Biallelic variants in <i>COX18</i> cause a mitochondrial disorder primarily manifesting as peripheral neuropathy

Defects in mitochondrial dynamics are a common cause of Charcot-Marie-Tooth disease (CMT), whereas primary deficiencies in the mitochondrial respiratory chain (MRC) are rare and atypical for this aetiology. This study aims to report COX18 as a novel CMT-causing gene. This gene encodes an assembly factor of mitochondrial Complex IV that translocates the C-terminal tail of MTCO2 across the mitochondrial inner membrane. Exome sequencing was performed in four affected individuals from three families. The patients and available family members underwent thorough neurological and electrophysiological assessment. The impact of one of the identified variants on splicing, protein levels and mitochondrial bioenergetics was investigated in patient-derived lymphoblasts. The functionality of the mutant protein was assessed using a proteinase K protection assay and immunoblotting. Neuronal relevance of COX18 was assessed in a Drosophila melanogaster knockdown model. Exome sequencing coupled with homozygosity mapping revealed a homozygous splice variant c.435-6A>G in COX18 in two siblings with early-onset progressive axonal sensorimotor peripheral neuropathy. By querying external databases, we identified two additional families with rare deleterious biallelic variants in COX18. All eight affected individuals presented with axonal CMT, and some patients also exhibited CNS symptoms, such as dystonia and spasticity. Functional characterization of the c.435-6A>G variant demonstrated that it leads to the expression of an alternative transcript that lacks exon 2, resulting in a stable but defective COX18 isoform. The mutant protein impairs Complex IV assembly and activity, leading to a reduction in mitochondrial membrane potential. Downregulation of the COX18 homologue in D. melanogaster resulted in signs of neurodegeneration, including locomotor deficit and progressive axonal degeneration of sensory neurons. Our study presents genetic and functional evidence that supports COX18 as a newly identified gene candidate for autosomal recessive axonal CMT with or without CNS involvement. These findings emphasize the significance of peripheral neuropathy within the spectrum of primary mitochondrial disorders, in addition to the role of mitochondrial Complex IV in the development of CMT. Our research has important implications for the diagnostic work-up of CMT patients.