Fatal neonatal encephalopathy and lactic acidosis caused by a homozygous loss-of-function variant in <em>COQ9</em>.

Coenzyme Q10 (CoQ10) has an important role in mitochondrial energy metabolism by way of its functioning as an electron carrier in the respiratory chain. Genetic defects disrupting the endogenous biosynthesis pathway of CoQ10 may lead to severe metabolic disorders with onset in early childhood. Using exome sequencing in a child with fatal neonatal lactic acidosis and encephalopathy, we identified a homozygous loss-of-function variant in COQ9. Functional studies in patient fibroblasts showed that the absence of the COQ9 protein was concomitant with a strong reduction of COQ7, leading to a significant accumulation of the substrate of COQ7, 6-demethoxy ubiquinone10. At the same time, the total amount of CoQ10 was severely reduced, which was reflected in a significant decrease of mitochondrial respiratory chain succinate-cytochrome c oxidoreductase (complex II/III) activity. Lentiviral expression of COQ9 restored all these parameters, confirming the causal role of the variant. Our report on the second COQ9 patient expands the clinical spectrum associated with COQ9 variants, indicating the importance of COQ9 already during prenatal development. Moreover, the rescue of cellular CoQ10 levels and respiratory chain complex activities by CoQ10 supplementation points to the importance of an early diagnosis and immediate treatment