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Leber hereditary optic neuropathy is a mitochondrial disease mainly due to pathologic mutations in mitochondrial genes related to the respiratory complex I of the oxidative phosphorylation system. Genetic, physiological, and environmental factors modulate the penetrance of these mutations. We report two patients suffering from this disease and harboring a m.15950G > A mutation in the mitochondrial DNA-encoded gene for the threonine transfer RNA. We also provide evidences supporting the pathogenicity of this mutation.

Mitochondrial oxidative phosphorylation disorders are extremely heterogeneous conditions. Their clinical and genetic variability makes the identification of reliable and specific biomarkers very challenging. Until now, only a few studies have focused on the effect of a defective oxidative phosphorylation functioning on the cell’s secretome, although it could be a promising approach for the identification and pre-selection of potential circulating biomarkers for mitochondrial diseases. Here, we review the insights obtained from secretome studies with regard to oxidative phosphorylation dysfunction, and the biomarkers that appear, so far, to be promising to identify mitochondrial diseases. We propose two new biomarkers to be taken into account in future diagnostic trials.

The frequency of dermatological manifestations in diseases due to mitochondrial DNA mutations is not well known, although multiple symmetric lipomatosis has been repeatedly associated to mitochondrial DNA mutations. Here, we present a patient suffering from multiple symmetric lipomatosis and other skin signs. We found a new mitochondrial DNA mutation, m.8357T>C, in the tRNALys -coding gene and, using a cybrid approach, confirmed its pathogenicity. A meta-analysis of the dermatological signs of the patient shows that they are not common in patients with confirmed mitochondrial DNA mutations and suggests that, in these cases, lipomatosis is not related to the oxidative phosphorylation dysfunction, but to an alteration of an additional function associated to particular mitochondrial tRNAs.

Oxidative phosphorylation dysfunction has been found in many different disorders. This biochemical pathway depends on mitochondrial protein synthesis. Thus, mutations in components of the mitochondrial translation system can be responsible for some of these pathologies. We identified a new homozygous missense mutation in the mitochondrial translation elongation factor Ts gene in a patient suffering from slowly progressive childhood ataxia and hypertrophic cardiomyopathy. Using cell, biochemical and molecular-genetic protocols, we confirm it as the etiologic factor of this phenotype. Moreover, as an important functional confirmation, we rescued the normal molecular phenotype by expression of the wild-type TSFM cDNA in patient''s fibroblasts. Different TSFM mutations can produce the same or very different clinical phenotypes, going from abortions to moderately severe presentations. On the other hand, the same TSFM mutation can also produce same or different phenotypes within the same range of presentations, therefore suggesting the involvement of unknown factors.

The association between mitochondrial DNA (mtDNA) haplogroup and risk of type 2 diabetes (T2D) is undetermined and controversial. This study aims to evaluate the impact of the main mtDNA haplogroups on glycaemic control and renal function in a Spanish population of 303 T2D patients and 153 healthy controls. Anthropometrical and metabolic parameters were assessed and mtDNA haplogroup was determined in each individual. Distribution of the different haplogroups was similar in diabetic and healthy populations and, as expected, T2D patients showed poorer glycaemic control and renal function than controls. T2D patients belonging to the JT haplogroup (polymorphism m.4216T&gt, C) displayed statistically significant higher levels of fasting glucose and HbA1c than those of the other haplogroups, suggesting a poorer glycaemic control. Furthermore, diabetic patients with the JT haplogroup showed a worse kidney function than those with other haplogroups, evident by higher levels of serum creatinine, lower estimated glomerular filtration rate (eGFR), and slightly higher (although not statistically significant) urinary albumin-to-creatinine ratio. Our results suggest that JT haplogroup (in particular, change at position 4216 of the mtDNA) is associated with poorer glycaemic control in T2D, which can trigger the development of diabetic nephropathy.

Mitochondrial DNA mutations in genes encoding respiratory complex I polypeptides can cause Leber hereditary optic neuropathy. Toxics affecting oxidative phosphorylation system can also cause mitochondrial optic neuropathy. Some complex I inhibitors found in edible plants might differentially interact with these pathologic mutations and modify their penetrance. To analyze this interaction, we have compared the effect of rotenone, capsaicin and rolliniastatin-1 on cybrids harboring the most frequent Leber hereditary optic neuropathy mutations and found that m.3460G > A mutation increases rotenone resistance but capsaicin and rolliniastatin-1 susceptibility. Thus, to explain the pathogenicity of mitochondrial diseases due to mitochondrial DNA mutations, their potential interactions with environment factors will have to be considered.

Leber hereditary optic neuropathy (LHON) is a rare, inherited mitochondrial disease. No treatment has shown a clear-cut benefit on a clinically meaningful end-point. Primary open-angle glaucoma (POAG) is a frequent, acquired optic neuropathy. Lowering intraocular pressure (IOP) reduces disease progression. However, current methods to decelerate this progression are recognized as being inadequate. Therefore, there is a clear need to look for new therapeutic approaches. The growing evidence indicates that POAG can also be a mitochondrial optic neuropathy (MON). Several risk elements are common for both diseases and all of them decrease mitochondrial (mt)DNA content. Based on these susceptibility factors and their molecular mechanism, we suggest herein pharmacological therapies targeted to increase mtDNA levels, oxidative phosphorylation (OXPHOS) capability, and mitochondrial energy production as treatments for MONs.

Mitochondrial DNA mutations at MT-ATP6 gene are relatively common in individuals suffering from striatal necrosis syndromes. These patients usually do not show apparent histochemical and/or biochemical signs of oxidative phosphorylation dysfunction. Because of this, MT-ATP6 is not typically analyzed in many other mitochondrial disorders that have not been previously associated to mutations in this gene. To correct this bias, we have performed a screening of the MT-ATP6 gene in a large collection of patients suspected of suffering different mitochondrial DNA (mtDNA) disorders. In three cases, biochemical, molecular-genetics and other analyses in patient tissues and cybrids were also carried out. We found three new pathologic mutations. Two of them in patients showing phenotypes that have not been commonly associated to mutations in the MT-ATP6 gene. These results remark the importance of sequencing the MT-ATP6 gene in patients with striatal necrosis syndromes, but also within other mitochondrial pathologies. This gene should be sequenced at least in all those patients suspected of suffering an mtDNA disorder disclosing normal results for histochemical and biochemical analyses of respiratory chain.

Some ribosomal antibiotics used in clinical practice to fight pathogenic bacteria can provoke serious adverse drug reactions in patients. Sensitivity to the antibiotics is a multifactorial trait but the genetic variation of sensitive individuals to off-target effects of the drugs might be one of the factors contributing to this condition. Thus, the protein synthesis apparatus of mitochondria is similar to that of bacteria because of its endosymbiotic origin and, therefore, mitochondrial ribosomes are frequently unintended off-targets of these antibiotics. Because of the limitations of epidemiologic studies of pharmacogenomics, we constructed 25 transmitochondrial cell lines using platelets from individuals belonging to high-frequency European mitochondrial DNA (mtDNA) haplogroups and grew them in the absence or presence of commonly used ribosomal antibiotics. Next, we analyzed the mitochondrial synthesis of proteins and the mitochondrial oxygen consumption to ascertain whether some side effects of ribosomal drugs are due to their interaction with particular mtDNA haplogroup-defining polymorphisms. The amount of mitochondrial translation products, the p.MT-CO1/succinate dehydrogenase subunit A ratio and the ratio of respiratory complex IV quantity to citrate synthase (CS)-specific activity were significantly lower, after the treatment with linezolid, in cybrids harboring the highly frequent m.3010A allele. These results suggest that mitochondrial antibiograms should be implemented for at least the most frequent mitochondrial ribosomal RNA (rRNA) polymorphisms and combinations of polymorphisms and the most frequently used ribosomal antibiotics. In this way, we would obtain individualized barcodes for antibiotic therapy, avoid the side effects of the antibiotics and enable appropriate personalized medicine.

Leber's hereditary optic neuropathy is a maternally inherited optic atrophy caused by mitochondrial DNA point mutations. Previous epidemiological studies have shown that individuals from mitochondrial genetic backgrounds (haplogroups) J/Uk and H have a higher and a lower risk, respectively, of suffering this disorder. To analyze the bases of these associations at cellular and molecular levels, functional studies with cybrids provide high quality evidence. Cybrids from haplogroup J contain less mitochondrial deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and synthesize a smaller amount of mitochondrial DNA-encoded polypeptides than those from haplogroup H. Haplogroup J cybrids also display lower oxygen consumption, mitochondrial inner membrane potential and total adenosine‐5′‐triphosphate (ATP) levels. Moreover, mitochondrial DNA levels correlate with many parameters of the oxidative phosphorylation system. These results suggest that the mitochondrial DNA amount determines oxidative phosphorylation capacity and, along with other recently published observations, support the possibility that mitochondrial DNA levels may be responsible for the bias of the disorder toward males, for the incomplete penetrance of mutations causing Leber's hereditary optic neuropathy and for the association of the disease with particular mitochondrial DNA haplogroups.

Tumor microenvironment promotes mtDNA mutations. A number of these mutations will affect cell metabolism and increase cell survival. These mutations are positively selected and contribute to other tumor features, such as extracellular matrix remodeling and angiogenic processes, thus favoring metastases. Like somatic mutations, although with less marked effects, some mtDNA population polymorphisms will affect OXPHOS function, cell metabolism, and homeostasis. Thus, they could behave as inherited susceptibility factors for cancer. However, in addition to epidemiological evidence, other more direct clues are required. The cybrid approach can help to clarify this issue. This article is part of a Special Issue entitled: Bioenergetics of Cancer.

Mitochondrial diseases (MD) are a group of genetic and acquired disorders which present significant diagnostic challenges. Here we report the disease characteristics of a large cohort of pediatric MD patients (n = 95) with a definitive genetic diagnosis, giving special emphasis on clinical muscle involvement, biochemical and histopathological features. Of the whole cohort, 51 patients harbored mutations in nuclear DNA (nDNA) genes and 44 patients had mutations in mitochondrial DNA (mtDNA) genes. The nDNA patients were more likely to have a reduction in muscle fiber succinate dehydrogenase (SDH) stains and in SDH-positive blood vessels, while a higher frequency of mtDNA patients had ragged red (RRF) and blue fibers. The presence of positive histopathological features was associated with ophthalmoplegia, myopathic facies, weakness and exercise intolerance. In 17 patients younger than two years of age, RRF and blue fibers were observed only in one case, six cases presented cytochrome c oxidase (COX) reduction/COX-fibers, SDH reduction was observed in five and all except one presented SDH-positive blood vessels. In conclusion, muscle involvement was a frequent finding in our series of MD patients, especially in those harboring mutations in mtDNA genes.

Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is a devastating disorder of infancy caused by a significant reduction of the number of copies of mitochondrial DNA in one or more tissues. We report a Spanish patient with the myopathic form of MDS, harboring two mutations in the thymidine kinase 2 gene (TK2): a previously reported deletion (p.K244del) and a novel nucleotide duplication in the exon 2, generating a frameshift and premature stop codon. Sensorineural hearing loss was a predominant symptom in the patient and a novel feature of MDS due to TK2 mutations. The patient survived up to the age of 8.5 y, which confirms that survival above the age of 5 y is not infrequent in patients with MDS due to TK2 deficiency.

Many epidemiologic studies have associated human mitochondrial haplogroups to rare mitochondrial diseases like Leber's hereditary optic neuropathy or to more common age-linked disorders such as Parkinson's disease. However, cellular, biochemical and molecular-genetic evidence that is able to explain these associations is very scarce. The etiology of multifactorial diseases is very difficult to sort out because such diseases are due to a combination of genetic and environmental factors that individually only contribute in small part to the development of the illness. Thus, the haplogroup-defining mutations might behave as susceptibility factors, but they could have only a small effect on oxidative phosphorylation (OXPHOS) function. Moreover, these effects would be highly dependent on the 'context' in which the genetic variant is acting. To homogenize this 'context' for mitochondrial DNA (mtDNA) mutations, a cellular approach is available that involves the use of what is known as 'cybrids'. By using this model, we demonstrate that mtDNA and mtRNA levels, mitochondrial protein synthesis, cytochrome oxidase activity and amount, normalized oxygen consumption, mitochondrial inner membrane potential and growth capacity are different in cybrids from the haplogroup H when compared with those of the haplogroup Uk. Thus, these inherited basal differences in OXPHOS capacity can help to explain why some individuals more quickly reach the bioenergetic threshold below which tissue symptoms appear and progress toward multifactorial disorders. Hence, some population genetic variants in mtDNA contribute to the genetic component of complex disorders. The existence of mtDNA-based OXPHOS differences opens possibilities for the existence of a new field, mitochondrial pharmacogenomics. New sequence accession nos: HM103354-HM103363.

Despite the strong purifying selection that occurs during embryonic development, the particular location and features of mitochondrial DNA make it especially susceptible to accumulating point mutations, giving rise to a large number of mitochondrial DNA variants. Many of these will have moderate or no phenotypic effects but others will be the cause of very dramatic diseases, usually known as mitochondriopathies. Because of the abundance of different mitochondrial DNA variants, it is not easy to determine whether a new mutation is pathogenic. To facilitate this task, different criteria have been proposed, but they are often either too severely or too loosely applied. Citing examples from the literature, in this paper we discuss some critical aspects of these criteria.

A disorder of mitochondrial energy metabolism may be missed in children with a very mild phenotype. Here, we described a patient with a moderate mental retardation and a mild exercise intolerance. This child harboured a mtDNA transition (m.6955G>A) in the subunit I of the cytochrome oxidase (MT-CO1) that fulfils most of the requirements to be pathologic. Despite this subunit is the second longest polypeptide encoded in the mtDNA, only one other missense mutation associated with a myopathy has been described. This suggests that we are missing other phenotypes and that the mitochondrial pathology field is broader that previously thought.

Background: The oxidative phosphorylation system (OXPHOS) includes nuclear chromosome (nDNA)– and mitochondrial DNA (mtDNA)–encoded polypeptides. Many rare OXPHOS disorders, such as striatal necrosis syndromes, are caused by genetic mutations. Despite important advances in sequencing procedures, causative mutations remain undetected in some patients. It is possible that etiologic factors, such as environmental toxins, are the cause of these cases. Indeed, the inhibition of a particular enzyme by a poison could imitate the biochemical effects of pathological mutations in that enzyme. Moreover, environmental factors can modify the penetrance or expressivity of pathological mutations. Objectives: We studied the interaction between mitochondrially encoded ATP synthase 6 (p.MT-ATP6) subunit and an environmental exposure that may contribute phenotypic differences between healthy individuals and patients suffering from striatal necrosis syndromes or other mitochondriopathies. Methods: We analyzed the effects of the ATP synthase inhibitor tributyltin chloride (TBTC), a widely distributed environmental factor that contaminates human food and water, on transmitochondrial cell lines with or without an ATP synthase mutation that causes striatal necrosis syndrome. Doses were selected based on TBTC concentrations previously reported in human whole blood samples. Results: TBTC modified the phenotypic effects caused by a pathological mtDNA mutation. Interestingly, wild-type cells treated with this xenobiotic showed similar bioenergetics when compared with the untreated mutated cells. Conclusions: In addition to the known genetic causes, our findings suggest that environmental exposure to TBTC might contribute to the etiology of striatal necrosis syndromes. Citation: López-Gallardo E, Llobet L, Emperador S, Montoya J, Ruiz-Pesini E. 2016. Effects of tributyltin chloride on cybrids with or without an ATP synthase pathologic mutation. Environ Health Perspect 124:1399–1405; http://dx.doi.org/10.1289/EHP182

CoQ deficiency study group: et al., We evaluated the coenzyme Q₁₀ (CoQ) levels in patients who were diagnosed with mitochondrial oxidative phosphorylation (OXPHOS) and non-OXPHOS disorders (n = 72). Data from the 72 cases in this study revealed that 44.4% of patients showed low CoQ concentrations in either their skeletal muscle or skin fibroblasts. Our findings suggest that secondary CoQ deficiency is a common finding in OXPHOS and non-OXPHOS disorders. We hypothesize that cases of CoQ deficiency associated with OXPHOS defects could be an adaptive mechanism to maintain a balanced OXPHOS, although the mechanisms explaining these deficiencies and the pathophysiological role of secondary CoQ deficiency deserves further investigation., This work was supported by grants from the Instituto de Salud Carlos III (FIS: PI12/01683, PI14/00005, PI14/00028 and PI14/01962), the Explora Ciencia Program (SAF2013-50139-EXP), the Departamento de Ciencia, Tecnología, from Universidad del Gobierno de Aragón (Grupos Consolidados B33), the Fondo Europeo de Desarrollo Regional (FEDER Funding Program) from the European Union. The CIBERER is an initiative of the ISCIII. UCLH/UCL received a proportion of funding from the Department of Health sNIHR Biomedical Research Centers funding scheme.

Background Mutations in the gene encoding thymidine kinase 2 (TK2) result in the myopathic form of mitochondrial DNA depletion syndrome which is a mitochondrial encephalomyopathy presenting in children. In order to unveil some of the mechanisms involved in this pathology and to identify potential biomarkers and therapeutic targets we have investigated the gene expression profile of human skeletal muscle deficient for TK2 using cDNA microarrays. Results We have analysed the whole transcriptome of skeletal muscle from patients with TK2 mutations and compared it to normal muscle and to muscle from patients with other mitochondrial myopathies. We have identified a set of over 700 genes which are differentially expressed in TK2 deficient muscle. Bioinformatics analysis reveals important changes in muscle metabolism, in particular, in glucose and glycogen utilisation, and activation of the starvation response which affects aminoacid and lipid metabolism. We have identified those transcriptional regulators which are likely to be responsible for the observed changes in gene expression. Conclusion Our data point towards the tumor suppressor p53 as the regulator at the centre of a network of genes which are responsible for a coordinated response to TK2 mutations which involves inflammation, activation of muscle cell death by apoptosis and induction of growth and differentiation factor 15 (GDF-15) in muscle and serum. We propose that GDF-15 may represent a potential novel biomarker for mitochondrial dysfunction although further studies are required.

Coenzyme Q10 deficiency study group et al., We evaluated coenzyme Q10 (CoQ) levels in patients studied under suspicion of mitochondrial DNA depletion syndromes (MDS) (n = 39). CoQ levels were quantified by HPLC, and the percentage of mtDNA depletion by quantitative real-time PCR. A high percentage of MDS patients presented with CoQ deficiency as compared to other mitochondrial patients (Mann–Whitney-U test: p = 0.001). Our findings suggest that MDS are frequently associated with CoQ deficiency, as a possible secondary consequence of disease pathophysiology. Assessment of muscle CoQ status seems advisable in MDS patients since the possibility of CoQ supplementation may then be considered as a candidate therapy., This work has been supported by grant from the Instituto de Salud Carlos III, Ministerio de Ciencia e Innovación, Spain (FIS: PI11/00078, PI10/00662, PI11/01301, PI08/0307 and PI08/0663 and a Fellowship to IMR PI07/00184). The Centro de Investigaciones Biomédicas en Red de Enfermedades Raras (CIBERER) is an initiative of the Instituto de Salud Carlos III by Departamento de Ciencia, Tecnología y Universidad del Gobierno de Aragón y Fondo Social Europeo (Grupo B33) and Asociación de Enfermos de Patología Mitocondrial (AEPMI). Part of this work was undertaken at UCLH which received a proportion of funding from the Department of Health's NIHR Biomedical Research Centres funding scheme.

A human mitochondrial DNA (mtDNA) transition, m.1555A>G, in the 12S rRNA gene causes non-syndromic hearing loss. However, this pathological mutation is the wild-type allele in orangutan mtDNA. Here we rule out different genetic factors as the reason for its fixation in orangutans and show that aminoglycosides negatively affect the oxidative phosphorylation function by decreasing the synthesis of mtDNA-encoded proteins and the amount and activity of respiratory complex IV. These drugs also diminish the growth rate of orangutan cells. The m.1555G nucleotide is also the wild-type allele in other mammal species and they might be at risk of suffering a mitochondrial disorder if treated with aminoglycosides. Therefore, pharmacogenomic approaches should be used to confirm this possibility. These observations are important for human health. Due to the fact that old age and high frequency are criteria widely used in mitochondrial medicine to rule out a genetic change as being a pathological mutation, our results prevent against simplistic genetic approaches that do not consider the potential effect of environmental conditions. Hence, these results suggest that some ancient and highly frequent human population polymorphisms, such as those defining mtDNA haplogroups, in mitochondrial rRNA genes can be deleterious in association with new environmental conditions. Therefore, as the discovery of ribosomal antibiotics has allowed to fight infectious diseases and this breakthrough can be considered an important scientific advance or ‘progress’, our results suggest that ‘progress’ can also have a negative counterpart and render detrimental many of these mtDNA genotypes.

There is increasing evidence regarding the association between mitochondrial DNA (mtDNA) and aerobic capacity; however, whether mtDNA haplogroups are associated with the status of being an elite endurance athlete is more controversial. We compared the frequency distribution of mtDNA haplogroups among the following groups of Spanish (Caucasian) men: 102 elite endurance athletes (professional road cyclists, endurance runners), 51 elite power athletes (jumpers, throwers and sprinters), and 478 non-athletic controls. We observed a significant difference between endurance athletes and controls (Fisher exact test = 17.89, P = 0.015; Bonferroni's significant threshold = 0.017). yet not between power athletes and controls (Fisher exact test = 47.99, P = 0.381) or between endurance and power athletes (Fisher exact test = 5.53, P = 0.597). We observed that the V haplogroup was overrepresented in endurance athletes (15.7%) compared with controls (7.5%) (odds ratio: 2.284; 95% confidence interval: 1.237, 4.322). In conclusion, our findings overall support the idea that mtDNA variations could be among the numerous contributors to the status of being an elite endurance athlete, whereas no association was found with elite power athletic status. (C) 2011 Elsevier B.V. and Mitochondria Research Society. All rights reserved.