Combined Histological and Proteomic Analysis Reveals Muscle Denervation in KMT5B-Related Neurodevelopmental Disorder: A Case Report

<b>Background</b>: Patients with neurodevelopmental and neuromuscular disorders often show overlapping clinical phenotypes. Pathogenic variants in <i>KMT5B</i>, a histone lysine methyltransferase, have been linked to neurodevelopmental disorders, yet their effects on human skeletal muscle remain unexplored. We report on a patient with <i>KMT5B</i>-linked disease who presented to a neuromuscular specialty clinic with significant involvement of skeletal muscle, where a multi-omics approach established the genetic diagnosis and revealed neuromuscular findings relevant for diagnosis, care and rehabilitation. <b>Methods</b>: Whole-exome sequencing was performed from blood and data was analyzed using the RD-Connect Genome Phenome Analysis Platform. Histological analysis and proteomic profiling were performed on muscle tissue. <b>Results</b>: Whole-exome sequencing revealed a pathogenic heterozygous variant (c.554_557del, p.Tyr185Cysfs*27) in <i>KMT5B</i>. Histological examination revealed fiber-type grouping, angular fibers, increased fast-twitch fiber proportion, and lipid droplet accumulation, indicative of muscle denervation. Proteomic profiling identified 77 dysregulated proteins, including upregulation of sarcomeric proteins, mitochondrial and glycolytic enzymes, acute-phase and complement factors, and extracellular matrix components, reflecting structural remodeling, metabolic adaptation, and inflammatory activation. These findings align with the role types observed in <i>Kmt5b</i> mouse models, supporting a role of <i>KMT5B</i> in neuromuscular function. <b>Conclusions</b>: We present the first combined histological and proteomic analysis of quadriceps muscle from a patient carrying a pathogenic <i>KMT5B</i> variant with a neuromuscular phenotype. The convergence of histological and proteomic alterations suggests that <i>KMT5B</i> haploinsufficiency may be associated with fiber-type shifts, denervation, and metabolic stress in human skeletal muscle. Understanding these processes provides mechanistic insight into motor deficits and informs targeted therapeutic strategies, including physiotherapeutic interventions, and early compensatory measures.