Myotonic dystrophy type 2: molecular, diagnostic and clinical spectrum

Clinical Characteristics and Analysis of CLCN1 in Patients with "EMG Disease"

Background and Purpose

While the etiology and clinical features of "EMG disease" - which is characterized by diffusely increased insertional activity on needle electromyography (EMG) in the absence of neuromuscular disease - are not well known, some authorities believe it may be a form of myotonia congenita (MC). The aims of this study were to determine the clinical features of EMG disease and its relationship with CLCN1 mutations in patients.

Methods

The detailed clinical and electrophysiological features of EMG disease were evaluated in six patients. All 23 coding exons and exon-intron boundaries in CLCN1 gene were analyzed by direct sequencing to detect nucleotide changes.

Results

The common clinical symptoms of EMG disease were chronic muscle stiffness or generalized myalgia, which were aggravated in a cold environment. Four patients complained of action myotonia several times a year. Short trains of provoked positive sharp waves were documented on needle EMG, but myotonic discharges, fibrillation potentials, and fasciculations were not. Increased insertional activity was identified at the asymptomatic muscles studied. One novel heterozygous mutation was identified in one patient following genetic testing for CLCN1 mutations (c.1679T>C, p.Met560Thr).

Conclusions

The clinical features of EMG disease might be quite similar to those of MC, but CLCN1 mutation was found in only one subject. It is thus difficult to accept that EMG disease lies within the phenotypic spectrum of MC. Additional testing is needed to verify the pathogenetic cause of the diffusely increased insertional activity associated with this condition.

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

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

Deregulated microRNAs in myotonic dystrophy type 2

Myotonic Dystrophy Type-2 (DM2) is an autosomal dominant disease caused by the expansion of a CCTG tetraplet repeat. It is a multisystemic disorder, affecting skeletal muscles, the heart, the eye, the central nervous system and the endocrine system. Since microRNA (miRNA) expression is disrupted in Myotonic Dystrophy Type-1 and many other myopathies, miRNAs deregulation was studied in skeletal muscle biopsies of 13 DM2 patients and 13 controls. Eleven miRNAs were deregulated: 9 displayed higher levels compared to controls (miR-34a-5p, miR-34b-3p, miR-34c-5p, miR-146b-5p, miR-208a, miR-221-3p and miR-381), while 4 were decreased (miR-125b-5p, miR-193a-3p, miR-193b-3p and miR-378a-3p). To explore the relevance of DM2 miRNA deregulation, the predicted interactions between miRNA and mRNA were investigated. Global gene expression was analyzed in DM2 and controls and bioinformatic analysis identified more than 1,000 miRNA/mRNA interactions. Pathway and function analysis highlighted the involvement of the miRNA-deregulated mRNAs in multiple aspects of DM2 pathophysiology. In conclusion, the observed miRNA dysregulations may contribute to DM2 pathogenetic mechanisms.

The unstable CCTG repeat responsible for myotonic dystrophy type 2 originates from an AluSx element insertion into an early primate genome

Myotonic dystrophy type 2 (DM2) is a subtype of the myotonic dystrophies, caused by expansion of a tetranucleotide CCTG repeat in intron 1 of the zinc finger protein 9 (ZNF9) gene. The expansions are extremely unstable and variable, ranging from 75–11,000 CCTG repeats. This unprecedented repeat size and somatic heterogeneity make molecular diagnosis of DM2 difficult, and yield variable clinical phenotypes. To better understand the mutational origin and instability of the ZNF9 CCTG repeat, we analyzed the repeat configuration and flanking regions in 26 primate species. The 3′-end of an AluSx element, flanked by target site duplications (5′-ACTRCCAR-3′or 5′-ACTRCCARTTA-3′), followed the CCTG repeat, suggesting that the repeat was originally derived from the Alu element insertion. In addition, our results revealed lineage-specific repetitive motifs: pyrimidine (CT)-rich repeat motifs in New World monkeys, dinucleotide (TG) repeat motifs in Old World monkeys and gibbons, and dinucleotide (TG) and tetranucleotide (TCTG and/or CCTG) repeat motifs in great apes and humans. Moreover, these di- and tetra-nucleotide repeat motifs arose from the poly (A) tail of the AluSx element, and evolved into unstable CCTG repeats during primate evolution. Alu elements are known to be the source of microsatellite repeats responsible for two other repeat expansion disorders: Friedreich ataxia and spinocerebellar ataxia type 10. Taken together, these findings raise questions as to the mechanism(s) by which Alu-mediated repeats developed into the large, extremely unstable expansions common to these three disorders.

Nuclear ribonucleoprotein-containing foci increase in size in non-dividing cells from patients with myotonic dystrophy type 2

Myotonic dystrophies (DM) are genetically based neuromuscular disorders characterized by the accumulation of mutant transcripts into peculiar intranuclear foci, where different splicing factors (among which the alternative splicing regulator muscleblind-like 1 protein, MBNL1) are ectopically sequestered. The aim of the present investigation was to describe the dynamics of the DM-specific intranuclear foci in interphase nuclei and during mitosis, as well as after the exit from the cell cycle. Primary cultures of skin fibroblasts from DM2 patients were used, as a model system to reproduce in vitro, as accurately as possible, the in vivo conditions. Cycling and resting fibroblasts were investigated by immunocytochemical and morphometric techniques, and the relative amounts of MBNL1 were also estimated by western blotting. MBNL1-containing foci were exclusively found in the nucleus during most of the interphase, while being observed in the cytoplasm during mitosis when they never associate with the chromosomes; the foci remained in the cytoplasm at cytodieresis, and underwent disassembly in early G1 to be reformed in the nucleus at each cell cycle. After fibroblasts had stopped dividing in late-passage cultures, the nuclear foci were observed to progressively increase in size. Interestingly, measurements on muscle biopsies taken from the same DM2 patients at different ages demonstrated that, in the nuclei of myofibers, the MBNL1-containing foci become larger with increasing patient's age. As a whole, these results suggest that in non-dividing cells of DM2 patients the sequestration in the nuclear foci of factors needed for RNA processing would be continuous and progressive, eventually leading to the onset (and the worsening with time) of the pathological traits. This is consistent with the evidence that in DM patients the most affected organs or tissues are those where non-renewing cells are mainly present, i.e., the central nervous system, heart and skeletal muscle.

Cerebral and muscle MRI abnormalities in myotonic dystrophy

Pathophysiological mechanisms underlying the clinically devastating CNS features of myotonic dystrophy (DM) remain more enigmatic and controversial than do the muscle abnormalities of this common form of muscular dystrophy. To better define CNS and cranial muscle changes in DM, we used quantitative volumetric and diffusion tensor MRI methods to measure cerebral and masticatory muscle differences between controls (n=5) and adults with either congenital (n=5) or adult onset (n=5) myotonic dystrophy type 1 and myotonic dystrophy type 2 (n=5). Muscle volumes were diminished in DM1 and strongly correlated with reduced white matter integrity and gray matter volume. Moreover, correlation of reduced fractional anisotropy (white matter integrity) and gray matter volume in both DM1 and DM2 suggests that these abnormalities may share a common underlying pathophysiological mechanism. Further quantitative temporal and spatial characterization of these features will help delineate developmental and progressive neurological components of DM, and help determine the causative molecular and cellular mechanisms.

Best practice guidelines and recommendations on the molecular diagnosis of myotonic dystrophy types 1 and 2

Myotonic dystrophy is an autosomal dominant, multisystem disorder that is characterized by myotonic myopathy. The symptoms and severity of myotonic dystrophy type l (DM1) ranges from severe and congenital forms, which frequently result in death because of respiratory deficiency, through to late-onset baldness and cataract. In adult patients, cardiac conduction abnormalities may occur and cause a shorter life span. In subsequent generations, the symptoms in DM1 may present at an earlier age and have a more severe course (anticipation). In myotonic dystrophy type 2 (DM2), no anticipation is described, but cardiac conduction abnormalities as in DM1 are observed and patients with DM2 additionally have muscle pain and stiffness. Both DM1 and DM2 are caused by unstable DNA repeats in untranslated regions of different genes: A (CTG)n repeat in the 3'-UTR of the DMPK gene and a (CCTG)n repeat in intron 1 of the CNBP (formerly ZNF9) gene, respectively. The length of the (CTG)n repeat expansion in DM1 correlates with disease severity and age of onset. Nevertheless, these repeat sizes have limited predictive values on individual bases. Because of the disease characteristics in DM1 and DM2, appropriate molecular testing and reporting is very important for the optimal counseling in myotonic dystrophy. Here, we describe best practice guidelines for clinical molecular genetic analysis and reporting in DM1 and DM2, including presymptomatic and prenatal testing.

If you build a rare disease registry, will they enroll and will they use it? Methods and data from the National Registry of Myotonic Dystrophy (DM) and Facioscapulohumeral Muscular Dystrophy (FSHD)

INTRODUCTION

Registries are becoming increasingly important for rare diseases as experimental therapies develop. This report describes the methodology behind the National Registry of Myotonic Dystrophy (DM) and Facioscapulohumeral Muscular Dystrophy (FSHD) Patients and Family Members to facilitate the development of other rare disease registries. We also highlight data about the pathophysiology and select burdens of DM and FSHD reported at baseline and longitudinally.

METHODS

The Registry consists of de-identified, patient reported information collected at baseline and annually and information from review of medical records. Investigators can use the Registry to analyze de-identified data and to facilitate recruitment into clinical studies.

RESULTS

To date, the Registry has enrolled 1611 members, facilitated 24 studies, and collected data annually for up to 8 years. Genetic test results were obtained in 56.2% of enrollees. Approximately one-third of members used assistive devices and another one-third reported psychological problems at baseline. Wheelchair use was reported for both short and long distances by 7.0% of DM and 18.1% of FSHD members. Approximately 60% of members reported their employment was affected by their disease.

CONCLUSIONS

Strengths of the Registry include large sample sizes, stringent review of clinical and molecular data, annually updated information, and regular interactions between patients and investigators. Registry data provide new insights into the burdens of DM and FSHD, such as, psychological problems and reduced employment. Opportunities abound for investigators to utilize Registry resources to assess the impact of these and other burdens on health care costs, progression of symptoms, and quality of life.

Laboratory abnormalities in patients with myotonic dystrophy type 2

BACKGROUND

Myotonic dystrophy type 2 (DM2) is a recently discovered adult muscular dystrophy. Similar to DM1, this disease causes progressive debilitating weakness, clinical myotonia, and early cataracts and is thought to cause widespread physiologic dysfunction of multiple organ systems.

OBJECTIVE

To analyze and compile the laboratory abnormalities of patients with DM2.

DESIGN

Baseline DM2 laboratory data were compiled representing 68 different types of laboratory tests and 1442 total studies.

SETTING

University medical center.

PATIENTS

Eighty-three adults with genetically confirmed or clinically probable DM2 were identified. Of these patients, 49 had documented baseline laboratory screening.

MAIN OUTCOME MEASURES

The individual frequencies of abnormal laboratory values in the population with DM2 studied.

RESULTS

Of the 1442 studies, results for 359 (24.9%) were outside of their standard reference ranges. Of the 68 types of laboratory tests studied, 43 had values from 15 or more different patients with DM2. The relative frequency of an abnormally elevated laboratory value was greater than 50% in several tests, including the levels of creatine kinase, total cholesterol, lactate dehydrogenase, and alanine aminotransferase. In addition, serum levels of IgG were low in 75% of all patients with DM2 tested, and absolute lymphocyte counts were low in 54% of all patients with DM2 tested.

CONCLUSIONS

There is a high frequency of laboratory abnormalities in patients with DM2. These abnormalities provide insight into the widespread pathologic manifestations of DM2 and may form a basis for clinical monitoring and disease screening.

Myotonic dystrophy protein kinase is critical for nuclear envelope integrity

Myotonic dystrophy 1 (DM1) is a multisystemic disease caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the gene coding for myotonic dystrophy protein kinase (DMPK). DMPK is a nuclear envelope (NE) protein that promotes myogenic gene expression in skeletal myoblasts. Muscular dystrophy research has revealed the NE to be a key determinant of nuclear structure, gene regulation, and muscle function. To investigate the role of DMPK in NE stability, we analyzed DMPK expression in epithelial and myoblast cells. We found that DMPK localizes to the NE and coimmunoprecipitates with Lamin-A/C. Overexpression of DMPK in HeLa cells or C2C12 myoblasts disrupts Lamin-A/C and Lamin-B1 localization and causes nuclear fragmentation. Depletion of DMPK also disrupts NE lamina, showing that DMPK is required for NE stability. Our data demonstrate for the first time that DMPK is a critical component of the NE. These novel findings suggest that reduced DMPK may contribute to NE instability, a common mechanism of skeletal muscle wasting in muscular dystrophies.

Cultured myoblasts from patients affected by myotonic dystrophy type 2 exhibit senescence-related features: ultrastructural evidence

Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by the expansion of the tetranucleotidic repeat (CCTG)n in the first intron of the Zinc Finger Protein-9 gene. In DM2 tissues, the expanded mutant transcripts accumulate in nuclear focal aggregates where splicing factors are sequestered, thus affecting mRNA processing. Interestingly, the ultrastructural alterations in the splicing machinery observed in the myonuclei of DM2 skeletal muscles are reminiscent of the nuclear changes occurring in age-related muscle atrophy. Here, we investigated in vitro structural and functional features of satellite cell-derived myoblasts from biceps brachii, in the attempt to investigate cell senescence indices in DM2 patients by ultrastructural cytochemistry. We observed that in satellite cell-derived DM2 myoblasts, cell-senescence alterations such as cytoplasmic vacuolization, reduction of the proteosynthetic apparatus, accumulation of heterochromatin and impairment of the pre-mRNA maturation pathways occur earlier than in myoblasts from healthy patients. These results, together with preliminary in vitro observations on the early onset of defective structural features in DM2 myoblast derived-myotubes, suggest that the regeneration capability of DM2 satellite cells may be impaired, thus contributing to the muscular dystrophy in DM2 patients.

Targeting RNA to treat neuromuscular disease

The development of effective therapies for neuromuscular disorders such as Duchenne muscular dystrophy (DMD) is hampered by considerable challenges: skeletal muscle is the most abundant tissue in the body, and many neuromuscular disorders are multisystemic conditions. However, despite these barriers there has recently been substantial progress in the search for novel treatments. In particular, the use of antisense oligonucleotides, which are designed to target RNA and modulate pre-mRNA splicing to restore functional protein isoforms or directly inhibit the toxic effects of pathogenic RNAs, offers great promise and these approaches are now being tested in the clinic. Here, we review recent advances in the development of such antisense oligonucleotides and other promising novel approaches, including the induction of readthrough nonsense mutations.

Selective inhibition of MBNL1-CCUG interaction by small molecules toward potential therapeutic agents for myotonic dystrophy type 2 (DM2)

Myotonic dystrophy type 2 (DM2) is an incurable neuromuscular disease caused by expanded CCUG repeats that may exhibit toxicity by sequestering the splicing regulator MBNL1. A series of triaminotriazine- and triaminopyrimidine-based small molecules (ligands 1–3) were designed, synthesized and tested as inhibitors of the MBNL1–CCUG interaction. Despite the structural similarities of the triaminotriazine and triaminopyrimidine units, the triaminopyrimidine-based ligands bind with low micromolar affinity to CCUG repeats (K_d ∼ 0.1–3.6 µM) whereas the triaminotriazine ligands do not bind CCUG repeats. Importantly, these simple and small triaminopyrimidine ligands exhibit both strong inhibition (K_i ∼ 2 µM) of the MBNL1–CCUG interaction and high selectivity for CCUG repeats over other RNA targets. These experiments suggest these compounds are potential lead agents for the treatment of DM2.

Molecular diagnosis of myopathies

Neuromuscular diseases (NMD) constitute a group of phenotypically and genetically heterogeneous disorders, characterized by (progressive) weakness and atrophy of proximal and/or distal muscles. The objective of molecular testing is to confirm the pathogenicity of a relevant sequence variation by correlating an individual's phenotype with what is expected in a given condition. Within the last two decades the application of molecular genetic strategies has led to a delineation of subgroups of clinically indistinguishable NMDs and has disclosed marked disease overlap. The expanding number of molecular defined NMDs requires new strategies to classify overlapping and clinical indistinguishable phenotypes.

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

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

Population frequency of myotonic dystrophy: higher than expected frequency of myotonic dystrophy type 2 (DM2) mutation in Finland

Myotonic dystrophy (DM) is the most common adult-onset muscular dystrophy with an estimated prevalence of 1/8000. There are two genetically distinct types, DM1 and DM2. DM2 is generally milder with more phenotypic variability than the classic DM1. Our previous data on co-segregation of heterozygous recessive CLCN1 mutations in DM2 patients indicated a higher than expected DM2 prevalence. The aim of this study was to determine the DM2 and DM1 frequency in the general population, and to explore whether the DM2 mutation functions as a modifier in other neuromuscular diseases (NMD) to account for unexplained phenotypic variability. We genotyped 5535 Finnish individuals: 4532 normal blood donors, 606 patients with various non-myotonic NMD, 221 tibial muscular dystrophy patients and their 176 healthy relatives for the DM2 and DM1 mutations. We also genotyped an Italian idiopathic non-myotonic proximal myopathy cohort (n = 93) for the DM2 mutation. In 5496 samples analyzed for DM2, we found three DM2 mutations and two premutations. In 5511 samples analyzed for DM1, we found two DM1 mutations and two premutations. In the Italian cohort, we identified one patient with a DM2 mutation. We conclude that the DM2 mutation frequency is significantly higher in the general population (1/1830; P-value = 0.0326) than previously estimated. The identification of DM2 mutations in NMD patients with clinical phenotypes not previously associated with DM2 is of particular interest and is in accord with the high overall prevalence. On the basis of our results, DM2 appears more frequent than DM1, with most DM2 patients currently undiagnosed with symptoms frequently occurring in the elderly population.

Dutch myotonic dystrophy type 2 patients and a North-African DM2 family carry the common European founder haplotype

Myotonic dystrophy type 2 (DM2) is a progressive multisystem disease with muscle weakness and myotonia as main characteristics. The disease is caused by a repeat expansion in the zinc-finger protein 9 (ZNF9) gene on chromosome 3q21. Several reports show that patients from European ancestry share an identical haplotype surrounding the ZNF9 gene. In this study, we investigated whether the Dutch DM2 population carries the same founder haplotype. In all, 40 Dutch DM2 patients from 16 families were genotyped for eight short tandem repeat markers surrounding the ZNF9 gene. In addition, the single-nucleotide polymorphism (SNP) rs1871922 located in the first intron of DM2 was genotyped. Results were compared with previously published haplotypes from unrelated Caucasian patients. The repeat lengths identified in this study were in agreement with existing literature. In 36 patients of our population, we identified three common haplotypes. One patient showed overlap with the common haplotype for only one marker closest to the ZNF9 gene. The haplotype from a family originating from Morocco showed overlap with that of the patients of European descent for a region of 222 kb. All patients carried at least one C allele of SNP rs1871922 indicating that all patients carry the European founder haplotype. We conclude that DM2 patients from the Netherlands, including a North-African family, harbor a common haplotype surrounding the ZNF9 gene. This data show that the Dutch patients carry the common founder haplotype and strongly suggest that DM2 mutations in Europe and North Africa originate from a single ancestral founder.

Repeat-primed polymerase chain reaction in myotonic dystrophy type 2 testing

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are the most common autosomal dominant neuromuscular disorders in adults. DM1 is caused by an unstable expansion of the (CTG)(n) repeat tract in the DMPK gene, whereas DM2 is caused by an unstable expansion of the (CCTG)(n) repeat tract in the ZNF9 gene. The (CCTG)(n) repeat is a part of a complex repetitive motif (TG)(n)(TCTG)(n)(CCTG)(n), in which each of the elements is highly polymorphic. Repeat-primed polymerase chain reaction (PCR) is a commonly used technique for the determination of the presence or absence of the expanded alleles in both DM1 and DM2. Besides the expansion detection, it can be used for the determination of the repeat structure (repeat number, presence of interruptions, and their localization) in healthy-range alleles. Because the (CCTG)(n) part of the motif in DM2 is generally interrupted with other sequences, "tetraplet" repeat-primed PCR (TP-PCR) results interpretation is more complicated than for DM1. Most of the studies, published so far, used TP-PCR in a direction such that they amplified through the (TG)(n)(TCTG)(n) part of the motif. We compared the features of TP-PCR performed in the commonly used direction with the results obtained by TP-PCR performed in the opposite direction. Our results suggest that the direction that does not include the (TG)(n)(TCTG)(n) tract leads to better quality and more informative results in comparison with the direction containing the (TG)(n)(TCTG)(n) tract.

Autosomal dominant late-onset quadriceps myopathy: three patients of a Taiwanese kindred

OBJECTIVE

Primary quadriceps weakness/atrophy is a rare disorder with variable etiologies; therefore, this disorder has been regarded as a clinical syndrome rather than a distinct entity. However, three affected patients of a Taiwanese family demonstrate a uniform pattern of quadriceps weakness and atrophy, their clinical manifestations and pattern of inheritance may suggest a new disease entity.

PATIENTS AND METHODS

Three patients in a Taiwanese kindred with selective quadriceps weakness and atrophy, which began after age 40 years, were examined. To disclose the confines of this disorder, muscle CT scans, electromyography, nerve conduction studies and muscle biopsies were performed; and to unravel and better understand the nature of this disorder, histopathological, ultrastructural, immunocytochemical and genetic studies were carried out.

RESULTS

In two patients with long-standing disease, muscle imaging showed marked atrophy and fat replacement of the anterior thigh muscles and electromyography showed a mixture of myopathic and neuropathic changes. Muscle histopathology on the mildly affected tibialis anterior showed myopathic changes with myofibrillar degeneration and secondary neurogenic alterations. Immunocytochemical staining was not diagnostic but excluded the dystrophinopathies and other well-known muscular dystrophies.

CONCLUSION

All previously identified diseases resulting in quadriceps weakness and atrophy have been ruled out and the present disorder appears to be a new disease entity of autosomal dominant late onset quadriceps myopathy.

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.

The expanding world of myotonic dystrophies: how can they be detected?

Myotonic dystrophy (DM) comprises at least two genetically distinct forms, both of which are caused by expansions of microsatellite repeats. The expansion of a CTG repeat in the DMPK gene leads to the first genetic form (DM type 1), and the expansion of a CCTG repeat in the ZNF9 gene causes the second genetic form of the disease (DM type 2). In both cases, the repeat units may expand to several thousand repeats, and the number of repeats in the expanded alleles shows a high degree of meiotic and somatic instability. The unprecedented size of expansions and their dynamic nature still represents a diagnostic challenge, which has been facilitated using different methods and modifications since the identification of the underlying mutations of these disorders. Here, we present an overview of the basic methods described for the purpose of identification of the DM type 1 and DM type 2 expansions and discuss particular modifications and improvements implemented to extend the detection ranges of these methods. Our review focuses on the advantages and disadvantages of the methods based on Southern blot analysis, polymerase chain reaction amplification, and in situ hybridization techniques and also on the possibilities of preimplantation and prenatal genetic testing.

Validation of sensitivity and specificity of tetraplet-primed PCR (TP-PCR) in the molecular diagnosis of myotonic dystrophy type 2 (DM2)

Myotonic dystrophy type 2 (DM2, OMIM #602688) is a multisystemic hereditary degenerative disease caused by a tetranucleotide CCTG expansion in the ZNF9 gene. Routine testing strategies for DM2 require the use of Southern blot or long-range PCR, but the presence of very large expansions and wide somatic mosaicism greatly reduce the sensitivity of these reference techniques. We therefore developed and validated a tetraplet-primed PCR (TP-PCR) method to detect the DM2 mutation by testing 87 DM2-positive and 76 DM2-negative previously characterized patients. The specificity of this technique was evaluated including DNA samples from 39 DM1-positive patients. We then attempted a prospective analysis of 50 patients with unknown genotype who referred to our center for diagnostic or presymptomatic tests. Results show that TP-PCR is a fast, reliable, and flexible technique, whose specificity and sensitivity is almost 100%, with no false positive or negative results either in retrospective and prospective applications. We therefore conclude that using this technique, in combination with the short-range PCR, is sufficient to correctly establish the presence or the absence of ZNF9 expanded alleles in the molecular diagnosis of DM2.

Absent, unrecognized, and minimal myotonic discharges in myotonic dystrophy type 2

The purpose of this study was to describe the frequency of absent, unrecognized, or minimal myotonic discharges (MDs) in myotonic dystrophy type 2 (DM2). We performed a retrospective review of needle electromyography (EMG) data prior to genetic diagnosis in 49 DM2 patients at the Mayo Clinic. MDs were not reported on first or repeat EMG studies (n = 8) and not found in archived recordings of 4 patients (8%); archived EMG recordings (n = 4) confirmed the absence of MDs (n = 2), including 1 patient with normal insertional activity in all muscles, and misinterpretation of MDs as slow fibrillation potentials (n = 1) and complex repetitive discharge (CRD) activity (n = 1). Eight (16%) patients had minimal classic MDs with diffusely increased insertional activity, including waning-only MDs in all patients in this group with archived EMG recordings (n = 5). Diffuse MDs were found in 33 (67%) patients. Absent or minimal MDs do not exclude DM2. Over-reliance on diffuse MDs in patients who present with myopathy may lead to delay in genetic diagnosis of DM2.

Analysis of MTMR1 expression and correlation with muscle pathological features in juvenile/adult onset myotonic dystrophy type 1 (DM1) and in myotonic dystrophy type 2 (DM2)

Among genes abnormally expressed in myotonic dystrophy type1 (DM1), the myotubularin-related 1 gene (MTMR1) was related to impaired muscle differentiation. Therefore, we analyzed MTMR1 expression in correlation with CUG-binding protein1 (CUG-BP1) and muscleblind-like1 protein (MBNL1) steady-state levels and with morphological features in muscle tissues from DM1 and myotonic dystrophy type 2 (DM2) patients. Semi-quantitative RT-PCR for MTMR1 was done on muscle biopsies and primary muscle cultures. The presence of impaired muscle fiber maturation was evaluated using immunochemistry for neural cell adhesion molecule (NCAM), Vimentin and neonatal myosin heavy chain. CUG-BP1 and MBNL1 steady-state levels were estimated by Western blot. RNA-fluorescence in situ hybridization combined with immunochemistry for CUG-BP1, MBNL1 and NCAM were performed on serial muscle sections. An aberrant splicing of MTMR1 and a significant amount of NCAM-positive myofibers were detected in DM1 and DM2 muscle biopsies; these alterations correlated with DNA repeat expansion size only in DM1. CUG-BP1 levels were increased only in DM1 muscles, while MBNL1 levels were similar among DM1, DM2 and controls. Normal and NCAM-positive myofibers displayed no differences either in the amount of ribonuclear foci and the intracellular distribution of MBNL1 and CUG-BP1. In conclusion, an aberrant MTMR1 expression and signs of altered myofiber maturation were documented in both DM1 and in DM2 muscle tissues. The more severe dysregulation of MTMR1 expression in DM1 versus DM2, along with increased CUG-BP1 levels only in DM1 tissues, suggests that the mutual antagonism between MBNL1 and CUG-BP1 on alternative splicing is more unbalanced in DM1.

Differences in aberrant expression and splicing of sarcomeric proteins in the myotonic dystrophies DM1 and DM2

Aberrant transcription and mRNA processing of multiple genes due to RNA-mediated toxic gain-of-function has been suggested to cause the complex phenotype in myotonic dystrophies type 1 and 2 (DM1 and DM2). However, the molecular basis of muscle weakness and wasting and the different pattern of muscle involvement in DM1 and DM2 are not well understood. We have analyzed the mRNA expression of genes encoding muscle-specific proteins and transcription factors by microarray profiling and studied selected genes for abnormal splicing. A subset of the abnormally regulated genes was further analyzed at the protein level. TNNT3 and LDB3 showed abnormal splicing with significant differences in proportions between DM2 and DM1. The differential abnormal splicing patterns for TNNT3 and LDB3 appeared more pronounced in DM2 relative to DM1 and are among the first molecular differences reported between the two diseases. In addition to these specific differences, the majority of the analyzed genes showed an overall increased expression at the mRNA level. In particular, there was a more global abnormality of all different myosin isoforms in both DM1 and DM2 with increased transcript levels and a differential pattern of protein expression. Atrophic fibers in DM2 patients expressed only the fast myosin isoform, while in DM1 patients they co-expressed fast and slow isoforms. However, there was no increase of total myosin protein levels, suggesting that aberrant protein translation and/or turnover may also be involved.

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.

ZNF9 activation of IRES-mediated translation of the human ODC mRNA is decreased in myotonic dystrophy type 2

Myotonic dystrophy types 1 and 2 (DM1 and DM2) are forms of muscular dystrophy that share similar clinical and molecular manifestations, such as myotonia, muscle weakness, cardiac anomalies, cataracts, and the presence of defined RNA-containing foci in muscle nuclei. DM2 is caused by an expansion of the tetranucleotide CCTG repeat within the first intron of ZNF9, although the mechanism by which the expanded nucleotide repeat causes the debilitating symptoms of DM2 is unclear. Conflicting studies have led to two models for the mechanisms leading to the problems associated with DM2. First, a gain-of-function disease model hypothesizes that the repeat expansions in the transcribed RNA do not directly affect ZNF9 function. Instead repeat-containing RNAs are thought to sequester proteins in the nucleus, causing misregulation of normal cellular processes. In the alternative model, the repeat expansions impair ZNF9 function and lead to a decrease in the level of translation. Here we examine the normal in vivo function of ZNF9. We report that ZNF9 associates with actively translating ribosomes and functions as an activator of cap-independent translation of the human ODC mRNA. This activity is mediated by direct binding of ZNF9 to the internal ribosome entry site sequence (IRES) within the 5′UTR of ODC mRNA. ZNF9 can activate IRES-mediated translation of ODC within primary human myoblasts, and this activity is reduced in myoblasts derived from a DM2 patient. These data identify ZNF9 as a regulator of cap-independent translation and indicate that ZNF9 activity may contribute mechanistically to the myotonic dystrophy type 2 phenotype.

Hypothesis: neoplasms in myotonic dystrophy

Tumorigenesis is a multi-step process due to an accumulation of genetic mutations in multiple genes in diverse pathways which ultimately lead to loss of control over cell growth. It is well known that inheritance of rare germline mutations in genes involved in tumorigenesis pathways confer high lifetime risk of neoplasia in affected individuals. Furthermore, a substantial number of multiple malformation syndromes include cancer susceptibility in their phenotype. Studies of the mechanisms underlying these inherited syndromes have added to the understanding of both normal development and the pathophysiology of carcinogenesis. Myotonic dystrophy (DM) represents a group of autosomal dominant, multisystemic diseases that share the clinical features of myotonia, muscle weakness, and early-onset cataracts. Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) result from unstable nucleotide repeat expansions in their respective genes. There have been multiple reports of tumors in individuals with DM, most commonly benign calcifying cutaneous tumors known as pilomatricomas. We provide a summary of the tumors reported in DM and a hypothesis for a possible mechanism of tumorigenesis. We hope to stimulate further study into the potential role of DM genes in tumorigenesis, and help define DM pathogenesis, and facilitate developing novel treatment modalities.

Scaled-down genetic analysis of myotonic dystrophy type 1 and type 2

Types 1 and 2 myotonic dystrophy are neuromuscular disorders caused by genomic expansions of simple sequence repeats. These mutations are unstable in somatic cells, which leads to an age-dependent increase of expansion length. Studies to determine whether changes in repeat size may influence disease severity are limited by the small amount of DNA that can be recovered from tissue biopsies samples. Here we used locked nucleic acid oligonucleotide probes and rolling circle amplification to determine length of the expanded repeat in sub-microgram quantities of genomic DNA. These methods can facilitate genetic analysis in cells and tissues obtained from individuals with myotonic dystrophy.