RNA pathogenesis of the myotonic dystrophies

Muscleblind-like proteins: similarities and differences in normal and myotonic dystrophy muscle

In myotonic dystrophy, muscleblind-like protein 1 (MBNL1) protein binds specifically to expanded CUG or CCUG repeats, which accumulate as discrete nuclear foci, and this is thought to prevent its function in the regulation of alternative splicing of pre-mRNAs. There is strong evidence for the role of the MBNL1 gene in disease pathology, but the roles of two related genes, MBNL2 and MBNL3, are less clear. Using new monoclonal antibodies specific for each of the three gene products, we found that MBNL2 decreased during human fetal development and myoblast culture, while MBNL1 was unchanged. In Duchenne muscular dystrophy muscle, MBNL2 was elevated in immature, regenerating fibres compared with mature fibres, supporting some developmental role for MBNL2. MBNL3 was found only in C2C12 mouse myoblasts. Both MBNL1 and MBNL2 were partially sequestered by nuclear foci of expanded repeats in adult muscle and cultured cells from myotonic dystrophy patients. In adult muscle nucleoplasm, both proteins were reduced in myotonic dystrophy type 1 compared with an age-matched control. In normal human myoblast cultures, MBNL1 and MBNL2 always co-distributed but their distribution could change rapidly from nucleoplasmic to cytoplasmic. Functional differences between MBNL1 and MBNL2 have not yet been found and may prove quite subtle. The dominance of MBNL1 in mature, striated muscle would explain why ablation of the mouse mbnl1 gene alone is sufficient to cause a myotonic dystrophy.

The effect of myotonic dystrophy transcript levels and location on muscle differentiation

In myotonic dystrophy type I (DM1), nuclear retention of mutant DMPK transcripts compromises muscle cell differentiation. Although several reports have identified molecular defects in myogenesis, it remains still unclear how exactly the retention of the mutant transcripts induces this defect. We have recently created a novel cellular model in which the mutant DMPK 3' UTR transcripts were released to the cytoplasm of myoblasts by using the WPRE genetic element. As a result, muscle cell differentiation was repaired. In this paper, this cellular model was further exploited to investigate the effect of the levels and location of the mutant transcripts on muscle differentiation. Results show that the levels of these transcripts were proportional to the inhibition of both the initial fusion of myoblasts and the maturity of myotubes. Moreover, the cytoplasmic export of the mutant RNAs to the cytoplasm caused less inhibition only in the initial fusion of myoblasts.

Zebrafish muscleblind-like genes: identification, structural features and expression

Muscleblind-like (MBNL) proteins are a family of RNA-binding proteins that participate in the regulation of tissue-specific alternative splicing. Misregulation of MBNL activity in humans leads to pathogenesis. Here, we report upon the identification and characterization of three muscleblind-like genes in zebrafish (zmbnl1, zmbnl2 and zmbnl3). Alternative splicing of the three zmbnl primary transcripts gives rise to at least four protein isoforms for zmbnl1, four for zmbnl2 and five for zmbnl3, respectively. All of the zmbnl proteins contain the characteristic CCCH zinc fingers required for RNA binding. In addition, several structural motifs, including a C-terminal Ser/Thr-rich region, are conserved among Mbnl orthologs in vertebrates, but not invertebrates. These genes are broadly expressed in most adult tissues. However, the relative expression levels of specific spliceforms vary across different tissues. During embryogenesis, zmbnl1 and zmbnl2 are both maternally and zygotically expressed. In contrast, zmbnl3 transcripts are not detected until the late pharyngula stage. Our results reveal the expression pattern of various mbnl spliceforms for the first time and suggest that they may play specific roles during fish development.

Defining early steps in mRNA transport: mutant mRNA in myotonic dystrophy type I is blocked at entry into SC-35 domains

In myotonic dystrophy type 1 (DM1), triplet repeat expansion in the 3' untranslated region of dystrophia myotonica protein kinase (DMPK) causes the nuclear retention of mutant messenger RNA (mRNA). Although the DMPK gene locus positions precisely at the outer edge of a factor-rich SC-35 domain, the normal mRNA consistently accumulates within the domain, and this RNA is depleted upon transcriptional inhibition. In DM1, mutant transcripts detach from the gene but accumulate in granules that abut but do not enter SC-35 domains, suggesting that RNA entry into the domain is blocked. Despite their exclusion from these compartments, mutant transcripts are spliced. MBNL1 (muscleblind-like protein 1) is an alternative splicing factor that becomes highly concentrated with mutant RNA foci. Small interfering RNA-mediated knockdown of MBNL1 promotes the accumulation or entry of newly synthesized mutant transcripts in the SC-35 domain. Collectively, these data suggest that an initial step in the intranuclear path of some mRNAs is passage from the gene into an SC-35 domain and implicate these structures in postsplicing steps before export.

Neuromuscular excitability properties in myotonic dystrophy type 1

OBJECTIVE

To study neuromuscular excitability in patients with dystrophia myotonica type 1 (DM1).

METHODS

The neuromuscular recovery cycle following motor nerve stimulation was assessed in 16 DM1 patients who had no sign of peripheral neuropathy or diabetes. Compound muscle action potentials were recorded from the adductor digiti minimi muscle to ulnar nerve stimulation at the wrist. Paired pulses were delivered, consisting of a conditioning stimulus of supramaximal intensity, followed by a submaximal test stimulus. Interstimuli intervals (ISIs) ranged between 1 and 8ms. Durations of the absolute and relative refractory periods (ARP, RRP) and percentages of refractoriness and supernormality at ISIs of 2.6 and 7ms, respectively, were computed using a subtraction method. The results obtained in the series of DM1 patients were compared to those obtained in six patients with other forms of myotonia and to normative values established in a series of age-matched healthy subjects. Correlations were made between excitability parameters, the number of cytosine-thymine-guanine (CTG) repeats, and the severity of myotonia, scored clinically.

RESULTS

Compared to controls, DM1 patients presented prolonged durations of ARP and RRP, increased refractoriness and reduced supernormality. The decrease in refractoriness correlated with both the number of CTG repeats and the severity of myotonia.

CONCLUSIONS

Changes in the recovery cycle following supramaximal motor nerve stimulation revealed the existence of subtle alterations of neuromuscular excitability in DM1 patients.

SIGNIFICANCE

Increase in refractoriness together with a reduced supernormality was consistent with a process of membrane depolarization. Such a depolarization may be related to the loss of chloride channels or to alterations in sodium conductance in the motor axon or the muscle fiber.

Myotonic dystrophy CTG repeat expansion alters Ca2+channel functional expression in PC12 cells

We previously reported that expression of myotonic dystrophy (DM1) expanded CUG repeats impedes NGF-induced differentiation in a PC12 clone (CTG90 cells). Here, we present evidence for changes in the fractional contribution of distinct voltage-gated Ca(2+) channels, key elements in neurotrophin-promoted differentiation, to the total Ca(2+) current in the CTG90 cells. Patch-clamp recordings showed that the relative proportion of pharmacologically isolated Ca(2+) channel types differed between control and CTG90 cells. Particularly, the functional expression of N-type channels was significantly reduced. Though quantitative real-time RT-PCR revealed that transcripts for the pore-forming subunit encoding the N-type channels remained unchanged, the protein level analyzed by semi-quantitative Western blotting was down-regulated in the CTG90 cells. These data suggest modifications in the processing of N-type Ca(2+) channels in PC12 cells expressing the DM1 mutation.

MBNL3/CHCR prevents myogenic differentiation by inhibiting MyoD-dependent gene transcription

Muscle differentiation is controlled by positive and negative signals. While much attention has been placed on proteins that promote muscle formation, the importance of negative regulators has been underemphasized. MBNL3/CHCR belongs to the muscleblind family of Cys3His zinc finger proteins implicated in myotonic dystrophy. MBNL3 is expressed in myoblasts, muscle precursor cells, and during the early stages of myogenesis, but is detected at very low levels in terminally differentiated myotubes. Constitutive expression of MBNL3 inhibits myotube formation and antagonizes myogenin and myosin heavy chain expression. To identify MBNL3 target genes, we compared the expression profile of C2C12 mouse myoblasts that constitutively express MBNL3 with control cells. From the 15,247 genes represented on the DNA microarray, classification by biological function indicated that genes involved in muscle development/contraction and cell adhesion were down-regulated by MBNL3 expression. mRNA and protein levels for the muscle transcription factor MyoD and E-box regulated transcription were reduced in C2C12-MBNL3 expressing cells. We hypothesize that MBNL3 serves to antagonize muscle differentiation by suppressing MyoD expression levels to prevent unwanted myogenic gene transcription. These findings are the first indication that a mammalian muscleblind-like (MBNL) protein plays a regulatory role in muscle differentiation under nonpathogenic conditions.

Expression pattern of muscleblind-like proteins differs in differentiating myoblasts

Muscleblind-like (MBNL) proteins are believed to be regulators of myogenesis and are implicated in myotonic dystrophy. While Drosophila melanogaster muscleblind is required for terminal muscle differentiation, mammalian MBNL3 functions as an inhibitor of myogenesis. In this study, we analyzed the expression pattern of MBNL3 in different adult mouse tissues and tissue culture cells. MBNL3 transcript is enriched in the lung, spleen, and testis and not in heart and skeletal muscle. By Western blotting, we found that MBNL3 was expressed in C2C12 myoblasts and ts13 myofibroblasts, but was detected at significantly lower levels in fibroblasts. MBNL3 protein levels decreased when cells were shifted to muscle differentiation conditions, but the closely related MBNL1 protein was unaffected. These results suggest that myoblasts and fibroblasts respond to differentiation conditions by activating signaling pathways that repress MBNL3 but not MBNL1 expression.

Ribonuclease dicer cleaves triplet repeat hairpins into shorter repeats that silence specific targets

Ribonuclease Dicer functions in cells to excise microRNAs from their precursors and process long double-stranded RNAs into short interfering RNAs. We show that transcripts containing long hairpin structures composed of CNG repeats are another class of Dicer targets. The cellular levels of transcripts from mutant genes involved in triplet repeat expansion diseases such as myotonic dystrophy type 1, Huntington's disease, and spinocerebellar ataxia type 1 are under Dicer control. The Dicer-induced downregulation of the mutant transcript in myotonic dystrophy cells is accompanied by the downregulation of transcripts containing long complementary repeats. Short CUG repeats generated from long repeat hairpins act as siRNAs and use the RNA interference pathway to trigger the downstream silencing effects. We demonstrate that synthetic oligonucleotides composed of repeats are highly specific in the silencing of mutant transcripts containing complementary repeats and may be considered as potential therapeutic agents.

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.

MicroRNAs regulate the expression of the alternative splicing factor nPTB during muscle development

Alternative pre-mRNA splicing determines many changes in gene expression during development. Two regulators known to control splicing patterns during neuron and muscle differentiation are the polypyrimidine tract-binding protein (PTB) and its neuronal homolog nPTB. These proteins repress certain exons in early myoblasts, but upon differentiation of mature myotubes PTB/nPTB expression is reduced, leading to increased inclusion of their target exons. We show here that the repression of nPTB expression during myoblast differentiation results from its targeting by the muscle-restricted microRNA miR-133. During differentiation of C2C12 myoblasts, nPTB protein but not mRNA expression is strongly reduced, concurrent with the up-regulation of miR-133 and the induction of splicing for several PTB-repressed exons. Introduction of synthetic miR-133 into undifferentiated C2C12 cells leads to a decrease in endogenous nPTB expression. Both the miR-133 and the coexpressed miR-1/206 microRNAs are extremely conserved across animal species, and PTB proteins are predicted targets for these miRNAs in Drosophila, mice, and humans. There are two potential miR-133-responsive elements (MRE) within the nPTB 3' untranslated region (UTR), and a luciferase reporter carrying this 3' UTR is repressed by miR-133 in an MRE-dependent manner. Transfection of locked nucleic acid (LNA) oligonucleotides designed to block the function of miR-133 and miR-1/206 increases expression of nPTB and decreases the inclusion of PTB dependent exons. These results indicate that miR-133 directly down-regulates a key splicing factor during muscle development and establishes a role for microRNAs in the control of a developmentally dynamic splicing program.

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.

Molecular mechanisms of muscular dystrophies: old and new players

The study of the muscle cell in the muscular dystrophies (MDs) has shown that mutant proteins result in perturbations of many cellular components. MDs have been associated with mutations in structural proteins, signalling molecules and enzymes as well as mutations that result in aberrant processing of mRNA or alterations in post-translational modifications of proteins. These findings have not only revealed important insights for cell biologists, but have also provided unexpected and exciting new approaches for therapy.

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.

Myotonic dystrophies type 1 and 2: a summary on current aspects

Myotonic dystrophies (DMs) encompass at least 2 forms: myotonic dystrophy type 1 and 2. In general, DMs are late-onset autosomal dominant disorders characterized by a variety of multisystemic features including myotonia, muscular dystrophy, cardiac conduction defects, dilated cardiomyopathy, posterior iridescent cataracts, frontal balding, insulin-resistance and disease-specific serological abnormalities such as gamma-glutamyltransferase and creatine kinase elevations, hyperglycemia, hypotestosteronism, and reduced immunoglobulin (Ig) G and IgM levels. Beyond the adult forms, in the classic DM1, a congenital form and an early-onset form is recognized. Here we summarize current aspects of the myotonic dystrophy pathogenesis and review the core features of both types of myotonic dystrophies, including the congenital DM1.

Cognitive deficits and CTG repeat expansion size in classical myotonic dystrophy type 1 (DM1)

Background

This study was designed to investigate cognitive abilities and their correlations with CTG repeat expansion size in classical Myotonic dystrophy type 1 (DM1), given that earlier studies have indicated cognitive deficits, possibly correlating with blood CTG repeats expansion size.

Methods

A measurement of CTG repeat expansion in lymphocytes and an extensive neuropsychological examination was made in 47 patients (25 women and 22 men). Individual results in the examination were compared with normative data.

Results

A substantial proportion of patients with DM1 (> 40%) scored worse in comparison to normative collectives on tests measuring executive, arithmetic, attention, speed and visuospatial abilities. We found significant correlations between longer CTG repeat expansion size and lower results on most tests associated with these abilities. Furthermore, the association between executive (frontal) deficits and DM1 were strengthened when considering both test results and correlations with CTG repeat expansion size in lymphocytes.

Conclusion

This study showed deficits in several cognitive abilities when patients with DM1 are compared to normative collectives. Some of the neuropsychological tests associated with these abilities are correlated to CTG repeat expansion size in blood. These data highlight the importance of considering cognitive deficits when seeing patients with classical DM1 in clinical practice, but also the utility of using blood CTG repeat expansion size as a broad predictor of finding cognitive deficit in DM1.

A systematic analysis of disease-associated variants in the 3' regulatory regions of human protein-coding genes I: general principles and overview

The 3' regulatory regions (3' RRs) of human genes play an important role in regulating mRNA 3' end formation, stability/degradation, nuclear export, subcellular localization and translation and are consequently rich in regulatory elements. Although 3' RRs contain only approximately 0.2% of known disease-associated mutations, this is likely to represent a rather conservative estimate of their actual prevalence. In an attempt to catalogue 3' RR-mediated disease and also to gain a greater understanding of the functional role of regulatory elements within 3' RRs, we have performed a systematic analysis of disease-associated 3' RR variants; 121 3' RR variants in 94 human genes were collated. These included 17 mutations in the upstream core polyadenylation signal sequence (UCPAS), 81 in the upstream sequence (USS) between the translational termination codon and the UCPAS, 6 in the left arm of the 'spacer' sequence (LAS) between the UCPAS and the pre-mRNA cleavage site (CS), 3 in the right arm of the 'spacer' sequence (RAS) or downstream core polyadenylation signal sequence (DCPAS) and 7 in the downstream sequence (DSS) of the 3'-flanking region, with 7 further mutations being treated as isolated examples. All the UCPAS mutations and the rather unusual cases of the DMPK, SCA8, FCMD and GLA mutations exert a significant effect on the mRNA phenotype and are usually associated with monogenic disease. By contrast, most of the remaining variants are polymorphisms that exert a comparatively minor influence on mRNA expression, but which may nevertheless predispose to or otherwise modify complex clinical phenotypes. Considerable efforts have been made to validate/elucidate the mechanisms through which the 3' untranslated region (3' UTR) variants affect gene expression. It is hoped that the integrative approach employed here in the study of naturally occurring variants of actual or potential pathological significance will serve to complement ongoing efforts to identify all functional regulatory elements in the human genome.

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.

Emerging roles of chloride channels in human diseases

In the past decade, there has been remarkable progress in understanding of the roles of Cl(-) channels in the development of human diseases. Genetic studies in humans have identified mutations in the genes encoding Cl(-) channels which lead to a loss of Cl(-) channel activity. These mutations are responsible for the development of a variety of deleterious diseases in muscle, kidney, bone and brain including myotonia congenita, dystrophia myotonica, cystic fibrosis, osteopetrosis and epilepsy. Recent studies indicate that some diseases may develop as a result of Cl(-) channel activation. There is growing evidence that the progression of glioma in the brain and the growth of the malaria parasite in red blood cells may be mediated through Cl(-) channel activation. These findings suggest that Cl(-) channels may be novel targets for the pharmacological treatment of a broad spectrum of diseases. This review discusses the proposed roles of abnormal Cl(-) channel activity in the pathogenesis of human diseases.

Distinct neuromuscular phenotypes in myotonic dystrophy types 1 and 2 : a whole body highfield MRI study

Myotonic Dystrophy Type 1 (DM1) and 2 (DM2) present with distinct though overlapping clinical phenotypes. Comparative imaging data on skeletal muscle involvement are not at present available. We used the novel technique of whole body 3.0 Tesla (T) Magnetic Resonance Imaging (MRI) to further characterize musculoskeletal features in DM2 and compared the results with DM1.MRI findings of 15 DM1 and 14 DM2 patients were evaluated with respect to patterns of skeletal muscle affection and clinical data using the Muscular Impairment Rating Scale (MIRS) and Medical Research Council scale (MRC). All DM1 patients had pathological MRI compared with only 5 DM2 patients. In contrast to DM2, DM1 patients showed a characteristic distribution of muscle involvement with frequent and early degeneration of the medial heads of gastrocnemius muscles, and a perifemoral semilunar pattern of quadriceps muscle affection sparing the rectus femoris. The most frequently affected muscles in DM1 were the medial heads of gastrocnemius, soleus, and vastus medialis muscles. In DM2, however, the erector spinae and gluteus maximus muscles were most vulnerable to degeneration. MRI data were in line with the clinical grading in 12 DM1 and 3 DM2 patients. In 3 DM1 and 5 DM2 patients, MRI detected subclinical muscle involvement. 9 DM2 patients with mild to moderate proximal muscle weakness and/or myalgias had normal MRI. Pathological MRI changes in DM2 emerged with increasing age and were restricted to women. Whole body 3.0T MRI is a sensitive imaging technique that demonstrated a characteristic skeletal muscle affection in DM1. In contrast, MRI was no reliable indicator for skeletal muscle involvement in mildly affected DM2 patients since myalgia and mild paresis were usually not reflected by MRI signal alterations.

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.

Chemical modifiers of unstable expanded simple sequence repeats: what goes up, could come down

A mounting number of inherited human disorders, including Huntington disease, myotonic dystrophy, fragile X syndrome, Friedreich ataxia and several spinocerebellar ataxias, have been associated with the expansion of unstable simple sequence DNA repeats. Despite a similar genetic basis, pathogenesis in these disorders is mediated by a variety of both loss and gain of function pathways. Thus, therapies targeted at downstream pathology are likely to be disease specific. Characteristically, disease-associated expanded alleles in these disorders are highly unstable in the germline and somatic cells, with a tendency towards further expansion. Whereas germline expansion accounts for the phenomenon of anticipation, tissue-specific, age-dependent somatic expansion may contribute towards the tissue-specificity and progressive nature of the symptoms. Thus, somatic expansion presents as a novel therapeutic target in these disorders. Suppression of somatic expansion should be therapeutically beneficial, whilst reductions in repeat length could be curative. It is well established that both cis- and trans-acting genetic modifiers play key roles in the control of repeat dynamics. Importantly, recent data have revealed that expanded CAG.CTG repeats are also sensitive to a variety of trans-acting chemical modifiers. These data provide an exciting proof of principle that drug induced suppression of somatic expansion might indeed be feasible. Moreover, as our understanding of the mechanism of expansion is refined more rational approaches to chemical intervention in the expansion pathway can be envisioned. For instance, the demonstration that expansion of CAG.CTG repeats is dependent on the Msh2, Msh3 and Pms2 genes, highlights components of the DNA mismatch repair pathway as therapeutic targets. In addition to potential therapeutic applications, the response of expanded simple repeats to genotoxic assault suggests such sequences could also have utility as bio-monitors of environmentally induced genetic damage in the soma.

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.

The structural basis of myotonic dystrophy from the crystal structure of CUG repeats

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

Oxidative stress in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is the most common form of muscular dystrophy affecting adults. The genetic basis of DM1 consists of a mutational expansion of a repetitive trinucleotide sequence (CTG). The number of triplets expansion divides patients in four categories related to the molecular changes (E1, E2, E3, E4). The pathogenic mechanisms of multi-systemic involvement of DM1 are still unclear. DM1 has been suspected to be due to premature aging, that is known to be sustained by increased free radicals levels and/or decreased antioxidants activities in neurodegenerative disorders. Recently, the gain-of-function at RNA level hypothesis has gained great attention, but oxidative stress might act in the disease progression. We have investigated 36 DM1 patients belonging to 22 unrelated families, 10 patients with other myotonic disorders (OMD) and 22 age-matched healthy controls from the clinical, biochemical and molecular point of view. Biochemical analysis detected blood levels of superoxide dismutase (SOD), malonilaldehyde (MDA), vitamin E (Vit E), hydroxyl radicals (OH) and total antioxidant system (TAS). Results revealed that DM1 patients showed significantly higher levels of SOD (+40%; MAL (+57%; RAD 2 (+106%; and TAS (+20%; than normal controls. Our data support the hypothesis of a pathogenic role of oxidative stress in DM1 and therefore confirm the detrimental role played by free radicals in this pathology and suggest the opportunity to undertake clinical trials with antioxidants in this disorder.