Bi-allelic MYMX variants cause a syndromic congenital myopathy with recognizable facial palsy, growth restriction, and dysmorphism.

Myogenic fusion, primarily regulated by the Myomaker and Myomixer proteins, is essential for skeletal muscle development, yet its mechanisms remain poorly understood. This study presents the clinical and molecular details of the third and fourth reported patients with biallelic variants in MYMX, the gene that encodes Myomixer. We identified a homozygous truncating variant [c.107 T > A (p.Leu36Ter)] and a homozygous stop-codon loss variant [c.255 A > G (p.Ter85TrpextTer41)] in MYMX, both associated with a complex neuromuscular syndrome characterized by generalized hypotonia, congenital myopathy, facial nerve palsy, growth restriction and facial dysmorphism. Additional variable features include hearing loss (confirmed in one patient, suspected in the other), scoliosis, joint contractures, cleft palate, hypoglossia, potentially contributing to Pierre Robin sequence, and abnormalities on neuroimaging studies including cerebellar atrophy and Chiari 1 deformity. Comparative analysis of patients with pathogenic variants in MYMK and MYMX, including our cases, reveals largely overlapping phenotypes, underscoring their synergistic role in myofiber formation and implicating their involvement in the etiology of neuromuscular conditions.