Nemaline Myopathy 4

A number sign (#) is used with this entry because of evidence that nemaline myopathy-4 (NEM4) and cap myopathy-2 (CAPM2) are caused by heterozygous mutation in the tropomyosin-2 gene (TPM2; 190990) on chromosome 9p13.

For a discussion of genetic heterogeneity of nemaline myopathy, see 161800.

Clinical Features

Donner et al. (2002) reported a woman with nemaline myopathy who presented at age 12 years with difficulty in walking. She had a mild form of the disorder with asymmetric limb involvement and mild facial and neck flexor weakness. She used a wheelchair beginning at age 48 years and died of respiratory causes at age 51. In a second affected family, of Bosnian origin, the proband was a 4-year-old boy who had feeding difficulties and severe hypotonia from birth. He had delayed motor development, but achieved ambulation. His affected mother had never been able to run. She had asymmetric limb involvement, myopathic facies, high-arched palate, and weak neck flexor muscles. One muscle biopsy from the son showed type 1 fiber predominance consistent with nemaline myopathy, but the finding of nemaline rods was equivocal; however, Donner et al. (2002) noted that the quantity of nemaline rods can vary substantially in affected patients.

Tajsharghi et al. (2007) reported a 66-year-old woman and her 35-year-old daughter with congenital, slowly progressive muscle weakness. Both had neonatal hypotonia, with respiratory insufficiency and feeding difficulties, respectively, delayed motor milestones, and moderate proximal muscle weakness at onset, which later progressed to distal muscle weakness. Other features included facial diplegia with ptosis, long narrow face, high-arched palate, micrognathia, and restricted ventilatory capacity with nocturnal hypoventilation. Serum creatine kinase was normal in both. Muscle biopsy in the daughter at age 2.5 years and the mother at age 32 years showed small type 1 fibers and fiber size variation with no inclusions. Muscle biopsy in the daughter at 26 years showed cap-like structures composed of disorganized myofibrils and thickened Z bands sharply demarcated from the rest of the fiber. There were no nemaline rods. Repeat biopsy in the mother at age 57 years showed numerous subsarcolemmal nemaline rods within cap-like structures and numerous ragged-red fibers with accumulation of abnormal mitochondria.

Mokbel et al. (2013) reported 8 patients from 5 unrelated families with nemaline myopathy, all of whom carried the same heterozygous TPM2 mutation (K7del; 190990.0009). The most common presentation was joint contractures affecting the distal and proximal joints in infancy or early childhood, although 3 patients from 1 family presented with congenital contractures of the elbows, knees, and ankles at birth. Most children had an awkward gait and many required release of the Achilles tendon. Many patients also developed contractures of the jaw, long finger flexors, shoulders, and hips. Most patients developed proximal lower-limb muscle weakness as adolescents or adults, and some also had weakness of the proximal upper limbs. Two patients lost ambulation in late adulthood. Only 1 patient was noted to have generalized muscle hypertonia. All had normal cardiac function. Creatine kinase was usually increased. Muscle biopsies showed nemaline bodies, often with subsarcolemmal accumulations, variation in myofiber size, and type 1 fiber predominance and atrophy. Three biopsies showed core-like areas with decreased mitochondrial density.

Davidson et al. (2013) reported 4 unrelated families with a congenital myopathy associated with distal contractures of the upper and lower limbs. In the first family, 5 males spanning 3 generations had childhood onset of distal lower extremity weakness and atrophy resulting in gait instability. The disorder was progressive, eventually resulting in loss of ambulation in the oldest patient. In addition, affected individuals had contractures of the wrist and Achilles tendon, pseudocamptodactyly, and difficulties with mouth opening (trismus). Muscle biopsy showed a myopathy with increased variation in fiber size, atrophic and hypertrophic fibers, nemaline rods, cap structures, and core-like disruptions as evidenced by oxidative staining. The 3 other families (families 2, 3, and 4), presented with a phenotype reminiscent of DA7 (158300) with limited jaw opening, but genetic analysis excluded mutation in the MYH8 gene (160741). Features included delayed motor development, childhood-onset gait difficulties, muscle weakness and atrophy, and distal contractures, with progression of the disorder. The severity was variable; only some patients lost independent ambulation. Additional more variable features included proximal contractures, kyphosis, and short trunk and neck. Serum creatine kinase was sometimes increased, and muscle biopsies showed myopathic changes, including atrophic fibers, type 1 fiber predominance, internal nuclei, fibrosis, nemaline rods, and sometimes core or minicore-like areas on oxidative staining. All patients from the 4 families carried the same heterozygous mutation in the TPM2 gene (Kdel7; 190990.0009).

Cap Myopathy 2

Fidzianska et al. (1981) first described cap myopathy in a 7-year-old boy with congenital myopathy. He never walked and showed generalized muscle wasting and weakness. Other features included high-arched palate, narrow face, elongated funnel chest, lordosis, and kyphoscoliosis. Skeletal muscle biopsy showed fiber size variability with about 70% of fibers containing a PAS-positive substance forming a partial ring around the fibers in the peripheral part of the fiber: so-called 'caps.' ATPase staining of the caps was negative. Electron microscopy showed abnormally arranged myofibrils with an abnormal sarcomere pattern and irregular or streaming Z lines. Fidzianska et al. (1981) postulated a defect in the fusion and synthesis of muscle protein.

Lehtokari et al. (2007) reported a 36-year-old man with muscle weakness since childhood. He had delayed motor development and was never able to run. At age 11 years, he showed myopathic facies, dysarthria, hyperlordosis, small muscle bulk, and generalized muscle weakness. At age 33 years, he developed acute respiratory insufficiency with pneumonia. Other features included high-arched palate and ptosis. Skeletal muscle biopsies as a child and adult showed fiber type disproportion with hypotrophy of type I fibers, hypertrophy of type II fibers, absence of type IIb fibers, and cap-like myofibrillar lesions mostly in type I fibers that stained positive for tropomyosin. Electron microscopy showed modified subsarcolemmal areas, disoriented myofibrils, and thickened Z line extensions with some short, rod-like structures. Lehtokari et al. (2007) noted the similarities of cap myopathy to nemaline myopathy.

Ohlsson et al. (2008) reported 3 unrelated patients with cap myopathy. Although the phenotypic severity was variable, all had hypotonia in infancy and showed difficulty in walking and running in childhood. Other features included kyphoscoliosis, Gowers sign, winged scapulae, and decreased pulmonary vital capacity. Skeletal muscle biopsies showed uniformity of type 1 fibers, cap structures, and a coarse irregular intermyofibrillar network. Nemaline rods were not identified. The authors noted that cap structures were only identified by electron microscopy in 1 patient and only on a second biopsy in adulthood in another.

Clarke et al. (2009) reported a 14-year-old Australian girl who had a history of hypotonia since infancy, delayed motor development, and slow running. She had generalized muscle wasting and weakness, particularly in the proximal muscles, and hyporeflexia. Other features included mild facial weakness, nasal voice, high-arched palate, long thin face, and mild micrognathia. Cardiac examination showed decreased systolic function, with an ejection fraction of 42%, mild mitral valve prolapse, and borderline aortic root dilatation. She also had central hypoventilation and restrictive lung disease with decreased forced vital capacity. Skeletal muscle biopsy at age 10 showed fiber type variability, and she received an initial diagnosis of congenital fiber-type disproportion (CFTD; 255310), a diagnosis of exclusion. However, about 4% of fibers were later noted to have caps, leading to a final histologic diagnosis of cap myopathy. There was type 1 fiber predominance. Electron microscopy showed irregular and thickened Z-lines, and the caps contained disorganized thin filaments. No classic nemaline rods were observed. Clarke et al. (2009) noted that the cardiac involvement in this patient was an unusual finding.

Inheritance

The transmission pattern of nemaline myopathy in the families reported by Mokbel et al. (2013) was consistent with autosomal dominant inheritance.

Molecular Genetics

In a woman with childhood-onset nemaline myopathy, Donner et al. (2002) identified a heterozygous mutation in the TPM2 gene (190990.0002). In another family, of Bosnian origin, they identified a heterozygous mutation in the TPM2 gene (190990.0003) in both mother and son. Donner et al. (2002) commented that mutations in the TPM2 gene are an uncommon cause of nemaline myopathy.

Tajsharghi et al. (2007) identified a heterozygous TPM2 mutation (190990.0005) in a mother and daughter with nemaline myopathy.

Lehtokari et al. (2007) identified a heterozygous deletion in the TPM2 gene (415delGAG; 190990.0006) in a patient with cap myopathy. In 3 unrelated patients with cap myopathy, Ohlsson et al. (2008) identified 3 different de novo heterozygous mutations in the TPM2 gene (see, e.g., 190990.0007 and 190990.0008). The findings confirmed that TPM2 mutations can cause cap myopathy.

In a girl with cap myopathy, Clarke et al. (2009) identified a heterozygous 415delGAG mutation in the TPM2 gene.

In 8 patients from 5 unrelated families with NEM4, Mokbel et al. (2013) identified the same 3-bp deletion in the TPM2 gene (lys7del, K7del; 190990.0009). The transmission pattern in 2 families was consistent with autosomal dominant inheritance; the mutation was suspected to have occurred de novo in 3 other families. Studies of patient muscle and differentiated myotubes transfected with the mutation suggested that the mutant protein incorporates poorly into sarcomeres and likely accumulates in nemaline bodies, and interferes with wildtype in a dominant-negative manner. Patient myofibers had normal force generation, but increased sensitivity to calcium in motility assays. The mutant protein also showed reduced binding affinity for actin and a decreased ability to polymerize into long tropomyosin filaments. Mokbel et al. (2013) noted that the presence of joint contractures was the most prominent clinical feature in these patients and that 1 patient had hypertonicity. These findings suggested that the joint contractures may result from impaired muscle relaxation and a tendency for muscles to slowly shorten. Progressive muscle weakness in adulthood may result from a combination of muscle degeneration and apoptosis due to cellular stress.

Davidson et al. (2013) identified a heterozygous K7del mutation in affected members of 4 families with NEM4 associated with distal arthrogryposis and limited jaw opening. The mutation in 1 family was found by whole-exome sequencing and confirmed by Sanger sequencing to segregate with the disorder; the mutation in the other families was found by candidate gene testing. Molecular modeling predicted that the mutation would alter protein-protein binding between TPM2 and other proteins as well as disturb the head-to-tail polymerization of TPM2 dimers. Expression of the mutation in developing zebrafish showed that the mutant protein did not localize properly within the thin filament compartment and altered sarcomere length, suggesting that it impaired sarcomeric structure. Areas of perimembranous accumulations of alpha-actinin were observed in mutant myofibers, consistent with findings in nemaline myopathy. These findings unified the phenotype of congenital myopathy with distal arthrogryposis (DA1A; 108120).