Impaired activity of the fusogenic micropeptide Myomixer causes myopathy resembling Carey-Fineman-Ziter syndrome

Abstract

Skeletal muscle fibers contain hundreds of nuclei, which increase the overall transcriptional activity of the tissue and perform specialized functions. Multinucleation occurs through myoblast fusion, mediated by the muscle fusogens Myomaker ( MYMK ) and Myomixer ( MYMX ). We describe a human pedigree harboring a recessive truncating variant of the MYMX gene that eliminates an evolutionarily conserved extracellular hydrophobic domain of MYMX, thereby impairing fusogenic activity. Homozygosity of this human variant resulted in a spectrum of abnormalities that mimicked the clinical presentation of Carey-Fineman-Ziter syndrome (CFZS), caused by hypomorphic MYMK variants. Myoblasts generated from patient-derived induced pluripotent stem cells displayed defective fusion, and mice bearing the human MYMX variant died perinatally due to muscle abnormalities. In vitro assays showed that the human MYMX variant conferred minimal cell-cell fusogenicity, which could be restored with CRISPR/Cas9–mediated base editing, thus providing therapeutic potential for this disorder. Our findings identify MYMX as a recessive, monogenic human disease gene involved in CFZS, and provide new insights into the contribution of myoblast fusion to neuromuscular diseases.

Skeletal muscle: molecular structure, myogenesis, biological functions, and diseases

“…Recently, researchers found that CFZS is associated with a recessive mutation in the myomaker gene through muscle biopsy and whole‐genome sequencing of patients, which was verified in a zebrafish animal experiment 176,177 . Mutations in myomergers are also involved in CFZS, and deletion of its extracellular region (MYMX R46* variant) can result in the loss of cell fusion ability and the occurrence of disease in mice 178 . Additionally, muscle atrophy and abnormal athletic ability caused by spinal muscular atrophy are associated with the decreased expression of myomakers and myomergers, leading to myofiber fusion disorders 179 .…”

“… 176 , 177 Mutations in myomergers are also involved in CFZS, and deletion of its extracellular region (MYMX R46* variant) can result in the loss of cell fusion ability and the occurrence of disease in mice. 178 Additionally, muscle atrophy and abnormal athletic ability caused by spinal muscular atrophy are associated with the decreased expression of myomakers and myomergers, leading to myofiber fusion disorders. 179 Recently, an interesting argument has been made that the genetic basis of human disease can be revealed through leverage comparisons among species.…”

Biochemistry and Protein Interactions of the CYREN Microprotein

“…To date, only a fraction of all microproteins have been characterized. Microproteins have been found to have important roles in muscle development and function, − DNA repair, metabolism, and cell growth. ,− Phospholamban was one of the first micropeptides to be studied, and it is involved in muscle function. , Phospholamban is a 52-amino acid single-pass transmembrane protein that interacts with sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2), a large transmembrane protein in the ER that regulates calcium transport in the muscle to mediate muscle contraction. Binding of phospholamban inhibits SERCA2 activity to reduce muscle performance, and mutations in phospholamban have been linked to cardiomyopathy and heart failure .…”