Mitochondrial Dysfunctions in Human Primary Coenzyme Q₁₀ Deficiencies

Coenzyme Q₁₀ (CoQ₁₀) is an essential lipid-soluble molecule that plays a central role in mitochondrial energy production as a mobile electron carrier. In addition to its bioenergetic function, CoQ₁₀ participates in antioxidant defense, redox homeostasis, lipid and nucleotide metabolism, and mitochondrial quality control. Primary CoQ₁₀ deficiencies are rare inherited mitochondrial disorders caused by pathogenic variants in nuclear genes involved in CoQ₁₀ biosynthesis. These defects lead to reduced CoQ₁₀ levels and impaired mitochondrial functions. Clinically, primary CoQ₁₀ deficiencies display remarkable phenotypic heterogeneity, ranging from isolated organ involvement, notably renal or cerebellar disease, to severe multisystemic disorders affecting the nervous system, skeletal muscle, heart, and other organs. Disease onset spans from the antenatal period to adulthood, and clinical severity varies widely, even among patients carrying variants in the same gene. This diversity cannot be fully explained by defective ATP production alone. Growing evidence indicates that disruption of non-bioenergetic functions of CoQ₁₀, including oxidative stress regulation and CoQ-dependent metabolic pathways, contributes significantly to disease pathophysiology and tissue vulnerability. In this review, we summarize current knowledge on CoQ₁₀ biology, biosynthesis, and the clinical spectrum of primary CoQ₁₀ deficiencies, and we discuss emerging mechanisms linking CoQ₁₀ depletion to mitochondrial dysfunctions and human diseases.