Monica Ohlsson

Mitochondrial abnormalities in inclusion-body myositis.

Mitochondrial changes are frequently encountered in sporadic inclusion-body myositis (s-IBM). Cytochrome c oxidase (COX)-deficient muscle fibers and large-scale mitochondrial DNA (mtDNA) deletions are more frequent in s-IBM than in age-matched controls. COX deficient muscle fibers are due to clonal expansion of mtDNA deletions and point mutations in segments of muscle fibers. Such segments range from 75 μm to more than 1,000 μm in length. Clonal expansion of the 4977 bp “common deletion” is a frequent cause of COX deficient muscle fiber segments, but many other deletions also occur. The deletion breakpoints cluster in a few regions that are similar to what is found in human mtDNA deletions in general. Analysis in s-IBM patients of three nuclear genes associated with multiple mtDNA deletions, POLG1, ANT1 and C10orf2, failed to demonstrate any mutations. In s-IBM patients with high number of COX-deficient fibers, the impaired mitochondrial function probably contribute to muscle weakness and wasting. Treatment that has positive effects in mitochondrial myopathies may be tried also in s-IBM.

Hereditary myopathy with early respiratory failure associated with a mutation in A-band titin

Hereditary myopathy with early respiratory failure and extensive myofibrillar lesions has been described in sporadic and familial cases and linked to various chromosomal regions. The mutated gene is unknown in most cases. We studied eight individuals, from three apparently unrelated families, with clinical and pathological features of hereditary myopathy with early respiratory failure. The investigations included clinical examination, muscle histopathology and genetic analysis by whole exome sequencing and single nucleotide polymorphism arrays. All patients had adult onset muscle weakness in the pelvic girdle, neck flexors, respiratory and trunk muscles, and the majority had prominent calf hypertrophy. Examination of pulmonary function showed decreased vital capacity. No signs of cardiac muscle involvement were found. Muscle histopathological features included marked muscle fibre size variation, fibre splitting, numerous internal nuclei and fatty infiltration. Frequent groups of fibres showed eosinophilic inclusions and deposits. At the ultrastructural level, there were extensive myofibrillar lesions with marked Z-disc alterations. Whole exome sequencing in four individuals from one family revealed a missense mutation, g.274375T>C; p.Cys30071Arg, in the titin gene (TTN). The mutation, which changes a highly conserved residue in the myosin binding A-band titin, was demonstrated to segregate with the disease in all three families. High density single nucleotide polymorphism arrays covering the entire genome demonstrated sharing of a 6.99 Mb haplotype, located in chromosome region 2q31 including TTN, indicating common ancestry. Our results demonstrate a novel and the first disease-causing mutation in A-band titin associated with hereditary myopathy with early respiratory failure. The typical histopathological features with prominent myofibrillar lesions and inclusions in muscle and respiratory failure early in the clinical course should be incentives for analysis of TTN mutations.

Defective regulation of contractile function in muscle fibres carrying an E41K β‐tropomyosin mutation

A novel E41K β‐tropomyosin (β‐Tm) mutation, associated with congenital myopathy and muscle weakness, was recently identified in a woman and her daughter. In both patients, muscle weakness was coupled with muscle fibre atrophy. It remains unknown, however, whether the E41K β‐Tm mutation directly affects regulation of muscle contraction, contributing to the muscle weakness. To address this question, we studied a broad range of contractile characteristics in skinned muscle fibres from the two patients and eight healthy controls. Results showed decreases (i) in speed of contraction at saturated Ca2+ concentration (apparent rate constant of force redevelopment (ktr) and unloaded shortening speed (V0)); and (ii) in contraction sensitivity to Ca2+ concentration, in fibres from patients compared with controls, suggesting that the mutation has a negative effect on contractile function, contributing to the muscle weakness. To investigate whether these negative impacts are reversible, we exposed skinned muscle fibres to the Ca2+ sensitizer EMD 57033. In fibres from patients, 30 μm of EMD 57033 (i) had no effect on speed of contraction (ktr and V0) at saturated Ca2+ concentration but (ii) increased Ca2+ sensitivity of contraction, suggesting a potential therapeutic approach in patients carrying the E41K β‐Tm mutation.

New morphologic and genetic findings in cap disease associated with β-tropomyosin (TPM2) mutations

Objective: Mutations in the β-tropomyosin gene (TPM2) are a rare cause of congenital myopathies with features of nemaline myopathy and cap disease and may also cause distal arthrogryposis syndromes without major muscle pathology. We describe the muscle biopsy findings in three patients with cap disease and novel heterozygous mutations in TPM2. Methods: Three unrelated patients with congenital myopathy were investigated by muscle biopsy and genetic analysis. Results: All three patients had early-onset muscle weakness of variable severity and distribution. Muscle biopsy demonstrated in all three patients near uniformity of type 1 fibers and an unusual irregular and coarse-meshed intermyofibrillar network. By electron microscopy, the myofibrils were broad and partly split, and the Z lines appeared jagged. In one of the patients caps structures were identified only by electron microscopy, and in one patient they were identified only in a second biopsy at adulthood. Three novel, de novo, heterozygous mutations in TPM2 were identified: a three–base pair deletion in-frame (p.Lys49del), a three-base pair duplication in-frame (p.Gly52dup), and a missense mutation (p.Asn202Lys). Conclusions: Mutations in TPM2 seem to be a frequent cause of cap disease. Because cap structures may be sparse, other prominent features, such as a coarse-meshed intermyofibrillar network and jagged Z lines, may be clues to correct diagnosis and also indicate that the pathogenesis involves defective assembly of myofilaments. GLOSSARY: ATPase = adenosine triphosphatase; CK = creatine kinase; DA = distal arthrogryposis; NADH-TR = nicotinamide adenine dinucleotide–tetrazolium reductase; Tm = tropomyosin.

TPM3 mutation in one of the original cases of cap disease.

Cap disease is a rare congenital skeletal muscle disorder characterized by the presence of cap-like structures which are well demarcated and peripherally located under the sarcolemma and show abnormal accumulation of sarcomeric proteins.1,2 Clinical features are early onset of hypotonia and nonprogressive or slowly progressive muscle weakness. Respiratory problems are common. Five dominant mutations have been reported in association with cap disease, all in the β-tropomyosin (βTm) gene ( TPM2 ).3-5 We report a mutation in the αTmslow (γTm) gene ( TPM3 ) in a patient with cap disease, supporting the concept that cap disease is genetically heterogeneous and closely related to nemaline myopathy.4 ### Case report. This 38-year-old woman was first described in 2002 (patient K.D.).2 There was no family history of neuromuscular disorder. She was born at term after an uncomplicated pregnancy, labor, and delivery. She had somewhat delayed motor milestones and at age 5 years mild motor difficulties were noted. She could not keep up with her peers and could not run. At age 18 years, a muscle biopsy was performed and the diagnosis of congenital myopathy (cap disease) was established (figure). Nemaline rods were not identified. EMG showed a myopathic pattern. She was admitted to hospital with pneumonia when she was 21 years old …

Myopathies associated with β-tropomyosin mutations

Mutations in TPM2, encoding β-tropomyosin, have recently been found to cause a range of muscle disorders. We review the clinical and morphological expression of the previously reported mutations illustrating the heterogeneity of β-tropomyosin-associated diseases and describe an additional case with a novel mutation. The manifestations of mutations in TPM2 include non-specific congenital myopathy with type 1 fibre predominance, nemaline myopathy, cap disease and distal arthrogryposis. In addition, Escobar syndrome with nemaline myopathy is a manifestation of homozygous truncating β-tropomyosin mutation. Cap disease appears to be the most common morphological manifestation. A coarse intermyofibrillar network and jagged Z lines are additional frequent changes. The dominant β-tropomyosin mutations manifest either as congenital myopathy or distal arthrogryposis. The various congenital myopathies are usually associated with moderate muscle weakness and no congenital joint contractures. The distal arthrogryposis syndromes associated with TPM2 mutations include the less severe forms, with congenital contractures mainly of the hands and feet and mild or no muscle weakness. The dominant TPM2 mutations include amino acid deletions/insertions and missense mutations. There is no clear relation between the type of mutations or the localisation of the mutated residue in the β-tropomyosin molecule and the clinical and morphological phenotype.

Follow-up of nemaline myopathy in two patients with novel mutations in the skeletal muscle alpha-actin gene (ACTA1)

Nemaline myopathy has been associated with mutations in five different genes, which all encode protein components of the sarcomeric thin filaments. We report follow-up studies in two children with mutations not previously described in skeletal muscle alpha-actin (ACTA1). Case 1 was a male patient who after birth suffered from pronounced muscle weakness and hypotonia. Muscle biopsy showed small fibers with numerous rods. He failed to achieve any motor milestones. At the age of 17 he required 24 h ventilator support. He could not lift his arms against gravity, but he could use his hands to control his electric wheelchair. The muscle biopsy showed marked replacement of muscle tissue by fat and connective tissue. Only few fibers showed nemaline rods. He had a de novo, heterozygous mutation, G268D in ACTA1. Case 2 was a female patient with feeding difficulties and mild hypotonia in the neonatal period. Muscle biopsy showed hypoplastic muscle fibers and numerous rods. At 11 years of age she walked and moved unhindered and could run fairly well. She had a de novo, heterozygous mutation, K373E, in ACTA1. These two patients illustrate the marked variability in the clinical features of nemaline myopathy in spite of similar muscle pathology in early childhood. The severe muscle atrophy with replacement of fat and connective tissue in case 1 demonstrates the progressive nature of nemaline myopathy in some cases. The described two mutations add to the previously reported mutations in ACTA1 associated with nemaline myopathy.