Coenzyme Q10 (CoQ10) plays a pivotal role in oxidative phosphorylation (OXPHOS) as it distributes electrons between the various dehydrogenases and the cytochrome segments of the respiratory chain. Primary coenzyme Q10 deficiency is a rare, but possibly treatable, autosomal recessive condition with four major clinical presentations, an encephalomyopathic form, a generalized infantile variant with severe encephalopathy and renal disease, a myopathic form and an ataxic form. The diagnosis of ubiquinone deficiency is supported by respiratory chain analysis and eventually by the quantification of CoQ10 in patient tissues. We review here the quinone deficiency diseases as well as the clinical improvement after oral CoQ10 therapy. The clinical heterogeneity of ubiquinone deficiency is suggestive of a genetic heterogeneity that should be related to the large number of enzymes, and corresponding genes, involved in ubiquinone biosynthesis.
Coenzyme Q (CoQ, ubiquinone, UQ) is a lipophilic component located in the inner mitochondrial membrane that has a pivotal role in oxidative phosphorylation (OXPHOS). Indeed, CoQ shuttles electrons from complex I and complex II to complex III. Being in large excess compared to any other component of the respiratory chain (RC), it forms a kinetically compartmentalized pool, the redox status of which tightly regulates the activity of the dehydrogenases. CoQ also plays a critical function in antioxidant defenses. Ubiquinone is composed by a redox active benzoquinone ring connected to a long isoprenoid side chain. Ubiquinone is present in prokaryotes and eukaryotes but the length of its isoprenoid chain varies among species. Humans and rodents produce mainly CoQ10 and CoQ9 respectively, whereas Saccharomyces cerevisiae synthesizes CoQ6 and Escherichia coli CoQ8. In human, CoQ10 is present in all tissues and cells but in variable amount. It ranges between 114 μg/g in heart to 8μg/g in lung1. However, small amount of CoQ9 (2-7%) are also synthesized in human tissues. Moreover, it has been shown in bovine brain that the concentration of ubiquinone varies among cells and regions of a specific organ.
Primary coenzyme Q10 deficiency is a rare, clinically heterogeneous disorder of the respiratory chain. The first patient was described more than twenty years ago2 and less than thirty patients have been further reported. CoQ10 deficiency is presumably inherited as an autosomal recessive trait as all enzymes involved in ubiquinone biosynthesis pathway are nuclearly encoded and as patients are born from healthy parents.
Four major phenotypes have been previously described (i) an encephalomyopathic form characterized by exercise intolerance, mitochondrial myopathy, myoglobinuria, epilepsy and ataxia; (ii) a generalized infantile variant with severe encephalopathy and renal disease; (iii) a myopathic form with exercise intolerance, myoglobinuria, and myopathy; and (iv) an ataxic form, dominated by ataxia, seizures and either cerebellar atrophy or anomalies of the basal ganglia. However, since this first classification of ubiquinone deficiency, ...