Biallelic SCO2 Variants Presenting as Motor-Predominant Axonal Neuropathy With Complex IV Deficiency.

BACKGROUND AND AIMS: SCO2 encodes a mitochondrial copper chaperone required for cytochrome c oxidase (COX) assembly and is classically associated with severe multisystem mitochondrial disease. We characterize a motor-predominant axonal neuropathy presentation associated with biallelic SCO2 variants.

METHODS

Clinical, genetic, and functional studies were performed in a 15-year-old female presenting with axonal neuropathy. Functional studies were conducted in patient-derived fibroblasts, including Western blot analysis and spectrophotometric cytochrome c oxidation assay. Structural modeling was performed using ChimeraX.

RESULTS

The patient presented with a motor-predominant axonal neuropathy consistent with Charcot-Marie-Tooth (CMT) disease. Clinical genetic testing identified compound heterozygous SCO2 variants of uncertain significance: a missense variant (p.Arg120Trp) and a frameshift variant (p.Asp252ValfsTer24). Structural modeling predicted disruption of protein stability for both variants. Functional studies in patient-derived fibroblasts demonstrated complete absence of SCO2 protein and reduced mitochondrial complex IV activity, supporting a loss-of-function mechanism.

INTERPRETATION: These findings demonstrate that SCO2-related disease can present as an isolated axonal neuropathy, a phenotype that remains rarely reported. Our study also highlights the value of integrating in silico prediction tools with functional assays to establish pathogenicity in rare sporadic cases of inherited neuropathy.