Mutations in KLHL40 Are a Frequent Cause of Severe Autosomal-Recessive Nemaline Myopathy

Abstract

Nemaline myopathy (NEM) is a common congenital myopathy. At the very severe end of the NEM clinical spectrum are genetically unresolved cases of autosomal-recessive fetal akinesia sequence. We studied a multinational cohort of 143 severe-NEM-affected families lacking genetic diagnosis. We performed whole-exome sequencing of six families and targeted gene sequencing of additional families. We identified 19 mutations in KLHL40 (kelch-like family member 40) in 28 apparently unrelated NEM kindreds of various ethnicities. Accounting for up to 28% of the tested individuals in the Japanese cohort, KLHL40 mutations were found to be the most common cause of this severe form of NEM. Clinical features of affected individuals were severe and distinctive and included fetal akinesia or hypokinesia and contractures, fractures, respiratory failure, and swallowing difficulties at birth. Molecular modeling suggested that the missense substitutions would destabilize the protein. Protein studies showed that KLHL40 is a striated-muscle-specific protein that is absent in KLHL40-associated NEM skeletal muscle. In zebrafish, klhl40a and klhl40b expression is largely confined to the myotome and skeletal muscle, and knockdown of these isoforms results in disruption of muscle structure and loss of movement. We identified KLHL40 mutations as a frequent cause of severe autosomal-recessive NEM and showed that it plays a key role in muscle development and function. Screening of KLHL40 should be a priority in individuals who are affected by autosomal-recessive NEM and who present with prenatal symptoms and/or contractures and in all Japanese individuals with severe NEM.

Congenital myopathies

“…At the severe end of the spectrum, dramatic neonatal onset with bulbar and respiratory complications was observed in all patients with MTM1 mutation, 18 but also in several patients with ACTA1 mutation, 35,36 a few RYR1 cases, mainly with recessive inheritance, 17,37 and some patients with KLHL40 mutation. 38 Despite severe neonatal onset in some cases, all patients with RYR1 mutation survived, in keeping with only a few lethal cases previously reported in the literature. 17,39,40,e1 We found that in RYR1-related myopathies, feeding difficulties can be transitory: the only 4 neonates in our cohort who were tube-fed, but did not subsequently require G/J placement, all had RYR1 mutations.…”

Leiomodin-3 dysfunction results in thin filament disorganization and nemaline myopathy

“…Further studies are required to confirm this hypothesis. In a recently published study, LMOD3 and nebulin were identified as major binding partners of KLHL40, loss of which results in a severe lethal form of NM associated with destabilization of thin filament proteins (19,20). KLHL40 was found to promote stability of LMOD3 by blocking its ubiquitination, and loss of KLHL40 was associated with almost complete absence of LMOD3 protein in skeletal muscles of mice and KLHL40-deficient patients.…”

Kelch Repeat and BTB Domain Containing Protein 5 (Kbtbd5) Regulates Skeletal Muscle Myogenesis through the E2F1-DP1 Complex

“…Kbtbd5 has been shown to be involved in protein ubiquitination (27,28). Mutation of Kbtbd5 in the human is frequently associated with nemaline myopathy (29,30). In this study, we identified DP1 as the substrate of Kbtbd5 and demonstrated that Kbtbd5 regulated E2F1-DP1 activity.…”