Coenzyme Q 10 defects may be associated with a deficiency of Q 10 -independent mitochondrial respiratory chain complexes

Background Coenzyme Q 10 (CoQ 10 or ubiquinone) deficiency can be due either to mutations in genes involved in CoQ 10 biosynthesis pathway, or to mutations in genes unrelated to CoQ 10 biosynthesis. CoQ 10 defect is the only oxidative phosphorylation disorder that can be clinically improved after oral CoQ 10 supplementation. Thus, early diagnosis, first evoked by mitochondrial respiratory chain (MRC) spectrophotometric analysis, then confirmed by direct measurement of CoQ 10 levels, is of critical importance to prevent irreversible damage in organs such as the kidney and the central nervous system. It is widely reported that CoQ 10 deficient patients present decreased quinone-dependent activities (segments I + III or G3P + III and II + III) while MRC activities of complexes I, II, III, IV and V are normal. We previously suggested that CoQ 10 defect may be associated with a deficiency of CoQ 10 -independent MRC complexes. The aim of this study was to verify this hypothesis in order to improve the diagnosis of this disease. Results To determine whether CoQ 10 defect could be associated with MRC deficiency, we quantified CoQ 10 by LC-MSMS in a cohort of 18 patients presenting CoQ 10 -dependent deficiency associated with MRC defect. We found decreased levels of CoQ 10 in eight patients out of 18 (45 %), thus confirming CoQ 10 disease. Conclusions Our study shows that CoQ 10 defect can be associated with MRC deficiency. This could be of major importance in clinical practice for the diagnosis of a disease that can be improved by CoQ 10 supplementation.