Cardiac arrhythmia and late-onset muscle weakness caused by a myofibrillar myopathy with unusual histopathological features due to a novel missense mutation in FLNC.

Myofibrillar myopathies (MFM) are mostly adult-onset diseases characterized by progressive morphological alterations of the muscle fibers beginning in the Z-disk and the presence of protein aggregates in the sarcoplasm. They are mostly caused by mutations in different genes that encode Z-disk proteins, including DES, CRYAB, LDB3, MYOT, FLNC and BAG3. A large family of French origin, presenting an autosomal dominant pattern, characterized by cardiac arrhythmia associated to late-onset muscle weakness, was evaluated to clarify clinical, morphological and genetic diagnosis. Muscle weakness began during adult life (over 30 years of age), and had a proximal distribution. Histology showed clear signs of a myofibrillar myopathy, but with unusual, large inclusions. Subsequently, genetic testing was performed in MFM genes available for screening at the time of clinical/histological diagnosis, and desmin (DES), αB-crystallin (CRYAB), myotilin (MYOT) and ZASP (LDB3), were excluded. LMNA gene screening found the p.R296C variant which did not co-segregate with the disease. Genome wide scan revealed linkage to 7q.32, containing the FLNC gene. FLNC direct sequencing revealed a heterozygous c.3646T>A p.Tyr1216Asn change, co-segregating with the disease, in a highly conserved amino acid of the protein. Normal filamin C levels were detected by Western-blot analysis in patient muscle biopsies and expression of the mutant protein in NIH3T3 showed filamin C aggregates. This is an original FLNC mutation in a MFM family with an atypical clinical and histopathological presentation, given the presence of significantly focal lesions and prominent sarcoplasmic masses in muscle biopsies and the constant heart involvement preceding significantly the onset of the myopathy. Though a rare etiology, FLNC gene should not be excluded in early-onset arrhythmia, even in the absence of myopathy, which occurs later in the disease course.

Diagnostic measures for sarcopenia and bone mineral density.

SUMMARY: Currently used diagnostic measures for sarcopenia utilize different measures of muscle mass, muscle strength, and physical performance. These diagnostic measures associate differently to bone mineral density (BMD), as an example of muscle-related clinical outcome. These differences should be taken into account when studying sarcopenia. INTRODUCTION: Diagnostic measures for sarcopenia utilize different measures of muscle mass, muscle strength, and physical performance. To understand differences between these measures, we determined the association with respect to whole body BMD, as an example of muscle-related clinical outcome. METHODS: In the European cross-sectional study MYOAGE, 178 young (18-30 years) and 274 healthy old participants (69-81 years) were recruited. Body composition and BMD were evaluated using dual-energy X-ray densitometry. Diagnostic measures for sarcopenia were composed of lean mass as percentage of body mass, appendicular lean mass (ALM) as percentage of body mass, ALM divided by height squared (ALM/height(2)), knee extension torque, grip strength, walking speed, and Timed Up and Go test (TUG). Linear regression models were stratified for sex and age and adjusted for age and country, and body composition in separate models. RESULTS: Lean mass and ALM/height(2) were positively associated with BMD (P < 0.001). Significance remained in all sex and age subgroups after further adjustment for fat mass, except in old women. Lean mass percentage and ALM percentage were inversely associated with BMD in old women (P < 0.001). These inverse associations disappeared after adjustment for body mass. Knee extension torque and handgrip strength were positively associated with BMD in all subgroups (P < 0.01), except in old women. Walking speed and TUG were not related to BMD. CONCLUSIONS: The associations between diagnostic measures of sarcopenia and BMD as an example of muscle-related outcome vary widely. Differences between diagnostic measures should be taken into account when studying sarcopenia.

Familial reducing body myopathy with cytoplasmic bodies and rigid spine revisited: identification of a second LIM domain mutation in FHL1.

OBJECTIVE: Reducing body myopathy (RBM) is a rare progressive disorder of muscle characterized by intracytoplasmic inclusions, which stain strongly with menadione-NBT (nitroblue tetrazolium). We recently identified the four and a half LIM domain gene FHL1 located on chromosome Xq26 as the causative gene for RBM. So far eight familial cases and 21 sporadic patients with RBM have been reported in the literature. METHODS: We ascertained a total of 8 members of a German family initially reported by Goebel et al. as a mixed myopathy with rigid spine myopathy and reducing as well as cytoplasmic bodies. Clinical findings in the original and additional family members have been reviewed. Mutation detection was performed by direct sequencing of FHL1 exons. RESULTS: We identified a novel mutation (p.C150R) in the second LIM domain of FHL1 in six family members (1 male, 5 females). The male index patient was the most affected member presenting with rigid spine, followed by rapidly progressive muscle weakness. He died from the consequences of respiratory insufficiency at the age of 29.5 years. His sister, mother, grandmother, aunt and female cousin all carried the mutation in the heterozygous state. The sister is clinically unaffected; their mother had myopathic changes in her muscle biopsy, while the grandmother showed first signs of weakness at 50 years of age. The 54-year-old aunt and her daughter are clinically asymptomatic. CONCLUSION: We report a novel LIM2 domain mutation in FHL1 in a previously reported family with RBM with cytoplasmic bodies and spinal rigidity. While the male index patient was significantly affected, female carriers show varying manifestations and may be asymptomatic, likely reflecting varying degrees of X-inactivation. RBM continues to be associated with mutations in the LIM2 domain of FHL1. We also confirm our earlier observation that mutations at the N-terminal end of the LIM2 domain seem to be milder compared to mutations seen at the C-terminal part of the domain which cause severe disease even in female carriers.

miR-379 links glucocorticoid treatment with mitochondrial response in Duchenne muscular dystrophy.

Duchenne Muscular Dystrophy (DMD) is a lethal muscle disorder, caused by mutations in the DMD gene and affects approximately 1:5000-6000 male births. In this report, we identified dysregulation of members of the Dlk1-Dio3 miRNA cluster in muscle biopsies of the GRMD dog model. Of these, we selected miR-379 for a detailed investigation because its expression is high in the muscle, and is known to be responsive to glucocorticoid, a class of anti-inflammatory drugs commonly used in DMD patients. Bioinformatics analysis predicts that miR-379 targets EIF4G2, a translational factor, which is involved in the control of mitochondrial metabolic maturation. We confirmed in myoblasts that EIF4G2 is a direct target of miR-379, and identified the DAPIT mitochondrial protein as a translational target of EIF4G2. Knocking down DAPIT in skeletal myotubes resulted in reduced ATP synthesis and myogenic differentiation. We also demonstrated that this pathway is GC-responsive since treating mice with dexamethasone resulted in reduced muscle expression of miR-379 and increased expression of EIF4G2 and DAPIT. Furthermore, miR-379 seric level, which is also elevated in the plasma of DMD patients in comparison with age-matched controls, is reduced by GC treatment. Thus, this newly identified pathway may link GC treatment to a mitochondrial response in DMD.

Muscular dystrophy and neuronal migration disorder caused by mutations in a glycosyltransferase, POMGnT1.

Muscle-eye-brain disease (MEB) is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, and lissencephaly. Mammalian O-mannosyl glycosylation is a rare type of protein modification that is observed in a limited number of glycoproteins of brain, nerve, and skeletal muscle. Here we isolated a human cDNA for protein O-mannose beta-1,2-N-acetylglucosaminyltransferase (POMGnT1), which participates in O-mannosyl glycan synthesis. We also identified six independent mutations of the POMGnT1 gene in six patients with MEB. Expression of most frequent mutation revealed a great loss of the enzymatic activity. These findings suggest that interference in O-mannosyl glycosylation is a new pathomechanism for muscular dystrophy as well as neuronal migration disorder.

Direct visualization of the dystrophin network on skeletal muscle fiber membrane.

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene locus, is expressed on the muscle fiber surface. One key to further understanding of the cellular function of dystrophin would be extended knowledge about its subcellular organization. We have shown that dystrophin molecules are not uniformly distributed over the humen, rat, and mouse skeletal muscle fiber surface using three independent methods. Incubation of single-teased muscle fibers with antibodies to dystrophin revealed a network of denser transversal rings (costameres) and finer longitudinal interconnections. Double staining of longitudinal semithin cryosections for dystrophin and alpha-actinin showed spatial juxtaposition of the costameres to the Z bands. Where peripheral myonuclei precluded direct contact of dystrophin to the Z bands the organization of dystrophin was altered into lacunae harboring the myonucleus. These lacunae were surrounded by a dystrophin ring and covered by a more uniform dystrophin veil. Mechanical skinning of single-teased fibers revealed tighter mechanical connection of dystrophin to the plasma membrane than to the underlying internal domain of the muscle fiber. The entire dystrophin network remained preserved in its structure on isolated muscle sarcolemma and identical in appearance to the pattern observed on teased fibers. Therefore, connection of defined areas of plasma membrane or its constituents such as ion channels to single sarcomeres might be a potential function exerted by dystrophin alone or in conjunction with other submembrane cytoskeletal proteins.

Respiratory and upper limb function as outcome measures in ambulant and non-ambulant subjects with Duchenne muscular dystrophy: A prospective multicentre study.

The field of translational research in Duchenne muscular dystrophy (DMD) has been transformed in the last decade by a number of therapeutic targets, mostly studied in ambulant patients. A paucity of studies focus on measures that capture the non-ambulant stage of the disease, and the transition between the ambulant and non-ambulant phase. In this prospective natural history study, we report the results of a comprehensive assessment of respiratory, upper limb function and upper limb muscle strength in a group of 89 DMD boys followed in 3 European countries, 81 receiving corticosteroids, spanning a wide age range (5-18 years) and functional abilities, from ambulant (n = 60) to non-ambulant (n = 29). Respiratory decline could be detected in the early ambulatory phase using Peak Expiratory Flow percentage predicted (PEF%), despite glucocorticoid use (mean annual decline: 4.08, 95% CI [-7.44,-0.72], p = 0.02 in ambulant; 4.81, 95% CI [-6.79,-2.82], p < 0.001 in non-ambulant). FVC% captured disease progression in non-ambulant DMD subjects, with an annual loss of 5.47% (95% CI [-6.48,-4.45], p < 0.001). Upper limb function measured with the Performance of Upper Limb (PUL 1.2) showed an annual loss of 4.13 points (95% CI [-4.79,3.47], p < 0.001) in the non-ambulant cohort. Measures of upper limb strength (MyoGrip and MyoPinch) showed a continuous decline independent of the ambulatory status, when reported as percentage predicted (grip force -5.51%, 95% CI [-6.54,-4.48], p < 0.001 in ambulant and a slower decline -2.86%; 95% CI -3.29,-2.43, p < 0.001, in non-ambulant; pinch force: -2.66%, 95% CI [-3.82,-1.51], p < 0.001 in ambulant and -2.23%, 95% CI [-2.92,-1.53], p < 0.001 in non-ambulant). Furthermore, we also explored the novel concept of a composite endpoint by combining respiratory, upper limb function and force domains: we were able to identify clear clinical progression in patients in whom an isolated measurement of only one of these domains failed to appreciate the yearly change. Our study contributes to the field of natural history of DMD, linking the ambulant and non-ambulant phases of the disease, and suggests that composite scores should be explored further.

Electron microscopy in myofibrillar myopathies reveals clues to the mutated gene.

We studied the ultrastructural characteristics in patients with myofibrillar myopathy (MFM) and differentiated between MFM-subtypes using electron microscopic (EM) findings. The ultrastructural findings in 19 patients with different genetically proven MFMs (9 desmin, 5 alphaB-crystallin, 3 ZASP, 2 myotilin) were analyzed. In one ZASPopathy, we additionally performed an immunoEM study, using antibodies against desmin, alphaB-crystallin, ZASP and myotilin. The ultrastructural findings in desminopathies and alphaB-crystallinopathies were very similar and consisted of electrondense granulofilamentous accumulations and sandwich formations. They differed in the obvious presence of early apoptotic nuclear changes in alphaB-crystallinopathies. ZASPopathies were characterized by filamentous bundles (labeled with the myotilin antibody on immunoEM), and floccular accumulations of thin filamentous material. Tubulofilamentous inclusions in sarcoplasm and myonuclei in combination with filamentous bundles were characteristic for myotilinopathies. We conclude that MFMs ultrastructural findings can direct diagnostic efforts towards the causal gene mutated, and that EM should be included in the diagnostic workup of MFMs.

Non-invasive ventilation reduces respiratory tract infections in children with neuromuscular disorders.

Non-invasive ventilation (NIV) has been shown to improve gas exchange and survival in patients with chronic neuromuscular disorders. Little is known about its influence on respiratory tract infections (RTIs). Twenty-four patients with regular use of NIV and 11 patients without NIV with neuromuscular disorders answered a questionnaire concerning the use of NIV and assisted coughing techniques, the status of influenza and pneumococcus vaccination, and the frequency and severity of RTIs. Additionally, we performed a retrospective chart review of twelve patients who were ventilated over a period of at least 5 years. In the first year of NIV consultations of a general practitioner due to RTI decreased from 9.2+/-20.8 to 3.2+/-5.3 per year (P<0.005), the number of antibiotic treatment due to RTI decreased from 4.1+/-3.4 to 1.9+/-2.2 per year (P<0.005) and the number of hospital admissions due to RTI decreased from 1.6+/-1.7 to 0.7+/-1.3 per year (P<0.005). Vaccinations against influenza and/or pneumococcus did not have a significant influence on the rate of infections. In 12 patients using NIV for more than 5 years the incidence of RTI requiring hospital admission decreased from 0.54+/-0.41/year in the pre-ventilation period to 0.12+/-0.09/year in the NIV period (P<0.005). NIV had a favorable impact on respiratory infectious complications in children with neuromuscular disorders.

Predictors of severe chest infections in pediatric neuromuscular disorders.

Chest infections are serious complications in neuromuscular disorders. The predictive values of lung and respiratory muscle function including peak cough flow still remain unclear. We performed retrospective analysis of 46 children and adolescents (12.7+/-3.7 years) in whom lung function, respiratory muscle function and peak cough flows had been obtained. Data were related to: (1). number of chest infections and days of antibiotic treatment the year prior to the study and (2). history of severe chest infection requiring hospital admission. The number of chest infections and the number of days treated with antibiotics correlated with Inspiratory Vital Capacity IVC, peak cough flow PCF and Peak Expiratory Pressure PEP. Twenty-two patients were hospitalized at least once due to severe chest infection. IVC (0.65 vs. 1.44 l; P<0.0001) and PCF (116 vs. 211 l/min; P<0.0005) in these patients were significantly lower than in the non-hospitalized group. IVC<1.1l and PCF<160 l/min were specific and sensitive thresholds to discriminate between patients who had already suffered severe chest infections and those who had not. Therefore, spirometry and peak cough flow are reliable tests to identify patients at high risk for severe chest infections. Patients with IVC below 1.1l and/or PCF below 160 l/min should be well monitored and introduced to assisted coughing techniques.

Genetic analysis of the first 4 patients with beta-ureidopropionase deficiency.

beta-Ureidopropionase is the third enzyme of the pyrimidine degradation pathway and it catalyses the irreversible hydrolysis of N-carbamyl-ss-aminoisobutyric acid or N-carbamyl-ss-alanine to beta-aminoisobutyric acid or ss-alanine, ammonia, and CO2. Analysis of the beta-ureidopropionase gene (UPB1) of the first 4 patients presenting with a complete enzyme deficiency, revealed the presence of 2 splice-site mutations (IVS1-2A>G and IVS8-1G>A) and one missense mutation (A85E). RT-PCR analysis of the complete beta-ureidopropionase cDNA suggested that both splice-site mutations lead to a variety of alternative splice variants, with deletions of a single or several exons. The alanine at position 85 was not conserved in other eukaryotic beta-ureidopropionase protein sequences.

Childhood Pompe disease: clinical spectrum and genotype in 31 patients.

BACKGROUND: As little information is available on children with non-classic presentations of Pompe disease, we wished to gain knowledge of specific clinical characteristics and genotypes. We included all patients younger than 18 years, who had been evaluated at the Pompe Center in Rotterdam, the Netherlands, between 1975 and 2012, excluding those with the classic-infantile form. None were treated with enzyme replacement therapy at the time of evaluation. We collected information on first symptoms, diagnosis, use of a wheelchair and/or respirator, and enzyme and mutation analysis and assessed muscle strength, pulmonary function, and cardiac parameters. RESULTS: Thirty-one patients participated. Median age at symptom onset was 2.6 years (range 0.5-13y) and at diagnosis 4.0 years. Most first problems were delayed motor development and problems related to limb-girdle weakness. Fatigue, persistent diarrhea and problems in raising the head in supine position were other first complaints. Ten patients were asymptomatic at time of diagnosis. Five of them developed symptoms before inclusion in this study. Over 50 % of all patients had low or absent reflexes, a myopathic face, and scoliosis; 29 % were underweight. Muscle strength of the neck flexors, hip extensors, hip flexors, and shoulder abductors were most frequently reduced. Pulmonary function was decreased in over 48 % of the patients; 2 patients had cardiac hypertrophy. Patients with mutations other than the c.-32-13T > G were overall more severely affected, while 18 out of the 21 patients (86 %) with the c.-32-13T > G/'null' genotype were male. CONCLUSIONS: Our study shows that Pompe disease can present with severe mobility and respiratory problems during childhood. Pompe disease should be considered in the differential diagnosis of children with less familiar signs such as disproportional weakness of the neck flexors, unexplained fatigue, persistent diarrhea and unexplained high CK/ASAT/ALAT. Disease presentation appears to be different from adult patients. The majority of affected children with GAA genotype c.-32-13T > G/'null' appeared to be male.

Safety and efficacy of recombinant acid alpha-glucosidase (rhGAA) in patients with classical infantile Pompe disease: results of a phase II clinical trial.

Pompe disease is an autosomal recessive muscle-wasting disorder caused by the deficiency of the lysosomal enzyme acid alpha-glucosidase. Due to virtual absence of acid alpha-glucosidase, patients with classical infantile Pompe disease develop progressive cardiomyopathy, skeletal muscle weakness and respiratory insufficiency leading to death in early infancy. We report on the results of a phase II clinical trial including two patients with classical infantile Pompe disease receiving enzyme replacement therapy over a period of 48 weeks by weekly infusions. Recombinant acid alpha-glucosidase was derived from the milk of transgenic rabbits. Safety was evaluated by recording adverse events while clinical efficacy was evaluated by ventilator-free survival, left ventricular mass index, motor development as well as histologic and biochemical analysis of muscle biopsies. This therapy was in general well-tolerated. There was an overall improvement in left ventricular mass, cardiac function, skeletal muscle function and histological appearance of skeletal muscle.

Adult polyglucosan body myopathy.

This report describes a sporadic late-onset myopathy in two unrelated adults which was marked by polyglucosan inclusions surrounded by abnormally structured mitochondria, the latter finding a localized, possibly reactive phenomenon. The polyglucosan material was characterized by a battery of histochemical and enzyme histochemical techniques; revealed common antigenicity with Lafora bodies, corpora amylacea and muscle fiber inclusions in types IV and VII glycogenoses; and contained ubiquitin. Additional lectin histochemical and associated digestion preparations disclosed the presence of alpha-glycosyl residues as apparently the sole carbohydrate component in polyglucosan bodies while the above mentioned common antigenicity with Lafora bodies and other inclusions suggests an additional, so far unidentified, protein component.

Functional consequences of the autosomal dominant G272A mutation in the human GLUT1 gene.

The first autosomal dominant missense mutation (G272A) reported within the human GLUT1 gene and shared by three affected family members was investigated in respect to functional consequences. Substitution of glycine-91 by site-directed mutagenesis with either aspartate or alanine resulted in a significant decrease in transport activity of GLUT1 expressed in Xenopus oocytes. Expression of mutant transporters was confirmed by immunoblot, 2-deoxy-glucose uptake and confocal laser microscopy. The data agree with 3-O-methyl-glucose uptake into patient erythrocytes and indicate that the loss of glycine rather than a hydrophilic side chain (Gly91Asp) defines the functional consequences of this mutation.

A homologue of dystrophin is expressed at the neuromuscular junctions of normal individuals and DMD patients, and of normal and mdx mice. Immunological evidence.

Polyclonal and monoclonal antibodies, which recognize different regions and epitopes of the dystrophin molecule, bind to a protein of Mr 400,000 which is present in extracts of mdx muscle from regions which contain neuromuscular junctions (NMJ) and is absent from those which do not. This NMJ-associated homologue of dystrophin has at least 2 epitopes which are different to usual Xp21 form of dystrophin expressed along the sarcolemma of muscle fibres in normal muscles. This protein is also expressed at the NMJ of a DMD patient who lacks the first 52 exons of the Xp21 dystrophin gene and it must therefore be translated from a different gene transcript.

Laminin alpha2 chain-deficient congenital muscular dystrophy: variable epitope expression in severe and mild cases.

OBJECTIVE: To characterize the expression of distinct fragments of laminin alpha2 chain in patients with partial laminin alpha2 chain deficiency and variable clinical severity. BACKGROUND: Deficiency of laminin alpha2 chain caused by mutations of the LAMA2 gene on chromosome 6q2 account for approximately 50% of cases of congenital muscular dystrophy (CMD) in white patients. The complete absence of laminin alpha2 is usually associated with a severe phenotype affecting skeletal muscle and the peripheral and central nervous systems. METHODS: Quantitative assessment of immunofluorescence to study the expression of C- and N-terminal portions of laminin alpha2 chain in five patients with partial laminin alpha2 chain deficiency and variable phenotype. All five patients showed abnormal T2 signal on brain MRI. RESULTS: Immunohistochemistry of muscle specimens showed preserved or minimally reduced expression of the C-terminal region of the laminin alpha2 chain (67 to 74%), but a marked reduction of the N-terminal region in four patients (13 to 19%). One patient with a mild phenotype had a partial reduction (45%) of the C-terminal and the N-terminal (51%) portions of the laminin alpha2 chain. Two patients were unable to walk or sit, although the C-terminal portion of the laminin alpha2 chain was expressed at significant levels (67 to 74%). In contrast, two patients with a similar expression of the C-terminus (67 to 70%) had a milder phenotype and became ambulatory. It was impossible to predict the phenotypes in these four patients with a strong expression of the C-terminus and with low levels of the N-terminus based on the amount of protein expressed. In addition, the laminin beta2 chain was moderately reduced (54 to 75%) in all patients with laminin alpha2 chain deficiency. A strong correlation between the amount of the C-terminus but not for the N-terminus and laminin beta2 reduction could be observed. CONCLUSIONS: N-terminal antibodies to the laminin alpha2 chain provide a more precise immunohistochemical detection of partially laminin alpha2 chain-deficient CMD. The secondary reduction of laminin beta2 chain may better define laminin alpha2 chain-deficient CMD. More data are needed to predict which portions of C-terminus and midrod region of the laminin alpha2 chain result in a semifunctional protein and a milder phenotype.

Sleep-disordered breathing and respiratory failure in acid maltase deficiency.

BACKGROUND: Sleep-disordered breathing (SDB) and respiratory failure (RF) are complications of acid maltase deficiency (AMD), a rare hereditary myopathy. OBJECTIVE: To define the relationship between lung and respiratory muscle function, to establish incidence and patterns of SDB, and to determine daytime predictors of SDB. METHODS: Sitting and supine lung and respiratory muscle function tests were obtained in 27 subjects with juvenile and adult AMD (aged 39 +/- 19 years) and compared with outcomes of polysomnography. RESULTS: Ventilatory restriction was present in 17/27 subjects. Inspiratory vital capacity (IVC) correlated (p < 0.005) with peak inspiratory muscle pressure (PIP, R = 0.61), respiratory muscle strain (P(0.1)/P(0.1max), R = -0.68), and gas exchange by day (PaO(2): R = 0.71; PaCO(2): R = -0.64) and night (SaO(2): R = 0.73; P(tc)CO(2): R = -0.75). Diaphragm weakness (DW) was present in 13 subjects, 10 of whom had hypercapnic RF (PaCO(2) 65 +/- 7 mm Hg), and was associated with longer disease course. SDB was found in 13 subjects, 12 with DW. It was characterized by REM-sleep hypopneas that, as ventilatory restriction worsened, were complemented by hypoventilation (P(tc)CO(2) > 50 mm Hg) first in REM sleep, then in non-REM sleep (p < 0.005). SDB was predicted by DW (sensitivity 80%, specificity 86%) and nocturnal hypoventilation by IVC < 40% (sensitivity 80%, specificity 93%). Noninvasive ventilation, instituted for daytime respiratory failure or nocturnal hypoventilation, normalized daytime and nocturnal gas exchange (p < 0.005). CONCLUSION: Vital capacity correlates with respiratory muscle function in AMD. Diaphragm weakness is the major cause of SDB and RF. SDB and nocturnal hypoventilation are predictable from daytime function tests.

Secondary calpain3 deficiency in 2q-linked muscular dystrophy: titin is the candidate gene.

BACKGROUND: Tibial muscular dystrophy (TMD), a late-onset dominant distal myopathy, is caused by yet unknown mutations on chromosome 2q, whereas MD with myositis (MDM) is a muscular dystrophy of the mouse, also progressing with age and linked to mouse chromosome 2. For both disorders, linkage studies have implicated titin as a potential candidate gene. METHODS: The authors analyzed major candidate regions in the titin gene by sequencing and Southern blot hybridization, and performed titin immunohistochemistry on TMD patient material to identify the underlying mutation. Western blot studies were performed on the known titin ligands in muscle samples of both disorders and controls, and analysis of apoptosis was also performed. RESULTS: The authors identified almost complete loss of calpain3, a ligand of titin, in the patient with limb-girdle MD (LGMD) with a homozygous state of TMD haplotype when primary calpain3 gene defect was excluded. Apoptotic myonuclei with altered distribution of transcription factor NF-kB and its inhibitor IkBalpha were encountered in muscle samples of patients with either heterozygous or homozygous TMD haplotype. Similar findings were confirmed in the MDM mouse. CONCLUSIONS: These results imply that titin mutations may be responsible for TMD, and that the pathophysiologic pathway following calpain3 deficiency may overlap with LGMD2A. The loss of calpain3 could be a downstream effect of the deficient TMD gene product. The significance of the secondary calpain3 defect for the pathogenesis of TMD was emphasized by similar calpain3 deficiency in the MDM mouse, which is suggested to be a mouse model for TMD. Homozygous mutation at the 2q locus may thus be capable of producing yet another LGMD.