Nemaline Myopathy 2

A number sign (#) is used with this entry because of evidence that nemaline myopathy-2 (NEM2) is caused by homozygous or compound heterozygous mutation in the nebulin gene (NEB; 161650) on chromosome 2q23.

Description

Nemaline myopathy-2 is an autosomal recessive skeletal muscle disorder with a wide range of severity. The most common clinical presentation is early-onset (in infancy or childhood) muscle weakness predominantly affecting proximal limb muscles. Muscle biopsy shows accumulation of Z-disc and thin filament proteins into aggregates named 'nemaline bodies' or 'nemaline rods,' usually accompanied by disorganization of the muscle Z discs. The clinical and histologic spectrum of entities caused by variants in the NEB gene is a continuum, ranging in severity from the severe form with perinatal onset and fetal death to milder forms with later onset. The distribution of weakness can vary from generalized muscle weakness, more pronounced in proximal limb muscles, to distal-only involvement, although neck flexor weakness appears to be rather consistent. Histologic patterns range from a severe usually nondystrophic disturbance of the myofibrillar pattern to an almost normal pattern, with or without nemaline bodies, sometimes combined with cores (summary by Lehtokari et al., 2014).

For a discussion of genetic heterogeneity of nemaline myopathy, see NEM3 (161800).

Mutations in the NEB gene are the most common cause of nemaline myopathy (Lehtokari et al., 2006).

Clinical Features

Wallgren-Pettersson et al. (1999) reported 41 unrelated families with NEM2 suggested by linkage analysis. All were consistent with autosomal recessive inheritance. Wallgren-Pettersson et al. (1999) noted that phenotypic classification of nemaline myopathy is difficult because the disease spectrum forms a continuum; however, the authors described 2 main forms, 'typical' and 'severe.' Typical patients had generalized hypotonia at birth, with particular involvement of the bulbar, neck flexor, and respiratory muscles. Proximal limb muscles were weaker initially, but distal limb muscle weakness eventually occurred. The extraocular muscles were spared. The facies was myopathic with a high-arched palate. The spine was hyperlordotic, sometimes rigid, and scoliosis sometimes developed at puberty. Deep tendon reflexes were decreased or absent, and the gag reflex was often absent. Chest deformities were common, but there was no cardiac involvement. Imaging showed decreased muscle density with increased fatty infiltration. Serum creatine kinase was normal or mildly elevated. Muscle biopsies characteristically showed nemaline bodies and type 1 fiber predominance. Although myopathic changes were observed, there were no dystrophic or inflammatory changes. Despite delayed motor development and waddling gait, many patients with typical disease remained ambulatory as adults. Intelligence was normal. A subset of patients had severe disease, characterized by absence of spontaneous movements or respiration at birth. These patients often had contractures or fractures at birth and usually never achieved independent sitting or ambulation.

Wallgren-Pettersson et al. (2002) reported 7 patients from 5 families with severe NEM2 confirmed by mutation in the NEB gene. Four pregnancies were complicated by polyhydramnios, and 4 patients had arthrogryposis. Most affected infants did show show spontaneous respiration or antigravity movements. Several previous infant deaths or miscarriages were reported in the families. Two affected brothers in 1 family had large fontanels, low-set ears, adducted thumbs, hypospadias, and micropenis. Another affected child had cleft palate, rocker-bottom feet, and undescended testes. Death occurred between 30 minutes and 19 months of age. Muscle biopsies showed nemaline bodies in up to 90% of muscle fibers.

Romero et al. (2009) reported a 27-year-old man with NEM2 confirmed by genetic analysis. He presented at birth with generalized hypotonia, poor spontaneous movements, and restrictive respiratory insufficiency requiring mechanical ventilation. He showed diffuse muscle weakness with axial predominance, never acquired independent ambulation, and developed multiple joint contractures. As an adult, he was quadriplegic with mild facial weakness. No cardiac symptoms were observed. Muscle biopsy showed numerous fibers with distinctive rods and well-delineated cores in type 1 fibers. Electron microscopy showed rods with characteristic striation and cores with some sarcomeric disorganization and depletion of mitochondria. The histologic findings indicated that cores, in addition to rods, may be found in patients with NEM2.

In a detailed review and update of 159 families with mutations in the nebulin gene associated with myopathies, Lehtokari et al. (2014) noted that the most common associated phenotype was nemaline myopathy (90% of families). Within this broad category, there was a range of severity. In addition, some families with NEB mutations had more diverse manifestations, including early-onset distal myopathy without nemaline bodies (4 families), a distal form of nemaline myopathy (3 families), core-rod myopathy with generalized muscle weakness (3 families), a childhood-onset distal myopathy with rods and cores (3 families), and fetal akinesia/lethal multiple pterygium syndrome (3 families).

Fetal Akinesia/Lethal Multiple Pterygia Syndrome

Yonath et al. (2012) reported 4 unrelated pregnancies with abnormal prenatal ultrasound findings in fetuses with NEM2. In each family, 1 or both of the parents was of Ashkenazi Jewish descent, and the common exon 55 deletion (161650.0007) in the NEB gene was found in the heterozygous state in the patients and in unaffected parents. A second pathogenic NEB mutation was found in 3 of the patients; a second mutation could not be identified in 1 of the patients. Prenatal ultrasound showed polyhydramnios, decreased fetal movements, clubfoot, and clenched hands. All patients showed severe hypotonia after birth, and all died within the first months of life.

Todd et al. (2015) reported twin male fetuses, conceived of consanguineous parents, with NEM2 presenting as fetal akinesia with lethal multiple pterygia syndrome. Prenatal ultrasound showed severe hydrops in both fetuses, and the pregnancy was terminated at 16 weeks' gestation. Postmortem examination showed joint contractures consistent with arthrogryposis multiplex congenita (AMC), bilateral talipes, multiple pterygia, hypertelorism, and cystic hygromas. Muscle tissue was not obtained. A previous fetus was therapeutically aborted due to hydrops fetalis at 19 weeks' gestation.

Abdalla et al. (2017) reported 2 male fetuses, conceived of consanguineous Egyptian parents, with NEM2 presenting as fetal akinesia with lethal multiple pterygia syndrome. Both pregnancies were complicated by polyhydramnios and hydrops fetalis. Prenatal ultrasound of the second fetus at 18 weeks' gestation showed growth restriction, fetal akinesia, cystic hygroma, edema, pericardial effusion, hydrothorax, and flexion deformities. The pregnancy was terminated, and postmortem examination confirmed the findings of multiple pterygia and contractures; dysmorphic features, including hypertelorism, downslanting palpebral fissures, long philtrum, and low-set ears were also noted. Muscle biopsy was not performed.

Distal Myopathy

Wallgren-Pettersson et al. (2007) reported 7 patients from 4 unrelated Finnish families with NEM2 presenting as distal myopathy with foot drop between the first and third decades after normal early motor development. Muscle weakness predominantly affected the ankle dorsiflexors, finger extensors, and neck flexors, resulting in walking difficulties and increased falls in some patients. The patients were unable to walk on their heels. Additional variable features included high-arched palate, slight facial weakness, dysarthria, pseudohypertrophy of the calves, and mild atrophy of proximal muscles of the upper and lower limbs. Radiographic imaging showed fatty replacement of the muscles of the anterior compartment of the lower legs. None of the patients had breathing problems or dysphagia, and serum creatine kinase was not elevated. Muscle biopsy showed myopathic changes with marked fiber size variability, fibrosis, and large hypertrophic fibers with increased internal nuclei. Rare nemaline rods or bodies were observed, although these were often apparent only on electron microscopy or with special staining. The patients were initially thought to have tibial muscular dystrophy (TMD; 600334), but molecular analysis excluded that diagnosis.

Inheritance

The transmission pattern of NEM2 in the families reported by Yonath et al. (2012) was consistent with autosomal recessive inheritance.

Mapping

Wallgren-Pettersson et al. (1995) used genetic linkage analysis from 7 European families with autosomal recessive nemaline myopathy to map a candidate disease locus, designated NEM2, to a 13-cM region on chromosome 2q21.2-q22 between markers D2S150 and D2S142 (maximum lod score of 5.34 at marker D2S151).

By radiation hybrid mapping, Pelin et al. (1997) mapped the nebulin gene to a position close to the microsatellite marker D2S2236 on 2q22. They also mapped the titin gene (188840) to the vicinity of markers D2S384 and D2S364 on 2q24.3. Pelin et al. (1997) concluded that of these 2 giant muscle proteins, the gene for nebulin resides within the candidate region for NEM2, whereas the titin gene is located outside this region.

Molecular Genetics

Pelin et al. (1999) demonstrated mutations in the NEB gene in autosomal recessive typical nemaline myopathy (161650.0001-161650.0006).

Wallgren-Pettersson et al. (2002) identified mutations in the NEB gene in patients with severe congenital nemaline myopathy (see, e.g., 161650.0003 and 161650.0004).

In 5 affected individuals from 5 Ashkenazi Jewish families with autosomal recessive typical nemaline myopathy, Anderson et al. (2004) identified a 2,502-bp deletion in the NEB gene (161650.0007), resulting in removal of exon 55.

Using denaturing high-performance liquid chromatography (DHPLC), Lehtokari et al. (2006) identified 45 novel mutations in the NEB gene in affected members of 44 unrelated families with nemaline myopathy. Mutations were identified in patients representing all clinical categories of disease severity. The majority (55%) of mutations were frameshift or nonsense mutations resulting in premature termination of the protein. Mutations were distributed throughout the gene, with no obvious hotspots. Lehtokari et al. (2006) concluded that mutations in the NEB gene are the most common cause of nemaline myopathy.

In 7 patients from 4 unrelated Finnish families, 2 of whom were consanguineous, with NEM2 manifest as distal myopathy with onset in childhood or adulthood, Wallgren-Pettersson et al. (2007) identified homozygous missense mutations in the NEB (T5681P, 161650.0008 and S4665I, 161650.0009). The mutations segregated with the disorder in the families from whom DNA was available. Functional studies of the mutation were not performed, but Wallgren-Pettersson et al. (2007) noted that both missense mutations had been found in compound heterozygosity with more disruptive NEB mutations in other families with the more severe typical form of NEM2. The findings suggested that homozygosity for a missense NEB variant may result in a milder myopathic phenotype.

In a detailed review and update of 159 families with mutations in the nebulin gene associated with myopathies, Lehtokari et al. (2014) found that the most common types of mutations were splice-site mutations (34%), followed by frameshift (32%), nonsense (23%), and finally missense (7%). The vast majority of patients had compound heterozygous mutations. There were no apparent genotype/phenotype correlations.

In twin male fetuses, conceived of consanguineous parents, with NEM2 presenting as fetal akinesia with lethal multiple pterygia syndrome, Todd et al. (2015) identified a homozygous nonsense mutation in the NEB gene (R974X; 161650.0010). The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed. In affected patients from 3 additional families diagnosed with fetal akinesia, a heterozygous splice site, frameshift, or truncating mutation in the NEB gene was found. A second pathogenic mutation was not found in these patients, but there was evidence for autosomal recessive inheritance, suggesting that the affected fetuses carried a second pathogenic variant. Todd et al. (2015) noted the difficulty in screening the nebulin gene for mutations because of its large size and because next-generation sequencing data may not accurately identify pathogenic small copy number variations. These 4 families were ascertained from a cohort of 38 families with severe neuromuscular disease apparent before or at birth.

Pathogenesis

Ottenheijm et al. (2009) studied the muscular phenotype of nemaline myopathy (NM) patients with a well-defined (161650.0007) nebulin mutation (NM-NEB), resulting in deletion of exon 55 from the transcript. SDS-PAGE and Western blot analysis revealed greatly reduced nebulin levels in skeletal muscle of NM-NEB patients, with the most prominent reduction at nebulin's N-terminal end. Muscle mechanical studies indicated an approximately 60% reduced force generating capacity of NM-NEB muscle and a leftward shift of the force-sarcomere length relation in NM-NEB muscle fibers. This indicates that the mechanism for the force reduction is likely to include shorter and nonuniform thin filament lengths in NM-NEB muscle compared with control muscle. The average thin filament length was reduced from approximately 1.3-micrometer in control muscle to approximately 0.75-micrometer in NM-NEB muscle. Ottenheijm et al. (2009) hypothesized that dysregulated thin filament length may contribute to muscle weakness in nemaline myopathy patients with nebulin mutations.

Population Genetics

Anderson et al. (2004) screened for 2,503-bp deletion in a random sample of 4,090 Ashkenazi Jewish individuals, revealing a carrier frequency of 1 in 108. Lehtokari et al. (2009) identified the 2,502-bp deletion in 14 of 355 probands with nemaline myopathy from around the world; 2 of the probands had been reported by Anderson et al. (2004). Seven probands were homozygous for the deletion, and 7 carried the mutation in heterozygosity. Two of the families were not of known Ashkenazi Jewish descent, but carried the common haplotype identified in Ashkenazi Jews. The findings were consistent with a founder effect.