Mutations in MYLPF Cause a Novel Segmental Amyoplasia that Manifests as Distal Arthrogryposis

Abstract

We identified ten persons in six consanguineous families with distal arthrogryposis (DA) who had congenital contractures, scoliosis, and short stature. Exome sequencing revealed that each affected person was homozygous for one of two different rare variants (c.470G>T [p.Cys157Phe] or c.469T>C [p.Cys157Arg]) affecting the same residue of myosin light chain, phosphorylatable, fast skeletal muscle (MYLPF). In a seventh family, a c.487G>A (p.Gly163Ser) variant in MYLPF arose de novo in a father, who transmitted it to his son. In an eighth family comprised of seven individuals with dominantly inherited DA, a c.98C>T (p.Ala33Val) variant segregated in all four persons tested. Variants in MYLPF underlie both dominant and recessively inherited DA. Mylpf protein models suggest that the residues associated with dominant DA interact with myosin whereas the residues altered in families with recessive DA only indirectly impair this interaction. Pathological and histological exam of a foot amputated from an affected child revealed complete absence of skeletal muscle (i.e., segmental amyoplasia). To investigate the mechanism for this finding, we generated an animal model for partial MYLPF impairment by knocking out zebrafish mylpfa. The mylpfa mutant had reduced trunk contractile force and complete pectoral fin paralysis, demonstrating that mylpf impairment most severely affects limb movement. mylpfa mutant muscle weakness was most pronounced in an appendicular muscle and was explained by reduced myosin activity and fiber degeneration. Collectively, our findings demonstrate that partial loss of MYLPF function can lead to congenital contractures, likely as a result of degeneration of skeletal muscle in the distal limb.

Anin vivoapproach to characterize novel variants associated with musculoskeletal disorders

“…The TPM2 muscle morphogenesis defects we have observed in Drosophila, zebrafish, and cultured cells are likely the result of improperly regulated actin dynamics in migrating myoblasts or in elongating myotubes that expressed pathogenic variants. DA and amyoplasia (absence of muscle) were often thought to be distinct clinical diagnoses, but amyoplasia was recently reported in a case of congenital DA (Chong et al, 2020). Our studies provide additional support for a model in which DA variants disrupt muscle development.…”

Models of Distal Arthrogryposis and Lethal Congenital Contracture Syndrome

“…Exome sequencing recently identified MYLPF, a phosphorylatable fast skeletal muscle regulatory light chain, as a cause of DA [23]. Some affected individuals were homozygous for rare variants in the gene, while other individuals have autosomal dominant disease, a finding similar to what was described for MYH3-related disorders.…”

“…However, unlike MYH3-related disorders, the phenotypes for MYLPF autosomal dominant and recessive conditions are apparently indistinguishable. Protein modeling of MYLPF alleles suggested that the autosomal dominant pathogenic variants cause disease through their direct interaction with myosin, while the recessive alleles only indirectly affect the interaction with myosin [23].…”

“…Therefore, to further model hypomorphic alleles of MYLPF, a zebrafish mylpfa mutant was characterized. Because zebrafish have 2 MYLPF genes, mylpfa and mylpfb, the more prominently expressed gene (mlfpfa) was chosen to model hypomorphic MYLPF alleles [23]. Of note, zebrafish had an evolutionary genome duplication event that has resulted in many human genes being represented twice in the zebrafish genome.…”

Further delineation of van den Ende‐Gupta syndrome: Genetic heterogeneity and overlap with congenital heart defects and skeletal malformations syndrome

“…Genomes were sequenced to >95% at >10× coverage and >90% at 20× coverage. In brief, data were annotated using Variant Effect Predictor v95.3 (McLaren et al, 2016) and analyzed using GEMINI v0.30.1 (Paila et al, 2013) as previously described (Chong et al, 2020). All variants were filtered to remove low‐quality sites, requiring a minimum read depth of 6, a minimum genotyping quality of 20, and the absence of any Genome Analysis Tookit filter flags.…”