Characterization of CoQ₁₀ biosynthesis in fibroblasts of patients with primary and secondary CoQ₁₀ deficiency.

Primary coenzyme Q₁₀ (CoQ₁₀) deficiencies are associated with mutations in genes encoding enzymes important for its biosynthesis and patients are responsive to CoQ₁₀ supplementation. Early treatment allows better prognosis of the disease and therefore, early diagnosis is desirable. The complex phenotype and genotype and the frequent secondary CoQ₁₀ deficiencies make it difficult to achieve a definitive diagnosis by direct quantification of CoQ₁₀. We developed a non-radioactive methodology for the quantification of CoQ₁₀ biosynthesis in fibroblasts that allows the identification of primary deficiencies. Fibroblasts were incubated 72 h with 28 μmol/L (2)H₃-mevalonate or 1.65 mmol/L (13)C₆-p-hydroxybenzoate. The newly synthesized (2)H₃- and (13)C₆- labelled CoQ₁₀ were analysed by high performance liquid chromatography-tandem mass spectrometry. The mean and the reference range for (13)C₆-CoQ₁₀ and (2)H₃-CoQ₁₀ biosynthesis were 0.97 (0.83-1.1) and 0.13 (0.09-0.17) nmol/Unit of citrate synthase, respectively. We validated the methodology through the study of one patient with COQ2 mutations and six patients with CoQ₁₀ deficiency secondary to other inborn errors of metabolism. Afterwards we investigated 16 patients' fibroblasts and nine showed decreased CoQ₁₀ biosynthesis. Therefore, the next step is to study the COQ genes in order to reach a definitive diagnosis in these nine patients. In the patients with normal rates the deficiency is probably secondary. In conclusion, we have developed a non-invasive non-radioactive method suitable for the detection of defects in CoQ₁₀ biosynthesis, which offers a good tool for the stratification of patients with these treatable mitochondrial diseases.