Background: Coenzyme Q(10) (CoQ(10)) is an important component of the mitochondrial respiratory chain (RC) and is critical for energy production. Although the prevalence of CoQ(10) deficiency is still unknown, the general consensus is that the condition is under-diagnosed. The aim of this study was to retrospectively investigate CoQ(10) deficiency in frozen muscle specimens in a cohort of ethnically diverse patients who received muscle biopsies for the investigation of a possible RC deficiency (RCD). Methods: Muscle samples were homogenized whereby 600 x g supernatants were used to analyze RC enzyme activities, followed by quantification of CoQ(10) by stable isotope dilution liquid chromatography tandem mass spectrometry. The experimental group consisted of 156 patients of which 76 had enzymatically confirmed RCDs. To further assist in the diagnosis of CoQ(10) deficiency in this cohort, we included sequencing of 18 selected nuclear genes involved with CoQ(10) biogenesis in 26 patients with low CoQ(10) concentration in muscle samples. Results: Central 95% reference intervals (RI) were established for CoQ(10) normalized to citrate synthase (CS) or protein. Nine patients were considered CoQ(10) deficient when expressed against CS, while 12 were considered deficient when expressed against protein. In two of these patients the molecular genetic cause could be confirmed, of which one would not have been identified as CoQ(10) deficient if expressed only against protein content. Conclusion: In this retrospective study, we report a central 95% reference interval for 600 x g muscle supernatants prepared from frozen samples. The study reiterates the importance of including CoQic quantification as part of a diagnostic approach to study mitochondrial disease as it may complement respiratory chain enzyme assays with the possible identification of patients that may benefit from CoQ(10) supplementation. However, the anomaly that only a few patients were identified as CoQ(10) deficient against both markers (CS and protein), while the majority of patients where only CoQic deficient against one of the markers (and not the other), remains problematic. We therefore conclude from our data that, to prevent possibly not diagnosing a potential CoQ(10) deficiency, the expression of CoQ(10) levels in muscle on both CS as well as protein content should be considered.
