Myotonic dystrophy: the role of RNA CUG triplet repeats

Development of Therapeutic Approaches for Myotonic Dystrophies Type 1 and Type 2

Myotonic Dystrophies type 1 (DM1) and type 2 (DM2) are complex multisystem diseases without disease-based therapies. These disorders are caused by the expansions of unstable CTG (DM1) and CCTG (DM2) repeats outside of the coding regions of the disease genes: DMPK in DM1 and CNBP in DM2. Multiple clinical and molecular studies provided a consensus for DM1 pathogenesis, showing that the molecular pathophysiology of DM1 is associated with the toxicity of RNA CUG repeats, which cause multiple disturbances in RNA metabolism in patients' cells. As a result, splicing, translation, RNA stability and transcription of multiple genes are misregulated in DM1 cells. While mutant CCUG repeats are the main cause of DM2, additional factors might play a role in DM2 pathogenesis. This review describes current progress in the translation of mechanistic knowledge in DM1 and DM2 to clinical trials, with a focus on the development of disease-specific therapies for patients with adult forms of DM1 and congenital DM1 (CDM1).

Correction of RNA-Binding Protein CUGBP1 and GSK3β Signaling as Therapeutic Approach for Congenital and Adult Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a complex genetic disease affecting many tissues. DM1 is caused by an expansion of CTG repeats in the 3′-UTR of the DMPK gene. The mechanistic studies of DM1 suggested that DMPK mRNA, containing expanded CUG repeats, is a major therapeutic target in DM1. Therefore, the removal of the toxic RNA became a primary focus of the therapeutic development in DM1 during the last decade. However, a cure for this devastating disease has not been found. Whereas the degradation of toxic RNA remains a preferential approach for the reduction of DM1 pathology, other approaches targeting early toxic events downstream of the mutant RNA could be also considered. In this review, we discuss the beneficial role of the restoring of the RNA-binding protein, CUGBP1/CELF1, in the correction of DM1 pathology. It has been recently found that the normalization of CUGBP1 activity with the inhibitors of GSK3 has a positive effect on the reduction of skeletal muscle and CNS pathologies in DM1 mouse models. Surprisingly, the inhibitor of GSK3, tideglusib also reduced the toxic CUG-containing RNA. Thus, the development of the therapeutics, based on the correction of the GSK3β-CUGBP1 pathway, is a promising option for this complex disease.

Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a progressive neuromuscular disease caused by expanded CUG repeats, which misregulate RNA metabolism through several RNA-binding proteins, including CUG-binding protein/CUGBP1 elav-like factor 1 (CUGBP1/CELF1) and muscleblind 1 protein. Mutant CUG repeats elevate CUGBP1 and alter CUGBP1 activity via a glycogen synthase kinase 3β (GSK3β)-cyclin D3-cyclin D-dependent kinase 4 (CDK4) signaling pathway. Inhibition of GSK3β corrects abnormal activity of CUGBP1 in DM1 mice [human skeletal actin mRNA, containing long repeats ( HSA^LR) model]. Here, we show that the inhibition of GSK3β in young HSA^LR mice prevents development of DM1 muscle pathology. Skeletal muscle in 1-yr-old HSA^LR mice, treated at 1.5 mo for 6 wk with the inhibitors of GSK3, exhibits high fiber density, corrected atrophy, normal fiber size, with reduced central nuclei and normalized grip strength. Because CUG-GSK3β-cyclin D3-CDK4 converts the active form of CUGBP1 into a form of translational repressor, we examined the contribution of CUGBP1 in myogenesis using Celf1 knockout mice. We found that a loss of CUGBP1 disrupts myogenesis, affecting genes that regulate differentiation and the extracellular matrix. Proteins of those pathways are also misregulated in young HSA^LR mice and in muscle biopsies of patients with congenital DM1. These findings suggest that the correction of GSK3β-CUGBP1 pathway in young HSA^LR mice might have a positive effect on the myogenesis over time.-Wei, C., Stock, L., Valanejad, L., Zalewski, Z. A., Karns, R., Puymirat, J., Nelson, D., Witte, D., Woodgett, J., Timchenko, N. A., Timchenko, L. Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1.

Emergence of synthetic mRNA: In vitro synthesis of mRNA and its applications in regenerative medicine

The field of gene therapy has evolved over the past two decades after the first introduction of nucleic acid drugs, such as plasmid DNA (pDNA). With the development of in vitro transcription (IVT) methods, synthetic mRNA has become an emerging class of gene therapy. IVT mRNA has several advantages over conventional pDNA for the expression of target proteins. mRNA does not require nuclear localization to mediate protein translation. The intracellular process for protein expression is much simpler and there is no potential risk of insertion mutagenesis. Having these advantages, the level of protein expression is far enhanced as comparable to that of viral expression systems. This makes IVT mRNA a powerful alternative gene expression system for various applications in regenerative medicine. In this review, we highlight the synthesis and preparation of IVT mRNA and its therapeutic applications. The article includes the design and preparation of IVT mRNA, chemical modification of IVT mRNA, and therapeutic applications of IVT mRNA in cellular reprogramming, stem cell engineering, and protein replacement therapy. Finally, future perspectives and challenges of IVT mRNA are discussed.

Reduction of toxic RNAs in myotonic dystrophies type 1 and type 2 by the RNA helicase p68/DDX5

Myotonic dystrophies type 1 (DM1) and type 2 (DM2) are neuromuscular diseases, caused by accumulation of CUG and CCUG RNAs in toxic aggregates. Here we report that the increased stability of the mutant RNAs in both types of DM is caused by deficiency of RNA helicase p68. We have identified p68 by studying CCUG-binding proteins associated with degradation of the mutant CCUG repeats. Protein levels of p68 are reduced in DM1 and DM2 biopsied skeletal muscle. Delivery of p68 in DM1/2 cells causes degradation of the mutant RNAs, whereas delivery of p68 in skeletal muscle of DM1 mouse model reduces skeletal muscle myopathy and atrophy. Our study shows that correction of p68 may reduce toxicity of the mutant RNAs in DM1 and in DM2.

GSK3β is a new therapeutic target for myotonic dystrophy type 1

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

A defective Krab-domain zinc-finger transcription factor contributes to altered myogenesis in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is an RNA-mediated disorder caused by a non-coding CTG repeat expansion that, in particular, provokes functional alteration of CUG-binding proteins. As a consequence, several genes with misregulated alternative splicing have been linked to clinical symptoms. In our search for additional molecular mechanisms that would trigger functional defects in DM1, we took advantage of mutant gene-carrying human embryonic stem cell lines to identify differentially expressed genes. Among the different genes found to be misregulated by DM1 mutation, one strongly downregulated gene encodes a transcription factor, ZNF37A. In this paper, we show that this defect in expression, which derives from a loss of RNA stability, is controlled by the RNA-binding protein, CUGBP1, and is associated with impaired myogenesis-a functional defect reminiscent of that observed in DM1. Loss of the ZNF37A protein results in changes in the expression of the subunit α1 of the receptor for the interleukin 13. This suggests that the pathological molecular mechanisms linking ZNF37A and myogenesis may involve the signaling pathway that is known to promote myoblast recruitment during development and regeneration.

Daytime sleepiness and myotonic dystrophy

Myotonic dystrophy type 1 (DM1) represents the 1 chronic neuromuscular disease with the most prominent sleep disorders, including excessive daytime sleepiness (EDS), sleep apneas, periodic leg movements during sleep, and rapid eye movement sleep dysregulation. The large majority of DM1 patients complain about EDS, which may have a deleterious impact on work, domestic responsibilities, social life, and quality of life. Here, we review the extant literature and report that studies are largely supportive of the view that DM1-related EDS is primarily caused by a central dysfunction of sleep regulation rather than by sleep-related disordered breathing (SRDB) or sleep fragmentation. The pathogenesis of EDS in DM1 still remains unclear but several arguments favor a model in which brain/brainstem nuclear accumulations of toxic expanded DM protein kinase (DMPK) gene are responsible for aberrant genes expression in modifying alternative splicing. Regarding management, early recognition, and treatment of SRDB with nocturnal noninvasive mechanical ventilation is first mandatory. However, despite its appropriate management, EDS often persists and may require a psychostimulant but no consensus has been yet established. Further studies are needed to clarify the discrepancies between daytime sleepiness/fatigue complaints and subjective/objective measurement of daytime sleepiness, the role of cognitive impairment and apathy in this relationship, and its reversibility with appropriate management.

Molecular mechanisms of muscle atrophy in myotonic dystrophies

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

Molecular, physiological, and motor performance defects in DMSXL mice carrying >1,000 CTG repeats from the human DM1 locus

Myotonic dystrophy type 1 (DM1) is caused by an unstable CTG repeat expansion in the 3′UTR of the DM protein kinase (DMPK) gene. DMPK transcripts carrying CUG expansions form nuclear foci and affect splicing regulation of various RNA transcripts. Furthermore, bidirectional transcription over the DMPK gene and non-conventional RNA translation of repeated transcripts have been described in DM1. It is clear now that this disease may involve multiple pathogenic pathways including changes in gene expression, RNA stability and splicing regulation, protein translation, and micro–RNA metabolism. We previously generated transgenic mice with 45-kb of the DM1 locus and >300 CTG repeats (DM300 mice). After successive breeding and a high level of CTG repeat instability, we obtained transgenic mice carrying >1,000 CTG (DMSXL mice). Here we described for the first time the expression pattern of the DMPK sense transcripts in DMSXL and human tissues. Interestingly, we also demonstrate that DMPK antisense transcripts are expressed in various DMSXL and human tissues, and that both sense and antisense transcripts accumulate in independent nuclear foci that do not co-localize together. Molecular features of DM1-associated RNA toxicity in DMSXL mice (such as foci accumulation and mild missplicing), were associated with high mortality, growth retardation, and muscle defects (abnormal histopathology, reduced muscle strength, and lower motor performances). We have found that lower levels of IGFBP-3 may contribute to DMSXL growth retardation, while increased proteasome activity may affect muscle function. These data demonstrate that the human DM1 locus carrying very large expansions induced a variety of molecular and physiological defects in transgenic mice, reflecting DM1 to a certain extent. As a result, DMSXL mice provide an animal tool to decipher various aspects of the disease mechanisms. In addition, these mice can be used to test the preclinical impact of systemic therapeutic strategies on molecular and physiological phenotypes.

Myotonic dystrophy type 1 (DM1) is caused by the abnormal expansion of a CTG repeat located in the DM protein kinase (DMPK) gene. DMPK transcripts carrying CUG expansions form toxic nuclear foci that affect other RNAs. DM1 involve multiple pathogenic pathways including changes in gene expression, RNA stability and splicing regulation, protein translation, and micro–RNA metabolism. We previously generated transgenic mice carrying the human DM1 locus and very large expansions >1,000 CTG (DMSXL mice). Here we described for the first time, the expression pattern of the DMPK sense transcripts in DMSXL and human tissues. We also demonstrate that DMPK antisense transcripts are expressed in various tissues from DMSXL mice and human. Both sense and antisense transcripts form nuclear foci. DMSXL mice showed molecular DM1 features such as foci and mild splicing defects as well as muscles defects, reduced muscle strength, and lower motor performances. These mice recapitulate some molecular features of DM1 leading to physiological abnormalities. DMSXL are not only a tool to decipher various mechanisms involved in DM1 but also to test the preclinical impact of systemic therapeutic strategies.

GSK3β mediates muscle pathology in myotonic dystrophy

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

Myotonic dystrophy type 1, daytime sleepiness and REM sleep dysregulation

Myotonic dystrophy type 1 (DM1), or Steinert's disease, is the most common adult-onset form of muscular dystrophy. DM1 also constitutes the neuromuscular condition with the most significant sleep disorders including excessive daytime sleepiness (EDS), central and obstructive sleep apneas, restless legs syndrome (RLS), periodic leg movements in wake (PLMW) and periodic leg movements in sleep (PLMS) as well as nocturnal and diurnal rapid eye movement (REM) sleep dysregulation. EDS is the most frequent non-muscular complaint in DM1, being present in about 70-80% of patients. Different phenotypes of sleep-related problems may mimic several sleep disorders, including idiopathic hypersomnia, narcolepsy without cataplexy, sleep apnea syndrome, and periodic leg movement disorder. Subjective and objective daytime sleepiness may be associated with the degree of muscular impairment. However, available evidence suggests that DM1-related EDS is primarily caused by a central dysfunction of sleep regulation rather than by sleep fragmentation, sleep-related respiratory events or periodic leg movements. EDS also tends to persist despite successful treatment of sleep-disordered breathing in DM1 patients. As EDS clearly impacts on physical and social functioning of DM1 patients, studies are needed to identify the best appropriate tools to identify hypersomnia, and clarify the indications for polysomnography (PSG) and multiple sleep latency test (MSLT) in DM1. In addition, further structured trials of assisted nocturnal ventilation and randomized trials of central nervous system (CNS) stimulant drugs in large samples of DM1 patients are required to optimally treat patients affected by this progressive, incurable condition.

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

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

Daytime sleepiness and REM sleep characteristics in myotonic dystrophy: a case-control study

STUDY OBJECTIVES

Excessive daytime sleepiness (EDS) and high daytime REM sleep pressure are important sleep features of myotonic dystrophy (DM1). Small and uncontrolled studies have focused on EDS phenotype; none have focused on nocturnal REM sleep characteristics in DM1. Our objectives were to compare polysomnographic and multiple sleep latency test (MSLT) parameters, and both tonic and phasic components of REM sleep between DM1 and controls.

DESIGN AND PATIENTS

Forty consecutive DM1 patients and 40 sex- and age-matched controls were included. All subjects underwent overnight polysomnography followed by a MSLT.

RESULTS

About 80% of DM1 patients complained of EDS through clinical interview: 31.4% had Epworth scores > 10, and 12.5% had objective sleepiness (latency < 8 min). Higher apnea and central apnea indexes, and a greater proportion of subjects with severe apnea/hypopnea syndrome were found in DM1. The number of SOREMP differed between DM1 and controls, one and two SOREMPs being present in 47.5% and 32.5%, and one control had one SOREMP. Higher percentages of slow wave sleep and REM sleep were found in DM1. DM1 patients had significantly more PLMW, PLMS in both NREM and REM sleep, and PLMS-associated microarousals. Higher REM density was found in DM1 with similar tendencies for either REM sleep without atonia or phasic EMG activity.

CONCLUSIONS

This is the first case-control sleep study in DM1 to demonstrate higher frequency of daytime sleepiness and abnormalities in REM sleep regulation, with an increased daytime and nighttime REM sleep propensity, REM density, and PLMS. These data suggest a primary central sleep regulation dysfunction in DM1.

CCUG repeats reduce the rate of global protein synthesis in myotonic dystrophy type 2

Expansion of non-coding CTG and CCTG repeats in the 3' UTR of the myotonin protein kinase (DMPK) gene in Myotonic Dystrophy type 1 (DM1) and in the intron 1 of Zinc Finger Protein 9 (ZNF9) in Myotonic Dystrophy type 2 (DM2) represent typical non-coding mutations that cause the diseases mainly through transdominant effect on the RNA metabolism (splicing, translation and RNA stability). The commonly recognized RNA gain-of-function mechanism of DM1 and DM2 suggests that the mutant CUG and CCUG RNAs play a critical role in myotonic dystrophies (DMs) without a significant role of DMPK and ZNF9. Recent studies have shown that the molecular pathogenesis of DM2 also involves the protein product of the ZNF9 gene. CCUG repeats reduce ZNF9 protein, a translational regulator of the terminal oligo-pyrimidine tract (TOP) mRNAs encoding proteins of translational apparatus. Thus, in DM2 cells, expansion of CCUG repeats affects not only multiple RNAs, but also down-regulates ZNF9 which in turn reduces translation of the TOP-containing mRNAs and diminishes the rate of global protein synthesis. In this review, we discuss the role of expansion of CCUG repeats in the reduction of ZNF9-mediated regulation of the rate of protein synthesis in DM2 and the consequences of this reduction in the multi-systemic phenotype of DM2.

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

The purpose of this study was to investigate the role of the mutant CUGn RNA in the induction of stress in type 1 myotonic dystrophy (DM1) cells and in the stress-mediated inhibition of protein translation in DM1. To achieve our goals, we performed HPLC-based purification of stress granules (SGs), immunoanalysis of SGs with stress markers TIA-1, CUGBP1, and ph-eIF2, site-specific mutagenesis, and examinations of RNA-protein and protein-protein interactions in myoblasts from control and DM1 patients. The cause-and-effect relationships were addressed in stable cells expressing mutant CUG repeats. We found that the mutant CUGn RNA induces formation of SGs through the increase of the double-stranded RNA-dependent protein kinase (PKR) and following inactivation of eIF2α, one of the substrates of PKR. We show that SGs trap mRNA coding for the DNA repair and remodeling factor MRG15 (MORF4L1), translation of which is regulated by CUGBP1. As the result of the trapping, the levels of MRG15 are reduced in DM1 cells and in CUG-expressing cells. These data show that CUG repeats cause stress in DM1 through the PKR-ph-eIF2α pathway inhibiting translation of certain mRNAs, such as MRG15 mRNA. The repression of protein translation by stress might contribute to the progressive muscle loss in DM1.

Myotonic dystrophies 1 and 2: complex diseases with complex mechanisms

Two multi-system disorders, Myotonic Dystrophies type 1 and type 2 (DM1 and DM2), are complex neuromuscular diseases caused by an accumulation of expanded, non-coding RNAs, containing repetitive CUG and CCUG elements. Similarities of these mutations suggest similar mechanisms for both diseases. The expanded CUGn and CCUGn RNAs mainly target two RNA binding proteins, MBNL1 and CUGBP1, elevating levels of CUGBP1 and reducing levels of MBNL1. These alterations change processing of RNAs that are regulated by these proteins. Whereas overall toxicity of CUGn/CCUGn RNAs on RNA homeostasis in DM cells has been proven, the mechanisms which make these RNAs toxic remain illusive. A current view is that the toxicity of RNA CUGn and CCUGn is associated exclusively with global mis-splicing in DM patients. However, a growing number of new findings show that the expansion of CUGn and CCUGn RNAs mis-regulates several additional pathways in nuclei and cytoplasm of cells from patients with DM1 and DM2. The purpose of this review is to discuss the similarities and differences in the clinical presentation and molecular genetics of both diseases. We will also discuss the complexity of the molecular abnormalities in DM1 and DM2 caused by CUG and CCUG repeats and will summarize the outcomes of the toxicity of CUG and CCUG repeats.

Repeat expansion disease: progress and puzzles in disease pathogenesis

Repeat expansion mutations cause at least 22 inherited neurological diseases. The complexity of repeat disease genetics and pathobiology has revealed unexpected shared themes and mechanistic pathways among the diseases, such as RNA toxicity. Also, investigation of the polyglutamine diseases has identified post-translational modification as a key step in the pathogenic cascade and has shown that the autophagy pathway has an important role in the degradation of misfolded proteins--two themes that are likely to be relevant to the entire neurodegeneration field. Insights from repeat disease research are catalysing new lines of study that should not only elucidate molecular mechanisms of disease but also highlight opportunities for therapeutic intervention for these currently untreatable disorders.

Reduction of the rate of protein translation in patients with myotonic dystrophy 2

Myotonic dystrophy 2 (DM2) is an autosomal dominant, multisystem disease, which primarily affects skeletal muscle. DM2 is caused by CCTGn expansion in the intron 1 of the ZNF9 gene. Expression of the mutant CCUGn RNA changes RNA processing in patients with DM2; however, the role of ZNF9 protein in DM2 pathology has been not elucidated. ZNF9 has been shown to regulate cap-dependent and cap-independent translation. We have examined a possible role of ZNF9 in the regulation of translation in DM2 patients. We found that ZNF9 interacts with the 5' UTRs of terminal oligopyrimidine (TOP) tract mRNAs encoding human ribosomal protein, RPS17, poly(A)-binding protein 1 (PABP1), and the elongation factors, eEF1A and eEF2. The binding activity of ZNF9 toward these TOP-containing 5' UTRs is reduced in DM2 muscle. Consistent with the reduction of this activity, the levels of RPS17, PABP, eEF1A, and eEF2 proteins are also diminished in DM2 muscle. The reduction of ZNF9 RNA-binding activity in DM2 correlates with a decrease of ZNF9 protein levels in cytoplasm of DM2 muscle cells. We found that the reduction of ZNF9 is caused by expression of the mutant CCUG repeats. This decrease of proteins of translational apparatus in DM2 correlates with a reduction of a rate of protein synthesis in myoblasts from DM2 patients. We found that the ectopic expression of ZNF9 in DM2 myoblasts corrects rate of protein synthesis, suggesting that the alterations in CCUG-ZNF9-TOP mRNAs pathway are responsible for the reduction of the rate of protein translation in DM2 muscle cells.

Structural basis for the sequence-specific RNA-recognition mechanism of human CUG-BP1 RRM3

The CUG-binding protein 1 (CUG-BP1) is a member of the CUG-BP1 and ETR-like factors (CELF) family or the Bruno-like family and is involved in the control of splicing, translation and mRNA degradation. Several target RNA sequences of CUG-BP1 have been predicted, such as the CUG triplet repeat, the GU-rich sequences and the AU-rich element of nuclear pre-mRNAs and/or cytoplasmic mRNA. CUG-BP1 has three RNA-recognition motifs (RRMs), among which the third RRM (RRM3) can bind to the target RNAs on its own. In this study, we solved the solution structure of the CUG-BP1 RRM3 by hetero-nuclear NMR spectroscopy. The CUG-BP1 RRM3 exhibited a noncanonical RRM fold, with the four-stranded β-sheet surface tightly associated with the N-terminal extension. Furthermore, we determined the solution structure of the CUG-BP1 RRM3 in the complex with (UG)₃ RNA, and discovered that the UGU trinucleotide is specifically recognized through extensive stacking interactions and hydrogen bonds within the pocket formed by the β-sheet surface and the N-terminal extension. This study revealed the unique mechanism that enables the CUG-BP1 RRM3 to discriminate the short RNA segment from other sequences, thus providing the molecular basis for the comprehension of the role of the RRM3s in the CELF/Bruno-like family.

Cuff tear arthropathy: evidence of functional variation in pyrophosphate metabolism genes

We investigated the role of two genes, ANKH and TNAP, in patients with cuff tear arthropathy. These genes encode proteins which regulate the extracellular concentration of inorganic pyrophosphate, fluctuations of which can lead to calcium crystal formation. Variants were detected by direct sequencing of DNA and their frequencies compared with healthy controls. The effect of variants on protein function was further studied by in vitro approaches. Variant genotypes were observed more frequently in the cases when compared with controls in ANKH (45% and 20%) and TNAP (32% and 9%). Variants in ANKH altered inorganic pyrophosphate (PPi) concentrations in transfected human chondrocytes. There was a higher mean serum concentration of TNAP detected in female patients compared with normal ranges. Cuff tear arthropathy is associated with variants in ANKH and TNAP that alter extracellular inorganic pyrophosphate concentrations causing calcium crystal deposition. This supports a theory that genetic variants predispose patients to primary crystal deposition which when combined with a massive rotator cuff tear leads to the development of arthritis.

Inherited Conduction System Abnormalities-One Group of Diseases, Many Genes

The cardiac conduction system can be anatomically, developmentally, and molecularly distinguished from the working myocardium. Abnormalities in cardiac conduction can occur due to a variety of factors, including developmental and congenital defects, acquired injury or ischemia of portions of the conduction system, or less commonly due to inherited diseases that alter cardiac conduction system function. So called "idiopathic" conduction system degeneration may have familial clustering, and therefore is consistent with a hereditary basis. This "Molecular Perspectives" will highlight several diverse mechanisms of isolated conduction system disease as well as conduction system degeneration associated with other cardiac and non-cardiac disorders. The first part of this review focuses on channelopathies associated with conduction system disease. Human genetic studies have identified mutations in the sodium channel SCN5A gene causing tachyarrhythmia disorders, as well as progressive cardiac conduction system diseases, or overlapping syndromes. Next, the importance of embryonic developmental genes such as homeobox and T-box transcription factors are highlighted in conduction system development and function. Conduction system diseases associated with multisystem disorders, such as muscular and myotonic dystrophies, will be described. Last, a new glycogen storage cardiomyopathy associated with ventricular preexcitation and progressive conduction system degeneration will be reviewed. There are a myriad of mutations identified in genes encoding cardiac transcription factors, ion channels, gap junctions, energy metabolism regulators, lamins and other structural proteins. Understanding of the molecular and ionic mechanisms underlying cardiac conduction is essential for the appreciation of the pathogenesis of conduction abnormalities in structurally normal and altered hearts.

The expression of migration stimulating factor, a potent oncofetal cytokine, is uniquely controlled by 3'-untranslated region-dependent nuclear sequestration of its precursor messenger RNA

Migration stimulating factor (MSF) is a truncated oncofetal fibronectin isoform expressed by fetal and tumor-associated cells. MSF mRNA is distinguished from other fibronectin isoforms by its size (2.1 kb) and the inclusion of a specific intronic sequence at its 3' end. Initial Northern blot analysis with a MSF-specific probe indicated the presence of this 2.1-kb transcript and an additional unexpected 5.9-kb RNA present in both MSF-secreting (fetal) and nonsecreting (adult) fibroblasts. Our investigations into the nature of these transcripts and their relationship to MSF protein secretion revealed that the 5.9-kb mRNA is a second MSF-encoding transcript. Both these mRNAs have identical coding sequence and differ only in the length of their intron-derived 3'-untranslated region (UTR). The 5.9-kb MSF mRNA is retained in the nucleus whereas the 2.1-kb mRNA is not. MSF-secreting fetal fibroblasts have significantly lower nuclear levels of the 5.9-kb mRNA and correspondingly higher cytoplasmic levels of the 2.1-kb transcript than their nonsecreting adult counterparts. Adult fibroblasts induced to secrete MSF by treatment with transforming growth factor-beta1 displayed similar changes in their respective levels of MSF mRNA, but not those of a control gene. When cloned downstream of a reporter gene, only the longer 3'-UTR retained coding sequence within the nucleus. We conclude that expression of MSF protein is regulated by 3'-UTR truncation of the 5.9-kb nuclear-sequestered "precursor" MSF mRNA and nuclear export of mature 2.1-kb message. Inducible 3'-UTR processing represents a novel regulatory mechanism involved in cancer pathogenesis that may open new avenues for therapeutic gene delivery.

Some flavonoids and DHEA-S prevent the cis-effect of expanded CTG repeats in a stable PC12 cell transformant

Expanded CUG triplet repeats carrying mRNA seem to be responsible for myotonic dystrophy type 1 (DM1). To study the pathogenesis of DM1, we constructed a DM1 cell culture model using a PC12 neuronal cell line and screened flavonoids that ameliorate this mRNA gain of function. The expanded 250 CTG repeat was subcloned into the 3'-untranslated region of the luciferase gene yielding a stable transformant of PC12 (CTG-250). The cytotoxicity of CTG-250 was evaluated by intracellular LDH activity, and the cis-effect by luciferase activity. To find agents that alter CTG-250 toxic effects, 235 bioflavonoids were screened. An increased cis-effect and cytotoxicity were found when CTG-250 was treated with nerve growth factor to induce differentiation. Western blotting with anti-caspase-3 antibody suggested that cell death was caused by apoptosis. Screening analysis confirmed that a flavone (toringin), an isoflavones (genistein and formononetin), a flavanone (isosakuranetin), and DHEA-S prevent both the cytotoxicity and cis-effect of CTG-250 and that a flavanone (naringenin), isoflavone (ononin), and xanthylatin strongly inhibit the cis-effect of CTG repeats. In conclusion, we found that this neuronal cell line, which expresses the CUG repeat-bearing mRNA, showed cis-effects through the reporter gene and neuronal death after cell differentiation in vitro. However, some flavonoids and DHEA-S inhibit both the cis-effect and cytotoxicity, indicating that their chemical structures work to ameliorate both these toxic effects. This system makes it easy to evaluate the toxic effects of expanded CTG repeats and therefore should be useful for screening other DM1 treatments for their efficacies.

Congenital myotonic dystrophy: assisted ventilation duration and outcome

OBJECTIVE

To clarify the relationship between initial assisted ventilation duration and outcome for patients with congenital myotonic dystrophy (CDM).

METHODS

A retrospective chart review was conducted of cases of CDM that presented to the Children's Hospital of Eastern Ontario (Ottawa, Ontario, Canada) between 1980 and 2000. Inclusion criteria were conclusive testing for CDM and clinical presentation in the first 30 days of life. Duration of assisted ventilation, morbidity, mortality, and developmental outcome were measured.

RESULTS

A total of 23 children met the inclusion criteria. One child died at 5 days of age, and 2 others had withdrawal of ventilation. The remaining 20 children were divided into 2 groups on the basis of whether they needed > or <30 days of ventilation. In the first year of life, 25% mortality was noted in the children with prolonged ventilation, whereas no child in the short ventilation duration group died. After 1 year of age, 1 child in each group died with follow-up of 2 to 16 years. The children with prolonged ventilation needed more hospitalizations. Delays were noted in development in both groups of children at ages 1, 3, and 6 years; however, there was an improvement in motor and language scores over time in all children. Children who required ventilation for <30 days had better motor, language, and activities of daily living scores at all ages.

CONCLUSIONS

Children with CDM with prolonged ventilation experienced 25% mortality in the first year. The use of a specific time period of ventilation to decide on withdrawal of therapy must be reconsidered given these findings. Prolonged ventilation was followed by greater morbidity and developmental delay than children with shorter ventilation duration.

Overexpression of CUG triplet repeat-binding protein, CUGBP1, in mice inhibits myogenesis

Accumulation of RNA CUG repeats in myotonic dystrophy type 1 (DM1) patients leads to the induction of a CUG-binding protein, CUGBP1, which increases translation of several proteins that are required for myogenesis. In this paper, we examine the role of overexpression of CUGBP1 in DM1 muscle pathology using transgenic mice that overexpress CUGBP1 in skeletal muscle. Our data demonstrate that the elevation of CUGBP1 in skeletal muscle causes overexpression of MEF2A and p21 to levels that are significantly higher than those in skeletal muscle of wild type animals. A similar induction of these proteins is observed in skeletal muscle of DM1 patients with increased levels of CUGBP1. Immunohistological analysis showed that the skeletal muscle from mice overexpressing CUGBP1 is characterized by a developmental delay, muscular dystrophy, and myofiber-type switch: increase of slow/oxidative fibers and the reduction of fast fibers. Examination of molecular mechanisms by which CUGBP1 up-regulates MEF2A shows that CUGBP1 increases translation of MEF2A via direct interaction with GCN repeats located within MEF2A mRNA. Our data suggest that CUGBP1-mediated overexpression of MEF2A and p21 inhibits myogenesis and contributes to the development of muscle deficiency in DM1 patients.

RNA structure of trinucleotide repeats associated with human neurological diseases

The tandem repeats of trinucleotide sequences are present in many human genes and their expansion in specific genes causes a number of hereditary neurological disorders. The normal function of triplet repeats in transcripts is barely known and the role of expanded RNA repeats in the pathogenesis of Triplet Repeat Expansion Diseases needs to be more fully elucidated. Here we have described the structures formed by transcripts composed of AAG, CAG, CCG, CGG and CUG repeats, which were determined by chemical and enzymatic structure probing. With the exception of the repeated AAG motif, all studied repeats form hairpin structures and these hairpins show several alternative alignments. We have determined the molecular architectures of these co-existing hairpin structures by using transcripts with GC-clamps which imposed single alignments of hairpins. We have provided experimental evidence that CCUG repeats implicated in myotonic dystrophy type 2 also form hairpin structures with properties similar to that composed of the CUG repeats.

Selectable system for monitoring the instability of CTG/CAG triplet repeats in mammalian cells

Despite substantial progress in understanding the mechanism by which expanded CTG/CAG trinucleotide repeats cause neurodegenerative diseases, little is known about the basis for repeat instability itself. By taking advantage of a novel phenomenon, we have developed a selectable assay to detect contractions of CTG/CAG triplets. When inserted into an intron in the APRT gene or the HPRT minigene, long tracts of CTG/CAG repeats (more than about 33 repeat units) are efficiently incorporated into mRNA as a new exon, thereby rendering the encoded protein nonfunctional, whereas short repeat tracts do not affect the phenotype. Therefore, contractions of long repeats can be monitored in large cell populations, by selecting for HPRT(+) or APRT(+) clones. Using this selectable system, we determined the frequency of spontaneous contractions and showed that treatments with DNA-damaging agents stimulate repeat contractions. The selectable system that we have developed provides a versatile tool for the analysis of CTG/CAG repeat instability in mammalian cells. We also discuss how the effect of long CTG/CAG repeat tracts on splicing may contribute to the progression of polyglutamine diseases.

Cardiac complications of childhood myopathies

The management of individuals with a neuromuscular disorder is usually focused on the skeletal muscle weakness and resulting complications, such as respiratory failure. Long-term prognosis of a number of neuromuscular conditions is, however, also determined by the type and severity of cardiac involvement. Early recognition and treatment of the cardiovascular complications are part of the task of the multidisciplinary team involved in the care of these patients. Although for several of the common conditions, there is general consensus on the cardiac investigations and treatments, in the rarer disorders, evidence-based recommendations are not available, and suggestions from experts provide an acceptable solution. This review summarizes the recent advances in our understanding of the pathogenesis and phenotypic diversity of cardiac complications associated with pediatric myopathies and provides a rational framework for planning the monitoring and therapeutic intervention in individual conditions.

Expanded CTG repeats inhibit neuronal differentiation of the PC12 cell line

Myotonic dystrophy (DM) is a dominant neuromuscular disorder caused by the expansion of trinucleotide CTG repeats in the 3-untranslated region (3'-UTR) of the MtPK gene. Although DM-associated mental retardation suggests that neuronal functions are disturbed by the expansion mutation, the effect of this alteration in neuronal cells has not been approached. In this study we established stable transfectans of PC12 neuronal cell line expressing the reporter gene CAT alone (empty-vector clone) or fused to the MtPK 3'-UTR with 5, 60, or 90 CTG repeats (CTG5, CTG60, and CTG90 clones, respectively). CTG90 cells exhibited a suppression of NGF-induced neuronal differentiation while empty-vector, CTG5 and CTG60 clones differentiated normally. CTG90 cells displayed normal activation of early differentiation markers, ERK1/2, but the up-regulation of the late marker MAP2 was dramatically reduced. Our neuronal cell system provides the first information of how the mutant MtPK 3'-UTR mRNA affects neuronal functions.

Untranslated regions of mRNAs

Gene expression is finely regulated at the post-transcriptional level. Features of the untranslated regions of mRNAs that control their translation, degradation and localization include stem-loop structures, upstream initiation codons and open reading frames, internal ribosome entry sites and various cis-acting elements that are bound by RNA-binding proteins.

Novel role for RNA-binding protein CUGBP2 in mammalian RNA editing. CUGBP2 modulates C to U editing of apolipoprotein B mRNA by interacting with apobec-1 and ACF, the apobec-1 complementation factor

Mammalian apolipoprotein B (apoB) mRNA editing is mediated by a multicomponent holoenzyme containing apobec-1 and ACF. We have now identified CUGBP2, a 54-kDa RNA-binding protein, as a component of this holoenzyme. CUGBP2 and ACF co-fractionate in bovine liver S-100 extracts, and addition of recombinant apobec-1 leads to assembly of a holoenzyme. Immunodepletion of CUGBP2 co-precipitates ACF, and these proteins co-localize the nucleus of transfected cells, suggesting that CUGBP2 and ACF are bound in vivo. CUGBP2 binds apoB RNA, specifically an AU-rich sequence located immediately upstream of the edited cytidine. ApoB RNA from McA cells, bound to CUGBP2, was more extensively edited than the unbound fraction. However, addition of recombinant CUGBP2 to a reconstituted system demonstrated a dose-dependent inhibition of C to U RNA editing, which was rescued with either apobec-1 or ACF. Antisense CUGBP2 knockout increased endogenous apoB RNA editing, whereas antisense knockout of either apobec-1 or ACF expression eliminated apoB RNA editing, establishing the absolute requirement of these components of the core enzyme. These data suggest that CUGBP2 plays a role in apoB mRNA editing by forming a regulatory complex with the three components of the minimal editing enzyme, apobec-1, ACF, and apoB RNA.

Follicular cyst in a patient with myotonic dystrophy: a case of cyst with differentiation toward follicular infundibulum, isthmus, inner root sheath, and hair

We report a case of follicular cyst, which developed in a patient with myotonic dystrophy (MyD). Histopathologically, the cyst showed infundibular and trichilemmal keratinization, inner root sheath differentiation, aggregation of basaloid cells, and pilomatricoma-like changes in the pericystic connective tissue. These findings have been reported in follicular cysts with Gardner's syndrome (GS). Interestingly, pilomatricoma is known as one of the skin diseases associated with MyD, though there have been no reported cases of cyst formation with differentiation toward portions of hair follicle in a MyD patient. In our case, we hypothesized that the cyst might be derived from embryonic follicular germinative cells or follicular stem cells under the genetic influence of the MyD gene, as observed in follicular cysts in patients with GS.

Highlights of alternative splicing regulation session: yes, no, maybe--a history of paradigm shifts

Highlights from the Sixth Annual Meeting of the RNA Society, Banff, Alberta, Canada, 29 May to 3 June 2001. Cooper summarizes the discussions and presentations from the session entitled "Control of Splice Site Selection" held at the Sixth Annual Meeting of the RNA Society. Paradigms are shifting as experiments show that some of the proteins involved in regulating splicing can act as splicing enhancers or repressors, depending on the cellular context. The complex interactions among the ribonucleoproteins (RNPs) and proteins, and the role of cis elements, in controlling cell-specific splicing are highlighted. The importance of properly regulated splicing is emphasized by examples of disease pathologies in which alternative splicing is aberrant.

Triplet repeats, RNA secondary structure and toxic gain-of-function models for pathogenesis

Ten years after the discovery of human diseases caused by trinucleotide repeat expansions, searching for mechanistic links between gene mutation and pathological phenotype remains a fundamental and unsolved issue. Evidence accumulated so far indicates that the pathogenesis of repeat disorders is complex and multi-factorial. Diseases caused by CAG expansions coding for polyglutamine tracts have been extensively studied, and in most cases a toxic gain-of-function of the mutant protein was demonstrated. Most recently, tracking the effects of repeats along the pathway of gene expression is providing additional clues to understand how a triplet repeat expansion can cause disease. Expanded repeats form DNA secondary structures that confer genetic instability, and most likely contribute to alter the local chromatin configuration leading to transcriptional silencing. At the level of RNA, the expanded repeat may either interfere with processing of the primary transcript, resulting in deficit of the corresponding protein, or interact with RNA-binding proteins altering their normal activity. The latter mechanism, termed RNA gain-of-function, has no precedents in human genetics. Recent evidence suggests that expanded RNAs and associated RNA-binding proteins are potential contributors to the pathogenesis of several triplet repeat diseases.

Molecular basis for impaired muscle differentiation in myotonic dystrophy

Differentiation of skeletal muscle is affected in myotonic dystrophy (DM) patients. Analysis of cultured myoblasts from DM patients shows that DM myoblasts lose the capability to withdraw from the cell cycle during differentiation. Our data demonstrate that the expression and activity of the proteins responsible for cell cycle withdrawal are altered in DM muscle cells. Skeletal muscle cells from DM patients fail to induce cytoplasmic levels of a CUG RNA binding protein, CUGBP1, while normal differentiated cells accumulate CUGBP1 in the cytoplasm. In cells from normal patients, CUGBP1 up-regulates p21 protein during differentiation. Several lines of evidence show that CUGBP1 induces the translation of p21 via binding to a GC-rich sequence located within the 5' region of p21 mRNA. Failure of DM cells to accumulate CUGBP1 in the cytoplasm leads to a significant reduction of p21 and to alterations of other proteins responsible for the cell cycle withdrawal. The activity of cdk4 declines during differentiation of cells from control patients, while in DM cells cdk4 is highly active during all stages of differentiation. In addition, DM cells do not form Rb/E2F repressor complexes that are abundant in differentiated cells from normal patients. Our data provide evidence for an impaired cell cycle withdrawal in DM muscle cells and suggest that alterations in the activity of CUGBP1 causes disruption of p21-dependent control of cell cycle arrest.

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

Myotonic dystrophy type 1 (DM1) is caused by a CTG trinucleotide expansion in the 3' untranslated region of the DM protein kinase gene. People with DM1 have an unusual form of insulin resistance caused by a defect in skeletal muscle. Here we demonstrate that alternative splicing of the insulin receptor (IR) pre-mRNA is aberrantly regulated in DM1 skeletal muscle tissue, resulting in predominant expression of the lower-signaling nonmuscle isoform (IR-A). IR-A also predominates in DM1 skeletal muscle cultures, which exhibit a decreased metabolic response to insulin relative to cultures from normal controls. Steady-state levels of CUG-BP, a regulator of pre-mRNA splicing proposed to mediate some aspects of DM1 pathogenesis, are increased in DM1 skeletal muscle; overexpression of CUG-BP in normal cells induces a switch to IR-A. The CUG-BP protein mediates this switch through an intronic element located upstream of the alternatively spliced exon 11, and specifically binds within this element in vitro. These results support a model in which increased expression of a splicing regulator contributes to insulin resistance in DM1 by affecting IR alternative splicing.

Electrophysiological evaluation in myotonic dystrophy: correlation with CTG length expansion

In myotonic dystrophy (MD), disease severity has been correlated with expansion of CTG repeats in chromosome 19. The aims of this study were to evaluate efficacy of electromyography in the diagnosis of MD, access the frequency and the characteristics of peripheral involvement in the disease and to verify whether the CTG repeats correlated with the electrophysiological abnormalities. Twenty-five patients and six relatives at risk of carrying the MD gene were examined. Electrical myotonia (EM) was scored. Sensory and motor conduction velocity (CV) were studied in five nerves. Leukocyte DNA analysis was done in 26 subjects. Myopathy and myotonia were found in 27 cases. EM was most frequent in muscles of hand and in tibialis anterior. No significant correlation was found between EM scores and length of CTG expansions. EM scores correlated significantly with the degree of clinical myopathy, expressed by a muscular disability scale. Peripheral neuropathy was found in eight subjects and was not restricted to those who were diabetics.

RNA CUG repeats sequester CUGBP1 and alter protein levels and activity of CUGBP1

An RNA CUG triplet repeat binding protein, CUGBP1, regulates splicing and translation of various RNAs. Expansion of RNA CUG repeats in the 3'-untranslated repeat of the mutant myotonin protein kinase (DMPK) mRNA in myotonic dystrophy (DM) is associated with alterations in binding activity of CUGBP1. To investigate whether CUGBP1 is directly affected by expansion of CUG repeats in DM tissues, we examined the intracellular status of CUGBP1 in DM patients as well as in cultured cells over expressing RNA CUG repeats. The analysis of RNA-protein complexes showed that, in control tissues, the majority of CUGBP1 is free of RNA, whereas in DM patients the majority of CUGBP1 is associated with RNA containing CUG repeats. Similarly to DM patients, overexpression of RNA CUG repeats in cultured cells results in the re-allocation of CUGBP1 from a free state to the RNA.protein complexes containing CUG repeats. CUG repeat-dependent translocation of CUGBP1 into RNA-protein complexes is associated with increased levels of CUGBP1 protein and its binding activity. Experiments with cyclohexamide-dependent block of protein synthesis showed that the half-life of CUGBP1 is increased in cells expressing CUG repeats. Alteration of CUGBP1 in DM is accompanied by alteration in translation of a transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta), which has been previously described to be a target of CUGBP1. Analysis of C/EBPbeta isoforms in DM patients with altered levels of CUGBP1 showed that translation of a dominant negative isoform, LIP, is induced by CUGBP1. Results of this paper demonstrate that the expansion of CUG repeats in DM affects RNA-binding proteins and leads to alteration in RNA processing.

The intrinsically unstable life of DNA triplet repeats associated with human hereditary disorders

Expansions of specific DNA triplet repeats are the cause of an increasing number of hereditary neurological disorders in humans. In some diseases, such as Huntington's and several spinocerebellar ataxias, the repetitive DNA sequences are translated into long tracts of the same amino acid (usually glutamine), which alters interactions with cellular constituents and leads to the development of disease. For other disorders, including common genetic disorders such as myotonic dystrophy and fragile X syndrome, the DNA repeat is located in noncoding regions of transcribed sequences and disease is probably caused by altered gene expression. In studies in lower organisms, mammalian cells, and transgenic mice, high frequencies of length changes (increases and decreases) occur in long DNA triplet repeats. These observations are similar to other types of repetitive DNA sequences, which also undergo frequent length changes at genomic loci. A variety of processes acting on DNA influence the genetic stability of DNA triplet repeats, including replication, recombination, repair, and transcription. It is not yet known how these different multienzyme systems interact to produce the genetic mutation of expanded repeats. In vitro studies have identified that DNA triplet repeats can adopt several unusual DNA structures, including hairpins, triplexes, quadruplexes, slipped structures, and highly flexible and writhed helices. The formation of stable unusual structures within the cell is likely to disturb DNA metabolism and be a critical intermediate in the molecular mechanism(s) leading to genetic instabilities of DNA repeats and, hence, to disease pathogenesis.

Myotonic dystrophies

This review demonstrates genetic and phenotypic heterogeneity in all of the multisystemic myotonic disorders collectively called 'myotonic dystrophies' according to the new nomenclature: myotonic dystrophy type 1, myotonic dystrophy type 2, proximal myotonic myopathy and proximal myotonic dystrophy. Only two loci have so far been assigned (19q 13.3 in myotonic dystrophy type 1, and 3q 21.3 in myotonic dystrophy type 2). Although the diagnosis of these disorders may be suspected clinically, it needs to be confirmed by DNA analysis.

Omega speckles - a novel class of nuclear speckles containing hnRNPs associated with noncoding hsr-omega RNA in Drosophila

Fluorescence RNA:RNA in situ hybridization studies in various larval and adult cell types of Drosophila melanogaster showed that the noncoding hsr-omega nuclear (hsromega-n) transcripts were present in the form of many small speckles. These speckles, which we name ‘omega speckles’, were distributed in the interchromatin space in close proximity to the chromatin. The only chromosomal site where hsromega-n transcripts localized was the 93D locus or the hsromega gene itself. The number of nucleoplasmic speckles varied in different cell types. Heat shock, which inhibits general chromosomal transcription, caused the individual speckles to coalesce into larger but fewer clusters. In extreme cases, only a single large cluster of hsromega-n transcripts localizing to the hsromega locus was seen in each nucleus. In situ immunocytochemical staining using antibodies against heterogenous nuclear RNA binding proteins (hnRNPs) like HRB87F, Hrp40, Hrb57A and S5 revealed that, in all cell types, all the hnRNPs gave a diffuse staining of chromatin areas and in addition, were present as large numbers of speckles. Colocalization studies revealed an absolute colocalization of the hnRNPs and the omegaspeckles. Heat shock caused all the hnRNPs to cluster together exactly, following the hsromega-n transcripts. Immunoprecipitation studies using the hnRNP antibodies further demonstrated a physical association of hnRNPs and hsromega transcripts. The omegaspeckles are distinct from interchromatin granules since nuclear speckles containing serine/arginine-rich SR-proteins like SC35 and SRp55 did not colocalize with the ω speckles. The speckled distribution of hnRNPs was completely disrupted in hsromega nullosomics. We conclude that the hsromega-n transcripts play essential structural and functional roles in organizing and establishing the hnRNP-containing omega speckles and thus regulate the trafficking and availability of hnRNPs and other related RNA binding proteins in the cell nucleus.

Skeletal muscle sodium channel gating in mice deficient in myotonic dystrophy protein kinase

Myotonic dystrophy, a progressive autosomal dominant disorder, is associated with an expansion of a CTG repeat tract located in the 3'-untranslated region of a serine/threonine protein kinase, DMPK. DMPK modulates skeletal muscle Na channels in vitro, and thus we hypothesized that mice deficient in DMPK would have altered muscle Na channel gating. We measured macroscopic and single channel Na currents from cell-attached patches of skeletal myocytes from mice heterozygous (DMPK(+/-)) and homozygous (DMPK(-/-)) for DMPK loss. In DMPK(-/-) myocytes, Na current amplitude was reduced because of reduced channel number. Single channel recordings revealed Na channel reopenings, similar to the gating abnormality of human myotonic muscular dystrophy (DM), which resulted in a plateau of Na current. The gating abnormality deteriorated with increasing age. In DMPK(+/-) muscle there was reduced Na current amplitude and increased Na channel reopenings identical to those in DMPK(-/-) muscle. Thus, these mouse models of complete and partial DMPK deficiency reproduce the Na channel abnormality of the human disease, providing direct evidence that DMPK deficiency underlies the Na channel abnormality in DM.

Deconstructing myotonic dystrophy

Triplet repeat diseases are disorders in which there is expansion of a repeat sequence of three nucleotides in the affected gene. Although the pathology usually results from production of a defective protein, myotonic dystrophy (DM) has proved to be a puzzle because the expanded repeats appear in a non-coding region of the affected DMPK gene. In a Perspective, Tapscott explains how findings from a new mouse model of DM (Mankodi et al.) could solve this paradox.

The 3' untranslated region of messenger RNA: A molecular 'hotspot' for pathology?

The role of the 3' untranslated region in posttranscriptional regulation of mRNA expression is being elucidated. Here we describe diseases arising from anomalies in this region, that affect the expression of one or more genes.

Mice deficient in Six5 develop cataracts: implications for myotonic dystrophy

Expansion of a CTG trinucleotide repeat in the 3' UTR of the gene DMPK at the DM1 locus on chromosome 19 causes myotonic dystrophy, a dominantly inherited disease characterized by skeletal muscle dystrophy and myotonia, cataracts and cardiac conduction defects. Targeted deletion of Dm15, the mouse orthologue of human DMPK, produced mice with a mild myopathy and cardiac conduction abnormalities, but without other features of myotonic dystrophy, such as myotonia and cataracts. We, and others, have demonstrated that repeat expansion decreases expression of the adjacent gene SIX5 (refs 7,8), which encodes a homeodomain transcription factor. To determine whether SIX5 deficiency contributes to the myotonic dystrophy phenotype, we disrupted mouse Six5 by replacing the first exon with a beta-galactosidase reporter. Six5-mutant mice showed reporter expression in multiple tissues, including the developing lens. Homozygous mutant mice had no apparent abnormalities of skeletal muscle function, but developed lenticular opacities at a higher rate than controls. Our results suggest that SIX5 deficiency contributes to the cataract phenotype in myotonic dystrophy, and that myotonic dystrophy represents a multigenic disorder.

ETR-1, a homologue of a protein linked to myotonic dystrophy, is essential for muscle development in Caenorhabditis elegans

Post-transcriptional gene processing by RNA-binding proteins (RBPs) has crucial roles during development [1] [2]. Here, we report the identification of ETR-1 (ELAV-type RNA-binding protein), a muscle-specific RBP in the nematode Caenorhabditis elegans. ETR-1 is related to the family of RBPs defined by the protein ELAV, which is essential for neurogenesis in the fruit fly Drosophila; members of the family possess two consecutive RNA recognition motifs (RRMs) separated from a third, carboxy-terminal RRM by a tether region of variable length [3] [4] [5] [6]. Its closest homologue, CUG-binding protein (CUG-bp), is a human RBP that has been implicated in the disease myotonic dystrophy and binds CUG repeats in the 3' untranslated region (UTR) of the mRNA for myotonic dystrophy protein kinase (DMPK) [7] [8]. Inactivation of etr-1 by RNA-mediated interference resulted in embryonic lethality. Embryos failed to elongate and became paralysed, a phenotype characteristic of C. elegans Pat mutants, which are defective in muscle formation and function [9]. The data indicate that etr-1 is essential for muscle development in C. elegans, perhaps by playing a role in post-transcriptional processing of some muscle component, and thus suggesting a possible conservation of gene function with human CUG-bp.