Biallelic variants in COQ7 cause distal hereditary motor neuropathy with upper motor neuron signs

COQ7 encodes a hydroxylase responsible for the penultimate step of coenzyme Q10 (CoQ10) biosynthesis in mitochondria. CoQ10 is essential for multiple cellular functions, including mitochondrial oxidative phosphorylation, lipid metabolism, and reactive oxygen species homeostasis. Mutations in COQ7 have been previously associated with primary CoQ10 deficiency, a clinically heterogeneous multisystemic mitochondrial disorder. We identified COQ7 biallelic variants in nine families diagnosed with distal hereditary motor neuropathy with upper neuron involvement, expending the clinical phenotype associated with defects in this gene. A recurrent p.Met1? change was identified in five families from Brazil with evidence of a founder effect. Fibroblasts isolated from patients revealed a substantial depletion of COQ7 protein levels, indicating protein instability leading to loss of enzyme function. High-performance liquid chromatography assay showed that fibroblasts from patients had reduced levels of CoQ10, and abnormal accumulation of the biosynthetic precursor DMQ10. Accordingly, fibroblasts from patients displayed significantly decreased oxygen consumption rates in patients, suggesting mitochondrial respiration deficiency. Induced pluripotent stem cell-derived motor neurons from patient fibroblasts showed significantly increased levels of extracellular neurofilament light protein, indicating axonal degeneration. Our findings indicate a molecular pathway involving CoQ10 biosynthesis deficiency and mitochondrial dysfunction in patients with distal hereditary motor neuropathy. Further studies will be important to evaluate the potential benefits of CoQ10 supplementation in the clinical outcome of the disease.