Myotonic dystrophy is associated with a reduced level of RNA from the DMWD allele adjacent to the expanded repeat

Immortalized human myotonic dystrophy type 1 muscle cell lines to address patient heterogeneity

Historically, cellular models have been used as a tool to study myotonic dystrophy type 1 (DM1) and the validation of therapies in said pathology. However, there is a need for in vitro models that represent the clinical heterogeneity observed in patients with DM1 that is lacking in classical models. In this study, we immortalized three DM1 muscle lines derived from patients with different DM1 subtypes and clinical backgrounds and characterized them at the genetic, epigenetic, and molecular levels. All three cell lines display DM1 hallmarks, such as the accumulation of RNA foci, MBNL1 sequestration, splicing alterations, and reduced fusion. In addition, alterations in early myogenic markers, myotube diameter and CTCF1 DNA methylation were also found in DM1 cells. Notably, the new lines show a high level of heterogeneity in both the size of the CTG expansion and the aforementioned molecular alterations. Importantly, these immortalized cells also responded to previously tested therapeutics. Altogether, our results show that these three human DM1 cellular models are suitable to study the pathophysiological heterogeneity of DM1 and to test future therapeutic options.

DNA methylation changes and increased mRNA expression of coagulation proteins, factor V and thrombomodulin in Fuchs endothelial corneal dystrophy

Late-onset Fuchs endothelial corneal dystrophy (FECD) is a disease affecting the corneal endothelium (CE), associated with a cytosine-thymine-guanine repeat expansion at the CTG18.1 locus in the transcription factor 4 (TCF4) gene. It is unknown whether CTG18.1 expansions affect global methylation including TCF4 gene in CE or whether global CE methylation changes at advanced age. Using genome-wide DNA methylation array, we investigated methylation in CE from FECD patients with CTG18.1 expansions and studied the methylation in healthy CE at different ages. The most revealing DNA methylation findings were analyzed by gene expression and protein analysis. 3488 CpGs had significantly altered methylation pattern in FECD though no substantial changes were found in TCF4. The most hypermethylated site was in a predicted promoter of aquaporin 1 (AQP1) gene, and the most hypomethylated site was in a predicted promoter of coagulation factor V (F5 for gene, FV for protein). In FECD, AQP1 mRNA expression was variable, while F5 gene expression showed a ~ 23-fold increase. FV protein was present in both healthy and affected CE. Further gene expression analysis of coagulation factors interacting with FV revealed a ~ 34-fold increase of thrombomodulin (THBD). THBD protein was detected only in CE from FECD patients. Additionally, we observed an age-dependent hypomethylation in elderly healthy CE.Thus, tissue-specific genome-wide and gene-specific methylation changes associated with altered gene expression were discovered in FECD. TCF4 pathological methylation in FECD because of CTG18.1 expansion was ruled out.

Genetic and Epigenetic Interplay Define Disease Onset and Severity in Repeat Diseases

Repeat diseases, such as fragile X syndrome, myotonic dystrophy, Friedreich ataxia, Huntington disease, spinocerebellar ataxias, and some forms of amyotrophic lateral sclerosis, are caused by repetitive DNA sequences that are expanded in affected individuals. The age at which an individual begins to experience symptoms, and the severity of disease, are partially determined by the size of the repeat. However, the epigenetic state of the area in and around the repeat also plays an important role in determining the age of disease onset and the rate of disease progression. Many repeat diseases share a common epigenetic pattern of increased methylation at CpG islands near the repeat region. CpG islands are CG-rich sequences that are tightly regulated by methylation and are often found at gene enhancer or insulator elements in the genome. Methylation of CpG islands can inhibit binding of the transcriptional regulator CTCF, resulting in a closed chromatin state and gene down regulation. The downregulation of these genes leads to some disease-specific symptoms. Additionally, a genetic and epigenetic interplay is suggested by an effect of methylation on repeat instability, a hallmark of large repeat expansions that leads to increasing disease severity in successive generations. In this review, we will discuss the common epigenetic patterns shared across repeat diseases, how the genetics and epigenetics interact, and how this could be involved in disease manifestation. We also discuss the currently available stem cell and mouse models, which frequently do not recapitulate epigenetic patterns observed in human disease, and propose alternative strategies to study the role of epigenetics in repeat diseases.

Molecular Therapies for Myotonic Dystrophy Type 1: From Small Drugs to Gene Editing

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy affecting many different body tissues, predominantly skeletal and cardiac muscles and the central nervous system. The expansion of CTG repeats in the DM1 protein-kinase (DMPK) gene is the genetic cause of the disease. The pathogenetic mechanisms are mainly mediated by the production of a toxic expanded CUG transcript from the DMPK gene. With the availability of new knowledge, disease models, and technical tools, much progress has been made in the discovery of altered pathways and in the potential of therapeutic intervention, making the path to the clinic a closer reality. In this review, we describe and discuss the molecular therapeutic strategies for DM1, which are designed to directly target the CTG genomic tract, the expanded CUG transcript or downstream signaling molecules.

Overview of the Complex Relationship between Epigenetics Markers, CTG Repeat Instability and Symptoms in Myotonic Dystrophy Type 1

Among the trinucleotide repeat disorders, myotonic dystrophy type 1 (DM1) is one of the most complex neuromuscular diseases caused by an unstable CTG repeat expansion in the DMPK gene. DM1 patients exhibit high variability in the dynamics of CTG repeat instability and in the manifestations and progression of the disease. The largest expanded alleles are generally associated with the earliest and most severe clinical form. However, CTG repeat length alone is not sufficient to predict disease severity and progression, suggesting the involvement of other factors. Several data support the role of epigenetic alterations in clinical and genetic variability. By highlighting epigenetic alterations in DM1, this review provides a new avenue on how these changes can serve as biomarkers to predict clinical features and the mutation behavior.

Time-controlled and muscle-specific CRISPR/Cas9-mediated deletion of CTG-repeat expansion in the DMPK gene

CRISPR/Cas9-mediated therapeutic gene editing is a promising technology for durable treatment of incurable monogenic diseases such as myotonic dystrophies. Gene-editing approaches have been recently applied to in vitro and in vivo models of myotonic dystrophy type 1 (DM1) to delete the pathogenic CTG-repeat expansion located in the 3′ untranslated region of the DMPK gene. In DM1-patient-derived cells removal of the expanded repeats induced beneficial effects on major hallmarks of the disease with reduction in DMPK transcript-containing ribonuclear foci and reversal of aberrant splicing patterns. Here, we set out to excise the triplet expansion in a time-restricted and cell-specific fashion to minimize the potential occurrence of unintended events in off-target genomic loci and select for the target cell type. To this aim, we employed either a ubiquitous promoter-driven or a muscle-specific promoter-driven Cas9 nuclease and tetracycline repressor-based guide RNAs. A dual-vector approach was used to deliver the CRISPR/Cas9 components into DM1 patient-derived cells and in skeletal muscle of a DM1 mouse model. In this way, we obtained efficient and inducible gene editing both in proliferating cells and differentiated post-mitotic myocytes in vitro as well as in skeletal muscle tissue in vivo.

Nuclear Envelope Alterations in Myotonic Dystrophy Type 1 Patient-Derived Fibroblasts

Myotonic dystrophy type 1 (DM1) is a hereditary and multisystemic disease characterized by myotonia, progressive distal muscle weakness and atrophy. The molecular mechanisms underlying this disease are still poorly characterized, although there are some hypotheses that envisage to explain the multisystemic features observed in DM1. An emergent hypothesis is that nuclear envelope (NE) dysfunction may contribute to muscular dystrophies, particularly to DM1. Therefore, the main objective of the present study was to evaluate the nuclear profile of DM1 patient-derived and control fibroblasts and to determine the protein levels and subcellular distribution of relevant NE proteins in these cell lines. Our results demonstrated that DM1 patient-derived fibroblasts exhibited altered intracellular protein levels of lamin A/C, LAP1, SUN1, nesprin-1 and nesprin-2 when compared with the control fibroblasts. In addition, the results showed an altered location of these NE proteins accompanied by the presence of nuclear deformations (blebs, lobes and/or invaginations) and an increased number of nuclear inclusions. Regarding the nuclear profile, DM1 patient-derived fibroblasts had a larger nuclear area and a higher number of deformed nuclei and micronuclei than control-derived fibroblasts. These results reinforce the evidence that NE dysfunction is a highly relevant pathological characteristic observed in DM1.

The dystrophia myotonica WD repeat-containing protein DMWD and WDR20 differentially regulate USP12 deubiquitinase

Despite its potential clinical relevance, the product of the DMWD (dystrophia myotonica, WD repeat containing) gene is a largely uncharacterized protein. The DMWD amino acid sequence is similar to that of WDR20, a known regulator of the USP12 and USP46 deubiquitinases (DUBs). Here, we apply a combination of in silico and experimental methods to investigate several aspects of DMWD biology. Molecular evolution and phylogenetic analyses reveal that WDR20 and DMWD, similar to USP12 and USP46, arose by duplication of a common ancestor during the whole genome duplication event in the vertebrate ancestor lineage. The analysis of public human gene expression datasets suggests that DMWD expression is positively correlated with USP12 expression in normal tissues and negatively correlated with WDR20 expression in tumors. Strikingly, a survey of the annotated interactome for DMWD and WDR20 reveals a largely nonoverlapping set of interactors for these proteins. Experimentally, we first confirmed that DMWD binds both USP12 and USP46 through direct coimmunoprecipitation of epitope-tagged proteins. We found that DMWD and WDR20 share the same binding interface in USP12, suggesting that their interaction with the DUB may be mutually exclusive. Finally, we show that both DMWD and WDR20 promote USP12 enzymatic activity, but they differentially modulate the subcellular localization of the DUB. Altogether, our findings suggest a model whereby mutually exclusive binding of DMWD and WDR20 to USP12 may lead to formation of deubiquitinase complexes with distinct subcellular localization, potentially targeting different substrate repertoires.

Application of CRISPR-Cas9-Mediated Genome Editing for the Treatment of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a debilitating multisystemic disorder, caused by expansion of a CTG microsatellite repeat in the 3' untranslated region of the DMPK (dystrophia myotonica protein kinase) gene. To date, novel therapeutic approaches have focused on transient suppression of the mutant, repeat-expanded RNA. However, recent developments in the field of genome editing have raised the exciting possibility of inducing permanent correction of the DM1 genetic defect. Specifically, repurposing of the prokaryotic CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 (CRISPR-associated protein 9) system has enabled programmable, site-specific, and multiplex genome editing. CRISPR-based strategies for the treatment of DM1 can be applied either directly to patients, or indirectly through the ex vivo modification of patient-derived cells, and they include excision of the repeat expansion, insertion of synthetic polyadenylation signals upstream of the repeat, steric interference with RNA polymerase II procession through the repeat leading to transcriptional downregulation of DMPK, and direct RNA targeting of the mutant RNA species. Potential obstacles to such therapies are discussed, including the major challenge of Cas9 and guide RNA transgene/ribonuclear protein delivery, off-target gene editing, vector genome insertion at cut sites, on-target unintended mutagenesis (e.g., repeat inversion), pre-existing immunity to Cas9 or AAV antigens, immunogenicity, and Cas9 persistence.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Dosage effect of multiple genes accounts for multisystem disorder of myotonic dystrophy type 1

Multisystem manifestations in myotonic dystrophy type 1 (DM1) may be due to dosage reduction in multiple genes induced by aberrant expansion of CTG repeats in DMPK, including DMPK, its neighboring genes (SIX5 or DMWD) and downstream MBNL1. However, direct evidence is lacking. Here, we develop a new strategy to generate mice carrying multigene heterozygous mutations to mimic dosage reduction in one step by injection of haploid embryonic stem cells with mutant Dmpk, Six5 and Mbnl1 into oocytes. The triple heterozygous mutant mice exhibit adult-onset DM1 phenotypes. With the additional mutation in Dmwd, the quadruple heterozygous mutant mice recapitulate many major manifestations in congenital DM1. Moreover, muscle stem cells in both models display reduced stemness, providing a unique model for screening small molecules for treatment of DM1. Our results suggest that the complex symptoms of DM1 result from the reduced dosage of multiple genes.

Abnormalities in Skeletal Muscle Myogenesis, Growth, and Regeneration in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) and 2 (DM2) are autosomal dominant degenerative neuromuscular disorders characterized by progressive skeletal muscle weakness, atrophy, and myotonia with progeroid features. Although both DM1 and DM2 are characterized by skeletal muscle dysfunction and also share other clinical features, the diseases differ in the muscle groups that are affected. In DM1, distal muscles are mainly affected, whereas in DM2 problems are mostly found in proximal muscles. In addition, manifestation in DM1 is generally more severe, with possible congenital or childhood-onset of disease and prominent CNS involvement. DM1 and DM2 are caused by expansion of (CTG•CAG)n and (CCTG•CAGG)n repeats in the 3' non-coding region of DMPK and in intron 1 of CNBP, respectively, and in overlapping antisense genes. This critical review will focus on the pleiotropic problems that occur during development, growth, regeneration, and aging of skeletal muscle in patients who inherited these expansions. The current best-accepted idea is that most muscle symptoms can be explained by pathomechanistic effects of repeat expansion on RNA-mediated pathways. However, aberrations in DNA replication and transcription of the DM loci or in protein translation and proteome homeostasis could also affect the control of proliferation and differentiation of muscle progenitor cells or the maintenance and physiological integrity of muscle fibers during a patient's lifetime. Here, we will discuss these molecular and cellular processes and summarize current knowledge about the role of embryonic and adult muscle-resident stem cells in growth, homeostasis, regeneration, and premature aging of healthy and diseased muscle tissue. Of particular interest is that also progenitor cells from extramuscular sources, such as pericytes and mesoangioblasts, can participate in myogenic differentiation. We will examine the potential of all these types of cells in the application of regenerative medicine for muscular dystrophies and evaluate new possibilities for their use in future therapy of DM.

CpG Methylation, a Parent-of-Origin Effect for Maternal-Biased Transmission of Congenital Myotonic Dystrophy

CTG repeat expansions in DMPK cause myotonic dystrophy (DM1) with a continuum of severity and ages of onset. Congenital DM1 (CDM1), the most severe form, presents distinct clinical features, large expansions, and almost exclusive maternal transmission. The correlation between CDM1 and expansion size is not absolute, suggesting contributions of other factors. We determined CpG methylation flanking the CTG repeat in 79 blood samples from 20 CDM1-affected individuals; 21, 27, and 11 individuals with DM1 but not CDM1 (henceforth non-CDM1) with maternal, paternal, and unknown inheritance; and collections of maternally and paternally derived chorionic villus samples (7 CVSs) and human embryonic stem cells (4 hESCs). All but two CDM1-affected individuals showed high levels of methylation upstream and downstream of the repeat, greater than non-CDM1 individuals (p = 7.04958 × 10⁻¹²). Most non-CDM1 individuals were devoid of methylation, where one in six showed downstream methylation. Only two non-CDM1 individuals showed upstream methylation, and these were maternally derived childhood onset, suggesting a continuum of methylation with age of onset. Only maternally derived hESCs and CVSs showed upstream methylation. In contrast, paternally derived samples (27 blood samples, 3 CVSs, and 2 hESCs) never showed upstream methylation. CTG tract length did not strictly correlate with CDM1 or methylation. Thus, methylation patterns flanking the CTG repeat are stronger indicators of CDM1 than repeat size. Spermatogonia with upstream methylation may not survive due to methylation-induced reduced expression of the adjacent SIX5, thereby protecting DM1-affected fathers from having CDM1-affected children. Thus, DMPK methylation may account for the maternal bias for CDM1 transmission, larger maternal CTG expansions, age of onset, and clinical continuum, and may serve as a diagnostic indicator.

Genome-wide identification of microRNA-related variants associated with risk of Alzheimer's disease

MicroRNAs (miRNAs) serve as key post-transcriptional regulators of gene expression. Genetic variation in miRNAs and miRNA-binding sites may affect miRNA function and contribute to disease risk. Here, we investigated the extent to which variants within miRNA-related sequences could constitute a part of the functional variants involved in developing Alzheimer’s disease (AD), using the largest available genome-wide association study of AD. First, among 237 variants in miRNAs, we found rs2291418 in the miR-1229 precursor to be significantly associated with AD (p-value = 6.8 × 10⁻⁵, OR = 1.2). Our in-silico analysis and in-vitro miRNA expression experiments demonstrated that the variant’s mutant allele enhances the production of miR-1229-3p. Next, we found miR-1229-3p target genes that are associated with AD and might mediate the miRNA function. We demonstrated that miR-1229-3p directly controls the expression of its top AD-associated target gene (SORL1) using luciferase reporter assays. Additionally, we showed that miR-1229-3p and SORL1 are both expressed in the human brain. Second, among 42,855 variants in miRNA-binding sites, we identified 10 variants (in the 3′ UTR of 9 genes) that are significantly associated with AD, including rs6857 that increases the miR-320e-mediated regulation of PVRL2. Collectively, this study shows that miRNA-related variants are associated with AD and suggests miRNA-dependent regulation of several AD genes.

Methyl-Arginine Profile of Brain from Aged PINK1-KO+A53T-SNCA Mice Suggests Altered Mitochondrial Biogenesis

Hereditary Parkinson's disease can be triggered by an autosomal dominant overdose of alpha-Synuclein (SNCA) or the autosomal recessive deficiency of PINK1. We recently showed that the combination of PINK1-knockout with overexpression of A53T-SNCA in double mutant (DM) mice potentiates phenotypes and reduces survival. Now we studied brain hemispheres of DM mice at age of 18 months in a hypothesis-free approach, employing a quantitative label-free global proteomic mass spectrometry scan of posttranslational modifications focusing on methyl-arginine. The strongest effects were documented for the adhesion modulator CMAS, the mRNA decapping/deadenylation factor PATL1, and the synaptic plasticity mediator CRTC1/TORC1. In addition, an intriguing effect was observed for the splicing factor PSF/SFPQ, known to interact with the dopaminergic differentiation factor NURR1 as well as with DJ-1, the protein responsible for the autosomal recessive PARK7 variant of PD. CRTC1, PSF, and DJ-1 are modulators of PGC1alpha and of mitochondrial biogenesis. This pathway was further stressed by dysregulations of oxygen sensor EGLN3 and of nuclear TMPO. PSF and TMPO cooperate with dopaminergic differentiation factors LMX1B and NURR1. Further dysregulations concerned PRR18, TRIO, HNRNPA1, DMWD, WAVE1, ILDR2, DBNDD1, and NFM. Thus, we report selective novel endogenous stress responses in brain, which highlight early dysregulations of mitochondrial homeostasis and midbrain vulnerability.

Muscle wasting in myotonic dystrophies: a model of premature aging

Myotonic dystrophy type 1 (DM1 or Steinert’s disease) and type 2 (DM2) are multisystem disorders of genetic origin. Progressive muscular weakness, atrophy and myotonia are the most prominent neuromuscular features of these diseases, while other clinical manifestations such as cardiomyopathy, insulin resistance and cataracts are also common. From a clinical perspective, most DM symptoms are interpreted as a result of an accelerated aging (cataracts, muscular weakness and atrophy, cognitive decline, metabolic dysfunction, etc.), including an increased risk of developing tumors. From this point of view, DM1 could be described as a progeroid syndrome since a notable age-dependent dysfunction of all systems occurs. The underlying molecular disorder in DM1 consists of the existence of a pathological (CTG) triplet expansion in the 3′ untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene, whereas (CCTG)n repeats in the first intron of the Cellular Nucleic acid Binding Protein/Zinc Finger Protein 9(CNBP/ZNF9) gene cause DM2. The expansions are transcribed into (CUG)n and (CCUG)n-containing RNA, respectively, which form secondary structures and sequester RNA-binding proteins, such as the splicing factor muscleblind-like protein (MBNL), forming nuclear aggregates known as foci. Other splicing factors, such as CUGBP, are also disrupted, leading to a spliceopathy of a large number of downstream genes linked to the clinical features of these diseases. Skeletal muscle regeneration relies on muscle progenitor cells, known as satellite cells, which are activated after muscle damage, and which proliferate and differentiate to muscle cells, thus regenerating the damaged tissue. Satellite cell dysfunction seems to be a common feature of both age-dependent muscle degeneration (sarcopenia) and muscle wasting in DM and other muscle degenerative diseases. This review aims to describe the cellular, molecular and macrostructural processes involved in the muscular degeneration seen in DM patients, highlighting the similarities found with muscle aging.

Muscular myopathies other than myotonic dystrophy also associated with (CTG)n expansion at the DMPK locus

Objective:

Assess triplet repeat expansion (CTG)_n at the ‘dystrophia-myotonica protein kinase’ (DMPK) locus in muscular myopathies to elucidate its role in myopathic symptoms and enable genetic counseling and prenatal diagnosis in families.

Methods and Results:

Individuals with symptoms of myopathy, hypotonia and controls selected randomly from the population were evaluated for triplet repeat expansion of (CTG)_n repeats in the 3’untranslated region (UTR) of DMPK gene, the causative mutation in myotonic dystrophy (DM). DNA was isolated from peripheral blood of 40 individuals; they presented symptoms of muscle myopathy (n = 11), muscle hypotonia (n = 4), members of their families (n = 5) and control individuals from random population (n = 20). Molecular analysis of genomic DNA by polymerase chain reaction (PCR) using primers specific for the DMPK gene encompassing the triplet repeat expansion, showed that all controls (n = 20) gave a 2.1 kb band indicating normal triplet repeat number. Three out of 11 cases (two clinically diagnosed DM and one muscular dystrophy) had an expansion of the (CTG)_n repeat in the range of 1000-2100 repeats corresponding to the repeat number in cases of severe DM. Other two of these 11 cases, showed a mild expansion of ~ 66 repeats. Three samples, which included two cases of hypotonia and the father of a subject with muscular dystrophy, also gave a similar repeat expansion (~66 repeats).

Conclusion:

Results suggest a role of (CTG)_n expansion at the DMPK locus in unexplained hypotonias and muscular myopathies other than DM. This calls for screening of the triplet repeat expansion at the DMPK locus in cases of idiopathic myopathies and hypotonia.

Molecular Effects of the CTG Repeats in Mutant Dystrophia Myotonica Protein Kinase Gene

Myotonic Dystrophy type 1 (DM1) is a multi-system disorder characterized by muscle wasting, myotonia, cardiac conduction defects, cataracts, and neuropsychological dysfunction. DM1 is caused by expansion of a CTG repeat in the 3´untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene. A body of work demonstrates that DMPK mRNAs containing abnormally expanded CUG repeats are toxic to several cell types. A core mechanism underlying symptoms of DM1 is that mutant DMPK RNA interferes with the developmentally regulated alternative splicing of defined pre-mRNAs. Expanded CUG repeats fold into ds(CUG) hairpins that sequester nuclear proteins including human Muscleblind-like (MBNL) and hnRNP H alternative splicing factors. DM1 cells activate CELF family member CUG-BP1 protein through hyperphosphorylation and stabilization in the cell nucleus. CUG-BP1 and MBNL1 proteins act antagonistically in exon selection in several pre-mRNA transcripts, thus MBNL1 sequestration and increase in nuclear activity of CUG-BP1 both act synergistically to missplice defined transcripts. Mutant DMPK-mediated effect on subcellular localization, and defective phosphorylation of cytoplasmic CUG-BP1, have additionally been linked to defective translation of p21 and MEF2A in DM1, possibly explaining delayed differentiation of DM1 muscle cells. Mutant DMPK transcripts bind and sequester transcription factors such as Specificity protein 1 leading to reduced transcription of selected genes. Recently, transcripts containing long hairpin structures of CUG repeats have been shown to be a Dicer ribonuclease target and Dicer-induced downregulation of the mutant DMPK transcripts triggers silencing effects on RNAs containing long complementary repeats. In summary, mutant DMPK transcripts alter gene transcription, alternative splicing, and translation of specific gene transcripts, and have the ability to trigger gene-specific silencing effects in DM1 cells. Therapies aimed at reversing these gene expression alterations should prove effective ways to treat DM1.

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

Repeat expansion diseases: when a good RNA turns bad

An increasing number of dominantly inherited diseases have now been linked with expansion of short repeats within specific genes. Although some of these expansions affect protein function or result in haploinsufficiency, a significant portion cause pathogenesis through production of toxic RNA molecules that alter cellular metabolism. In this review, we examine the criteria that influence toxicity of these mutant RNAs and discuss new developments in therapeutic approaches.

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.

Clinical and molecular aspects of the myotonic dystrophies: a review

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

Myotonia and muscle contractile properties in mice with SIX5 deficiency

Myotonic dystrophy (DM1) is an autosomal-dominant multisystem disease characterized by progressive skeletal muscle weakness, myotonia, cataracts, cardiac arrhythmias, mild mental retardation, and endocrinopathies. Heterozygous loss of SIX5 in mice causes cataracts and cardiac conduction disease, and homozygous loss also leads to sterility and decreased testicular mass, reminiscent of DM1 in humans. The effect of SIX5 deficiency in muscle is unknown. In this study, we found that muscle contractile properties, electromyographic insertional activity, and muscle histology were normal in SIX5 deficient mice. The implications of these findings for the pathogenesis of DM1 are discussed.

RNA pathogenesis of the myotonic dystrophies

Myotonic dystrophy (dystrophia myotonica, DM) is the most common form of muscular dystrophy in adults. The presence of two genetic forms of this complex multisystemic disease (DM1 and DM2) was unrecognized until the genetic cause of DM1 was identified in 1992. The fact that the DM1 mutation is an untranslated CTG expansion led to extended controversy about the molecular pathophysiology of this disease. When the DM2 mutation was identified in 2001 as being a similarly untranslated CCTG expansion, the molecular and clinical parallels between DM1 and DM2 substantiated the role of a novel mechanism in generating the unusual constellation of clinical features seen in these diseases: the repeat expansions expressed at the RNA level alter RNA processing, at least in part by interfering with alternative splicing of other genes. For example, in both DM1 and DM2, altered splicing of chloride channel and insulin receptor transcripts leads to myotonia and insulin resistance, respectively. Although other mechanisms may underlie the differences between DM1 and DM2, the pathogenic effects of the RNA mechanism are now clear, which will facilitate development of appropriate treatments.

UNC-39, the C. elegans homolog of the human myotonic dystrophy-associated homeodomain protein Six5, regulates cell motility and differentiation

Mutations in the unc-39 gene of C. elegans lead to migration and differentiation defects in a subset of mesodermal and ectodermal cells, including muscles and neurons. Defects include mesodermal specification and differentiation as well a neuronal migration and axon pathfinding defects. Molecular analysis revealed that unc-39 corresponds to the previously named gene ceh-35 and that the UNC-39 protein belongs to the Six4/5 family of homeodomain transcription factors and is similar to human Six5, a protein implicated in the pathogenesis of type I myotonic dystrophy (DM1). We show that human Six5 and UNC-39 are functional homologs, suggesting that further characterization of the C. elegans unc-39 gene might provide insight into the etiology of DM1.

Anticipation and CAG*CTG repeat expansion in schizophrenia and bipolar affective disorder

The genetic contribution to the etiologies of schizophrenia and bipolar affective disorder (BPAD) has been considered for many decades, with twin, family, and adoption studies indicating consistently that the familial clustering of affected individuals is accounted for mainly by genetic factors. Despite the strong evidence for a genetic component, very little is understood about the underlying genetic and molecular mechanisms for schizophrenia and BPAD. In the early 1990s, after the discovery of "dynamic mutation" or "unstable DNA" as a molecular basis for the genetic anticipation observed in Huntington's disease, myotonic dystrophy, and many others, and the recently rediscovered, albeit still controversial, evidence for genetic anticipation in major psychoses, the genetic epidemiology of schizophrenia and BPAD was re-evaluated to demonstrate strong endorsement for the unstable DNA model. Many of the non-Mendelian genetic features of schizophrenia and BPAD could be explained by the behaviour of unstable DNA, and several molecular genetic approaches became available for testing the unstable DNA hypothesis. However, despite promising findings in the mid-1990s, no trinucleotide repeat expansion has yet been identified as a cause of idiopathic schizophrenia or BPAD.

The DMWD protein from the myotonic dystrophy (DM1) gene region is developmentally regulated and is present most prominently in synapse-dense brain areas

The DMWD gene is located in the myotonic dystrophy (DM1) gene cluster on 19q, just upstream of the DMPK gene. RNA and protein products of this gene are ubiquitously expressed in all adult tissues, but occur most abundant in testes and brain. Altered expression of DMWD mRNA in DM1 patients has been observed, suggesting a role of the DMWD gene products in disease manifestation. Here we focussed on DMWD expression in mouse brain and followed mRNA and protein levels and (intra)cellular location in developing brain in vivo as well as in differentiating neuronal cell cultures in vitro. In the interval between postnatal days P7 and P21, the steady-state level of DMWD mRNA remained constant, whereas the DMWD protein (doublet of 70 kDa) level gradually increased during the same period. The DMWD protein was expressed throughout the brain, at a low level in glial cells, more prominently in neurons and specifically in the neuropil of brain areas with a high density of synaptic connections. Intracellularly, DMWD was dispersed in a punctuate fashion throughout the neural cell body, the nucleus and the dendrites with their synapses, but was excluded from axons. Based on these findings and on new literature data concerning the role of DMWD homologs in lower eukaryotes, we discuss the possible role of DMWD in the brain-related symptoms seen in DM1 patients.

Hammerhead ribozyme-mediated destruction of nuclear foci in myotonic dystrophy myoblasts

Myotonic dystrophy type 1 (DM1) is caused by an unstable CTG expansion in the 3' untranslated region (3'UTR) of the myotonic dystrophy protein kinase gene (DMPK). Transcripts from this altered gene harbor large CUG expansions that are retained in the nucleus of DM1 cells and form foci. It is believed that the formation of these foci is closely linked to DM1 muscle pathogenesis. Here we investigated the possibility of using a nuclear-retained hammerhead ribozyme expressed from a modified tRNAmeti promoter to target and cleave mutant transcripts of DMPK. Accessible ribozyme target sites were identified in the 3'UTR of the DMPK mRNA and a hammerhead ribozyme was designed to cut the most accessible site. Utilizing this system, we have achieved 50 and 63% reductions, respectively, of the normal and CUG expanded repeat-containing transcripts. We also observed a significant reduction in the number of DMPK mRNA-containing nuclear foci in human DM1 myoblasts. Reduction of mutant DMPK mRNA and nuclear foci also corroborates with partial restoration of insulin receptor isoform B expression in DM1 myoblasts. These studies demonstrate for the first time intracellular ribozyme-mediated cleavage of nuclear-retained mutant DMPK mRNAs, providing a potential gene therapy agent for the treatment of myotonic dystrophy.

Allelic alterations at the STR markers in the buccal tissue cells of oral cancer patients and the oral epithelial cells of healthy betel quid-chewers: an evaluation of forensic applicability

Although cancerous specimens are usually not used in forensic DNA typing, they might be forcibly employed under certain instances. On the other hand, though the oral epithelial samples have been applied to forensic identification, the great popularity of betel quid (BQ)-chewing in Taiwan, which is known to be a risk factor leading to an oral cancer, makes this application questionable. The DNA stability of nine short tandem repeat (STR) markers (the AmpFlSTR kit) was first investigated and then used to evaluate the forensic appropriateness of the oral samples of both healthy BQ-chewers and the archived clinical specimens from oral cancer patients. The analyses were performed on buccal samples from 100 BQ-chewers and 100 oral cancer patients, as well as their paired peripheral blood samples, and a group of 100 non-BQ-chewers were used for the control. In the group of 100 oral cancer patients, two types of DNA instability were found. They were major allelic imbalance, and allelic alterations including the expansion, the contraction and the un-classified type (i.e. can not be confirmed as the expansion or the contraction). The overall percentage of the cancerous subjects demonstrating DNA instability was 33% (five patients possessing both types of DNA instability). Both types of DNA instability showed a tendency of increasing with the severity of the pathological stage of oral cancer. Forty-four occurrences of major allelic imbalance were found from 21 cancer patients. The statistical result revealed that there was no significant difference in the allelic imbalanced occurrence among the nine STR loci. Allelic alterations were found in 17 patients, within which 12 individuals had the expansion, five had the contraction, and three were the un-classified type. Further, among these 17 patients, three were found to acquire multiple allelic alterations at multiple loci. In the group of 100 unrelated healthy BQ-chewers, two loci with major allelic imbalance were detected. However, the two imbalanced alleles were virtually half lost, and could still be recognized as heterozygous alleles. The statistical results of ANOVA, chi(2), and Scheffe tests indicated that the means of allelic imbalance at the nine STR loci of the oral cancerous group revealed a significant difference from those in the control group. Our results suggest that oral cancer tissues cannot be used as references for forensic purposes using the PCR-based STR systems, whereas the oral swabs from healthy BQ-chewers can be employed, but should be done with caution.

Dominantly inherited, non-coding microsatellite expansion disorders

Dominantly inherited diseases are generally caused by mutations resulting in gain of function protein alterations. However, a CTG expansion located in the 3' untranslated portion of a kinase gene was found to cause myotonic dystrophy type 1, a multisystemic dominantly inherited disorder. The recent discovery that an untranslated CCTG expansion causes the same constellation of clinical features in myotonic dystrophy type 2 (DM2), along with other recent discoveries on DM1 pathogenesis, have led to the understanding that both DM1 and DM2 mutations are pathogenic at the RNA level. These findings indicate the existence of a new category of disease wherein repeat expansions in RNA alter cellular function. Pathogenic repeat expansions in RNA may also be involved in spinocerebellar ataxia types 8, 10 and 12, and Huntington's disease-like type 2.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

Coenzyme Q10, exercise lactate and CTG trinucleotide expansion in myotonic dystrophy

Steinert's myotonic dystrophy (DM) is a genetic autosomal dominant disease and the most frequent muscular dystrophy in adulthood. Although causative mutation is recognized as a CTG trinucleotide expansion on 19q13.3, pathogenic mechanisms of multisystem involvement of DM are still under debate. It has been suggested that mitochondrial abnormalities can occur in this disease and deficiency of coenzyme Q 10 (CoQ10) has been considered one possible cause for this. The aim of this investigation was to evaluate, in 35 DM patients, CoQ10 blood levels and relate them to the degree of CTG expansion as well as to the amount of lactate production in exercising muscle as indicator of mitochondrial dysfunction. CoQ10 concentrations appeared significantly reduced with respect to normal controls: 0.85 +/- 0.25 vs. 1.58 +/- 0.28 microg/ml (p < 0.05). Mean values of blood lactate were significantly higher in DM patients than controls (p < 0.05) both in resting conditions (2.9 +/- 0.55 vs. 1.44 +/- 1.11 mmol/L) and at the exercise peak (6.77 +/- 1.79 vs. 4.90 +/- 0.59 mmol/L), while exercise lactate threshold was anticipated (30-50% vs. 60-70% of the predicted normal maximal power output, p < 0.05). Statistical analysis showed that serum CoQ10 levels were significantly (p < 0.05) inversely correlated with both CTG expansion degree and lactate values at exercise lactate threshold level. Our data indicates the occurrence of reduced CoQ10 levels in DM, possibly related to disease pathogenic mechanisms associated with abnormal CTG trinucleotide amplification.

Myotonic dystrophy--a multigene disorder

Myotonic dystrophy (DM1) is the most common form of adult muscular dystrophy with an estimated incidence of 1/8000 births. The mutation responsible for this condition is an expanded CTG repeat within the 3' untranslated region of the protein kinase gene DMPK. Strong nucleosome positioning signals created by this expanded repeat cause a reduction in gene expression within the region. This "field effect" is further confounded by the retention of DMPK expansion containing transcripts, which acquire a toxic gain of function. Thus, the various manifestations exhibited by DM1 patients can be explained as a result of gene silencing, nuclear retention and sequestration of nuclear factors by the CUG containing transcript.

Decreased DMPK transcript levels in myotonic dystrophy 1 type IIA muscle fibers

Myotonic dystrophy 1 is caused by the expansion of a CTG trinucleotide repeat on chromosome 19q13.3. The repeat lies in the 3' untranslated region of the myotonic dystrophy protein kinase gene (DMPK), and it has been hypothesised that the expansion alters the expression levels of DMPK and/or its neighbouring genes, DMWD and SIX5. Published data remain controversial, partly due to the mixed cell populations found in most tissues examined. We have microdissected human skeletal muscle biopsies from myotonic dystrophy 1 patients and controls and analysed gene expression at this locus for type I and type IIA fibres, using quantitative real-time reverse transcription-polymerase chain reaction. Levels of DMPK expression were specifically decreased in the type IIA fibres of myotonic dystrophy patients, below the levels found in controls. This suggests that DMPK expression is altered in this disease, suggesting significant pathological consequences.

Effect of triplet repeat expansion on chromatin structure and expression of DMPK and neighboring genes, SIX5 and DMWD, in myotonic dystrophy

Myotonic dystrophy (DM), an autosomal dominant neuromuscular disease, is associated with expansion of a polymorphic (CTG)n repeat in the 3'-untranslated region of the DM protein kinase (DMPK) gene. The repeat expansion results in decreased levels of DMPK mRNA and protein, but the mechanism for this decreased expression is unknown. Loss of a nuclease-hypersensitive site in the region of the repeat expansion has been observed in muscle and skin fibroblasts from DM patients, indicating a change in local chromatin structure. This change in chromatin structure has been proposed as a mechanism whereby the expression of DMPK and neighboring genes, sine oculis homeobox (Drosophila) homolog 5 (SIX5) and dystrophia myotonica-containing WD repeat motif (DMWD), might be affected. We have developed a polymerase chain reaction (PCR)-based method to assay the chromatin sensitivity of the region adjacent to the repeat expansion in somatic cell hybrids carrying either normal or affected DMPK alleles and show that hybrids carrying expanded alleles exhibit decreased sensitivity to PvuII digestion in this region. Semiquantitative multiplex reverse transcriptase PCR (RT/PCR) assays of gene expression from the chromosomes carrying the expanded alleles showed marked reduction of DMPK mRNA, partial inhibition of SIX5 expression from a congenital DM chromosome, and no reduction of DMWD mRNA. Nested RT/PCR analysis of DMPK mRNA from somatic cell hybrids carrying the repeat expansions revealed that most of the DMPK transcripts expressed from the expanded alleles lacked exons 13 and 14, whereas full-length transcripts were expressed predominantly from the normal alleles. These results suggest that the CTG repeat expansion leads to a decrease in DMPK mRNA levels by affecting splicing at the 3' end of the DMPK pre-mRNA transcript.

Myotonic dystrophy type 2 caused by a CCTG expansion in intron 1 of ZNF9

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, can be caused by a mutation on either chromosome 19q13 (DM1) or 3q21 (DM2/PROMM). DM1 is caused by a CTG expansion in the 3' untranslated region of the dystrophia myotonica-protein kinase gene (DMPK). Several mechanisms have been invoked to explain how this mutation, which does not alter the protein-coding portion of a gene, causes the specific constellation of clinical features characteristic of DM. We now report that DM2 is caused by a CCTG expansion (mean approximately 5000 repeats) located in intron 1 of the zinc finger protein 9 (ZNF9) gene. Parallels between these mutations indicate that microsatellite expansions in RNA can be pathogenic and cause the multisystemic features of DM1 and DM2.

Drosophila homolog of the myotonic dystrophy-associated gene, SIX5, is required for muscle and gonad development

SIX5 belongs to a family of highly conserved homeodomain transcription factors implicated in development and disease. The mammalian SIX5/SIX4 gene pair is likely to be involved in the development of mesodermal structures. Moreover, a variety of data have implicated human SIX5 dysfunction as a contributor to myotonic dystrophy type 1 (DM1), a condition characterized by a number of pathologies including muscle defects and testicular atrophy. However, this link remains controversial. Here, we investigate the Drosophila gene, D-Six4, which is the closest homolog to SIX5 of the three Drosophila Six family members. We show by mutant analysis that D-Six4 is required for the normal development of muscle and the mesodermal component of the gonad. Moreover, adult males with defective D-Six4 genes exhibit testicular reduction. We propose that D-Six4 directly or indirectly regulates genes involved in the cell recognition events required for myoblast fusion and the germline:soma interaction. While the exact phenotypic relationship between D-Six4 and SIX4/5 remains to be elucidated, the defects in D-Six4 mutant flies suggest that human SIX5 should be more strongly considered as being responsible for the muscle wasting and testicular atrophy phenotypes in DM1.

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.

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.

Reduced expression of DMAHP/SIX5 gene in myotonic dystrophy muscle

In myotonic dystrophy (DM), the expansion of CTG triplet repeats in the 3'-untranslated region of DM-protein kinase (DMPK) is a causal gene mutation. However, the pathogenic molecular mechanism of CTG repeat expansion for DM phenotypic expression is unclear. To investigate this issue, we examined the influence of CTG repeat expansion on the expression levels of DMPK gene and 3'-flanking DM locus-associated homeodomain protein (DMAHP)/SIX5 gene in the muscles of DM patients. We isolated RNA from muscle tissues of six DM patients and six controls, and performed a competitive reverse transcriptional polymerase chain reaction (RT-PCR) assay. The total mRNA level of DMAHP/SIX5 was significantly lower in DM than in controls, but the DMPK mRNA level was unchanged. Our results suggest that CTG repeat expansion influences the expression of genes other than DMPK to cause the DM phenotype.

Recruitment of human muscleblind proteins to (CUG)(n) expansions associated with myotonic dystrophy

Myotonic dystrophy (DM1) is an autosomal dominant neuromuscular disorder associated with a (CTG)(n) expansion in the 3'-untranslated region of the DM1 protein kinase (DMPK) gene. To explain disease pathogenesis, the RNA dominance model proposes that the DM1 mutation produces a gain-of-function at the RNA level in which CUG repeats form RNA hairpins that sequester nuclear factors required for proper muscle development and maintenance. Here, we identify the triplet repeat expansion (EXP) RNA-binding proteins as candidate sequestered factors. As predicted by the RNA dominance model, binding of the EXP proteins is specific for dsCUG RNAs and proportional to the size of the triplet repeat expansion. Remarkably, the EXP proteins are homologous to the Drosophila muscleblind proteins required for terminal differentiation of muscle and photoreceptor cells. EXP expression is also activated during mammalian myoblast differentiation, but the EXP proteins accumulate in nuclear foci in DM1 cells. We propose that DM1 disease is caused by aberrant recruitment of the EXP proteins to the DMPK transcript (CUG)(n) expansion.

Six family genes--structure and function as transcription factors and their roles in development

The members of the Six gene family were identified as homologues of Drosophila sine oculis which is essential for compound-eye formation. The Six proteins are characterized by the Six domain and the Six-type homeodomain, both of which are essential for specific DNA binding and for cooperative interactions with Eya proteins. Mammals possess six Six genes which can be subdivided into three subclasses, and mutations of Six genes have been identified in human genetic disorders. Characterization of Six genes from various animal phyla revealed the antiquity of this gene family and roles of its members in several different developmental contexts. Some members retain conserved roles as components of the Pax-Six-Eya-Dach regulatory network, which may have been established in the common ancestor of all bilaterians as a toolbox controlling cell proliferation and cell movement during embryogenesis. Gene duplications and cis-regulatory changes may have provided a basis for diverse functions of Six genes in different animal lineages.

Independent regulation of the myotonic dystrophy 1 locus genes postnatally and during adult skeletal muscle regeneration

Myotonic dystrophy is caused by a CTG(n) expansion in the 3'-untranslated region of a serine/threonine protein kinase gene (DMPK), which is flanked by two other genes, DMWD and SIX5. One hypothesis to explain the wide-ranging effects of this expansion is that, as the mutation expands, it alters the expression of one or more of these genes. The effects may vary in different tissues and developmental stages, but it has been difficult to develop these hypotheses as the normal postnatal developmental expression patterns of these genes have not been adequately investigated. We have developed accurate transcript quantification based on fluorescent real-time reverse transcription-polymerase chain reaction (TaqMan) to develop gene expression profiles during postnatal development in C57Bl/10 mice. Our results show extensive independent postnatal regulation of the myotonic dystrophy-locus genes in selected tissues and demonstrate which are the most highly expressed of the genes in each tissue. All three genes at the locus are expressed in the adult lens, questioning a previous model of cataractogenesis mediated solely by effects on Six5 expression. Additionally, using an in vivo model, we have shown that Dmpk levels decrease during the early stages of muscle regeneration. Our data provide a framework for investigation of tissue-specific pathological mechanisms in this disorder.