Cardiac Pathology in Myotonic Dystrophy Type 1.

Myotonic dystrophy type 1 (DM1), the most common muscular dystrophy affecting adults and children, is a multi-systemic disorder affecting skeletal, cardiac, and smooth muscles as well as neurologic, endocrine and other systems. This review is on the cardiac pathology associated with DM1. The heart is one of the primary organs affected in DM1. Cardiac conduction defects are seen in up to 75% of adult DM1 cases and sudden death due to cardiac arrhythmias is one of the most common causes of death in DM1. Unfortunately, the pathogenesis of cardiac manifestations in DM1 is ill defined. In this review, we provide an overview of the history of cardiac studies in DM1, clinical manifestations, and pathology of the heart in DM1. This is followed by a discussion of emerging data about the utility of cardiac magnetic resonance imaging (CMR) as a biomarker for cardiac disease in DM1, and ends with a discussion on models of cardiac RNA toxicity in DM1 and recent clinical guidelines for cardiologic management of individuals with DM1.

Daily activity performance in congenital and childhood forms of myotonic dystrophy type 1: a population-based study.

AIMS: To identify and describe the profile characterizing motor and process skills during daily activity performance in individuals with congenital and childhood forms of myotonic dystrophy type 1 (DM1) and to investigate differences in performance between subgroups. METHOD: Sixty participants (34 males, 26 females, mean age=17y 8mo, SD=6y 0mo, range 5y 8mo-29y 0mo) were divided into severe congenital (n=9), mild congenital (n=20), and childhood (n=31) DM1 subgroups. Daily activity performance was evaluated using a standardized observational instrument: the Assessment of Motor and Process Skills. RESULTS: Deficits in performance were more pronounced in process than motor skills. Performance more than 2 SDs below age-specific norms was seen in 65% of participants for process skills and 33% of participants for motor skills. The cut-off scores indicated a potential need for assistance in daily activities for 79% of participants older than 18 years of age (n=28) due to deficient process skills. INTERPRETATION: Extensive deficits in daily activity performance were found in congenital and childhood forms of DM1, mainly owing to deficient process skills. Such skills impact on the ability to perform daily activities and could explain dependency in individuals with DM1. Process skills should be considered when evaluating daily activity performance. WHAT THIS PAPER ADDS: Young people with myotonic dystrophy type 1 show deficits in motor and process skills when performing daily activities, compared with normative data. Deficits in process skills were more pronounced than deficits in motor skills.

Association of Sjögren's syndrome with myotonic dystrophy type 1.

A 47-year-old woman presented with sicca symptoms, polyarthralgias, polymyalgias and dysphagia. She was found to have positive antinuclear, anti-SSA-Ro and anti-SSB-La antibodies. Slit lamp exam confirmed the presence of keratoconjunctivitis sicca, and the patient was diagnosed with Sjögren's syndrome. Three years later, she was referred for evaluation of gait instability associated with recent falls. On physical examination, the patient was found to have bilateral ptosis, percussion myotonia, distal upper and lower extremity weakness, and a steppage gait. Electromyography demonstrated electrical myotonia. Genetic testing revealed expanded CTG repeats (733 and 533) in the myotonic dystrophy type 1 (DM1) protein kinase gene, confirming the diagnosis of DM1. Dysphagia, pain and eye discomfort may occur in both Sjögren's syndrome and DM1, and in this case, may have delayed the diagnosis of muscular dystrophy.

Systematic thyroid screening in myotonic dystrophy: link between thyroid volume and insulin resistance.

BACKGROUND: Myotonic dystrophy (DM1), a neuromuscular disease related to DMPK gene mutations, is associated to endocrine disorders and cancer. A routine endocrine work-up, including thyroid ultrasound (US), was conducted in 115 genetically-proven DM1 patients in a neuromuscular reference center. The aim of this study was to determine the prevalence and the causes of US thyroid abnormalities in DM1. RESULTS: In the whole population (age 45.1 ± 12.2 years, 61.7% female), palpable nodules or goiters were present in 29.2%. The percentage of US goiter (thyroid volume > 18 mL) and US nodules were, respectively, 38.3 and 60.9%. Sixteen of the 115 patients had a thyroidectomy, after 22 fine-needle aspiration cytology guided by thyroid imaging reporting and data system (TIRADS) classification. Six micro- (1/6 pT3) and 3 macro-papillary thyroid carcinoma (PTCs) (2/3 intermediate risk) were diagnosed (7.9% of 115). Thyroid US led to the diagnosis of 4 multifocal and 2 unifocal (including 1 macro-PTC) non-palpable PTCs. Ultrasound thyroid volume was positively correlated to body mass index (BMI) (p = 0.015) and parity (p = 0.036), and was inversely correlated to TSH (p < 0.001) and vitamin D levels (p = 0.023). The BMI, the frequencies of glucose intolerance and PTC were significantly higher in UsGoiter versus non-UsGoiter groups. CONCLUSION: In this systematically screened DM1 cohort, the frequency of UsGoiter, mainly associated to BMI, was about 40%, US nodules 60%, thyroidectomies 13-14%, and PTCs 8%, two-thirds of them being micro-PTCs with good prognosis. Therefore, a systematic screening remains debatable. A targeted US screening in case of clinical abnormality or high BMI seems more appropriate.

Myotonic dystrophy type 1 and pseudo-obstruction in a child with smooth muscle α-actin deficiency and eosinophilic myenteric plexitis.

Myotonic dystrophy (MD) frequently involves the gastrointestinal tract, where it can manifest as chronic intestinal pseudo-obstruction (CIPO), particularly in adults. This manifestation is quite uncommon in children. We report the case of a 12-year-old boy with MD type 1 and CIPO, in which a pathologic assessment revealed an association with smooth muscle α-actin deficiency in the external muscular layer of the ileum, and with features of eosinophilic plexitis and eosinophilic muscle infiltration in the colon. In this peculiar case, the clinical aspects of CIPO might have been due to the involvement of several neuromuscular mechanisms.

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.

Muscleblind-like 3 deficit results in a spectrum of age-associated pathologies observed in myotonic dystrophy.

Myotonic dystrophy type I (DM1) exhibits distinctive disease specific phenotypes and the accelerated onset of a spectrum of age-associated pathologies. In DM1, dominant effects of expanded CUG repeats result in part from the inactivation of the muscleblind-like (MBNL) proteins. To test the role of MBNL3, we deleted Mbnl3 exon 2 (Mbnl3(ΔE2)) in mice and examined the onset of age-associated diseases over 4 to 13 months of age. Accelerated onset of glucose intolerance with elevated insulin levels, cardiac systole deficits, left ventricle hypertrophy, a predictor of a later onset of heart failure and the development of subcapsular and cortical cataracts is observed in Mbnl3(ΔE2) mice. Retention of embryonic splice isoforms in adult organs, a prominent defect in DM1, is not observed in multiple RNAs including the Insulin Receptor (Insr), Cardiac Troponin T (Tnnt2), Lim Domain Binding 3 (Ldb3) RNAs in Mbnl3(ΔE2) mice. Although rare DM1-like splice errors underlying the observed phenotypes cannot be excluded, our data in conjunction with the reported absence of alternative splice errors in embryonic muscles of a similar Mbnl3(ΔE2) mouse by RNA-seq studies, suggest that mechanisms distinct from the adult retention of embryonic splice patterns may make important contributions to the onset of age-associated pathologies in DM1.

Overweight Is an Independent Risk Factor for Reduced Lung Volumes in Myotonic Dystrophy Type 1.

BACKGROUND: In this large observational study population of 105 myotonic dystrophy type 1 (DM1) patients, we investigate whether bodyweight is a contributor of total lung capacity (TLC) independent of the impaired inspiratory muscle strength. METHODS: Body composition was assessed using the combination of body mass index (BMI) and fat-free mass index. Pulmonary function tests and respiratory muscle strength measurements were performed on the same day. Patients were stratified into normal (BMI < 25 kg/m(2)) and overweight (BMI ≥ 25 kg/m(2)) groups. Multiple linear regression was used to find significant contributors for TLC. RESULTS: Overweight was present in 59% of patients, and body composition was abnormal in almost all patients. In overweight patients, TLC was significantly (p = 2.40×10(-3)) decreased, compared with normal-weight patients, while inspiratory muscle strength was similar in both groups. The decrease in TLC in overweight patients was mainly due to a decrease in expiratory reserve volume (ERV) further illustrated by a highly significant (p = 1.33×10(-10)) correlation between BMI and ERV. Multiple linear regression showed that TLC can be predicted using only BMI and the forced inspiratory volume in 1 second, as these were the only significant contributors. CONCLUSIONS: This study shows that, in DM1 patients, overweight further reduces lung volumes, as does impaired inspiratory muscle strength. Additionally, body composition is abnormal in almost all DM1 patients.

Therapeutic Approaches for Dominant Muscle Diseases: Highlight on Myotonic Dystrophy.

Myotonic Dystrophy (DM), one of the most common neuromuscular disorders in adults, comprises two genetically distinct forms triggered by unstable expanded repeats in non-coding regions. The most common DM1 is caused by expanded CTG repeats in the 3'UTR of the DMPK gene, whereas DM2 is due to large expanded CCTG repeats in the first intron of the CNBP gene. Both mutations induce a pathogenic RNA gain-of-function mechanism. Mutant RNAs containing CUG or CCUG expanded repeats, which are retained in the nuclei as aggregates alter activities of alternative splicing regulators such as MBNL proteins and CELF1. As a consequence, alternative splicing misregulations of several pre-mRNAs are associated with DM clinical symptoms. Currently, there is no available cure for this dominant neuromuscular disease. Nevertheless, promising therapeutic strategies have been developed in the last decade. Preclinical progress in DM research prompted the first DM1 clinical trial based on antisense oligonucleotides promoting a RNase-H-mediated degradation of the expanded CUG transcripts. The ongoing Phase 1/2a clinical trial will hopefully give further insights into the quest to find a bona fide cure for DM1. In this review, we will provide an overview of the different strategies that were developed to neutralize the RNA toxicity in DM1. Different approaches including antisense oligonucleotide technologies, gene therapies or small molecules have been tested and validated in cellular and animal models. Remaining challenges and additional avenues to explore will be discussed.

Hypogonadism Associated with Cyp19a1 (Aromatase) Posttranscriptional Upregulation in Celf1 Knockout Mice.

CELF1 is a multifunctional RNA-binding protein that controls several aspects of RNA fate. The targeted disruption of the Celf1 gene in mice causes male infertility due to impaired spermiogenesis, the postmeiotic differentiation of male gametes. Here, we investigated the molecular reasons that underlie this testicular phenotype. By measuring sex hormone levels, we detected low concentrations of testosterone in Celf1-null mice. We investigated the effect of Celf1 disruption on the expression levels of steroidogenic enzyme genes, and we observed that Cyp19a1 was upregulated. Cyp19a1 encodes aromatase, which transforms testosterone into estradiol. Administration of testosterone or the aromatase inhibitor letrozole partly rescued the spermiogenesis defects, indicating that a lack of testosterone associated with excessive aromatase contributes to the testicular phenotype. In vivo and in vitro interaction assays demonstrated that CELF1 binds to Cyp19a1 mRNA, and reporter assays supported the conclusion that CELF1 directly represses Cyp19a1 translation. We conclude that CELF1 downregulates Cyp19a1 (Aromatase) posttranscriptionally to achieve high concentrations of testosterone compatible with spermiogenesis completion. We discuss the implications of these findings with respect to reproductive defects in men, including patients suffering from isolated hypogonadotropic hypogonadism and myotonic dystrophy type I.

Increased autophagy and apoptosis contribute to muscle atrophy in a myotonic dystrophy type 1 Drosophila model.

Muscle mass wasting is one of the most debilitating symptoms of myotonic dystrophy type 1 (DM1) disease, ultimately leading to immobility, respiratory defects, dysarthria, dysphagia and death in advanced stages of the disease. In order to study the molecular mechanisms leading to the degenerative loss of adult muscle tissue in DM1, we generated an inducible Drosophila model of expanded CTG trinucleotide repeat toxicity that resembles an adult-onset form of the disease. Heat-shock induced expression of 480 CUG repeats in adult flies resulted in a reduction in the area of the indirect flight muscles. In these model flies, reduction of muscle area was concomitant with increased apoptosis and autophagy. Inhibition of apoptosis or autophagy mediated by the overexpression of DIAP1, mTOR (also known as Tor) or muscleblind, or by RNA interference (RNAi)-mediated silencing of autophagy regulatory genes, achieved a rescue of the muscle-loss phenotype. In fact, mTOR overexpression rescued muscle size to a size comparable to that in control flies. These results were validated in skeletal muscle biopsies from DM1 patients in which we found downregulated autophagy and apoptosis repressor genes, and also in DM1 myoblasts where we found increased autophagy. These findings provide new insights into the signaling pathways involved in DM1 disease pathogenesis.

Parental age effects, but no evidence for an intrauterine effect in the transmission of myotonic dystrophy type 1.

Myotonic dystrophy type 1 (DM1) is caused by the expansion of an unstable CTG repeat (g.17294_17296(45_1000)) with more repeats associated with increased disease severity and reduced age at onset. Expanded disease-associated alleles are highly unstable in both the germline and soma. Germline instability is expansion biased, providing a molecular explanation for anticipation. Somatic instability is expansion biased, size- and age-dependent, features that have compromised genotype-phenotype correlations and intergenerational studies. We corrected these confounding factors by estimating the progenitor allele length in 54 father-offspring and 52 mother-offspring pairs in Costa Rican DM1 families. Not surprisingly, we found major parental allele length effects on the size of the allele transmitted, the magnitude of the intergenerational length change, the age at onset in the next generation and the degree of anticipation in both male and female transmissions. We also detected, for the first time, an age-of-parent effect for both male and female transmission. Interestingly, we found no evidence for an intrauterine effect in the transmission of congenital DM1, suggesting previous reports may have been an artefact of age-dependent somatic instability and sampling bias. These data provide new insights into the germline dynamics of the CTG repeat and opportunities for providing additional advice and more accurate risk assessments to prospective parents in DM1 families.

[Current studies in myotonic dystrophy].

Myotonic dystrophy (DM) is a genetic, progressive, multisystemic disease with muscular disorder as its primary symptom. There are two types of DM (DM1 and DM2) caused by mutations in different genes, and in Japan, DM occurs with an incidence of approximately 1 in 20,000. The pathogenic mechanism underlying the disease is RNA toxicity caused by transcripts of aberrantly elongated CTG or CCTG repeats located in the 3' untranslated region or in the intron. The current treatments for DM is limited to symptomatic care. In this review, we will discuss several new therapeutic strategies based on recent studies of RNA toxicity.

Cell recovery from DM1 transgenic mouse tissue to study (CTG) n instability and DM1 pathogenesis.

Myotonic dystrophy type 1 results from an unstable expanded CTG repeat ((CTG) n ) in the 3' UTR of the DMPK gene. Transgenic mouse models have been developed to reproduce the (CTG) n instability seen in DM1 patients. These transgenic mice provide an excellent tool to study the disease mechanism as well as the molecular mechanisms underlying trinucleotide repeat instability. The propensity for somatic instability differs per tissue and cell type. Expansion of the (CTG) n over time in certain tissues is thought to underlie progression of the clinical picture. It is therefore crucial to understand what causes the (CTG) n to expand in certain cells and not in others, as well as to see possibly distinct downstream cellular effects of different (CTG) n lengths in different cell populations. We describe here an updated method to determine the genotype (homozygous, hemizygous, or non-transgenic) of the transgene, as well as length of the very long (CTG) n tracts now commonly obtained in our mouse model. Furthermore, in order to facilitate research into cell populations that show different degrees of instability, we present here a fast technique to recover cells from mouse tissues, which can serve as a basis for multiple downstream applications, including cell culture and biochemical or molecular studies.

Two is better than one: a case of homozygous myotonic dystrophy type 1.

Myotonic dystrophy type 1 is an autosomal dominant condition caused by a trinucleotide CTG repeat expansion in the 3' untranslated region of the dystrophia myotonica protein kinase gene. The phenotypic features of myopathic facies, generalized weakness, and myotonia are thought to be dependent on repeat number, with larger expansions generally leading to earlier and/or more severe disease. The vast majority of individuals are heterozygous for an expanded allele and an allele in the normal range. In this clinical report, we describe two brothers with congenital myotonic dystrophy type 1. The younger of the two siblings is one of only 13 homozygous patients ever reported in the literature. He carries two expanded alleles: one with 1,170 repeats and the other with >100 repeats. We present his clinical picture in relation to his more severely affected heterozygous brother as well as other published homozygous cases. Finally, we discuss the inconsistency between repeat size and symptomatic expression as it applies to the current proposed mechanisms of myotonic dystrophy type 1 pathogenicity.

Uninterrupted CCTG tracts in the myotonic dystrophy type 2 associated locus.

Myotonic dystrophy comprises at least two genetically distinct forms, DM1 and DM2. DM2 is caused by expansion of the (CCTG)n repeat tract in the CNBP gene. The CCTG tract is generally interrupted in healthy range alleles by GCTG, TCTG or ACTG motifs. However, alleles with uninterrupted tracts have been reported on expanded alleles, and occasionally on large-sized healthy range alleles. Therefore, these uninterrupted large-sized alleles have been considered to be possible DM2 premutations. In comparison to previous studies, we identified a wider range and a higher frequency of healthy range alleles containing uninterrupted CCTG tracts. They are most likely not restricted only to large alleles, as they constantly exist in the whole spectrum of healthy range alleles. Our results indicate that the boundary between stable and unstable uninterrupted healthy range alleles is approximately 30 CCTG repeats. This suggests a similar distribution of healthy range, premutation and mutation range uninterrupted DM2 alleles, which is also typical for DM1 alleles. Interrupted alleles, and those uninterrupted with less than approximately 30 CCTG repeats, appear stable, while instability increases with increasing length of uninterrupted parts above this threshold. Unstable DM2 premutation alleles can range from approximately 30 to 55 CCTG repeats.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies.

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

Somatic instability of the expanded CTG triplet repeat in myotonic dystrophy type 1 is a heritable quantitative trait and modifier of disease severity.

Deciphering the contribution of genetic instability in somatic cells is critical to our understanding of many human disorders. Myotonic dystrophy type 1 (DM1) is one such disorder that is caused by the expansion of a CTG repeat that shows extremely high levels of somatic instability. This somatic instability has compromised attempts to measure intergenerational repeat dynamics and infer genotype-phenotype relationships. Using single-molecule PCR, we have characterized more than 17 000 de novo somatic mutations from a large cohort of DM1 patients. These data reveal that the estimated progenitor allele length is the major modifier of age of onset. We find no evidence for a threshold above which repeat length does not contribute toward age at onset, suggesting pathogenesis is not constrained to a simple molecular switch such as nuclear retention of the DMPK transcript or haploinsufficiency for DMPK and/or SIX5. Importantly, we also show that age at onset is further modified by the level of somatic instability; patients in whom the repeat expands more rapidly, develop the symptoms earlier. These data establish a primary role for somatic instability in DM1 severity, further highlighting it as a therapeutic target. In addition, we show that the level of instability is highly heritable, implying a role for individual-specific trans-acting genetic modifiers. Identifying these trans-acting genetic modifiers will facilitate the formulation of novel therapies that curtail the accumulation of somatic expansions and may provide clues to the role these factors play in the development of cancer, aging and inherited disease in the general population.