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progressive congenital myopathy characterized by severe hypotonia and axial muscle weakness associated with previously unpublished biallelic variants in the RFC4 gene.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.

Department of Paediatric Neurology 2nd Faculty of Medicine Charles University and University Hospital Motol 5 Uvalu 84 Prague Full Member of the ERN Euro NMD Czech Republic

Dr John T Macdonald Foundation Department of Human Genetics University of Miami Miller School of Medicine Miami FL USA

National Institutes of Health Undiagnosed Diseases Program National Human Genome Research Institute National Institutes of Health Bethesda MD USA

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