clinical and molecular aspects of the myotonic dystrophies: a review

Type 1 myotonic dystrophy or DM1 (Steinert's disease), which is the commonest muscular dystrophy in adults, has intrigued physicians for over a century. Unusual features, compared with other dystrophies, include myotonia, anticipation, and involvement of other organs, notably the brain, eyes, smooth muscle, cardiac conduction apparatus, and endocrine system. Morbidity is high, with a substantial mortality relating to cardiorespiratory dysfunction. More recently a second form of multisystem myotonic disorder has been recognized and variously designated as proximal myotonic myopathy (PROMM), proximal myotonic dystrophy (PDM), or DM2. For both DM1 and DM2 the molecular basis is expansion of an unstable repeat sequence in a noncoding part of a gene (DMPK in DM1 and ZNF9 in DM2). There is accumulating evidence that the basic molecular mechanism is disruption of mRNA metabolism, which has far-reaching effects on many other genes, in part through the induction of aberrant splicing, explaining the multisystemic nature of the disease. The unstable nature of the expansion provides a molecular explanation for anticipation. This review emphasizes the clinical similarities and differences between DM1 and DM2. It examines current views about the molecular basis of these disorders, and contrasts them with other repeat expansion disorders that have increasingly been recognized as a cause of neurological disease.

Analysis of intronic conserved elements indicates that functional complexity might represent a major source of negative selection on non-coding sequences

The non-coding portion of human genome is punctuated by a large number of multispecies conserved sequence (MCS) elements with largely unknown function. We demonstrate that MCSs are unevenly distributed in human introns with the majority of relatively short introns (< 9 kb long) displaying no or a few MCSs and that MCS density reaching up to 10% of total size in longer introns. After correction for intron length, MCSs were found to be enriched within genes involved in development and transcription, whereas depleted in immune response loci. Moreover, many central nervous system tissues show a preferential expression of MCS-rich genes and MCS enrichment significantly correlates with gene functional complexity in terms of distinct protein domains. Analysis of human-mouse orthologous pairs indicated a significant association between intronic MCS density and conservation of protein sequence, promoter regions and untranslated sequences. Moreover, MCS density correlates with the predicted occurrence of human-mouse conserved alternative splicing events. These observations suggest that evolution acts on human genes as integrated units of coding and regulatory capacity and that functional complexity might represent a major source of negative selection on non-coding sequences. To substantiate our result, we also searched previously experimentally identified intronic regulatory elements and indicate that about half of these sequences map to an MCS; in particular, support to the notion whereby mutations in MCSs can result in human genetic diseases is provided, because three previously identified intronic pathological variations were found to occur within MCSs, and human disease and cancer genes were found significantly enriched in MCSs.

HnRNP H inhibits nuclear export of mRNA containing expanded CUG repeats and a distal branch point sequence

Myotonic dystrophy type 1 (DM1) is an autosomal dominant neuromuscular disorder associated with a (CUG)n expansion in the 3′-untranslated region of the DMPK (DM1 protein kinase) gene. Mutant DMPK mRNAs containing the trinucleotide expansion are retained in the nucleus of DM1 cells and form discrete foci. The nuclear sequestration of RNA binding proteins and associated factors binding to the CUG expansions is believed to be responsible for several of the splicing defects observed in DM1 patients and could ultimately be linked to DM1 muscular pathogenesis. Several RNA binding proteins capable of co-localizing with the nuclear-retained mutant DMPK mRNAs have already been identified but none can account for the nuclear retention of the mutant transcripts. Here, we have employed a modified UV crosslinking assay to isolate proteins bound to mutant DMPK-derived RNA and have identified hnRNP H as an abundant candidate. The specific binding of hnRNP H requires not only a CUG repeat expansion but also a splicing branch point distal to the repeats. Suppression of hnRNP H expression by RNAi rescued nuclear retention of RNA with CUG repeat expansions. The identification of hnRNP H as a factor capable of binding and possibly modulating nuclear retention of mutant DMPK mRNA may prove to be an important link in our understanding of the molecular mechanisms that lead to DM1 pathogenesis.

Recent progress in spinocerebellar ataxia type-10 (SCA10)

Spinocerebellar ataxia type 10 (SCA10) is a dominantly inherited ataxia caused by expansion of ATTCT pentanucleotide repeat in intron 9 of a novel gene, E46L, on chromosome 22q13.3. SCA10 is a complex neurodegenerative condition. Initial studies characterized SCA10 as pure cerebellar ataxia associated with seizures. Recent identification of new SCA10 families revealed more diverse phenotypes, including polyneuropathy, pyramidal signs, cognitive and neuropsychiatric impairment. Moreover, several families manifest with ataxia without seizures. Thus a complete clinical spectrum is emerging. Progress has also been made in understanding the molecular and genetic mechanisms of pathogenesis. The length of expanded ATTCT repeats is variable in different tissues and highly unstable during paternal transmission, revealing complex genetic and pathogenetic processes. Under torsional stress, ATTCT repeats form unpaired DNA structure and may serve as an erroneous DNA replication origin, potentially contributing to repeat instability and aberrant cell cycle entry. E46L is a cytoplasmic protein with unknown function. Reduced expression of E46L in primary neuronal cultures from cerebellum and cortex by small interfering RNAs (siRNAs) caused increased apoptosis, raising the possibility that reduced expression of E46L might also play an important role in SCA10 pathogenesis.

Altered mRNA splicing of the skeletal muscle ryanodine receptor and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder caused by a CTG repeat expansion in the DMPK gene. Aberrant splicing of several genes has been reported to contribute to some symptoms of DM1, but the cause of muscle weakness in DM1 and elevated Ca2+ concentrations in cultured DM muscle cells is unknown. Here, we investigated the alternative splicing of mRNAs of two major proteins of the sarcoplasmic reticulum, the ryanodine receptor 1 (RyR1) and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) 1 or 2. The fetal variants, ASI(-) of RyR1 which lacks residue 3481-3485, and SERCA1b which differs at the C-terminal were significantly increased in skeletal muscles from DM1 patients and the transgenic mouse model of DM1 (HSA(LR)). In addition, a novel variant of SERCA2 was significantly decreased in DM1 patients. The total amount of mRNA for RyR1, SERCA1 and SERCA2 in DM1 and the expression levels of their proteins in HSA(LR) mice were not significantly different. However, heterologous expression of ASI(-) in cultured cells showed decreased affinity for [3H]ryanodine but similar Ca2+ dependency, and decreased channel activity in single-channel recording when compared with wild-type (WT) RyR1. In support of this, RyR1-knockout myotubes expressing ASI(-) exhibited a decreased incidence of Ca2+ oscillations during caffeine exposure compared with that observed for myotubes expressing WT-RyR1. We suggest that aberrant splicing of RyR1 and SERCA1 mRNAs might contribute to impaired Ca2+ homeostasis in DM1 muscle.

Colocalization of muscleblind with RNA foci is separable from mis-regulation of alternative splicing in myotonic dystrophy

Myotonic dystrophy type I (DM1), which is caused by a non-coding CTG-repeat expansion in the dystrophia myotonica-protein kinase (DMPK) gene, is an RNA-mediated disease. Expanded CUG repeats in transcripts of mutant DMPK form nuclear foci that recruit muscleblind-like (MBNL) proteins, a family of alternative splicing factors. Although transcripts of mutant DMPK and MBNL proteins accumulate in nuclear RNA foci, it is not clear whether foci formation is required for splicing mis-regulation. Here, we use a co-transfection strategy to show that both CUG and CAG repeats form RNA foci that colocalize with green fluorescent protein (GFP)-MBNL1 and endogenous MBNL1. However, only CUG repeats alter splicing of the two tested pre-mRNAs, cardiac troponin T (cTNT) and insulin receptor (IR). Using FRAP, we demonstrate that GFP-MBNL1 in CUG and CAG foci have similar half-times of recovery and fractions of immobile molecules, suggesting that GFP-MBNL1 is bound by both CUG and CAG repeats. We also find an immobile fraction of GFP-MBNL1 in DM1 fibroblasts and a similar rapid exchange in endogenous CUG RNA foci. Therefore, formation of RNA foci and disruption of MBNL1-regulated splicing are separable events.

Identification of CELF splicing activation and repression domains in vivo

CUG-BP and ETR-3 like factor (CELF) proteins are regulators of pre-mRNA alternative splicing. We created a series of truncation mutants to identify the regions of CELF proteins that are required to activate and to repress alternative splicing of different exons. This analysis was performed in parallel on two CELF proteins, ETR-3 (CUG-BP2, NAPOR, BRUNOL3) and CELF4 (BRUNOL4). We identified a 20-residue region of CELF4 required for repression or activation, in contrast to ETR-3, for which the required residues are more disperse. For both ETR-3 and CELF4, distinct regions were required to activate splicing of two different alternative exons, while regions required for repression of an additional third exon overlapped with regions required for activation. Our results suggest that activation of different splicing events by individual CELF proteins requires separable regions, implying the nature of the protein–protein interactions required for activation are target-dependent. The finding that residues required for activation and repression overlap suggests either that the same region interacts with different proteins to mediate different effects or that interactions with the same proteins can have different effects on splicing due to yet-to-be defined downstream events. These results provide a foundation for identifying CELF-interacting proteins involved in activated and/or repressed splicing.

Transgenic mice expressing CUG-BP1 reproduce splicing mis-regulation observed in myotonic dystrophy

Myotonic dystrophy type I (DM1) is an RNA-mediated disease caused by a non-coding CTG repeat expansion. A key feature of the RNA-mediated pathogenesis model for DM is the disrupted splicing of specific pre-mRNA targets. A link has been established between splicing regulation by CUG-BP1, a member of the CELF family of proteins, and DM1 pathogenesis. To determine whether increased CUG-BP1 function was sufficient to model DM, transgenic mice overexpressing CUG-BP1 (MCKCUG-BP1) in heart and skeletal muscle, two tissues affected in DM1, were generated. Histological and electron microscopic analyses of skeletal muscle reveal common pathological features with DM tissues: chains of central nuclei, degenerating fibers and centralized NADH reactivity. MCKCUG-BP1 mice have disrupted splicing of three CELF target pre-mRNAs, cardiac troponin T (Tnnt2), myotubularin-related 1 gene (Mtmr1) and the muscle-specific chloride channel (Clcn1), consistent with that observed in DM heart and skeletal muscle. The results are consistent with a mechanism for DM pathogenesis in which expanded repeats result in increased CUG-BP1 activity and/or other CELF family members and have trans-dominant effects on specific pre-mRNA targets.

Low-dose metformin improves hyperglycaemia related to myotonic dystrophy

BACKGROUND

One of the clinical features of myotonic dystrophy is insulin resistance with non-obese diabetes mellitus (DM). Recently, the mechanism of insulin resistance in patients with myotonic dystrophy was revealed. The optimal treatment of DM with myotonic dystrophy has not been established. We report the effect of metformin in a patient with myotonic dystrophy without obesity.

CASE REPORT

A 58-year-old woman (BMI = 22.1 kg/m2) with myotonic dystrophy and DM was followed at our clinic. She had been treated with glimepiride for DM for the last 6 months, without achieving good control (HbA(1c) 9.3%). She was admitted with congestive heart failure and cholecystitis. She was treated with diuretics, antibiotics and insulin. As her blood glucose fell, we discontinued insulin and started glimepiride, but her glycaemic control had worsened. We started metformin instead of glimepiride. After 4 weeks of metformin, HbA(1c) was decreased to 7.4%, while HOMA-IR during glimepiride treatment was 4.9, and 3.7 with metformin. Three months later, HbA(1c) was maintained (7.5%).

CONCLUSION

It is important to choose the optimal treatment for DM in myotonic dystrophy, because the patients have hyperinsulinemia caused by specific mechanism and could not reduce the insulin resistance. Metformin improved hyperglycemia through increased insulin-independent glucose uptake in peripheral muscle. We believe metformin is the optimal agent for these patients.