Human Coenzyme Q10 Deficiency

Ubiquinone (coenzyme Q₁₀ or CoQ₁₀) is a lipid-soluble component of virtually all cell membranes and has multiple metabolic functions. Deficiency of CoQ₁₀ (MIM 607426) has been associated with five different clinical presentations that suggest genetic heterogeneity, which may be related to the multiple steps in CoQ₁₀ biosynthesis. Patients with all forms of CoQ₁₀ deficiency have shown clinical improvements after initiating oral CoQ₁₀ supplementation. Thus, early diagnosis is of critical importance in the management of these patients. This year, the first molecular defect causing the infantile form of primary human CoQ₁₀ deficiency has been reported. The availability of genetic testing will allow for a better understanding of the pathogenesis of this disease and early initiation of therapy (even presymptomatically in siblings of patients) in this otherwise life-threatening infantile encephalomyopathy. Special issue dedicated to John P. Blass.     

Treatment of CoQ10 Deficient Fibroblasts with Ubiquinone,CoQ Analogs,and Vitamin C: Time- and Compound-Dependent Effects

Background

Coenzyme Q₁₀ (CoQ₁₀) and its analogs are used therapeutically by virtue of their functions as electron carriers, antioxidant compounds, or both. However, published studies suggest that different ubiquinone analogs may produce divergent effects on oxidative phosphorylation and oxidative stress.

Methodology/Principal Findings

To test these concepts, we have evaluated the effects of CoQ₁₀, coenzyme Q₂ (CoQ₂), idebenone, and vitamin C on bioenergetics and oxidative stress in human skin fibroblasts with primary CoQ₁₀ deficiency. A final concentration of 5 µM of each compound was chosen to approximate the plasma concentration of CoQ₁₀ of patients treated with oral ubiquinone. CoQ₁₀ supplementation for one week but not for 24 hours doubled ATP levels and ATP/ADP ratio in CoQ₁₀ deficient fibroblasts therein normalizing the bioenergetics status of the cells. Other compounds did not affect cellular bioenergetics. In COQ2 mutant fibroblasts, increased superoxide anion production and oxidative stress-induced cell death were normalized by all supplements.

Conclusions/Significance

These results indicate that: 1) pharmacokinetics of CoQ₁₀ in reaching the mitochondrial respiratory chain is delayed; 2) short-tail ubiquinone analogs cannot replace CoQ₁₀ in the mitochondrial respiratory chain under conditions of CoQ₁₀ deficiency; and 3) oxidative stress and cell death can be counteracted by administration of lipophilic or hydrophilic antioxidants. The results of our in vitro experiments suggest that primary CoQ₁₀ deficiencies should be treated with CoQ₁₀ supplementation but not with short-tail ubiquinone analogs, such as idebenone or CoQ₂. Complementary administration of antioxidants with high bioavailability should be considered if oxidative stress is present.     

CoQ10 deficiency diseases in adults

Deficiency of Coenzyme Q₁₀ (CoQ₁₀) in muscle has been associated with a spectrum of diseases including infantile-onset multi-systemic diseases, encephalomyopathies with recurrent myobinuria, cerebellar ataxia, and pure myopathy. CoQ₁₀ deficiency predominantly affects children, but patients have presented with adult-onset cerebellar ataxia or myopathy. Mutations in the CoQ10 biosynthetic genes, COQ2 and PDSS2, have been identified in children with the infantile form of CoQ₁₀ deficiency; however, the molecular genetic bases of adult-onset CoQ₁₀ deficiency remains undefined.     

A water soluble CoQ10 formulation improves intracellular distribution and promotes mitochondrial respiration in cultured cells

Background

Mitochondria are both the cellular powerhouse and the major source of reactive oxygen species. Coenzyme Q₁₀ plays a key role in mitochondrial energy production and is recognized as a powerful antioxidant. For these reasons it can be argued that higher mitochondrial ubiquinone levels may enhance the energy state and protect from oxidative stress. Despite the large number of clinical studies on the effect of CoQ₁₀ supplementation, there are very few experimental data about the mitochondrial ubiquinone content and the cellular bioenergetic state after supplementation. Controversial clinical and in vitro results are mainly due to the high hydrophobicity of this compound, which reduces its bioavailability.

Principal Findings

We measured the cellular and mitochondrial ubiquinone content in two cell lines (T67 and H9c2) after supplementation with a hydrophilic CoQ₁₀ formulation (Qter®) and native CoQ₁₀. Our results show that the water soluble formulation is more efficient in increasing ubiquinone levels. We have evaluated the bioenergetics effect of ubiquinone treatment, demonstrating that intracellular CoQ₁₀ content after Qter supplementation positively correlates with an improved mitochondrial functionality (increased oxygen consumption rate, transmembrane potential, ATP synthesis) and resistance to oxidative stress.

Conclusions

The improved cellular energy metabolism related to increased CoQ₁₀ content represents a strong rationale for the clinical use of coenzyme Q₁₀ and highlights the biological effects of Qter®, that make it the eligible CoQ₁₀ formulation for the ubiquinone supplementation.     

Pediatric reference intervals for muscle coenzyme Q10

Coenzyme Q₁₀ (CoQ₁₀) is present in humans in both the reduced (ubiquinol, CoQ₁₀H₂) and oxidized (ubiquinone, CoQ₁₀) forms. CoQ₁₀ is an essential cofactor in mitochondrial oxidative phosphorylation, and is necessary for ATP production. Total, reduced and oxidized CoQ₁₀ levels in skeletal muscle of 148 children were determined by HPLC coupled with electrochemical detection, and we established three level thresholds for total CoQ₁₀ in muscle. We defined as “severe deficiency”, CoQ₁₀ levels falling in the range between 0.82 and 4.88 μmol/g tissue; as “intermediate deficiency”, those ranging between 5.40 and 9.80 μmol/g tissue, and as “mild deficiency”, the amount of CoQ₁₀ included between 10.21 and 19.10 μmol/g tissue. Early identification of CoQ₁₀ deficiency has important implications in children, not only for those with primary CoQ₁₀ defect, but also for patients with neurodegenerative disorders, in order to encourage earlier supplementation with this agent also in mild and intermediate deficiency.     

Redox status and pharmacokinetics of coenzyme Q10 in rat plasma after its single intravenous administration

The pharmacokinetics of the total pool of coenzyme Q₁₀ (CoQ₁₀), its oxidized (ubiquinone) and reduced (ubiquinol, CoQ₁₀H₂) forms have been investigated in rats plasma during 48 h after a single intravenous injection of a solution of solubilized CoQ₁₀ (10 mg/kg) to rats. Plasma levels of CoQ₁₀ were determined by HPLC with spectrophotometric and coulometric detection. In plasma samples taken during the first minutes after the CoQ₁₀ intravenous injection, the total pool of coenzyme Q₁₀ and proportion of CoQ₁₀H₂ remained unchanged during two weeks of storage at ?20°C. The kinetic curve of the total pool of coenzyme Q₁₀ corresponds to a one-compartment model (R ₂ = 0.9932), while the corresponding curve of its oxidized form fits to the two-compartment model. During the first minutes after the injection a significant portion of plasma ubiquinone undergoes reduction, and after 7 h the concentration of ubiquinol predominates. The decrease in total plasma coenzyme Q₁₀ content was accompanied by the gradual increase in plasma ubiquinol, which represented about 90% of total plasma CoQ₁₀ by the end of the first day. The results of this study demonstrate the ability of the organism to transform high concentrations of the oxidized form of CoQ₁₀ into the effective antioxidant (reduced) form and justify prospects of the development of parenteral dosage forms of CoQ₁₀ for the use in the treatment of acute pathological conditions.     

Effect of dietary coenzyme Q and fatty acids on the antioxidant status of rat tissues

Summary.  Wistar rats were fed with different diets with or without supplement coenzyme Q₁₀ (CoQ₁₀) and with oil of different sources (sunflower or virgin olive oil) for six or twelve months. Ubiquinone contents (CoQ₉ and CoQ₁₀) were quantified in homogenates of livers and brains from rats fed with the four diets. In the brain, younger rats showed a 3-fold higher amount of ubiquinone than older ones for all diets. In the liver, however, CoQ₁₀ supplementation increased the amount of CoQ₉ and CoQ₁₀ in both total homogenates and plasma membranes. Rats fed with sunflower oil as fat source showed higher amounts of ubiquinone content than those fed with olive oil, in total liver homogenates, but the total ubiquinone content in plasma membranes was similar with both fat sources. Older rats showed a higher amount of ubiquinone after diets supplemented with CoQ₁₀. Two ubiquinone-dependent antioxidant enzyme activities were measured. NADH-ferricyanide reductase activity in hepatocyte plasma membranes was unaltered by ubiquinone accumulation, but this activity increased slightly with age. Both cytosolic and membrane-bound dicumarol-sensitive NAD(P)H:(quinone acceptor) oxidoreductase (DT-diaphorase, EC 1.6.99.2) activities were decreased by diets supplemented with CoQ₁₀. Animals fed with olive oil presented lower DT-diaphorase activity than those fed with sunflower oil, suggesting that the CoQ₁₀ antioxidant protection is strengthened by olive oil as fat source. Received May 22, 2002; accepted September 20, 2002; published online May 21, 2003 RID="*" ID="*" Correspondence and reprints: Departamento de Biología Celular, Fisiología e Inmunología, Universidad de Córdoba Edificio Severo Ochoa, Campus de Rabanales, 14071 Córdoba, Spain.     

Effects of alkyl side chain modification of coenzyme Q10 on mitochondrial respiratory chain function and cytoprotection

The effect of the alkyl side chain length of coenzyme Q₁₀ on mitochondrial respiratory chain function has been investigated by the use of synthetic ubiquinone derivatives. Three analogues (3, 4 and 6) were identified that exhibited significantly improved effects on mitochondrial oxygen consumption and mitochondrial membrane potential, and also conferred significant cytoprotection on cultured mammalian cells in which glutathione had been depleted by treatment with diethyl maleate. The analogues also exhibited lesser inhibition of the electron transport chain than idebenone. The results obtained provide guidance for the design of CoQ₁₀ analogues with improved activity compared to that of idebenone (1), the latter of which is undergoing evaluation in the clinic as a therapeutic agent.     

Current state of coenzyme Q10 production and its applications

Coenzyme Q₁₀ (CoQ₁₀), an obligatory cofactor in the aerobic respiratory electron transfer for energy generation, is formed from the conjugation of a benzoquinone ring with a hydrophobic isoprenoid chain. CoQ₁₀ is now used as a nutritional supplement because of its antioxidant properties and is beneficial in the treatment of several human diseases when administered orally. Bioprocesses have been developed for the commercial production of CoQ₁₀ because of its increased demand, and these bioprocesses depend on microbes that produce high levels of CoQ₁₀ naturally. However, as knowledge of the biosynthetic enzymes and the regulatory mechanisms modulating CoQ₁₀ production increases, approaches arise for the genetic engineering of CoQ₁₀ production in Escherichia coli and Agrobacterium tumefaciens. This review focused on approaches for CoQ₁₀ production, strategies used to engineer CoQ₁₀ production in microbes, and potential applications of CoQ₁₀.     

CoQ10 deficiencies and MNGIE: Two treatable mitochondrial disorders

Background

Although causative mutations have been identified for numerous mitochondrial disorders, few disease-modifying treatments are available. Two examples of treatable mitochondrial disorders are coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) deficiency and mitochondrial neurogastrointestinal encephalomyopathy (MNGIE).

Scope of review

Here, we describe clinical and molecular features of CoQ₁₀ deficiencies and MNGIE and explain how understanding their pathomechanisms have led to rationale therapies. Primary CoQ₁₀ deficiencies, due to mutations in genes required for ubiquinone biosynthesis, and secondary deficiencies, caused by genetic defects not directly related to CoQ₁₀ biosynthesis, often improve with CoQ₁₀ supplementation. In vitro and in vivo studies of CoQ₁₀ deficiencies have revealed biochemical alterations that may account for phenotypic differences among patients and variable responses to therapy. In contrast to the heterogeneous CoQ₁₀ deficiencies, MNGIE is a single autosomal recessive disease due to mutations in the TYMP gene encoding thymidine phosphorylase (TP). In MNGIE, loss of TP activity causes toxic accumulations of the nucleosides thymidine and deoxyuridine that are incorporated by the mitochondrial pyrimidine salvage pathway and cause deoxynucleoside triphosphate pool imbalances, which, in turn cause mtDNA instability. Allogeneic hematopoetic stem cell transplantation to restore TP activity and eliminate toxic metabolites is a promising therapy for MNGIE.

Major conclusions

CoQ₁₀ deficiencies and MNGIE demonstrate the feasibility of treating specific mitochondrial disorders through replacement of deficient metabolites or via elimination of excessive toxic molecules.

General significance

Studies of CoQ₁₀ deficiencies and MNGIE illustrate how understanding the pathogenic mechanisms of mitochondrial diseases can lead to meaningful therapies. This article is part of a Special Issue entitled: Biochemistry of Mitochondria, Life and Intervention 2010.     

Identification of bottlenecks in Escherichia coli engineered for the production of CoQ(10)

In this work, Escherichia coli was engineered to produce a medically valuable cofactor, coenzyme Q₁₀ (CoQ₁₀), by removing the endogenous octaprenyl diphosphate synthase gene and functionally replacing it with a decaprenyl diphosphate synthase gene from Sphingomonas baekryungensis. In addition, by over-expressing genes coding for rate-limiting enzymes of the aromatic pathway, biosynthesis of the CoQ₁₀ precursor para-hydroxybenzoate (PHB) was increased. The production of isoprenoid precursors of CoQ₁₀ was also improved by the heterologous expression of a synthetic mevalonate operon, which permits the conversion of exogenously supplied mevalonate to farnesyl diphosphate. The over-expression of these precursors in the CoQ₁₀-producing E. coli strain resulted in an increase in CoQ₁₀ content, as well as in the accumulation of an intermediate of the ubiquinone pathway, decaprenylphenol (10P-Ph). In addition, the over-expression of a PHB decaprenyl transferase (UbiA) encoded by a gene from Erythrobacter sp. NAP1 was introduced to direct the flux of DPP and PHB towards the ubiquinone pathway. This further increased CoQ₁₀ content in engineered E. coli, but decreased the accumulation of 10P-Ph. Finally, we report that the combined over-production of isoprenoid precursors and over-expression of UbiA results in the decaprenylation of para-aminobenzoate, a biosynthetic precursor of folate, which is structurally similar to PHB.     

Coenzyme Q10 and statins: Biochemical and clinical implications

Statins are drugs of known and undisputed efficacy in the treatment of hypercholesterolemia, usually well tolerated by most patients. In some cases treatment with statins produces skeletal muscle complaints, and/or mild serum CK elevation; the incidence of rhabdomyolysis is very low. As a result of the common biosynthetic pathway Coenzyme Q (ubiquinone) and dolichol levels are also affected, to a certain degree, by the treatment with these HMG-CoA reductase inhibitors. Plasma levels of CoQ₁₀ are lowered in the course of statin treatment. This could be related to the fact that statins lower plasma LDL levels, and CoQ₁₀ is mainly transported by LDL, but a decrease is also found in platelets and in lymphocytes of statin treated patients, therefore it could truly depend on inhibition of CoQ₁₀ synthesis. There are also some indications that statin treatment affects muscle ubiquinone levels, although it is not yet clear to which extent this depends on some effect on mitochondrial biogenesis. Some papers indicate that CoQ₁₀ depletion during statin therapy might be associated with subclinical cardiomyopathy and this situation is reversed upon CoQ₁₀ treatment. We can reasonably hypothesize that in some conditions where other CoQ₁₀ depleting situations exist treatment with statins may seriously impair plasma and possible tissue levels of coenzyme Q₁₀. While waiting for a large scale clinical trial where patients treated with statins are also monitored for their CoQ₁₀ status, with a group also being given CoQ₁₀, physicians should be aware of this drug-nutrient interaction and be vigilant to the possibility that statin drugs may, in some cases, impair skeletal muscle and myocardial bioenergetics.     

CoQ10 supplementation rescues nephrotic syndrome through normalization of H2S oxidation pathway

Nephrotic syndrome (NS), a frequent chronic kidney disease in children and young adults, is the most common phenotype associated with primary coenzyme Q₁₀ (CoQ₁₀) deficiency and is very responsive to CoQ₁₀ supplementation, although the pathomechanism is not clear. Here, using a mouse model of CoQ deficiency-associated NS, we show that long-term oral CoQ₁₀ supplementation prevents kidney failure by rescuing defects of sulfides oxidation and ameliorating oxidative stress, despite only incomplete normalization of kidney CoQ levels and lack of rescue of CoQ-dependent respiratory enzymes activities. Liver and kidney lipidomics, and urine metabolomics analyses, did not show CoQ metabolites. To further demonstrate that sulfides metabolism defects cause oxidative stress in CoQ deficiency, we show that silencing of sulfide quinone oxido-reductase (SQOR) in wild-type HeLa cells leads to similar increases of reactive oxygen species (ROS) observed in HeLa cells depleted of the CoQ biosynthesis regulatory protein COQ8A. While CoQ₁₀ supplementation of COQ8A depleted cells decreases ROS and increases SQOR protein levels, knock-down of SQOR prevents CoQ₁₀ antioxidant effects. We conclude that kidney failure in CoQ deficiency-associated NS is caused by oxidative stress mediated by impaired sulfides oxidation and propose that CoQ supplementation does not significantly increase the kidney pool of CoQ bound to the respiratory supercomplexes, but rather enhances the free pool of CoQ, which stabilizes SQOR protein levels rescuing oxidative stress.     

Intracellular cholesterol accumulation and coenzyme Q10 deficiency in Familial Hypercholesterolemia

Familial Hypercholesterolemia (FH) is an autosomal co-dominant genetic disorder characterized by elevated low-density lipoprotein (LDL) cholesterol levels and increased risk for premature cardiovascular disease. Here, we examined FH pathophysiology in skin fibroblasts derived from FH patients harboring heterozygous mutations in the LDL-receptor.Fibroblasts from FH patients showed a reduced LDL-uptake associated with increased intracellular cholesterol levels and coenzyme Q₁₀ (CoQ₁₀) deficiency, suggesting dysregulation of the mevalonate pathway.Secondary CoQ₁₀ deficiency was associated with mitochondrial depolarization and mitophagy activation in FH fibroblasts. Persistent mitophagy altered autophagy flux and induced inflammasome activation accompanied by increased production of cytokines by mutant cells. All the pathological alterations in FH fibroblasts were also reproduced in a human endothelial cell line by LDL-receptor gene silencing.Both increased intracellular cholesterol and mitochondrial dysfunction in FH fibroblasts were partially restored by CoQ₁₀ supplementation. Dysregulated mevalonate pathway in FH, including increased expression of cholesterogenic enzymes and decreased expression of CoQ₁₀ biosynthetic enzymes, was also corrected by CoQ₁₀ treatment.Reduced CoQ₁₀ content and mitochondrial dysfunction may play an important role in the pathophysiology of early atherosclerosis in FH. The diagnosis of CoQ₁₀ deficiency and mitochondrial impairment in FH patients may also be important to establish early treatment with CoQ₁₀.     

Lactate increases coenzyme Q10 production by Agrobacterium tumefaciens

By the optimization of nitrogen source for coenzyme Q₁₀ (ubiquinone, CoQ₁₀) production in Agrobacterium tumefaciens KCCM 10413 culture, the highest CoQ₁₀ production was achieved in medium containing corn steep powder (CSP). Components for a stimulatory effect on the production of CoQ₁₀ in CSP were screened, and lactate was found to increase dry cell weight (DCW) and the specific CoQ₁₀ content. In a fed-batch culture of A. tumefaciens, supplementation with 1.5 g of lactate l⁻¹ further improved DCW, the specific CoQ₁₀ content, and CoQ₁₀ production by 16.0, 5.8, and 22.8%, respectively. It has been reported that lactate stimulates cell growth and acts as an accelerator driving the tricarboxylic acid (TCA) cycle (Roberto et al. 2002, Biotechnol Let 24:427–431; Matsuoka et al. 1996, Biosci Biotechnol Biochem 60:575–579). In this study, lactate supplementation increased DCW and the specific CoQ₁₀ content in A. tumefaciens culture, probably by accelerating TCA cycle and energy production as reported previously, leading to the increase of CoQ₁₀ production.     

Ubiquinol-10 ameliorates mitochondrial encephalopathy associated with CoQ deficiency

Coenzyme Q10 (CoQ₁₀) deficiency (MIM 607426) causes a mitochondrial syndrome with variability in the clinical presentations. Patients with CoQ₁₀ deficiency show inconsistent responses to oral ubiquinone-10 supplementation, with the highest percentage of unsuccessful results in patients with neurological symptoms (encephalopathy, cerebellar ataxia or multisystemic disease). Failure in the ubiquinone-10 treatment may be the result of its poor absorption and bioavailability, which may be improved by using different pharmacological formulations. In a mouse model (Coq9^X/X) of mitochondrial encephalopathy due to CoQ deficiency, we have evaluated oral supplementation with water-soluble formulations of reduced (ubiquinol-10) and oxidized (ubiquinone-10) forms of CoQ₁₀. Our results show that CoQ₁₀ was increased in all tissues after supplementation with ubiquinone-10 or ubiquinol-10, with the tissue levels of CoQ₁₀ with ubiquinol-10 being higher than with ubiquinone-10. Moreover, only ubiquinol-10 was able to increase the levels of CoQ₁₀ in mitochondria from cerebrum of Coq9^X/X mice. Consequently, ubiquinol-10 was more efficient than ubiquinone-10 in increasing the animal body weight and CoQ-dependent respiratory chain complex activities, and reducing the vacuolization, astrogliosis and oxidative damage in diencephalon, septum–striatum and, to a lesser extent, in brainstem. These results suggest that water-soluble formulations of ubiquinol-10 may improve the efficacy of CoQ₁₀ therapy in primary and secondary CoQ₁₀ deficiencies, other mitochondrial diseases and neurodegenerative diseases.     

Localization of coenzyme Q10 in the center of a deuterated lipid membrane by neutron diffraction

Quinones (e.g., coenzyme Q, CoQ₁₀) are best known as carriers of electrons and protons during oxidative phosphorylation and photosynthesis. A myriad of mostly more indirect physical methods, including fluorescence spectroscopy, electron-spin resonance, and nuclear magnetic resonance, has been used to localize CoQ₁₀ within lipid membranes. They have yielded equivocal and sometimes contradictory results. Seeking unambiguous evidence for the localization of ubiquinone within lipid bilayers, we have employed neutron diffraction. CoQ₁₀ was incorporated into stacked bilayers of perdeuterated dimyristoyl phosphatidyl choline doped with dimyristoyl phosphatidyl serine containing perdeuterated chains in the natural fluid-crystalline state. Our data show CoQ₁₀ at the center of the hydrophobic core parallel to the membrane plane and not, as might be expected, parallel to the lipid chains. This localization is of importance for its function as a redox shuttle between the respiratory complexes and, taken together with our recent result that squalane is in the bilayer center, may be interpreted to show that all natural polyisoprene chains lie in the bilayer center. Thus ubiquinone, in addition to its free radical scavenging and its well-known role in oxidative phosphorylation as a carrier of electrons and protons, might also act as an inhibitor of transmembrane proton leaks.     

The evidence basis for coenzyme Q therapy in oxidative phosphorylation disease

The evidence supporting a treatment benefit for coenzyme Q₁₀ (CoQ₁₀) in primary mitochondrial disease (mitochondrial disease) whilst positive is limited. Mitochondrial disease in this context is defined as genetic disease causing an impairment in mitochondrial oxidative phosphorylation (OXPHOS). There are no treatment trials achieving the highest Level I evidence designation. Reasons for this include the relative rarity of mitochondrial disease, the heterogeneity of mitochondrial disease, the natural cofactor status and easy ‘over the counter availability’ of CoQ₁₀ all of which make funding for the necessary large blinded clinical trials unlikely. At this time the best evidence for efficacy comes from controlled trials in common cardiovascular and neurodegenerative diseases with mitochondrial and OXPHOS dysfunction the etiology of which is most likely multifactorial with environmental factors playing on a background of genetic predisposition. There remain questions about dosing, bioavailability, tissue penetration and intracellular distribution of orally administered CoQ₁₀, a compound which is endogenously produced within the mitochondria of all cells. In some mitochondrial diseases and other commoner disorders such as cardiac disease and Parkinson’s disease low mitochondrial or tissue levels of CoQ₁₀ have been demonstrated providing an obvious rationale for supplementation. This paper discusses the current state of the evidence supporting the use of CoQ₁₀ in mitochondrial disease.     

COQ4 Mutations Cause a Broad Spectrum of Mitochondrial Disorders Associated with CoQ10 Deficiency

Primary coenzyme Q10 (CoQ₁₀) deficiencies are rare, clinically heterogeneous disorders caused by mutations in several genes encoding proteins involved in CoQ₁₀ biosynthesis. CoQ₁₀ is an essential component of the electron transport chain (ETC), where it shuttles electrons from complex I or II to complex III. By whole-exome sequencing, we identified five individuals carrying biallelic mutations in COQ4. The precise function of human COQ4 is not known, but it seems to play a structural role in stabilizing a multiheteromeric complex that contains most of the CoQ₁₀ biosynthetic enzymes. The clinical phenotypes of the five subjects varied widely, but four had a prenatal or perinatal onset with early fatal outcome. Two unrelated individuals presented with severe hypotonia, bradycardia, respiratory insufficiency, and heart failure; two sisters showed antenatal cerebellar hypoplasia, neonatal respiratory-distress syndrome, and epileptic encephalopathy. The fifth subject had an early-onset but slowly progressive clinical course dominated by neurological deterioration with hardly any involvement of other organs. All available specimens from affected subjects showed reduced amounts of CoQ₁₀ and often displayed a decrease in CoQ₁₀-dependent ETC complex activities. The pathogenic role of all identified mutations was experimentally validated in a recombinant yeast model; oxidative growth, strongly impaired in strains lacking COQ4, was corrected by expression of human wild-type COQ4 cDNA but failed to be corrected by expression of COQ4 cDNAs with any of the mutations identified in affected subjects. COQ4 mutations are responsible for early-onset mitochondrial diseases with heterogeneous clinical presentations and associated with CoQ10 deficiency.     

Detection of suppressed maturation of the human COQ5 protein in the mitochondria following mitochondrial uncoupling by an antibody recognizing both precursor and mature forms of COQ5

Yeast Coq5p is required for the biosynthesis of coenzyme Q₆ (CoQ₆), but its human homolog has not been studied. We purified soluble recombinant human COQ5 protein under native conditions and generated an antibody recognizing both precursor and mature forms of COQ5. Mitochondrial localization of the mature form in 143B cells was demonstrated with this antibody. Moreover, a chemical uncoupler in a dose that suppressed CoQ₁₀ levels downregulated the mature form but augmented the precursor form of COQ5. The results that knockdown of the COQ5 gene reduced CoQ₁₀ levels further indicated the critical role of COQ5 in the biosynthesis of CoQ₁₀.