Congenital myopathies

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Use of Clinical and Electrical Myotonia to Differentiate Childhood Myopathies

We retrospectively reviewed 2030 childhood electromyograms performed over an 11-year period (2004-2014). Twenty children (1%) with myotonic discharges were identified and placed into 2 groups. Group A (electrical and clinical myotonia) comprised 9 children (8 with myotonia congenita and 1 with paramyotonia congenita); all of them had diffuse myotonic discharges without clinical weakness or elevated creatine kinase. Group B (electrical myotonia without clinical myotonia) comprised 11 children (4 with inflammatory myopathy; 3, congenital myopathy, 3, muscular dystrophy; and 1, congenital muscular dystrophy). Clinical weakness was demonstrated in all of them and elevated creatine kinase in 6; all had a myopathic electromyogram and scattered myotonic discharges. We conclude that myotonic discharges are a rare but characteristic spontaneous discharge identified during electrodiagnostic studies in children. The presence of electrical and clinical myotonia provides helpful clues to differentiate between various muscle disorders in children.

Mild myopathy is associated with COMP but not MATN3 mutations in mouse models of genetic skeletal diseases

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are skeletal disorders resulting from mutations in COMP, matrilin-3 or collagen IX and are characterised by short-limbed dwarfism and premature osteoarthritis. Interestingly, recent reports suggest patients can also manifest with muscle weakness. Here we present a detailed analysis of two mouse models of the PSACH/MED disease spectrum; ΔD469 T3-COMP (PSACH) and V194D matrilin-3 (MED). In grip test experiments T3-COMP mice were weaker than wild-type littermates, whereas V194D mice behaved as controls, confirming that short-limbed dwarfism alone does not contribute to PSACH/MED-related muscle weakness. Muscles from T3-COMP mice showed an increase in centronuclear fibers at the myotendinous junction. T3-COMP tendons became more lax in cyclic testing and showed thicker collagen fibers when compared with wild-type tissue; matrilin-3 mutant tissues were indistinguishable from controls. This comprehensive study of the myopathy associated with PSACH/MED mutations enables a better understanding of the disease progression, confirms that it is genotype specific and that the limb weakness originates from muscle and tendon pathology rather than short-limbed dwarfism itself. Since some patients are primarily diagnosed with neuromuscular symptoms, this study will facilitate better awareness of the differential diagnoses that might be associated with the PSACH/MED spectrum and subsequent care of PSACH/MED patients.

The spectrum of myotonic and myopathic disorders in a pediatric electromyography laboratory over 12 years

This study assessed the spectrum of disorders associated with electrophysiologic myotonia in a pediatric electromyography laboratory. Records of 2234 patients observed in the Electromyography Laboratory at Boston Children's Hospital from 2000-2011 were screened retrospectively for electrophysiologic diagnoses of myotonia and myopathy. Based on electromyography, 11 patients manifested myotonic discharges alone, eight exhibited both myotonic discharges and myopathic motor unit potentials, and 54 demonstrated myopathic motor unit potentials alone. The final diagnoses of patients with myotonic discharges alone included myotonia congenita, paramyotonia congenita, congenital myopathy, and Pompe disease (acid maltase deficiency). The diagnoses of patients with both myotonic discharges and myopathic motor unit potentials included congenital myopathy and non-Pompe glycogen storage diseases. Myotonic discharges are rarely observed in a pediatric electromyography laboratory, but constitute useful findings when present. The presence or absence of concurrent myopathic motor unit potentials may help narrow the differential diagnosis further.

A mouse model offers novel insights into the myopathy and tendinopathy often associated with pseudoachondroplasia and multiple epiphyseal dysplasia

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are relatively common skeletal dysplasias belonging to the same bone dysplasia family. PSACH is characterized by generalized epi-metaphyseal dysplasia, short-limbed dwarfism, joint laxity and early onset osteoarthritis. MED is a milder disease with radiographic features often restricted to the epiphyses of the long bones. PSACH and some forms of MED result from mutations in cartilage oligomeric matrix protein (COMP), a pentameric glycoprotein found in cartilage, tendon, ligament and muscle. PSACH-MED patients often have a mild myopathy characterized by mildly increased plasma creatine kinase levels, a variation in myofibre size and/or small atrophic fibres. In some instances, patients are referred to neuromuscular clinics prior to the diagnosis of an underlying skeletal dysplasia; however, the myopathy associated with PSACH-MED has not previously been studied. In this study, we present a detailed study of skeletal muscle, tendon and ligament from a mouse model of mild PSACH harbouring a COMP mutation. Mutant mice exhibited a progressive muscle weakness associated with an increased number of muscle fibres with central nuclei at the perimysium and at the myotendinous junction. Furthermore, the distribution of collagen fibril diameters in the mutant tendons and ligaments was altered towards thicker collagen fibrils, and the tendons became more lax in cyclic strain tests. We hypothesize that the myopathy in PSACH-MED originates from an underlying tendon and ligament pathology that is a direct result of structural abnormalities to the collagen fibril architecture. This is the first comprehensive characterization of the musculoskeletal phenotype of PSACH-MED and is directly relevant to the clinical management of these patients.

The value of electromyography in the aetiological diagnosis of hypotonia in infants and toddlers

INTRODUCTION

During the first two years of life, hypotonia may be the only symptom of a central or peripheral nervous system disorder. We propose to assess the sensitivity of electroneuromyography (ENMG) in the aetiological diagnosis of hypotonia of neuromuscular origin in infants and toddlers.

METHOD

This is a retrospective, single-centre study with revision of the files of the 37 children aged between zero and 24 months who, between 1994 and 2006, underwent an ENMG in the etiological approach of their hypotonia and had a final diagnosis of neuromuscular disease.

RESULTS

All the 13 patients with spinal muscular atrophy or Charcot Marie-Tooth disease displayed neurogenic alterations on the electromyography (EMG). Among the 24 children ultimately diagnosed with myopathies, five only displayed myogenic alterations when tested before the age of two. Sixteen had normal EMG results and three showed neurogenic alterations.

DISCUSSION AND CONCLUSION

In infants presenting with hypotonia, ENMG is useful for the diagnosis of peripheral neuropathy. Normal ENMG is relatively common for confirmed muscle disorders in infants whereas myogenic alterations seem more unusual, so that muscle biopsy appears unquestionable. In a few cases, early onset myopathies may present with a neurogenic ENMG pattern. Such a result should not invalidate the clinically presumed diagnosis of myopathy and would indicate on the contrary the need for a muscle biopsy.

Comparison of clinical characteristics between congenital fiber type disproportion myopathy and congenital myopathy with type 1 fiber predominance

Congenital myopathies are clinical and genetic heterogeneous disorders characterized by skeletal muscle weakness and specific structural changes in muscle fiber. Congenital myopathy with fiber type disproportion (CFTD) is an established disorder of congenital myopathy. CFTD is characterized by non-progressive childhood neuromuscular disorders with a relatively good prognosis and type 1 fiber predominance and smallness. Congenital myopathy with type 1 fiber predominance (CMT1P) is also a distinct entity of congenital myopathy characterized by non-progressive childhood neuromuscular disorders and type 1 fiber predominance without smallness. Little is known about CMT1P. Clinical characteristics, including dysmorphic features such as hip dislocation, kyphoscoliosis, contracture, and high arch palate, were analyzed along with laboratory and muscle pathologies in six patients with CMT1P and three patients with CFTD. The clinical manifestations of CFTD and CMT1P were similar. However, the frequency of dysmorphic features is less in CMT1P than in CFTD. Long term observational studies of CMT1P are needed to determine if it will change to another form of congenital myopathy or if CMT1P is a distinct clinical entity.

Central core disease: atypical case with respiratory insufficiency in an intensive care unit

Central core disease is a rare congenital myopathy characterized by formation of typical cores in myofibrils. We report an atypical case of central core disease with respiratory insufficiency in the late stage of congenital myopathy. A 13-year-old girl was admitted to the intensive care unit with the diagnosis of respiratory distress syndrome. Ventilatory support was initiated. After 2 weeks of follow-up, the Division of Pediatric Neurology was consulted owing to the failure to wean her from the ventilator. Clinical and electromyographic features were in favor of primary muscle disease. Muscle biopsy revealed typical cores in type 1 muscle fibers, which were diagnostic for central core disease. This case was presented to emphasize that patients with respiratory distress who cannot be weaned from the ventilator should be evaluated for central core disease with an atypical presentation.

A new syndrome of myopathy with muscle spindle excess

Arthrogryposis may result from various neuromuscular or connective tissue disorders leading to in utero hypokinesia or akinesia and the prenatal development of joint contractures. We report the case of a preterm neonate born with arthrogryposis and flaccid quadriplegia that led to the diagnosis of myopathy with muscle spindle excess. The rare and unusual histopathologic abnormality associated with the myopathy illustrated in this case has been described in only three other cases in the medical literature. The concurrence of hypertrophic cardiomyopathy, arthrogryposis, and myopathy with muscle spindle excess suggests the presence of a newly described syndrome. This case clearly demonstrates that specific prenatal ultrasonographic findings combined with the presenting clinical manifestations should promptly raise the suspicion of a neuromuscular disorder.

EMG and nerve conduction studies in children with congenital muscular dystrophy

Motor and sensory nerve conduction velocities (NCVs) and needle electromyography (EMG) results were reviewed in 26 children with different types of congenital muscular dystrophy (CMD), including patients with mutations in the genes LAMA2, FKRP, and COL6A2. In every patient, at least one EMG examination detected myopathic changes that were predominant in proximal muscles, although EMG performed at birth was normal in two patients. Brief bursts of high-frequency repetitive discharges were electrically elicited in four patients. Uniformly slowed motor NCVs without signs of denervation were observed in seven patients: five merosin-deficient, one merosin-positive, and one with unavailable merosin status. The merosin-deficient neuropathy also involved sensory nerves in three patients and worsened with age in two. In conclusion, myopathic EMG changes were typical and early findings in all types of CMD. An associated neuropathy was detected in most patients with merosin-deficient CMD, and also in a child with normal merosin expression.

Congenital myopathies/dystrophies

The congenital myopathies and congenital muscular dystrophies are a group of relatively infrequent neuromuscular disorders. Ultimate understanding of these disorders, however, will undoubtedly shed considerable light on skeletal muscle development and function. Three classical congenital myopathies are central core disease, nemaline myopathy, and centronuclear myopathy. The congenital muscular dystrophies are often distinguished by whether or not they are associated with clinically evident cerebral involvement.

Electrodiagnostic approach to the patient with suspected myopathy

Uncovering the cause of a suspected myopathy may be challenging. However, a careful approach starts with utilizing the wealth of available information regarding the clinical and laboratory features of myopathy. Electrodiagnostic testing is then obtained (in most cases). Recognition of the pattern of EMG findings in light of the clinical and laboratory features should narrow the differential diagnosis and dictate the next steps in the evaluation. Histopathologic or molecular studies, or both may follow. Ultimately, this approach usually allows the clinician to make the correct diagnosis.

Myopathy with muscle spindle excess: A new congenital neuromuscular syndrome?

An infant presented with congenital weakness, hypotonia, arthrogryposis, atrial tachycardia, and a left intra-abdominal neuroblastoma. Muscle biopsy revealed marked excess of muscle spindles with atrophy of extrafusal fibers. The patient expired at age 14 months from progressive cardiorespiratory failure. Postmortem examination demonstrated muscle-spindle excess in other muscles, along with hypertrophic obstructive cardiomyopathy and organomegaly. Muscle spindle excess has previously been reported in two patients with Noonan syndrome and progressive hypertrophic cardiomyopathy. Muscle spindle excess with hypertrophic cardiomyopathy, organomegaly, and, possibly, congenital neuroblastoma suggests a syndromic association and may represent an unusual form of congenital myopathy.

[Anesthesia in a child presenting a anhydrotic ectodermic dysplasia associated with a multiminicore myopathy]

PURPOSE

To report the perioperative management of anesthesia and analgesia in a child presenting with the association of multiminicore myopathy (MMM) and anhydrotic ectodermic dysplasia (AED).

CLINICAL FEATURES

An eight-year-old girl was admitted for elective orthopedic surgery of the lower limbs. AED is a congenital dermatosis characterized by the absence of sweating and subsequent problems with thermoregulation; in addition, maxillary hypoplasia and abnormal teeth can render intubation difficult. MMM is a rare congenital myopathy characterized by proximal muscle weakness, stable in time or with a slow and progressive evolution. It can involve respiratory muscles and be associated with severe cardiomyopathy. Moreover, MMM shares some characteristics with Central Core Disease which is known to be associated with malignant hyperthermia. Since depolarizing muscle relaxants and halogenated agents could not be used, a combined propofol-based intravenous anesthesia with lumbar epidural analgesia was chosen. This combination provided stable anesthesia, smooth recovery and excellent analgesia during and after the operation, without complications. Temperature was monitored closely during surgery and in the postoperative period.

CONCLUSIONS

The association of MMM and AED requires rapid distinction between hyperthermia secondary to anhydrosis and malignant hyperthermia. The management should provide a "trigger-free" anesthetic and optimal postoperative analgesia without sedation. If appropriate for the surgical procedure, a combination of general with regional anesthesia is particularly attractive in achieving these objectives.

Congenital fiber type disproportion: severe form with marked improvement

A 30-month-old male exhibited marked hypotonia at birth accompanied by respiratory distress necessitating ventilator support. He subsequently demonstrated marked improvement in muscle power. He became independent of the respirator at 21 days of age and was able to sit without support at 11 months and walked alone at 24 months. Histopathologic analysis of the quadriceps femoris muscle confirmed the diagnosis of congenital fiber type of disproportion at 11 months of age. No other studies have described a patient with a severe neonatal form of congenital fiber type of disproportion who demonstrated such clear improvement. Physicians should be aware of this possibility when they interact with such patients and their families.

Myotubular myopathy: morphological, immunohistochemical and clinical variation

Myotubular myopathy frequently presents in male infants with severe generalised muscular hypotonia and weakness associated with ventilatory insufficiency, and is diagnosed on biopsy by the presence of many fibres with central nuclei and mitochondrial aggregation. In a 6-year period, we have investigated five unrelated patients with clinical and pathological features suggesting an X-linked myotubular myopathy, including one female patient. In one male infant, a biopsy of vastus lateralis showed less than 2% centrally-nucleated fibres, while biceps brachii showed up to 15% centrally-nucleated fibres. Immunohistochemical expression of the neural cell adhesion molecule (CD56) was more intense in the biceps muscle than in vastus lateralis, while expression of desmin and vimentin was similar. Morphometric evaluation of tissue from each of the patients revealed a wide spread of values for the number of centrally-nucleated fibres per microscopic field, and variation in the extent of immunohistochemical expression of NCAM, utrophin, laminin alpha 5 chain, vimentin and HLA1 antigen. These variations in the manifestations of myotubular myopathy have not been previously described, and will need to be correlated with the increasing knowledge of the mutations in the MTM1 gene coding for myotubularin.

Oculopharyngeal muscular dystrophy, other ocular myopathies, and progressive external ophthalmoplegia

Progressive external ophthalmoplegia comprises many different disorders. Those of childhood onset can be separated from juvenile or adult onset. Among those of later onset the most common causes are oculopharyngeal muscular dystrophy, oculopharyngodistal muscular dystrophy and the several mitochondrial disorders, especially those with large deletions of mitochondrial DNA (mtDNA) (sporadic), those with maternal inheritance (point mutations), or the autosomal dominant forms with multiple deletions of mtDNA. Ophthalmoplegia of presumably neurogenic origin is seen in some of the familial spinocerebellar ataxias. Advances in molecular genetics should provide information about affected gene products and, therefore, pathogenesis.

Familial mixed congenital myopathy with rigid spine phenotype

We describe a father and daughter with a rigid spine syndrome and proximal myopathy. The index patient was a 42-year-old man, who died from respiratory failure after a lifelong, slowly progressive proximal myopathy and a rigid spine phenotype. This was morphologically characterized by cytoplasmic bodies, increased desmin, features of reducing-body myopathy, and sarcoplasmic and intranuclear tubulofilamentous inclusions. These cases are characterized by an early onset and possibly autosomal-dominant inheritance, with associated complex structural hallmarks of both desmin-related and inclusion body myopathies. Together they may be defined as a complex mixed congenital myopathy with a rigid spine phenotype.

Congenital myopathy in Braunvieh x Brown Swiss calves

A hitherto unknown skeletal muscle disorder is described in six Braunvieh x Brown Swiss calves. The animals showed rapidly progressing muscular weakness and became recumbent within 2 weeks of birth. Histological examination of skeletal muscle revealed a marked variation in muscle fibre size, internally placed nuclei, segmental loss of cross-striation with disorganization of myofibrils, and accumulation of nemaline rods. The most distinctive histological finding was intracytoplasmic, homogeneous, mostly crescent-shaped areas at the periphery of numerous muscle fibres. Electron microscopically, accumulations of tightly packed, parallel filamentous structures, about 20 nm in diameter, were detected in these areas. Enzyme histochemistry showed that all muscle fibre types were affected. Vimentin and dystrophin immunohistochemistry revealed normal antigen distribution within connective tissue components and at the periphery of each muscle fibre, respectively. The lesions could be readily distinguished from other neurological and neuromuscular disorders previously described in Braunvieh x Brown Swiss or American Brown Swiss Cattle. The disease appears to be a novel congenital myopathy in this breed, and a hereditary aetiology is suspected.

Congenital myopathies

The congenital myopathies (CM) are a group of non or little progressive neuromuscular conditions, often hereditary, delineated by morphological techniques, ie, enzyme histochemistry and electron microscopy. The catalogue of CM entailing well known "classic" conditions as central core disease, nemaline myopathy, and centronuclear myopathy has continuously been expanded, now comprising some 40 conditions. Nosologic advances have occurred with immunohistochemical techniques that show generalized or focal protein abnormalities within muscle fibers of certain CM, but at much slower pace as to localization of CM genes. So far, only those for central core disease, nemaline myopathy, and myotubular myopathy have been reported. Epidemiological rarity and nosographic controversy of CM have contributed to this lack of molecular genetic progress in CM.

[Contribution of the electromyogram in the diagnosis of infantile spinal muscular atrophy in the neonatal period]

BACKGROUND

The acute form of Werdnig-Hoffman disease, infantile spinal muscular atrophy type I (SMA I), is characterized by severe paralytic hypotonia with neurogenic electromyographic (EMG) pattern and specific histologic features.

PATIENTS

Four cases of very severe SMA I suffering from generalized muscle weakness at birth were included in the study.

RESULTS

The neurogenic EMG pattern was observed at the first exam performed between D2 and D46. The muscular biopsy performed between D18 and D45 showed only a mild decrease of the muscle fiber size without grouping of fiber types.

CONCLUSION

In those forms of SMA I with a neonatal clinical onset, the diagnosis is assessed by clinical and EMG findings while early muscular biopsy can be misleading. EMG is the relevant diagnostic test which confirms the anterior horn cell disease and can justify the DNA study.

[Nemalinic myopathy with intracytoplasmic spheroid bodies: report of a case]

The authors report the case of a female patient, 18 years of age, with slowly progressing weakness in upper and lower limbs since childhood. There were no significant antecedents. The neurologic examination showed mild proximal and distal motor deficit with a slight muscular retraction at the level of shoulders, elbows, coxofemural joints, knees and ankles; muscular hypotrophy in the legs and feet; reflexes were present and sensitivity was normal. Creatinephosphokinase showed an increase of one and a half times the normal value. Electroneuromyography: decrease in the amplitude and duration of action potentials and excessive recruitment of motor units, compatible with a primary muscular disease. A muscle biopsy with frozen sections (HE, Gomori, PAS, ATPases, NADH, SDH, acid and alcaline phosphatases, cytochrome oxidase and Oil-red-o) revealed a primary muscular disease characterized by the presence of nemalinic and intracytoplasmic spheroid bodies. Nemalinic bodies have been described with different structural abnormalities of muscle fibers; however, such association is rare. This is the second case report of concomitant occurrence of nemalinic and spheroid bodies.

Acquired multifocal myofibrillar disruption selective of type II fibres

We report three cases of patients who complained of myalgia showing histological features similar to tubular aggregates in their muscle biopsies. All had an elevated erythrocyte sedimentation rate without any evidence of infectious or autoimmune disease. On electron microscopy, small areas of myofibrillar degeneration, selectively in type II fibres, were found in all patients, but no tubular aggregates were seen. Although the pathogenesis of these lesions is unclear, it does seem that this condition is acquired and transient.

Childhood-onset oculopharyngodistal myopathy with chronic intestinal pseudo-obstruction

Oculopharyngodistal myopathy is characterized by the adult onset of ptosis, external ophthalmoplegia, dysphagia, and distal weakness. Although dysphagia is common, other gastrointestinal involvement has not been described. We report a case with childhood onset who developed chronic intestinal pseudo-obstruction. Other myopathies associated with ophthalmoplegia and intestinal pseudo-obstruction such as mitochondrial cytopathies were excluded. Whether oculopharyngodistal myopathy is a variant of oculopharyngeal muscular dystrophy or a distinct neuromuscular disorder is unknown and requires further study.

Recent advances in muscular dystrophies and myopathies

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Neuromuscular disease: rehabilitation and electrodiagnosis. 3. Muscle disease

The purpose of this section of the Self-Directed Physiatric Education Program Study Guide on rehabilitation in diseases affecting nerve and muscle is to assist practitioners and trainees in physical medicine and rehabilitation by providing practical information about inherited and acquired muscle diseases. It emphasizes clinical management issues and new developments in commonly encountered muscle disorders.

Immunophenotyping of congenital myopathies: disorganization of sarcomeric, cytoskeletal and extracellular matrix proteins

We have studied the expression and distribution patterns of the intermediate filament proteins desmin and vimentin, the sarcomere components titin, nebulin and myosin, the basement membrane constituents collagen type IV and laminin, and the reticular layer component collagen type VI in skeletal muscle of patients with "classic" congenital myopathies (CM), using indirect immunofluorescence assays. In all biopsy specimens obtained from patients with central core disease (CCD), nemaline myopathy (NM), X-linked myotubular myopathy (XLMTM) and centronuclear myopathy (CNM), disease-specific desmin disturbances were observed. Vimentin was present in immature fibres in severe neonatal NM, and as sarcoplasmic aggregates in one case of CNM, while the amounts of vimentin and embryonic myosin, observed in XLMTM, decreased with age of the patients. Abnormal expression of myosin isoforms was found in several CM biopsies, although the organization of myosin and other sarcomere components was rarely disturbed. Basement membrane and reticular layer proteins were often prominently increased in severe cases of CM. We conclude that (i) desmin is a marker for individual types of CM and might be used for diagnostic purposes; (ii) the expression patterns of the differentiation markers desmin, vimentin and embryonic myosin in XLMTM, point either to a postnatal muscle fibre maturation or to a variable time-point of maturational arrest in individual patients; (iii) the correlation between the distribution patterns of extracellular matrix proteins and clinical presentation points to a role of these proteins in pathophysiology of CM.

A mutation in the alpha tropomyosin gene TPM3 associated with autosomal dominant nemaline myopathy

Nemaline myopathies are diseases characterized by the presence in muscle fibres of pathognomonic rod bodies. These are composed largely of alpha-actinin and actin. We have identified a missense mutation in the alpha-tropomyosin gene, TPM3, which segregates completely with the disease in a family whose autosomal dominant nemaline myopathy we had previously localized to chromosome 1p13-q25. The mutation substitutes an arginine residue for a highly conserved methionine in a putative actin-binding site near the N terminus of the alpha-tropomyosin. The mutation may strengthen tropomyosin - actin binding, leading to rod body formation, by adding a further basic residue to the postulated actin-binding motif.