New pathogenic variants in COQ4 cause ataxia and neurodevelopmental disorder without detectable CoQ₁₀ deficiency in muscle or skin fibroblasts

COQ4 is a component of an enzyme complex involved in the biosynthesis of coenzyme Q₁₀ (CoQ₁₀), a molecule with primary importance in cell metabolism. Mutations in the COQ4 gene are responsible for mitochondrial diseases showing heterogeneous age at onset, clinical presentations and association with CoQ₁₀ deficiency. We herein expand the phenotypic and genetic spectrum of COQ4-related diseases, by reporting two patients harboring bi-allelic variants but not showing CoQ₁₀ deficiency. One patient was found to harbor compound heterozygous mutations (specifically, c.577C>T/p.Pro193Ser and the previously reported c.718C>T/p.Arg240Cys) associated with progressive spasticity, while the other harbored two novel missense (c.284G>A/p.Gly95Asp and c.305G>A/p.Arg102His) associated with a neurodevelopmental disorder. Both patients presented motor impairment and ataxia. To further understand the role of COQ4, we performed functional studies in patient-derived fibroblasts, yeast and "crispant" zebrafish larvae. Micro-oxygraphy showed impaired oxygen consumption rates in one patient, while yeast complementation assays showed that all the mutations were presumably disease related. Moreover, characterization of the coq4 F0 CRISPR zebrafish line showed motor defects and cell reduction in a specific area of the hindbrain, a region reminiscent of the human cerebellum. Our expanded phenotype associated with COQ4 mutations allowed us to investigate, for the first time, the role of COQ4 in brain development in vivo.