Nemaline Myopathy 8

A number sign (#) is used with this entry because nemaline myopathy-8 (NEM8) is caused by homozygous or compound heterozygous mutation in the KLHL40 gene (615340) on chromosome 3p22.

Description

Nemaline myopathy-8 is a severe autosomal recessive muscle disorder characterized by fetal akinesia or hypokinesia, followed by contractures, fractures, respiratory failure, and swallowing difficulties apparent at birth. Most patients die in infancy. Skeletal muscle biopsy shows numerous small nemaline bodies, often with no normal myofibrils (summary by Ravenscroft et al., 2013).

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

Clinical Features

Ravenscroft et al. (2013) reported a large cohort of patients with severe nemaline myopathy. Many were of Japanese descent, although patients were ascertained from other areas as well. The phenotype was severe, and included fetal akinesia or hypokinesia, contractures, fractures, respiratory failure, and swallowing difficulties apparent at birth. Other features included facial weakness and chest deformity. The average age at death was 5 months. Skeletal muscle biopsy showed small nemaline bodies, and up to 20% of patients had virtually no normal myofibrils ('miliary NEM').

Inheritance

The transmission pattern of nemaline myopathy-8 in the families reported by Ravenscroft et al. (2013) was consistent with autosomal recessive inheritance.

Molecular Genetics

By whole-exome sequencing, Ravenscroft et al. (2013) identified homozygous or compound heterozygous mutations in the KLHL40 gene (see, e.g., 615340.0001-615340.0003) in affected members from 6 families with autosomal recessive severe nemaline myopathy-8. The mutations segregated with the disorder and were not found in several large control databases. Subsequent screening of KLHL40 by Sanger sequencing in additional probands with severe NEM resulted in the identification of a total of 19 variants in 28 (19.6%) of 143 families affected by severe NEM. There were 4 frameshifts, 12 missense mutations, 2 nonsense mutations, and 1 splice site mutation, and the mutations were scattered throughout all exons. One mutation (E528K; 615340.0001) was recurrent and was identified as a founder mutation among Japanese individuals. Structural analysis of the missense mutations indicated that they would most likely destabilize bonds or hydrophobic cores, resulting in protein instability. Skeletal muscle biopsy showed absence of KLHL40 even in patients with 2 missense mutations, consistent with a loss of function. In addition, 129 probands with a milder phenotype were screened, but no KLHL40 mutations were identified in this cohort, confirming that KLHL40 mutations are most likely exclusive to severe cases. Ravenscroft et al. (2013) suggested that the KLHL40 gene should be screened in individuals with autosomal recessive NEM who present with prenatal symptoms and/or contractures, as well as in all Japanese individuals with severe NEM.

Pathogenesis

By analyzing muscle samples from patients with nemalin myopathy and mutations in KLHL40, Garg et al. (2014) found that 2 patients with severe muscle pathology and complete deficiency of KLHL40 expression had marked reduction in the thin filament proteins LMOD3 (616112) and NEB (161650). A third patient with less severe muscle pathology and residual KLHL40 had relatively normal LMOD3 and NEB content.

Animal Model

The zebrafish genome contains 2 orthologs of KLHL40, Klhl40a and Klhl40b. Ravenscroft et al. (2013) found that morpholino-mediated knockdown of both Klhl40 genes in zebrafish resulted in a curved trunk and small head, with disrupted muscle patterning, gaps between myofibers, and widened Z-disks. Knockdown of either or both Klhl40 genes in zebrafish caused sporadic muscle tremors and absence of coordinated swimming.

Garg et al. (2014) found that Klhl40 -/- mice were indistinguishable from wildtype at birth, but they failed to grow, and none survived past 3 weeks of age. At 1 day of age, hindlimb skeletal muscle appeared normal, but it showed at least 50% less strength than wildtype. At 1 week of age, Klhl40 -/- skeletal muscle fibers appeared abnormal, with Z-line streaming, widened Z discs, and reduced content of Neb and Lmod3.