When muscles matter in SORD neuropathy.

Biallelic pathogenic variants in SORD (Sorbitoldehydrongenase gene), encoding sorbitol dehydrogenase, are a common cause of autosomal recessive axonal Charcot-Marie-Tooth disease type 2 (CMT2). Recent evidence suggests direct involvement of skeletal muscle in addition to peripheral nerve degeneration. We investigated muscle biopsies from 4 genetically confirmed CMT-SORD patients using an integrative approach. Histological evaluation revealed features of chronic denervation with grouped fiber atrophy, fiber-type grouping and central nuclei, ie, non-specific neurogenic muscle atrophy. Ultrastructural studies demonstrated mitochondrial abnormalities and expansion of the sarcoplasmic reticulum (SR). Proteomic profiling identified 220 significantly dysregulated proteins in CMT-SORD muscle, including alterations in mitochondrial complex I components, redox enzymes, and metabolic regulators distinct from changes observed in other rare recessive CMTs. Quantitative PCR validated increased levels of NNMT, POSTN, TACO1, as well as complement and immunomodulatory factors, suggesting mitochondrial stress, compensatory metabolic activation and tissue remodeling. Despite mitochondrial vulnerability, serum studies indicated that GDF-15 and FGF-21 did not appear to be suitable biomarkers for CMT-SORD. These findings demonstrate that SORD deficiency induces molecular and structural changes in skeletal muscle that extend beyond denervation, implicating impaired sorbitol metabolism, oxidative stress, and mitochondrial dysfunction as intrinsic myopathic features of SORD-related CMT2. They indicate the need for therapeutic strategies targeting both neuronal and muscular compartments.