Biallelic variants in the RFC4 gene cause a rapidly progressive congenital myopathy with severe hypotonia and axial weakness.

The RFC4 gene has recently been linked to a multisystemic disorder Morimoto-Ryu-Malicdan neuromuscular syndrome, with myopathy being one of the key symptoms described in nine patients. We report the case of two brothers with a rapidly progressive congenital myopathy characterized by severe hypotonia and axial muscle weakness associated with previously unpublished biallelic variants in the RFC4 gene. Whole exome sequencing revealed biallelic variants NM_002916.5:c.1019_1020insCAAA and NM_002916.5:c.982_983insACT, corresponding to the protein-level changes p.(Gly341Lysfs*4) and p.(Thr328delinsAsnSer) in both brothers. This case expands the phenotypic spectrum of Morimoto-Ryu-Malicdan neuromuscular syndrome, highlighting severe early-onset axial muscle weakness, severe hypotonia, and preserved intellectual development. We also provide novel insights into the clinical progression and potential multidisciplinary interventions for patients with Morimoto-Ryu-Malicdan neuromuscular syndrome. Our findings highlight the importance of advanced genetic diagnostics and international collaboration in identifying rare neuromuscular diseases and improving the clinical management of affected patients.