Myotonic Dystrophy type 1 (DM1) is a multi-system disorder characterized by muscle wasting, myotonia, cardiac conduction defects, cataracts, and neuropsychological dysfunction. DM1 is caused by expansion of a CTG repeat in the 3´untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene. A body of work demonstrates that DMPK mRNAs containing abnormally expanded CUG repeats are toxic to several cell types. A core mechanism underlying symptoms of DM1 is that mutant DMPK RNA interferes with the developmentally regulated alternative splicing of defined pre-mRNAs. Expanded CUG repeats fold into ds(CUG) hairpins that sequester nuclear proteins including human Muscleblind-like (MBNL) and hnRNP H alternative splicing factors. DM1 cells activate CELF family member CUG-BP1 protein through hyperphosphorylation and stabilization in the cell nucleus. CUG-BP1 and MBNL1 proteins act antagonistically in exon selection in several pre-mRNA transcripts, thus MBNL1 sequestration and increase in nuclear activity of CUG-BP1 both act synergistically to missplice defined transcripts. Mutant DMPK-mediated effect on subcellular localization, and defective phosphorylation of cytoplasmic CUG-BP1, have additionally been linked to defective translation of p21 and MEF2A in DM1, possibly explaining delayed differentiation of DM1 muscle cells. Mutant DMPK transcripts bind and sequester transcription factors such as Specificity protein 1 leading to reduced transcription of selected genes. Recently, transcripts containing long hairpin structures of CUG repeats have been shown to be a Dicer ribonuclease target and Dicer-induced downregulation of the mutant DMPK transcripts triggers silencing effects on RNAs containing long complementary repeats. In summary, mutant DMPK transcripts alter gene transcription, alternative splicing, and translation of specific gene transcripts, and have the ability to trigger gene-specific silencing effects in DM1 cells. Therapies aimed at reversing these gene expression alterations should prove effective ways to treat DM1.
Dystrophia myotonia type 1 (DM1; Steinert's disease; myotonic dystrophy) is an autosomal dominant disorder due to a large CTG expansion in the 3′-untranslated region (UTR) of the DM protein kinase (DMPK) gene. Transcription of this gene yields a long CUGn-containing mutant (mut) RNA, in which clinical disease is associated with repeats of n=100–5000. Phenomenologically, the expression of mut RNA is correlated with the morphologic observation of ribonucleoprotein precipitates (‘foci') in the nuclei of DMPK-expressing cells. The prevailing view is that the identification of proteins in these foci is the sine qua non of protein–mut RNA interactions. In this viewpoint, I contend that this is an unwarranted inference that falls short in explaining published data. A new model of mut RNA–protein interactions is proposed with distinct binding properties for soluble and insoluble (focus) mut RNA that accommodate these data without exclusions.
mutant RNA; RNA configuration; protein binding; CUGBP; MBNL; transcription factors
Myotonic dystrophy is also known as dystrophia myotonica (DM). The condition is composed of at least two clinical disorders with overlapping phenotypes and distinct molecular genetic defects: myotonic dystrophy type 1, the classic disease originally described by Steinert, and myotonic dystrophy type 2, also called proximal myotonic myopathy (PROMM). Mega cisterna magna is thought to be an anatomic variant with no clinical significance. We report a rare case of type 1 dystrophia myotonica in combination with mega cisterna magna.
Dystrophia myotonica; mega cisterna magna; congenital myotonic dystrophy
Myotonic dystrophy type 1 (Steinert's disease) is a multisystem disorder with autosomal dominant inheritance. This disease is associated with the presence of an abnormal expansion of a cytosine thymine-guanine (CTG) trinucleotide repeat on chromosome 19q13.3. Because of rhythmic complications, the place for systematic electrophysiological study (EPS) has to be discussed.
Serum insulin, blood sugar, and growth hormone levels were measured in response to a 50g oral glucose tolerance test in 10 patients with proven dystrophia myotonica. Three patients belonged to one family; seven patients had no known family history of the disease. One patient, a chronic invalid aged 56 years, produced a mild diabetic glucose tolerance curve and a delayed prolonged rise in serum insulin. Six of the group, including the three affected members from one family, exhibited normal glucose tolerance and fasting serum insulin values, but a markedly exaggerated rise in peripheral insulin levels maximal at 30 and 60 min. This abnormality showed no correlation with age of onset of the disease nor with severity of the muscle weakness. Growth hormone levels were normal in all of the patients studied. It is concluded that an excessive rise in circulating immunoreactive insulin in response to glucose is a common abnormality in dystrophia myotonica and reflects genetic heterogeneity in this condition. Futhermore, if the index patient in a family demostrates this abnormality, it is suggested that the 30- or 60-min blood insulin level during a glucose tolerance test is a useful methold of intra-family screen-ing for asymptomatic heterozygotes at an early stage before the development of physical defects.
Myotonic dystrophy of Steinert, DM1, is the most common adult muscular dystrophy and generally is not associated to development on multiple site neoplasm. Von Hippel-Lindau (VHL) disease is a dominantly inherited familial cancer syndrome that is associated to tumors such as hemangioblastoma of the retina or central nervous system, clear-cell renal carcinoma (RCC) and endocrine tumors, most commonly pheochromocytoma and non-secretory pancreatic islet cell cancers. No data exist in literature describing the coexistence of both DM1 and VHL.
PRESENTATION OF CASE
Herein we report a case of renal and pancreatic neoplasm in a young adult female affected by DM1 and VHL simultaneously.
DM1 is due to an unstable trinucleotide (CTG) expansion in the 30 antranslated region of the dystrophia myotonica-protein kinase (DMPK) gene, located on chromosome 19q13.3. Several molecular mechanisms thought to be determining the classical DM phenotype have been shown. VHL disease is characterized by marked phenotypic variability and the most common tumors are hemangioblastomas of the retina or central nervous system, clear-cell renal carcinoma (RCC) and endocrine tumors, most commonly pheochromocytoma and non-secretory pancreatic islet cell cancers. The pancreatic manifestations seen in patients with VHL disease are divided into 2 categories: pancreatic neuroendocrine tumor (PNET) as solid tumors, and cystic lesions, including a simple cyst and serous cystadenoma. The surgical approach for these cistic lesions is to consider as golden standard. Blansfield has proposed 3 criteria to predict metastatic disease of PNET in patients with VHL disease: (1) tumor size greater than or equal to 3 cm; (2) presence of a mutation in exon 3; and (3) tumor doubling time less than 500 d. If the patient has none of these criteria the patient could be followed with physical examination and radiological surveillance on a 2/3 years base.4 If the patient has 1 criterion, the patient should be followed more closely every 6 months to 1 year. If the patient has 2 or 3 criteria, the patient should be considered for surgery given the high risk of future malignancy. Our patient owned only one criterion but in presence of a second malignant tumor. Our hypothesis for this rare findings is that both DM and VHL might be derived from genetic aberration and these might be linked to a major cancer susceptibility. As far as we know this is the first confirmed case of RCC and neuroendocrine pancreatic cancer occurring concurrently with VHL and, at the same time, DM1. According to this case report and the literature data a VHL should be ruled out in the presence of RCC presenting along with pancreatic cysts/tumor.
As far as we know this is the first confirmed case of RCC and neuroendocrine pancreatic cancer occurring concurrently with VHL and, at the same time, DM1. Our hypothesis for the unusual findings is that both DM and VHL derived from genetic aberration and these are linked to a major cancer susceptibility.
Von Hippel–Lindau; Myotonic dystrophy of Steinert; Pancreatic neuroendocrine tumor; Renal clear cell carcinomal neoplasm
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.
Levels of immunoglobulins IgG, IgA, and IgM were measured in 38 patients with myotonic dystrophy, in normal members of their families, and in matched controls. Log IgG was significantly reduced in the patients. IgG investigation provides a further parameter to appraise the status of apparently unaffected members of myotonic dystrophy families.
Type 1 myotonic dystrophy (DM1) is an autosomal-dominant inherited disorder with a multisystem involvement, caused by an abnormal expansion of the CTG sequence of the dystrophic myotonia protein kinase (DMPK) gene. DM1 is a variable multisystem disorder with muscular and nonmuscular abnormalities. Increasingly, endocrine abnormalities, such as gonadal, pancreatic, and adrenal dysfunction are being reported. But, Electrolytes imbalance is a very rare condition in patients with DM1 yet. Herein we present a 42-yr-old Korean male of DM1 with abnormally elevated serum sodium and potassium. The patient had minimum volume of maximally concentrated urine without water loss. It was only cured by normal saline hydration. The cause of hypernatremia was considered by primary hypodipsia. Hyperkalemic conditions such as renal failure, pseudohyperkalemia, cortisol deficiency and hyperkalemic periodic paralysis were excluded. Further endocrine evaluation suggested selective hyperreninemic hypoaldosteronism as a cause of hyperkalemia.
Myotonic Dystrophy; Hypernatremia; Hyperkalemia
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.
Tumorigenesis; Myotonic dystrophy; Repeat expansion disorders; Pilomatricoma; β-Catenin
BACKGROUND--Breathlessness appears to be closely related to the perception of the outgoing motor command to breathe and should be increased in the presence of muscle weakness. However, breathlessness is not a common symptom in patients with chronic muscle disease who have weak respiratory muscles. The factors that determine the perception of respiratory effort in such patients have not been examined. METHODS--The inspiratory effort sensation during resting breathing and progressive hypercapnia was investigated in 12 patients with dystrophia myotonica with weak respiratory muscles (nine men and three women of mean (SD) age 41.1 (10.5) years; maximum inspiratory pressure 43.1 (17.2) cm H2O) and an age and sex matched control group of normal subjects of mean age 39.6 (10.6) years and a maximum inspiratory pressure of 123 (15.2) cm H2O. RESULTS--During resting breathing with a mouthpiece no differences were seen in inspiratory effort sensation, mouth occlusion pressure, or tidal volume, but inspiratory time and cycle duration were significantly shorter in the patients with dystrophia. Minute ventilation (VE) was significantly higher in the patients (15.8 (4.0) l/min v 12.5 (2.6) l/min), while resting breathing was no more variable in the patients than in controls. The ventilatory response to carbon dioxide (VE/PCO2) was not significantly lower in the patients (14.9 (6.9) l/min/kPa) than in the controls (17.4 (4.3) l/min/kPa). Effort sensation responses to carbon dioxide driven breathing were similar in the control subjects and the patients. With regression analysis of pooled data neither maximum inspiratory pressure nor disease state contributed to perceived inspiratory effort during hypercapnia. CONCLUSIONS--Moderately severe global respiratory muscle weakness does not appear to influence the ventilatory response to rising carbon dioxide tension or the perception of inspiratory effort in patients with dystrophia myotonica.
Somatosensory evoked potentials (SEPs) were recorded in a group of 21 patients with dystrophia myotonica and in a group of controls. Those with dystrophia myotonica had longer absolute peak latencies due to slower peripheral conduction. SEP abnormalities revealed peripheral and/or central conduction delays in 33% of the dystrophia myotonica subjects. There was no apparent relationship between the clinical severity of the disease and SEP abnormality.
A study has been performed on 124 first degree relatives of 38 index patients with dystrophia myotonica in order to assess means of detecting heterozygotes before neurological complaints. Some or all of the following tests have been performed on the relatives: clinical examination, electromyography, slit-lamp examination, radiography of the skull, electrocardiography, serum insulin, and serum immunoglobulin levels. There is evidence that abnormalities in symptomless heterozygotes may be detected by slit-lamp examination, electromyography, and immunoglobulin concentration, and this is probably the order of usefulness of the test in early recognition of the disease. In this study 13 previously undetected heterozygotes have been identified: six as a result of neurological examination, four by both electromyography and slit-lamp examination, and three by slit-lamp examination alone. Abnormalities detected by these tests appear to be independently manifest, so that they will probably be more useful in combination than singly. The family data give a maximum estimate for incidence of mutations among index cases of one quarter, lower than previously suggested. The estimation of immunoglobulins in 45 patients showed significant deficiency, as compared with controls, not only of IgG but also of IgM, and there was an insignificant trend for IgA to be low too. This suggests that the abnormally rapid catabolism of immunoglobulin, previously reported, is not specific for IgG.
Five preterm babies with the neonatal form of dystrophia myotonica are reported. In addition to the generally accepted signs and symptoms of the disease, two other features were present in these patients; oedema was notable in all 5 babies and 4 had unexplained haematomas. It is suggested that premature birth may be a result of severe involvement and that prematurity further aggravates the symptoms.
It is well known that myotonic dystrophy type 1 (DM1) - Curschmann-Steinert disease - is associated with white matter lesions in the brain. Further, DM1 patients may suffer from cardiac involvement and cardioembolic strokes. We report on the unique case of an adult-onset DM1 without cardiac or vascular abnormalities presenting with stroke-like episodes.
A 40 y old white female was admitted twice to our stroke unit with apoplectic dizziness, nausea, headaches, and numbness in the right arm. She was suffering from type 2 diabetes, cataract, and endometriosis. Magnetic resonance imaging (MRI) revealed confluent white matter lesions in all cerebral lobes. There was no hyperintensity on diffusion-weighted imaging (DWI) and no gadolinium enhancement. Cerebrospinal fluid was normal. Surprisingly, myotonic discharges were detected in electromyography (EMG). Genetic testing revealed 200 ± 10 CTG repeats in the dystrophia myotonica protein kinase (DMPK) gene on chromosome 19 and DM1 was diagnosed.
DM1 may be the cause of cerebral white matter lesions. This is the first case of DM1 presenting with stroke-like episodes.
Myotonic dystrophy type 1 (DM1) is caused by a CTG trinucleotide expansion in the 3′ untranslated region (3′ UTR) of DM protein kinase (DMPK). The key feature of DM1 pathogenesis is nuclear accumulation of RNA, which causes aberrant alternative splicing of specific pre-mRNAs by altering the functions of CUG-binding proteins (CUGBPs). Cardiac involvement occurs in more than 80% of individuals with DM1 and is responsible for up to 30% of disease-related deaths. We have generated an inducible and heart-specific DM1 mouse model expressing expanded CUG RNA in the context of DMPK 3′ UTR that recapitulated pathological and molecular features of DM1 including dilated cardiomyopathy, arrhythmias, systolic and diastolic dysfunction, and misregulated alternative splicing. Combined in situ hybridization and immunofluorescent staining for CUGBP1 and CUGBP2, the 2 CUGBP1 and ETR-3 like factor (CELF) proteins expressed in heart, demonstrated elevated protein levels specifically in nuclei containing foci of CUG repeat RNA. A time-course study demonstrated that colocalization of MBNL1 with RNA foci and increased CUGBP1 occurred within hours of induced expression of CUG repeat RNA and coincided with reversion to embryonic splicing patterns. These results indicate that CUGBP1 upregulation is an early and primary response to expression of CUG repeat RNA.
Myotonic dystrophy (DM) is the most common form of muscular dystrophy and is caused by expansion of a CTG trinucleotide repeat on human chromosome 19. Patients with DM develop atrioventricular conduction disturbances, the principal cardiac manifestation of this disease. The etiology of the pathophysiological changes observed in DM has yet to be resolved. Haploinsufficiency of myotonic dystrophy protein kinase (DMPK), DM locus-associated homeodomain protein (DMAHP) and/or titration of RNA-binding proteins by expanded CUG sequences have been hypothesized to underlie the multi-system defects observed in DM. Using an in vivo murine electrophysiology study, we show that cardiac conduction is exquisitely sensitive to DMPK gene dosage. DMPK–/– mice develop cardiac conduction defects which include first-, second-, and third-degree atrioventricular (A–V) block. Our results demonstrate that the A–V node and the His-Purkinje regions of the conduction system are specifically compromised by DMPK loss. Importantly, DMPK+/– mice develop first-degree heart block, a conduction defect strikingly similar to that observed in DM patients. These results demonstrate that DMPK dosage is a critical element modulating cardiac conduction integrity and conclusively link haploinsufficiency of DMPK with cardiac disease in myotonic dystrophy.
The neuromuscular disease myotonic dystrophy type I (DM1) affects multiple organ systems with the major symptoms being severe muscle weakness, progressive muscle wasting and myotonia. The causative mutation in DM1 is a CTG repeat expansion in the 3′-untranslated region of the DM protein kinase (DMPK) gene. RNA transcribed from the expanded allele contains the expanded CUG repeats and leads to the nuclear depletion of Muscleblind-like 1 (MBNL1) and to the increased steady-state levels of CUG-binding protein 1 (CUGBP1). The pathogenic effects of MBNL1 depletion have previously been tested by the generation of MBNL1 knockout mice, but the consequence of CUGBP1 overexpression in adult muscle is not known. In a DM1 mouse model expressing RNA containing 960 CUG repeats in skeletal muscle, CUGBP1 up-regulation is temporally correlated with severe muscle wasting. In this study, we generated transgenic mice with doxycycline-inducible and skeletal muscle-specific expression of CUGBP1. Adult mouse skeletal muscle overexpressing CUGBP1 reproduces molecular and physiological defects of DM1 tissue. The results from this study strongly suggest that CUGBP1 has a major role in DM1 skeletal muscle pathogenesis.
Myotonic dystrophy (MD) or Steinert's syndrome is a rare cause of chronic diarrhea and anal incontinence. In the presence of chronic diarrhea and fecal incontinence with muscle weakness, neuromuscular disorders such as myotonic dystrophy should be considered in the differential diagnosis.
We present the case of a 45-year-old Turkish man with Steinert's syndrome, who was not diagnosed until the age of 45.
In clinical practice, the persistence of diarrhea and fecal incontinence with muscle weakness should suggest that the physician perform an anal manometric study and electromyography. Neuromuscular disorders such as myotonic dystrophy should be considered in the differential diagnosis.
Twelve patients with dystrophia myotonica had cardiac electrphysiological study for conduction disturbances (five cases) or for syncope (seven cases). Conduction disturbances were found in each case, being intranodal in two cases, intra-Hisian in three cases, or diffuse in seven cases. These findings are in agreement with those previously reported, and may be related to the high incidence of sudden death in these patients. Pacemakers are advocated in symptomatic patients, and in some asymptomatic patients with severe and diffuse lesions.
Muscle degeneration and myotonia are clinical hallmarks of myotonic dystrophy type 1 (DM1), a multisystemic disorder caused by a CTG repeat expansion in the 3′ untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Transgenic mice engineered to express mRNA with expanded (CUG)250 repeats (HSALR mice) exhibit prominent myotonia and altered splicing of muscle chloride channel gene (Clcn1) transcripts. We used whole-cell patch clamp recordings and nonstationary noise analysis to compare and biophysically characterize the magnitude, kinetics, voltage dependence, and single channel properties of the skeletal muscle chloride channel (ClC-1) in individual flexor digitorum brevis (FDB) muscle fibers isolated from 1–3-wk-old wild-type and HSALR mice. The results indicate that peak ClC-1 current density at −140 mV is reduced >70% (−48.5 ± 3.6 and −14.0 ± 1.6 pA/pF, respectively) and the kinetics of channel deactivation increased in FDB fibers obtained from 18–20- d-old HSALR mice. Nonstationary noise analysis revealed that the reduction in ClC-1 current density in HSALR FDB fibers results from a large reduction in ClC-1 channel density (170 ± 21 and 58 ± 11 channels/pF in control and HSALR fibers, respectively) and a modest decrease in maximal channel open probability(0.91 ± 0.01 and 0.75 ± 0.03, respectively). Qualitatively similar results were observed for ClC-1 channel activity in knockout mice for muscleblind-like 1 (Mbnl1ΔE3/ΔE3), a second murine model of DM1 that exhibits prominent myotonia and altered Clcn1 splicing (Kanadia et al., 2003). These results support a molecular mechanism for myotonia in DM1 in which a reduction in both the number of functional sarcolemmal ClC-1 and maximal channel open probability, as well as an acceleration in the kinetics of channel deactivation, results from CUG repeat–containing mRNA molecules sequestering Mbnl1 proteins required for proper CLCN1 pre-mRNA splicing and chloride channel function.
Myotonic dystrophy (DM1), the most common muscular dystrophy in adults, is caused by an expanded (CTG)n tract in the 3′ UTR of the gene encoding myotonic dystrophy protein kinase (DMPK)1, which results in nuclear entrapment of the ‘toxic’ mutant RNA and interacting RNA-binding proteins (such as MBNL1) in ribonuclear inclusions2. It is unclear if therapy aimed at eliminating the toxin would be beneficial. To address this, we generated transgenic mice expressing the DMPK 3′ UTR as part of an inducible RNA transcript encoding green fluorescent protein (GFP). We were surprised to find that mice overexpressing a normal DMPK 3′ UTR mRNA reproduced cardinal features of myotonic dystrophy, including myotonia, cardiac conduction abnormalities, histopathology and RNA splicing defects in the absence of detectable nuclear inclusions. However, we observed increased levels of CUG-binding protein (CUG-BP1) in skeletal muscle, as seen in individuals with DM1. Notably, these effects were reversible in both mature skeletal and cardiac muscles by silencing transgene expression. These results represent the first in vivo proof of principle for a therapeutic strategy for treatment of myotonic dystrophy by ablating or silencing expression of the toxic RNA molecules.
Humoral and cellular immunity have been investigated in 15 patients with dystrophia myotonica. No abnormalities in total serum levels of the five major immunoglobulin classes were found but there was a rise in the mean serum level of β1A complement. Altogether, 54% of patients failed to make antibody to tetanus toxoid as compared with 1% of controls: 13% of patients failed to make antibody to Salmonella typhi H antigen as compared with no failure of this function in control subjects. There was a reduced uptake of tritiated thymidine by whole blood lymphocyte cultures spontaneously, while in the presence of phytohaemagglutinin (PHA) and both autologous and fetal calf serum the uptake was normal. It is suggested that there may be a wider derangement of immunological function in dystrophia myotonica than previously thought.
Seven patients with dystrophia myotonica were investigated using neurophysiological combined with histochemical techniques to elucidate motor unit properties in foot extensor muscles, which are often involved in the early stages of this disorder. For the 25 extensor digitorum brevis motor units studied the axonal conduction velocity, the axonal refractory period and the voluntary firing properties were within normal limits. However, high threshold motor units were not observed and the mean value of the axonal conduction velocities was lower (p less than 0.02) for the dystrophia myotonica motor units when compared with corresponding data from healthy subjects. There were also signs of impaired impulse propagation in the terminal part of the motor unit. In muscle biopsy specimens from the anterior tibial muscle, fibre type composition and structure were demonstrated using enzyme histochemical techniques for adenosine-triphosphate and immunohistochemical techniques for identification of the types of myosin isoform present. The histochemical findings indicated a type I fibre dominance, which was most obvious in the more seriously affected muscles. Neonatal myosin was observed preferentially in small but also in some normal sized fibres. Furthermore, some ring fibres were present and these showed staining with antineonatal myosin in their superficial portion. This indicates that an abnormal regeneration is one cause of the myopathic appearance of the muscle fibres in dystrophia myotonica. These investigations show that there is a reduced proportion of type II motor units in foot extensor muscles involved in the myopathy in dystrophia myotonica although it cannot definitely be established whether this is due to a loss of high threshold type II motor units or type II to type I transformation.
Myotonic dystrophy type 1 (DM1) is an autosomal dominant neuromuscular disorder associated with a (CUG)n expansion in the 3′-untranslated region of the DMPK (DM1 protein kinase) gene. Mutant DMPK mRNAs containing the trinucleotide expansion are retained in the nucleus of DM1 cells and form discrete foci. The nuclear sequestration of RNA binding proteins and associated factors binding to the CUG expansions is believed to be responsible for several of the splicing defects observed in DM1 patients and could ultimately be linked to DM1 muscular pathogenesis. Several RNA binding proteins capable of co-localizing with the nuclear-retained mutant DMPK mRNAs have already been identified but none can account for the nuclear retention of the mutant transcripts. Here, we have employed a modified UV crosslinking assay to isolate proteins bound to mutant DMPK-derived RNA and have identified hnRNP H as an abundant candidate. The specific binding of hnRNP H requires not only a CUG repeat expansion but also a splicing branch point distal to the repeats. Suppression of hnRNP H expression by RNAi rescued nuclear retention of RNA with CUG repeat expansions. The identification of hnRNP H as a factor capable of binding and possibly modulating nuclear retention of mutant DMPK mRNA may prove to be an important link in our understanding of the molecular mechanisms that lead to DM1 pathogenesis.