COQ4 encodes a protein that organises the multienzyme complex for the synthesis of coenzyme Q10 (CoQ10). A 3.9 Mb deletion of chromosome 9q34.13 was identified in a 3-year-old boy with mental retardation, encephalomyopathy and dysmorphic features. Because the deletion encompassed COQ4, the patient was screened for CoQ10 deficiency.
A complete molecular and biochemical characterisation of the patient’s fibroblasts and of a yeast model were performed.
The study found reduced COQ4 expression (48% of controls), CoQ10 content and biosynthetic rate (44% and 43% of controls), and activities of respiratory chain complex II+III. Cells displayed a growth defect that was corrected by the addition of CoQ10 to the culture medium. Knockdown of COQ4 in HeLa cells also resulted in a reduction of CoQ10. Diploid yeast haploinsufficient for COQ4 displayed similar CoQ deficiency. Haploinsufficency of other genes involved in CoQ10 biosynthesis does not cause CoQ deficiency, underscoring the critical role of COQ4. Oral CoQ10 supplementation resulted in a significant improvement of neuromuscular symptoms, which reappeared after supplementation was temporarily discontinued.
Mutations of COQ4 should be searched for in patients with CoQ10 deficiency and encephalomyopathy; patients with genomic rearrangements involving COQ4 should be screened for CoQ10 deficiency, as they could benefit from supplementation.
Autosomal dominant progressive external ophthalmoplegia due to PEO1 mutations is considered relatively benign, but no data about long-term progression of this disease have been reported. The aim of this study was to provide a 16-year clinical follow-up of autosomal dominant progressive external ophthalmoplegia due to the p.R357P gene mutation in PEO1.
Twenty-two members of an Irish-American family were examined in 1996, when PEO1 sequencing revealed a c.1071G>C/p.R357P mutation in 9 of them. We reexamined the family in 2012 using a standardized clinical protocol. Autosomal dominant progressive external ophthalmoplegia due to the p.R357P PEO1 mutation is a late-onset ocular myopathy beginning with ptosis and progressing slowly. Ophthalmoparesis, if present, is mild and evident only by neurological examination.
CONCLUSIONS AND RELEVANCE
Our results are important for prognosis and genetic counseling.
On November 9-12, 2006, the Friedreich’s Ataxia Research Alliance (FARA) and the National Institutes of Health (NIH) hosted the Third International Friedreich’s Ataxia (FRDA) Scientific Conference at the NIH in Bethesda, Maryland, highlighting the exciting research leading now to a variety of clinical trials that show promise of effective treatments for this devastating disorder. Nearly 150 leading FRDA scientists from around the world discussed their new insights and findings. The presence of six pharmaceutical and biotechnology companies underscored the importance of the public-private partnership that has grown in the past years. Some of these companies are already involved in advancing promising drug compounds into clinical trials, while others are eager to help take newer discoveries through drug development and into subsequent clinical trials. National Institute of Neurological Disorders and Stroke (NINDS) Director Dr. Story Landis noted in her opening remarks for the conference that there was a “palpable sense of energy, excitement, and enthusiasm” over the scientific progress made since the FRDA gene was discovered over 10 years ago.
Mutations in the human mitochondrial genome are known to cause an array of diverse disorders, most of which are maternally inherited, and all of which are associated with defects in oxidative energy metabolism. It is now emerging that somatic mutations in mitochondrial DNA (mtDNA) are also linked to other complex traits, including neurodegenerative diseases, ageing and cancer. Here we discuss insights into the roles of mtDNA mutations in a wide variety of diseases, highlighting the interesting genetic characteristics of the mitochondrial genome and challenges in studying its contribution to pathogenesis.
Prenatal diagnosis of disorders due to mitochondrial DNA (mtDNA) tRNA gene mutations is problematic. Experience in families harboring the protein-coding ATPase 6 m.8993T>G mutation suggests that the mutant load is homogeneous in different tissues, thus allowing prenatal diagnosis. We have encountered a novel protein-coding gene mutation, m.10198C>T in MT-ND3. A baby girl homoplasmic for this mutation died at 3 months after severe psychomotor regression and respiratory arrest. The mother had no detectable mutation in accessible tissues. The product of a second pregnancy showed only wild-type mt genomes in amniocytes, chorionic villi, and biopsied fetal muscle. This second girl is now 18 months old and healthy. Our observations support the concept that the pathogenic mutation in this patient appeared de novo and that fetal muscle biopsy is a useful aide in prenatal diagnosis.
mtDNA; complex I deficiency; prenatal diagnosis; mitochondria
Mitochondrial diseases involve the respiratory chain, which is under the dual control of nuclear and mitochondrial DNA (mtDNA). The complexity of mitochondrial genetics provides one explanation for the clinical heterogeneity of mitochondrial diseases, but our understanding of disease pathogenesis remains limited. Classification of Mendelian mitochondrial encephalomyopathies has been laborious, but whole-exome sequencing studies have revealed unexpected molecular aetiologies for both typical and atypical mitochondrial disease phenotypes. Mendelian mitochondrial defects can affect five components of mitochondrial biology: subunits of respiratory chain complexes (direct hits); mitochondrial assembly proteins; mtDNA translation; phospholipid composition of the inner mitochondrial membrane; or mitochondrial dynamics. A sixth category—defects of mtDNA maintenance—combines features of Mendelian and mitochondrial genetics. Genetic defects in mitochondrial dynamics are especially important in neurology as they cause optic atrophy, hereditary spastic paraplegia, and Charcot–Marie–Tooth disease. Therapy is inadequate and mostly palliative, but promising new avenues are being identified. Here, we review current knowledge on the genetics and pathogenesis of the six categories of mitochondrial disorders outlined above, focusing on their salient clinical manifestations and highlighting novel clinical entities. An outline of diagnostic clues for the various forms of mitochondrial disease, as well as potential therapeutic strategies, is also discussed.
The differential diagnosis of myoglobinuria includes multiple etiologies, such as infection, inflammation, trauma, endocrinopathies, drugs toxicity, and primary metabolic disorders. Metabolic myopathies can be due to inherited disorders of glycogen metabolism or to defects of fatty acid oxidation. Primary respiratory chain dysfunction is a rare cause of myoglobinuria but it has been described in sporadic cases with mutations in genes encoding cytochrome b or cytochrome c oxidase (COX) subunits, and in 4 cases with tRNA mutations. We describe a 39-year-old woman with myalgia and exercise-related recurrent myoglobinuria, who harbored a novel mitochondrial DNA mutation at nucleotide 4281 (m.4281A > G) in the tRNA-isoleucine gene. Her muscle biopsy revealed ragged-red and COX-deficient fibers. No deletions or duplication were detected by Southern blot analysis. The m.4281A > G mutation was present in the patient’s muscle with a mutation load of 46% and was detected in trace amounts in urine and cheek mucosa. Single-fiber analysis revealed significantly higher levels of the mutation in COX-deficient (65%) than in normal fibers (45%). This novel mutation has to be added to the molecular causes of recurrent myoglobinuria.
mitochondrial myopathy; myoglobinuria; mitochondrial DNA; tRNAIle; myalgia; elevated serum CK
Recessive mutations in the TK2 gene typically cause fatal infantile mitochondrial DNA (mtDNA) depletion syndromes (MDS).1–3 However, the progression of weakness may vary,4 as shown by recently described adult patients with late-onset myopathy.5,6 To date, only 5 adult patients with TK2-related MDS have been reported. Herein, we describe a man who had several unusual features. Clinically, he was weak as a child but sought medical attention as an adult. At the molecular level, multiple mtDNA deletions in muscle were more prominent than mtDNA depletion.
To identify the cause of an adult-onset multisystemic disease with multiple deletions of mitochondrial DNA (mtDNA).
A 65-year-old man with axonal sensorimotor peripheral neuropathy, ptosis, ophthalmoparesis, diabetes mellitus, exercise intolerance, steatohepatopathy, depression, parkinsonism, and gastrointestinal dysmotility.
Skeletal muscle biopsy revealed ragged-red and cytochrome-c oxidase–deficient fibers, and Southern blot analysis showed multiple mtDNA deletions. No deletions were detected in fibroblasts, and the results of quantitative polymerase chain reaction showed that the amount of mtDNA was normal in both muscle and fibroblasts. Exome sequencing using a mitochondrial library revealed compound heterozygous MPV17 mutations (p.LysMet88-89MetLeu and p.Leu143*), a novel cause of mtDNA multiple deletions.
In addition to causing juvenile-onset disorders with mtDNA depletion, MPV17 mutations can cause adult-onset multisystemic disease with multiple mtDNA deletions.
Pathogenic mutations in the tRNALeu(UCN) gene of mitochondrial DNA (mtDNA) have been invariably accompanied by skeletal myopathy with or without chronic progressive external ophthalmoplegia (CPEO). We report a young woman with a heteroplasmic m.12276G>A mutation in tRNALeu(UCN), who had childhood-onset and slowly progressive encephalopathy with ataxia, cognitive impairment, and hearing loss. Sequencing of the 22 tRNA mitochondrial genes is indicated in all unusual neurological syndromes, even in the absence of maternal inheritance.
Mitochondrial DNA depletion syndrome (MDS) is characterized by a reduction in mtDNA copy number and has been associated with mutations in eight nuclear genes, including enzymes involved in mitochondrial nucleotide metabolism (POLG, TK2, DGUOK, SUCLA2, SUCLG1, PEO1) and MPV17. Recently, mutations in The RRM2B gene, encoding the p53-controlled ribonucleotide reductase subunit, have been described in 7 infants from 4 families, who presented with various combinations of hypotonia, tubulopathy, seizures, respiratory distress, diarrhea, and lactic acidosis. All children died before 4 months of age.
We sequenced the RRM2B gene in three unrelated cases with unexplained severe mtDNA depletion. The first patient developed intractable diarrhea, profound weakness, respiratory distress, and died at three months. The other two unrelated patients had a much milder phenotype and are still alive at ages 27 and 36 months.
All three patients had lactic acidosis and severe depletion of mtDNA in muscle. Muscle histochemistry showed RRF and COX deficiency. Sequencing the RRM2B gene revealed three missense mutations and two single nucleotide deletions in exon 6, 8 and 9, confirming that RRM2B mutations are important causes of MDS and that the clinical phenotype is heterogeneous and not invariably fatal in infancy.
The human mitochondrial genome is replicated by DNA polymerase γ, which is encoded by polymerase γ gene (POLG1) on chromosome 15q25. Patients with POLG1 mutations usually present as Alpers’ syndrome or progressive external ophthalmoplegia. Our patient was a 48-year old woman with sensory ataxic neuropathy, dysarthria, ophthalmoplegia, and dysphagia. Sequence analysis revealed that she has two heterozygous missense mutations in the POLG1, a c.1774C>T substitution in exon 10, which results in a p.L591F amino acid change; and a c.3286C>T substitution in exon 21, which results in a p.R1096C amino acid change. The 1774C>T substitution is a novel mutation.
Previously described adult patients with one mutation in exon 10 and the other in exon 21 of POLG1 had presented with progressive external ophthalmoplegia. We now describe a patient with mutations in the same exons but suffering from the more complex clinical syndrome of sensory ataxic neuropathy, dysarthria, ophthalmoplegia.
Novel mutation; PEO; POLG1; SANDO
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.
CoQ10 deficiencies and MNGIE demonstrate the feasibility of treating specific mitochondrial disorders through replacement of deficient metabolites or via elimination of excessive toxic molecules.
Studies of CoQ10 deficiencies and MNGIE illustrate how understanding the pathogenic mechanisms of mitochondrial diseases can lead to meaningful therapies.
Coenzyme Q10 (CoQ10) deficiency syndrome is a rare condition that causes mitochondrial dysfunction and includes a variety of clinical presentations as encephalomyopathy, ataxia and renal failure. First, we sought to set up what all have in common, and then investigate why CoQ10 supplementation reverses the bioenergetics alterations in cultured cells but not all the cellular phenotypes.
Design Modelling study
This work models the transcriptome of human CoQ10 deficiency syndrome in primary fibroblast from patients and study the genetic response to CoQ10 treatment in these cells.
Four hospitals and medical centres from Spain, Italy and the USA, and two research laboratories from Spain and the USA.
Primary cells were collected from patients in the above centres.
We characterised by microarray analysis the expression profile of fibroblasts from seven CoQ10-deficient patients (three had primary deficiency and four had a secondary form) and aged-matched controls, before and after CoQ10 supplementation. Results were validated by Q-RT-PCR. The profile of DNA (CpG) methylation was evaluated for a subset of gene with displayed altered expression.
CoQ10-deficient fibroblasts (independently from the aetiology) showed a common transcriptomic profile that promotes cell survival by activating cell cycle and growth, cell stress responses and inhibiting cell death and immune responses. Energy production was supported mainly by glycolysis while CoQ10 supplementation restored oxidative phosphorylation. Expression of genes involved in cell death pathways was partially restored by treatment, while genes involved in differentiation, cell cycle and growth were not affected. Stably demethylated genes were unaffected by treatment whereas we observed restored gene expression in either non-methylated genes or those with an unchanged methylation pattern.
CoQ10 deficiency induces a specific transcriptomic profile that promotes cell survival, which is only partially rescued by CoQ10 supplementation.
Basic sciences; Genetics
Mitochondrial disease; Coenzyme Q10; Ubiquinone; Respiratory chain
Polymerase-γ (POLG) is a major human disease gene and may account for up to 25% of all mitochondrial diseases in the UK and in Italy. To date, >150 different pathogenic mutations have been described in POLG. Some mutations behave as both dominant and recessive alleles, but an autosomal recessive inheritance pattern is much more common. The most frequently detected pathogenic POLG mutation in the Caucasian population is c.1399G>A leading to a p.Ala467Thr missense mutation in the linker domain of the protein. Although many patients are homozygous for this mutation, clinical presentation is highly variable, ranging from childhood-onset Alpers-Huttenlocher syndrome to adult-onset sensory ataxic neuropathy dysarthria and ophthalmoparesis. The reasons for this are not clear, but familial clustering of phenotypes suggests that modifying factors may influence the clinical manifestation. In this study, we collected clinical, histological and biochemical data from 68 patients carrying the homozygous p.Ala467Thr mutation from eight diagnostic centres in Europe and the USA. We performed DNA analysis in 44 of these patients to search for a genetic modifier within POLG and flanking regions potentially involved in the regulation of gene expression, and extended our analysis to other genes affecting mitochondrial DNA maintenance (POLG2, PEO1 and ANT1). The clinical presentation included almost the entire phenotypic spectrum of all known POLG mutations. Interestingly, the clinical presentation was similar in siblings, implying a genetic basis for the phenotypic variability amongst homozygotes. However, the p.Ala467Thr allele was present on a shared haplotype in each affected individual, and there was no correlation between the clinical presentation and genetic variants in any of the analysed nuclear genes. Patients with mitochondrial DNA haplogroup U developed epilepsy significantly less frequently than patients with any other mitochondrial DNA haplotype. Epilepsy was reported significantly more frequently in females than in males, and also showed an association with one of the chromosomal markers defining the POLG haplotype. In conclusion, our clinical results show that the homozygous p.Ala467Thr POLG mutation does not cause discrete phenotypes, as previously suggested, but rather there is a continuum of clinical symptoms. Our results suggest that the mitochondrial DNA background plays an important role in modifying the disease phenotype but nuclear modifiers, epigenetic and environmental factors may also influence the severity of disease.
mitochondrial diseases; neuromuscular disorders; genetics; phenotype; molecular biology
Mitochondrial neurogastrointestinal encephalomyopathy is a rare multisystemic autosomic recessive disorder characterized by: onset typically before the age of 30 years; ptosis; progressive external ophthalmoplegia; gastrointestinal dysmotility; cachexia; peripheral neuropathy; and leucoencephalopathy. The disease is caused by mutations in the TYMP gene encoding thymidine phosphorylasethymine phosphorylase. Anecdotal reports suggest that allogeneic haematopoetic stem cell transplantation may be beneficial for mitochondrial neurogastrointestinal encephalomyopathy, but is associated with a high mortality. After selecting patients who fulfilled the clinical criteria for mitochondrial neurogastrointestinal encephalomyopathy and had severe thymidine phosphorylase deficiency in the buffy coat (<10% of normal activity), we reviewed their medical records and laboratory studies. We identified 102 patients (50 females) with mitochondrial neurogastrointestinal encephalomyopathy and an average age of 32.4 years (range 11–59 years). We found 20 novel TYMP mutations. The average age-at-onset was 17.9 years (range 5 months to 35 years); however, the majority of patients reported the first symptoms before the age of 12 years. The patient distribution suggests a relatively high prevalence in Europeans, while the mutation distribution suggests founder effects for a few mutations, such as c.866A>G in Europe and c.518T>G in the Dominican Republic, that could guide genetic screening in each location. Although the sequence of clinical manifestations in the disease varied, half of the patients initially had gastrointestinal symptoms. We confirmed anecdotal reports of intra- and inter-familial clinical variability and absence of genotype–phenotype correlation in the disease, suggesting genetic modifiers, environmental factors or both contribute to disease manifestations. Acute medical events such as infections often provoked worsening of symptoms, suggesting that careful monitoring and early treatment of intercurrent illnesses may be beneficial. We observed endocrine/exocrine pancreatic insufficiency, which had not previously been reported. Kaplan–Meier analysis revealed significant mortality between the ages of 20 and 40 years due to infectious or metabolic complications. Despite increasing awareness of this illness, a high proportion of patients had been misdiagnosed. Early and accurate diagnosis of mitochondrial neurogastrointestinal encephalomyopathy, together with timely treatment of acute intercurrent illnesses, may retard disease progression and increase the number of patients eligible for allogeneic haematopoetic stem cell transplantation.
mitochondrial disease; MNGIE; encephalomyopathy; TYMP; BMT
A 48-year-old man presented with a complex phenotype of myoclonus epilepsy with ragged-red fibers (MERRF) syndrome and Kearns-Sayre syndrome (KSS): progressive myoclonus epilepsy, cerebellar ataxia, hearing loss, myopathic weakness, ophthalmoparesis, pigmentary retinopathy, bifascicular heart block, and ragged-red fibers. The m.3291T>C mutation in the tRNALeu(UUR) gene was found with 92% heteroplasmy in muscle. This mutation has been reported with MELAS, myopathy, and deafness with cognitive impairment. This is the first description with a MERRF/KSS syndrome.
Mitochondrial DNA (mtDNA); Point mutation; tRNALeu(UUR); Myoclonus epilepsy; ragged-red fibers; Kearns-Sayre syndrome
Coenzyme Q10 (CoQ10) is a potent lipophilic antioxidant in cell membranes and a carrier of electrons in the mitochondrial respiratory chain. We previously characterized the effects of varying severities of CoQ10 deficiency on ROS production and mitochondrial bioenergetics in cells harboring genetic defects of CoQ10 biosynthesis. We observed a unimodal distribution of ROS production with CoQ10 deficiency: cells with <20% of CoQ10 and 50–70% of CoQ10 did not generate excess ROS while cells with 30–45% of CoQ10 showed increased ROS production and lipid peroxidation. Because our previous studies were limited to a small number of mutant cell lines with heterogeneous molecular defects, here, we treated 5 control and 2 mildly CoQ10 deficient fibroblasts with varying doses of 4-nitrobenzoate (4-NB), an analog of 4-hydroxybenzoate (4-HB) and inhibitor of 4-para-hydroxybenzoate:polyprenyl transferase (COQ2) to induce a range of CoQ10 deficiencies. Our results support the concept that the degree of CoQ10 deficiency in cells dictates the extent of ATP synthesis defects and ROS production and that 40–50% residual CoQ10 produces maximal oxidative stress and cell death.
CoQ10 deficiencies are clinically and genetically heterogeneous. This syndrome has been associated with five major clinical phenotypes: (1) encephalomyopathy, (2) severe infantile multisystemic disease, (3) cerebellar ataxia, (4) isolated myopathy, and (5) nephrotic syndrome. In a few patients, pathogenic mutations have been identified in genes involved in the biosynthesis of CoQ10 (primary CoQ10 deficiencies) or in genes not directly related to CoQ10 biosynthesis (secondary CoQ10 deficiencies). Respiratory chain defects, ROS production, and apoptosis variably contribute to the pathogenesis of primary CoQ10 deficiencies.
coenzyme Q10; respiratory chain activity; ROS; oxidative stress
Deficiency of thymidine kinase 2 (TK2) is a frequent cause of isolated myopathy or encephalomyopathy in children with mitochondrial DNA (mtDNA) depletion. To determine the bases of disease onset, organ specificity and severity of TK2 deficiency, we have carefully characterized Tk2 H126N knockin mice (Tk2−/−). Although normal until postnatal day 8, Tk2−/− mice rapidly develop fatal encephalomyopathy between postnatal days 10 and 13. We have observed that wild-type Tk2 activity is constant in the second week of life, while Tk1 activity decreases significantly between postnatal days 8 and 13. The down-regulation of Tk1 activity unmasks Tk2 deficiency in Tk2−/− mice and correlates with the onset of mtDNA depletion in the brain and the heart. Resistance to pathology in Tk2 mutant organs depends on compensatory mechanisms to the reduced mtDNA level. Our analyses at postnatal day 13 have revealed that Tk2−/− heart significantly increases mitochondrial transcript levels relative to the mtDNA content. This transcriptional compensation allows the heart to maintain normal levels of mtDNA-encoded proteins. The up-regulation in mitochondrial transcripts is not due to increased expression of the master mitochondrial biogenesis regulators peroxisome proliferator-activated receptor-gamma coactivator 1 alpha and nuclear respiratory factors 1 and 2, or to enhanced expression of the mitochondrial transcription factors A, B1 or B2. Instead, Tk2−/− heart compensates for mtDNA depletion by down-regulating the expression of the mitochondrial transcriptional terminator transcription factor 3 (MTERF3). Understanding the molecular mechanisms that allow Tk2 mutant organs to be spared may help design therapies for Tk2 deficiency.
Mammal adipose tissues require mitochondrial activity for proper development and differentiation. The components of the mitochondrial respiratory chain/oxidative phosphorylation system (OXPHOS) are encoded by both mitochondrial and nuclear genomes. The maintenance of mitochondrial DNA (mtDNA) is a key element for a functional mitochondrial oxidative activity in mammalian cells. To ascertain the role of mtDNA levels in adipose tissue, we have analyzed the alterations in white (WAT) and brown (BAT) adipose tissues in thymidine kinase 2 (Tk2) H126N knockin mice, a model of TK2 deficiency-induced mtDNA depletion. We observed respectively severe and moderate mtDNA depletion in TK2-deficient BAT and WAT, showing both tissues moderate hypotrophy and reduced fat accumulation. Electron microscopy revealed altered mitochondrial morphology in brown but not in white adipocytes from TK2-deficient mice. Although significant reduction in mtDNA-encoded transcripts was observed both in WAT and BAT, protein levels from distinct OXPHOS complexes were significantly reduced only in TK2-deficient BAT. Accordingly, the activity of cytochrome c oxidase was significantly lowered only in BAT from TK2-deficient mice. The analysis of transcripts encoding up to fourteen components of specific adipose tissue functions revealed that, in both TK2-deficient WAT and BAT, there was a consistent reduction of thermogenesis related gene expression and a severe reduction in leptin mRNA. Reduced levels of resistin mRNA were found in BAT from TK2-deficient mice. Analysis of serum indicated a dramatic reduction in circulating levels of leptin and resistin. In summary, our present study establishes that mtDNA depletion leads to a moderate impairment in mitochondrial respiratory function, especially in BAT, causes substantial alterations in WAT and BAT development, and has a profound impact in the endocrine properties of adipose tissues.
Coenzyme Q (CoQ) is an essential component of the respiratory chain but also participates in other mitochondrial functions such as regulation of the transition pore and uncoupling proteins. Furthermore, this compound is a specific substrate for enzymes of the fatty acids β–oxidation pathway and pyrimidine nucleotide biosynthesis. Furthermore, CoQ is an antioxidant that acts in all cellular membranes and lipoproteins. A complex of at least ten nuclear (COQ) genes encoded proteins synthesizes CoQ but its regulation is unknown. Since 1989, a growing number of patients with multisystemic mitochondrial disorders and neuromuscular disorders showing deficiencies of CoQ have been identified. CoQ deficiency caused by muta-tion(s) in any of the COQ genes is designated primary deficiency. Other patients have displayed other genetic defects independent on the CoQ biosynthesis pathway, and are considered to have secondary deficiencies. This review updates the clinical and molecular aspects of both types of CoQ deficiencies and proposes new approaches to understanding their molecular bases.
Mitochondria; Coenzyme Q deficiency; Neuromuscular diseases
Until even only a few years ago, the idea that effective therapies for human mitochondrial disorders resulting from dysfunction of the respiratory chain/oxidative phosphorylation system (OxPhos) could be developed was unimaginable. The obstacles to treating diseases caused by mutations in either mitochondrial DNA (mtDNA) or nuclear DNA (nDNA), and which had the potential to affect nearly every organ system, seemed overwhelming. However, while clinically applicable therapies still remain largely in the future, the landscape has changed dramatically; we can now envision the possibility of treating some of these disorders. Among these are techniques to upregulate mitochondrial biogenesis, to enhance organellar fusion and fission, to “shift heteroplasmy,” and to eliminate the burden of mutant mtDNAs via cytoplasmic transfer.
Coenzyme Q10 (CoQ10) is an essential electron carrier in the mitochondrial respiratory chain and an important antioxidant. Deficiency of CoQ10 is a clinically and molecularly heterogeneous syndrome, which, to date, has been found to be autosomal recessive in inheritance and generally responsive to CoQ10 supplementation. CoQ10 deficiency has been associated with five major clinical phenotypes: (1) encephalomyopathy, (2) severe infantile multisystemic disease, (3) cerebellar ataxia, (4) isolated myopathy, and (5) nephrotic syndrome. In a few patients, pathogenic mutations have been identified in genes involved in the biosynthesis of CoQ10 (primary CoQ10 deficiencies) or in genes not directly related to CoQ10 biosynthesis (secondary CoQ10 deficiencies). Respiratory chain defects, ROS production, and apoptosis contribute to the pathogenesis of primary CoQ10 deficiencies. In vitro and in vivo studies are necessary to further understand the pathogenesis of the disease and to develop more effective therapies.
coenzyme Q10; respiratory chain activity; ROS; oxidative stress