Proximal myotonic dystrophy--a family with autosomal dominant muscular dystrophy, cataracts, hearing loss and hypogonadism: heterogeneity of proximal myotonic syndromes?

Late-onset myopathies

PURPOSE OF REVIEW

Late-onset myopathies are defined as muscle diseases that begin after the age of 50 years. Some myopathies present classically in the elderly, whereas others may have a variable age of onset, including late-onset presentation. The purpose of this review is to summarize and comment on the most recent evidence regarding the main diagnosis of late-onset myopathies focusing on genetic causes.

RECENT FINDINGS

Although late-onset myopathies (LOM) are expected to be predominantly acquired myopathies, some common genetic myopathies, such as facioscapulohumeral muscular dystrophy (FSHD), can present late in life, usually with an atypical presentation. In addition, metabolic myopathies, which are classically early-onset diseases, are also diagnoses to be considered, particularly as they may be treatable. Late-onset multiple acyl-CoA dehydrogenase deficiency (MADD) has recently been identified as a cause of subacute LOM with a dramatic response to riboflavin supplementation.

SUMMARY

Inclusion body myositis is the most frequent of all LOM. Myotonic dystrophy type 2, FSHD and oculopharyngeal muscular dystrophy are the most frequent causes of genetic LOM. We summarize the major differential diagnoses and the clinical features on clinical examination that are suggestive of a genetic diagnosis to provide a diagnostic approach.

Neuropsychological Assessment Should Always be Considered in Myotonic Dystrophy Type 2

Myotonic dystrophies (DMs) are hereditary, multisystem, slowly progressive myopathies. One of the systems they affect is the CNS. In contrast to the well-established cognitive profile of myotonic dystrophy type 1 (DM1), only a few studies have investigated cognitive dysfunction in individuals with myotonic dystrophy type 2 (DM2), and their findings have been inconsistent. To identify the most commonly affected cognitive domains in individuals with DM2, we performed a formal comprehensive review of published DM2 studies. Using the terms "myotonic dystrophy type 2" AND "cognitive deficits," "cognitive," "cognition," "neuropsychological," "neurocognitive," and "neurobehavioral" in all fields, we conducted an advanced search on PubMed. We read and evaluated all of the available original research articles (13) and one case study, 14 in total, and included them in our review. Most of the research studies of DM2 reported primary cognitive deficits in executive functions (dysexecutive syndrome), memory (short-term nonverbal, verbal episodic memory), visuospatial/constructive-motor functions, and attention and processing speed; language was rarely reported to be affected. Based on the few neuroimaging and/or multimodal DM2 studies we could find, the cognitive profile of DM2 is associated with brain abnormalities in several secondary and high-order cortical and subcortical regions and associative white matter tracts. The limited sample size of individuals with DM2 was the most prominent limitation of these studies. The multifaceted profile of cognitive deficits found in individuals with DM2 highlights the need for routine neuropsychological assessment at both baseline and follow-up, which could unveil these individuals' cognitive strengths and deficits.

Myotonic dystrophy type 2: the 2020 update

The myotonic dystrophies are the commonest cause of adult-onset muscular dystrophy. Phenotypes of DM1 and DM2 are similar, but there are some important differences, including the presence or absence of congenital form, muscles primarily affected (distal vs proximal), involved muscle fiber types (type 1 vs type 2 fibers), and some associated multisystemic phenotypes. There is currently no cure for the myotonic dystrophies but effective management significantly reduces the morbidity and mortality of patients. For the enormous understanding of the molecular pathogenesis of myotonic dystrophy type 1 and myotonic dystrophy type 2, these diseases are now called "spliceopathies" and are mediated by a primary disorder of RNA rather than proteins. Despite clinical and genetic similarities, myotonic dystrophy type 1 and type 2 are distinct disorders requiring different diagnostic and management strategies. Gene therapy for myotonic dystrophy type 1 and myotonic dystrophy type 2 appears to be very close and the near future is an exciting time for clinicians and patients.

A Patient with Proximal Myotonic Myopathy and Parkinsonism

Introduction:

There are two case reports of patients who had proximal myotonic myopathy (PROMM) / myotonic dystrophy (DM) Type 1 and parkinsonism. The combination of myotonic myopathy and parkinsonism is so rare that it may appear to be just a coincidence. However, previous neuropathological examinations of patients who had myotonic dystrophy showed that there were intracytoplasmic inclusion bodies in the nigra and striatum, which raises the possibility that myotonic myopathy may be associated with parkinsonism. In this report we describe a patient with PROMM and a clinically definite parkinsonism to highlight this possibility.

Case Report:

A 65-year-old man developed proximal muscle weakness, myotonia and atrophy around the age of 55 and was diagnosed as having PROMM at the age of 62. Needle electromyography and muscle biopsy supported the diagnosis. A gene study of the DM Type 1 showed a normal CTG repeat length. At age 63, he developed rest tremor, bradykinesia, hypomimia, stooped posture, and gait disturbance. The postural instability worsened rapidly. The tremor and rigidity were much worse in his right side, where myotonia was more severe. Levodopa therapy was only partially effective.

Conclusion:

This is a case report of a patient with PROMM that shows an association with a rapidly progressive form of parkinsonism. We suggest that this may be a novel form of a neurodegenerative disorder, which we name ‘Parkinsonism- Myotonic Myopathy-Complex’.

Antisense oligonucleotides in neurological disorders

The introduction of genetics revolutionized the field of neurodegenerative and neuromuscular diseases and has provided considerable insight into the underlying pathomechanisms. Nevertheless, effective treatment options have been limited. This changed recently when antisense oligonucleotides (ASOs) could be translated from in vitro and experimental animal studies into clinical practice. In 2016, two ASOs were approved by the United States US Food and Drug Administration (FDA) and demonstrated remarkable efficacy in Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA). ASOs are synthetic single-stranded strings of nucleic acids. They selectively bind to specific premessenger ribonucleic acid (pre-mRNA)/mRNA sequences and alter protein synthesis by several mechanisms of action. Thus, apart from gene replacement, ASOs may therefore provide the most direct therapeutic strategy for influencing gene expression. In this review, we shall discuss basic mechanisms of ASO action, the role of chemical modifications needed to improve the pharmacodynamic and pharmacokinetic properties of ASOs, and we shall then focus on several ASOs developed for the treatment of neurodegenerative and neuromuscular disorders, including SMA, DMD, myotonic dystrophies, Huntington's disease, amyotrophic lateral sclerosis and Alzheimer's disease.

Hearing impairment in patients with myotonic dystrophy type 2

Objective

To systematically assess auditory characteristics of a large cohort of patients with genetically confirmed myotonic dystrophy type 2 (DM2).

Methods

Patients with DM2 were included prospectively in an international cross-sectional study. A structured interview about hearing symptoms was held. Thereafter, standardized otologic examination, pure tone audiometry (PTA; 0.25, 0.5, 1, 2, 4, and 8 kHz), speech audiometry, tympanometry, acoustic middle ear muscle reflexes, and brainstem auditory evoked potentials (BAEP) were performed. The ISO 7029 standard was used to compare the PTA results with established hearing thresholds of the general population according to sex and age.

Results

Thirty-one Dutch and 25 French patients with DM2 (61% female) were included with a mean age of 57 years (range 31–78). The median hearing threshold of the DM2 cohort was higher for all measured frequencies, compared to the 50th percentile of normal (p < 0.001). Hearing impairment was mild in 39%, moderate in 21%, and severe in 2% of patients with DM2. The absence of an air–bone gap with PTA, concordant results of speech audiometry with PTA, and normal findings of BAEP suggest that the sensorineural hearing impairment is located in the cochlea. A significant correlation was found between hearing impairment and age, even when corrected for presbycusis.

Conclusions

Cochlear sensorineural hearing impairment is a frequent symptom in patients with DM2, suggesting an early presbycusis. Therefore, we recommend informing about hearing impairment and readily performing audiometry when hearing impairment is suspected in order to propose early hearing rehabilitation with hearing aids when indicated.

The Dystrophic and Nondystrophic Myotonias

PURPOSE OF REVIEW

This article describes clinical and electrical myotonia and provides an update on the classification, diagnosis, and management of myotonic disorders.

RECENT FINDINGS

In the myotonic dystrophies, antisense oligonucleotides provide a general strategy to correct RNA gain of function and modulate the expression of CTG expanded repeats; they are currently being tested in a phase 1-2 randomized controlled trial in patients with adult-onset myotonic dystrophy type 1. New genetic mutations are continuously being identified in the nondystrophic myotonias involving sodium and chloride channels. This contributes to the difficulty in describing genotype-phenotype correlations as the same mutations can give rise to different phenotypes, and the same phenotypes can arise from different mutations. Pharmacologic therapy is moving toward mutation-targeted treatments.

SUMMARY

This article describes the clinical and diagnostic characteristics and management of the myotonic dystrophies and the nondystrophic myotonias. Clinical features of the congenital, juvenile, and classic adult forms of myotonic dystrophy type 1 are reviewed, and for the adult form, reference is made to the main diagnostic and follow-up tests for which general consensus exists. The different clinical presentations of myotonic dystrophy type 2 and its main differential diagnostic options are also discussed. The clinical spectrum of the sodium and chloride channelopathies is described, and clinical diagnostic clues to differentiate between these two groups are provided. Therapeutic options for patients with nondystrophic myotonias are also presented with reference to literature review and the author's personal experience.

Early onset posterior subscapular cataract in a series of myotonic dystrophy type 2 patients

PurposeEarly onset posterior subscapular cataract (<50 years of age) is a characteristic feature of myotonic dystrophy type 2 (DM2). Nevertheless, despite being operated at a young age, many patients remain undiagnosed for years. The purpose of this study was to assess the prevalence of early onset posterior subscapular cataract as a presenting symptom of the disease in a cohort of patients with DM2.Patients and methodsWe retrospectively reviewed medical records of DM2 patients followed in our institution for the presence of early onset posterior subscapular cataract, of any secondary causes of cataract, of the age of onset of muscle weakness and of final disease diagnosis.ResultsTwenty-eight patients were studied. Nine patients (32.1%) had presented early onset posterior subscapular cataract at a median age of 43 years (IQR=36-46) and seven (25%) reported it was the presenting sign. No patient was referred for neuromuscular evaluation due to the occurrence of early onset cataract. Median delay between cataract onset and referral for neuromuscular evaluation was 10 years (IQR=6.0-19.5) and final DM2 diagnosis was achieved after a median of 16 years (IQR=6.5-19.5).ConclusionThis study shows that early onset posterior subscapular cataract was the first symptom of the disease in 25% of our DM2 patients. Nevertheless, none was suspected of having cataract in the context of DM2, and referral for neuromuscular evaluation was made after a long delay and usually following the appearance of other symptoms. Ophthalmologists can be the first physicians encountering these patients and should have a low threshold for referring them for neuromuscular evaluation.

Assessing the influence of age and gender on the phenotype of myotonic dystrophy type 2

This study aims to provide a detailed clinical characterization of a large cohort of myotonic dystrophy type 2 (DM2) patients investigating the influence of age and gender as modifying factors of DM2 phenotype. A retrospective study was conducted on 307 patients with genetically confirmed DM2. The following data were analyzed: (1) demographics, (2) clinical features (first symptom, muscular complaints, and multisystemic involvement), (3) diagnostics (serological tests, electromyography, and muscle biopsy). In this cohort (186 females, 121 males), a proximal weakness was the leading symptom at onset (55.4%), followed by myalgia (35.5%) and myotonia (25.4%). Proximal weakness was more common in women than men (64.9 vs. 43.8%, p = 0.0006), whereas being male was associated with higher odds for developing myalgia [OR 2.94 (95% CI 1.53-5.67)]. Patients with muscle weakness at onset were older than those with myalgia and myotonia (p < 0.0001), while each additional disease year was associated with 10% decrease in the odds of developing myotonia [OR 0.9 (95% CI 0.87-0.93)] and 6% decrease of myalgia [OR 0.94 (95% CI 0.91-0.97)]. Cataract and thyroid diseases occurred more frequently in women (p = 0.002 and p = 0.002, respectively). Early onset of DM2 is an independent risk factor for the occurrence of multisystemic involvement [OR 0.94 (95% CI 0.90-0.98)]. In this updated clinical description of DM2 emerges a profound gender and age influence on the phenotype, emphasizing that female gender and ageing may be associated with a higher disease burden. These age- and gender-specific differences should be considered in diagnostics, management, and future clinical studies of DM2.

Biomolecular diagnosis of myotonic dystrophy type 2: a challenging approach

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are the most common adult form of muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia, and multiorgan involvement. The onset and symptoms of the myotonic dystrophies are diverse, complicating their diagnoses and limiting a comprehensive approach to their clinical care. Diagnostic delay in DM2 is due not only to the heterogeneous phenotype and the aspecific onset but also to the unfamiliarity with the disorder by most clinicians. Moreover, the DM2 diagnostic odyssey is complicated by the difficulties to develop an accurate, robust, and cost-effective method for a routine molecular assay. The aim of this review is to underline by challenging approach the diagnostic limits and pitfalls that could results in failure to recognize the presence of DM2 disease. Understanding and preventing delays in DM2 diagnosis may facilitate family planning, improve symptom management in the short term, and facilitate more specific treatment in the long term.

Marathoning with myotonic dystrophy type 2 (proximal myotonic myopathy) and leukopenia

Objectives:

A mild, slowly progressive course of proximal myotonic myopathy, also known as myotonic dystrophy type 2, over years allowing the patient to continue with extreme sport activity, has been only rarely reported.

Methods:

Case report.

Results:

The patient is a 54-year-old female sport teacher who developed myotonia of the distal upper limbs at the age of 32 years. Over the following 22 years, myotonia spreaded to the entire musculature. Myotonia did not prevent her from doing her job and from marathoning and improved with continuous exercise. Additionally, she had developed hypothyroidism, ovarial cysts, incipient cataract, motor neuropathy, hepatopathy, leukopenia, and mild hyper-CK-emia. A heterozygous CCTG-repeat expansion of 500–9500 was found in the CNBP/ZNF9 gene. At the age of 54 years, she was still performing sport, without presenting with myotonia on clinical examination or having developed other typical manifestations of proximal myotonic myopathy.

Conclusions:

This case shows that proximal myotonic myopathy may take a mild course over at least 22 years, that proximal myotonic myopathy with mild myotonia may allow a patient to continue strenuous sport activity, and that continuous physical activity may contribute to the mild course of the disease.

Myotonic dystrophy type 2 and modifier genes: an update on clinical and pathomolecular aspects

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia, and multiorgan involvement. To date, two distinct forms caused by similar mutations in two different genes have been identified: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (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 DM1 and DM2. However, despite clinical and genetic similarities, DM1 and DM2 may be considered as distinct disorders. This review is an update on the latest findings specific to DM2, including explanations for the differences in clinical manifestations and pathophysiology between the two forms of myotonic dystrophies.

Splicing misregulation of SCN5A contributes to cardiac-conduction delay and heart arrhythmia in myotonic dystrophy

Myotonic dystrophy (DM) is caused by the expression of mutant RNAs containing expanded CUG repeats that sequester muscleblind-like (MBNL) proteins, leading to alternative splicing changes. Cardiac alterations, characterized by conduction delays and arrhythmia, are the second most common cause of death in DM. Using RNA sequencing, here we identify novel splicing alterations in DM heart samples, including a switch from adult exon 6B towards fetal exon 6A in the cardiac sodium channel, SCN5A. We find that MBNL1 regulates alternative splicing of SCN5A mRNA and that the splicing variant of SCN5A produced in DM presents a reduced excitability compared with the control adult isoform. Importantly, reproducing splicing alteration of Scn5a in mice is sufficient to promote heart arrhythmia and cardiac-conduction delay, two predominant features of myotonic dystrophy. In conclusion, misregulation of the alternative splicing of SCN5A may contribute to a subset of the cardiac dysfunctions observed in myotonic dystrophy.

Patients with myotonic dystrophy (MD) suffer from severe cardiac issues of unknown aetiology. Freyermuth et al. show that fatal changes in cardiac electrophysiological properties in humans and mice with MD may arise from misregulation of the alternative splicing of the cardiac Na⁺ channel SCN5A transcript, resulting in expression of its fetal form.

Myotonic Dystrophy Type 2: An Update on Clinical Aspects, Genetic and Pathomolecular Mechanism

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in CNBP. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders. The pathogenesis of DM is explained by a common RNA gain-of-function mechanism in which the CUG and CCUG repeats alter cellular function, including alternative splicing of various genes. However additional pathogenic mechanism like changes in gene expression, modifier genes, protein translation and micro-RNA metabolism may also contribute to disease pathology and to clarify the phenotypic differences between these two types of myotonic dystrophies.This review is an update on the latest findings specific to DM2, including explanations for the differences in clinical manifestations and pathophysiology between the two forms of myotonic dystrophies.

Skin features in myotonic dystrophy type 1: an observational study

Poor data regarding skin involvement in Myotonic Dystrophy, also named Dystrophia Myotonica type 1, have been reported. This study aimed to investigate the prevalence and types of skin disorders in adult patients with Myotonic Dystrophy type 1. Fifty-five patients and one hundred age- and sex-matched healthy subjects were referred to a trained dermatologist for a complete skin examination to check for potential cutaneous hallmarks of disease. No difference in prevalence of preneoplastic, neoplastic, and cutaneous lesions was detected between the two groups. Among morphofunctional, proliferative and inflammatory lesions, focal hyperhidrosis (p < 0.0001), follicular hyperkeratosis (p = 0.0003), early androgenic alopecia (p = 0.01), nail pitting (p = 0.003), pedunculus fibromas (p = 0. 01), twisted hair (p = 0.01), seborrheic dermatitis (p = 0.02), macules of hyperpigmentation (p = 0.03) were significantly more frequent in patients compared with controls. In patients with Myotonic Dystrophy type 1 significant differences according to sex were found for: early androgenic alopecia, twisted hair and seborrheic dermatitis, whose prevalence was higher in males (p < 0.0001). Our preliminary results seem to rule out an increased prevalence of pre-neoplastic, and neoplastic skin lesions in Myotonic Dystrophy type 1. On the other hand, an increased prevalence of morphofunctional, inflammatory, and proliferative diseases involving adnexal structures seems to characterize adult patients with Myotonic Dystrophy type 1.

Progression of muscle histopathology but not of spliceopathy in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominant progressive disease involving skeletal and cardiac muscle and brain. It is caused by a tetranucleotide repeat within the first intron of the CNBP gene that leads to an alteration of the alternative splicing of several genes. To understand the molecular mechanisms that play a role in DM2 progression, the evolution of skeletal muscle histopathology and biomolecular findings in successive biopsies have been studied. Biceps brachii biopsies from 5 DM2 patients who underwent two successive biopsies at different years of age have been used. Muscle histopathology has been assessed on sections immunostained with fast or slow myosin. FISH in combination with MBNL1-immunofluorescence has been performed to evaluate ribonuclear inclusion and MBNL1 foci dimensions in myonuclei. Gene and protein expression and alteration of alternative splicing of several genes have been evaluated over time. All DM2 patients examined show a worsening of muscle histopathology and an increase of foci dimensions over time. The progressive worsening of myotonia in DM2 patients may be due to the decrease of CLCN1 mRNA observed in all patients examined. However, a worsening of alternative splicing alterations has not been evidenced over time. The data obtained in this study confirm that DM2 is a slow progression disease since histological and biomolecular alterations observed in skeletal muscle are minimal even after 10-year interval. The data indicate that muscle morphological alterations evolve more rapidly over time than the molecular changes thus indicating that muscle biopsy is a more sensitive tool than biomolecular markers to assess disease progression at muscle level.

Myotonic dystrophies: An update on clinical aspects, genetic, pathology, and molecular pathomechanisms

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. However, it is now clear that additional pathogenic mechanism like changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. This review is an update on the recent advances in the understanding of the molecular mechanisms behind myotonic dystrophies. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

Overexpression of CUGBP1 in skeletal muscle from adult classic myotonic dystrophy type 1 but not from myotonic dystrophy type 2

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are progressive multisystemic disorders caused by similar mutations at two different genetic loci. The common key feature of DM pathogenesis is nuclear accumulation of mutant RNA which causes aberrant alternative splicing of specific pre-mRNAs by altering the functions of two RNA binding proteins, MBNL1 and CUGBP1. However, DM1 and DM2 show disease-specific features that make them clearly separate diseases suggesting that other cellular and molecular pathways may be involved. In this study we have analysed the histopathological, and biomolecular features of skeletal muscle biopsies from DM1 and DM2 patients in relation to presenting phenotypes to better define the molecular pathogenesis. Particularly, the expression of CUGBP1 protein has been examined to clarify if this factor may act as modifier of disease-specific manifestations in DM. The results indicate that the splicing and muscle pathological alterations observed are related to the clinical phenotype both in DM1 and in DM2 and that CUGBP1 seems to play a role in classic DM1 but not in DM2. In conclusion, our results indicate that multisystemic disease spectrum of DM pathologies may not be explained only by spliceopathy thus confirming that the molecular pathomechanism of DM is more complex than that actually suggested.

Altered expression and splicing of Ca(2+) metabolism genes in myotonic dystrophies DM1 and DM2

AIMS

Myotonic dystrophy types 1 and 2 (DM1 and DM2) are multisystem disorders caused by similar repeat expansion mutations, with similar yet distinct clinical features. Aberrant splicing of multiple effector genes, as well as dysregulation of transcription and translation, has been suggested to underlie different aspects of the complex phenotypes in DM1 and DM2. Ca(2+) plays a central role in both muscle contraction and control of gene expression, and recent expression profiling studies have indicated major perturbations of the Ca(2+) signalling pathways in DM. Here we have further investigated the expression of genes and proteins involved in Ca(2+) metabolism in DM patients, including Ca(2+) channels and Ca(2+) binding proteins.

METHODS

We used patient muscle biopsies to analyse mRNA expression and splicing of genes by microarray expression profiling and RT-PCR. We studied protein expression by immunohistochemistry and immunoblotting.

RESULTS

Most of the genes studied showed mRNA up-regulation in expression profiling. When analysed by immunohistochemistry the Ca(2+) release channel ryanodine receptor was reduced in DM1 and DM2, as was calsequestrin 2, a sarcoplasmic reticulum lumen Ca(2+) storage protein. Abnormal splicing of ATP2A1 was more pronounced in DM2 than DM1.

CONCLUSIONS

We observed abnormal mRNA and protein expression in DM affecting several proteins involved in Ca(2+) metabolism, with some differences between DM1 and DM2. Our protein expression studies are suggestive of a post-transcriptional defect(s) in the myotonic dystrophies.

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.

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.

Mutant (CCTG)n expansion causes abnormal expression of zinc finger protein 9 (ZNF9) in myotonic dystrophy type 2

The mutation that underlies myotonic dystrophy type 2 (DM2) is a (CCTG)n expansion in intron 1 of zinc finger protein 9 (ZNF9). It has been suggested that ZNF9 is of no consequence for disease pathogenesis. We determined the expression levels of ZNF9 during muscle cell differentiation and in DM2 muscle by microarray profiling, real-time RT-PCR, splice variant analysis, immunofluorescence, and Western blotting. Our results show that in differentiating myoblasts, ZNF9 protein was localized primarily to the nucleus, whereas in mature muscle fibers, it was cytoplasmic and organized in sarcomeric striations at the Z-disk. In patients with DM2, ZNF9 was abnormally expressed. First, there was an overall reduction in both the mRNA and protein levels. Second, the subcellular localization of the ZNF9 protein was somewhat less cytoplasmic and more membrane-bound. Third, our splice variant analysis revealed retention of intron 3 in an aberrant isoform, and fourth quantitative allele-specific expression analysis showed the persistence of intron 1 sequences from the abnormal allele, further suggesting that the mutant allele is incompletely spliced. Thus, the decrease in total expression appears to be due to impaired splicing of the mutant transcript. Our data indicate that ZNF9 expression in DM2 patients is altered at multiple levels. Although toxic RNA effects likely explain overlapping phenotypic manifestations between DM1 and DM2, abnormal ZNF9 levels in DM2 may account for the differences 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.

Novel myosin heavy chain immunohistochemical double staining developed for the routine diagnostic separation of I, IIA and IIX fibers

The different histochemical ATPase properties of myosins separating the muscle fiber types have been utilized in diagnostic muscle biopsy routine for more than four decades. The ATPase staining method is rather laborious and has several disadvantages, such as weakening of staining over time and non-specific staining of capillaries, making the distinction of extremely atrophic muscle fibers difficult. We have developed a reliable and advanced immunohistochemical myosin double staining method for the identification of fiber types, including highly atrophic fibers in routine diagnostics. With this double staining method, we are able to distinguish among type I (ATPase type 1), IIA (ATPase type 2A), IIX (ATPase type 2B) and remodeled ATPase type 2C fibers expressing both fast and slow myosins using a one slide technique. Immunohistochemical double staining of myosin heavy chain isoforms can be used as an alternative for the conventional ATPase staining method in routine histopathology. The method provides even more detailed information of fast fiber subtypes and highly atrophic fibers on one single slide.

Brain involvement in myotonic dystrophies: neuroimaging and neuropsychological comparative study in DM1 and DM2

The objective of this study was to determine the degree of brain involvement in a cohort of myotonic dystrophy type 1 and type 2 (DM1, DM2) patients by brain studies and functional tests and to compare the results of the two groups. DM1, DM2 are multisystemic disorders due to polynucleotide expansions. Previous studies on brain involvement by neuroimaging and functional methods have led to contradictory results. Fifty molecularly defined DM1 patients and 14 DM2 patients, were recruited for the study. Age at recruitment, age at disease onset, disease duration and educational level were recorded. Neuromuscular assessment was done by MIRS. An extensive neuropsychological battery was performed in 48/50 DM1 and in a control group of 44 healthy matched subjects. Forty six of 50 DM1 and 12/14 DM2 underwent brain MRI; 21/50 DM1 and 9/14 DM2 underwent brain perfusion SPECT, with semiquantitative analysis of the results. MRI images were classified by ARWMC (age-related white matter changes) score, in order to quantify recurrence, localization and patterns of distribution of white matter hyperintense lesions (WMHLs) in our two cohorts. MRI results were matched to SPECT and to neuropsychological results. Thirty-seven of 46 DM1 and 10/12 DM2 had abnormal MRI imaging, showing scattered supratentorial, bilateral, symmetrical focal or diffuse WMHLs. A typical temporo-insular diffuse subcortical pattern was seen in DM1 subjects only, with no correlation with cognitive involvement. Major cognitive involvement was seen in the case of diffuse frontal lesions. A relationship with CTG expansion size was documented for DM1 subjects. SPECT showed minimal hypoperfusion in the posterior cortex planes in DM1 and, to a lesser extent, in DM2. Very mild degrees of involvement in the DM2 cohort were seen. Neuroimaging and functional investigations confirmed a more severe involvement of the brain in DM1 compared to DM2. A temporo-insular diffuse lesional pattern, specific for DM1, was found on MRI. This confirms greater expansion size as a risk factor for more extensive brain involvement in DM1.

Myotonic dystrophy type 2 found in two of sixty-three persons diagnosed as having fibromyalgia

Because of its high prevalence, fibromyalgia (FM) is a major general health issue. Myotonic dystrophy type 2 (DM2) is a recently described autosomal-dominant multisystem disorder. Besides variable proximal muscle weakness, myotonia, and precocious cataracts, muscle pain and stiffness are prominent presenting features of DM2. After noting that several of our mutation-positive DM2 patients had a previous diagnosis of FM, suggesting that DM2 may be misdiagnosed as FM, we invited 90 randomly selected patients diagnosed as having FM to undergo genetic testing for DM2. Of the 63 patients who agreed to participate, 2 (3.2%) tested positive for the DM2 mutation. Their cases are described herein. DM2 was not found in any of 200 asymptomatic controls. We therefore suggest that the presence of DM2 should be investigated in a large sample of subjects diagnosed as having FM, and clinicians should be aware of overlap in the clinical presentation of these 2 distinct disorders.

High frequency of co-segregating CLCN1 mutations among myotonic dystrophy type 2 patients from Finland and Germany

Based on previous reports the frequency of co-segregating recessive chloride channel (CLCN1) mutations in families with myotonic dystrophy type 2 (DM2) was suspected to be increased. We have studied the frequency of CLCN1 mutations in two separate patient and control cohorts from Germany and Finland, and for comparison in a German myotonic dystrophy type 1 (DM1) patient cohort. The frequency of heterozygous recessive chloride channel (CLCN1) mutations is disproportionally higher (5 %) in currently diagnosed DM2 patients compared to 1.6 % in the control population (p = 0.037), while the frequency in DM1 patients was the same as in the controls. Because the two genes segregate independently, the prevalence of CLCN1 mutations in the total DM2 patient population is, by definition, the same as in the control population. Our findings are, however, not based on the total DM2 population but on the currently diagnosed DM2 patients and indicate a selection bias in molecular diagnostic referrals. DM2 patients with co-segregating CLCN1 mutation have an increased likelihood to be referred for molecular diagnostic testing compared to DM2 patients without co-segregating CLCN1 mutation.

Severity, type, and distribution of myotonic discharges are different in type 1 and type 2 myotonic dystrophy

To characterize and compare electrical myotonia in myotonic dystrophy type 1 (DM1) and type 2 (DM2), 16 patients with genetically confirmed DM1 and 17 patients with DM2 underwent standardized concentric needle electromyography of deltoid, biceps, extensor digitorum communis, first dorsal interosseous, tensor fascia lata (TFL), vastus lateralis (VL), tibialis anterior, and thoracic paraspinal muscles. Eight needle insertions per muscle were made by electromyographers blinded to DM type who recorded the presence and type of myotonia (e.g., classic waxing-waning or less specific waning discharges). Manual muscle testing was performed by a physical therapist. Overall, myotonia was more elicitable in DM1 than DM2; only in VL and TFL was myotonia more elicitable in DM2 than DM1. The major type of myotonia was waxing-waning in DM1, and waning in DM2. Four DM2 (24%), but no DM1 patients had only waning myotonia. In the arms, myotonia was distally predominant in both DM1 and DM2. In the legs, it was distally predominant in DM1, but both proximal and distal in DM2. The severity of myotonia was positively correlated with muscle weakness and with the presence of waxing and waning discharges in DM1, but with neither in DM2. Thus, myotonia is qualitatively and quantitatively different in DM1 than DM2. Except for proximal leg muscles, myotonia is more evocable in DM1 than DM2. It tends to be waxing-waning in DM1 but waning in DM2, thus making electrodiagnosis of DM2 more challenging. Its severity correlates with muscle weakness and the presence of waxing-waning discharges in DM1 but not DM2.

Cerebral involvement in myotonic dystrophies

Myotonic dystrophy types 1 (DM1) and 2 (DM2) are similar yet distinct autosomal-dominant disorders characterized by muscle weakness, myotonia, cataracts, and multiple organ involvement, including the brain. One key difference between DM1 and DM2 is that a congenital form has been described for DM1 only. Expression of RNA transcripts containing pathogenic repeat lengths produces defects in alternative splicing of multiple RNAs, sequesters specific repeat-binding proteins, and ultimately leads to developmentally inappropriate splice products for a particular tissue. Whether brain pathology in its entirety in adult DM1 and DM2 is caused by interference in RNA processing remains to be determined. This review focuses on the similarities and differences between DM1 and DM2 with respect to neuropsychological, neuropathological, and neuroimaging data relating to cerebral involvement, with special emphasis on the clinical relevance and social consequences of such involvement.

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

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

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.

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.

Genetics and molecular pathogenesis of the myotonic dystrophies

Pathogenic repeat expansions were initially identified as causing either a loss of gene product, such as in fragile X mental retardation, or an expansion of a polyglutamine region of a protein, as was first shown in spinobulbar muscular atrophy (Kennedy's disease). The pathogenic effect of the repeat expansion in myotonic dystrophy type 1, however, has been controversial because it does not encode a protein but nonetheless results in a highly penetrant dominant disease. Clinical and molecular characterization of myotonic dystrophy types 1 and 2 have now demonstrated a novel disease mechanism involving pathogenic effects of repeat expansions that are expressed in RNA but are not translated into protein.

Myotonic dystrophy type 2 and related myotonic disorders

The myotonic dystrophies are a group of dominantly inherited disorders characterized by muscle wasting, myotonia, cataracts, hypogonadism and other system manifestations. Myotonic dystrophy type 1 (DM1) results from an unstable expansion of a CTG repeat in 3' UTR of the DM protein kinase (DMPK) gene on chromosome 19q 13.3. Myotonic dystrophy type 2 (DM2) is caused by an unstable expansion of a CCTG tetraplet repeat in intron 1 of the zinc finger 9 (ZFN9 gene) on chromosome 3q 21.3. However, the clinical diagnosis of DM2 is more complex than that of DM1, and conventional molecular genetic methods used for diagnosis of DM1 are not helpful for DM2. We here describe the detailed clinical, laboratory and biomolecular tests to identify DM2 and related myotonic disorders. At present, foci of accumulated noncoding CCTG repeat RNA (ribonuclear inclusions) in the cell nuclei are thought to interfere with the regulation and expression of several genes at the basis of multisystemic aspects of myotonic dystrophy type 2.

Clinical and genetic analysis of a family with PROMM

PROMM (proximal myotonic myopathy) and DM2 (myotonic dystrophy Type 2) are autosomal dominant multisystem disorders that have both been linked to chromosome 3q. Recently, the genetic basis of DM2 has been defined by a '(CCTG)(n)' expansion mutation in intron 1 of the ZNF9 gene. We identified and studied a multigenerational family in which five members had clinical features consistent with PROMM. Two affected members were available for detailed clinical, electrophysiological, radiological and genetic analysis. Our study confirms that the PROMM phenotype is associated with DM2-(CCTG)(n) expansion mutations. In addition, our results may extend the clinical spectrum of manifestations to include vestibular symptoms.

Myotonic dystrophy: RNA pathogenesis comes into focus

Myotonic dystrophy (DM)--the most common form of muscular dystrophy in adults, affecting 1/8000 individuals--is a dominantly inherited disorder with a peculiar and rare pattern of multisystemic clinical features affecting skeletal muscle, the heart, the eye, and the endocrine system. Two genetic loci have been associated with the DM phenotype: DM1, on chromosome 19, and DM2, on chromosome 3. In 1992, the mutation responsible for DM1 was identified as a CTG expansion located in the 3' untranslated region of the dystrophia myotonica-protein kinase gene (DMPK). How this untranslated CTG expansion causes myotonic dystrophy type 1(DM1) has been controversial. The recent discovery that myotonic dystrophy type 2 (DM2) is caused by an untranslated CCTG expansion, along with other discoveries on DM1 pathogenesis, indicate that the clinical features common to both diseases are caused by a gain-of-function RNA mechanism in which the CUG and CCUG repeats alter cellular function, including alternative splicing of various genes. We discuss the pathogenic mechanisms that have been proposed for the myotonic dystrophies, the clinical and molecular features of DM1 and DM2, and the characterization of murine and cell-culture models that have been generated to better understand these diseases.

New methods for molecular diagnosis and demonstration of the (CCTG)n mutation in myotonic dystrophy type 2 (DM2)

Myotonic dystrophy types 1 and 2 are autosomal dominant, multisystemic disorders with many similarities in their clinical manifestations. Myotonic dystrophy type 1 is caused by a (CTG)n expansion in the 3' untranslated region of the DMPK gene in 19q13.3 and myotonic dystrophy type 2 by a (CCTG)n expansion in intron 1 of ZNF9 in 3q21.3. However, the clinical diagnosis of myotonic dystrophy type 2 is more complex than that of myotonic dystrophy type 1, and conventional molecular genetic methods used for diagnosing myotonic dystrophy type 1 are insufficient for myotonic dystrophy type 2. Herein we describe two in situ hybridization protocols for the myotonic dystrophy type 2 mutation detection. Chromogenic in situ hybridization was used to detect both the genomic expansion and the mutant transcripts in muscle biopsy sections. Chromogenic in situ hybridization can be used in routine myotonic dystrophy type 2 diagnostics. Fluorescence in situ hybridization on extended DNA fibers was used to directly visualize the myotonic dystrophy type 2 mutation and to estimate the repeat expansion sizes.