Biochemical phenotypes associated with the mitochondrial ATP6 gene mutations at nt8993

Mitochondrial DNA 8993T>G mutation in a child with ornithine transcarbamylase deficiency and leigh syndrome: an unexpected association

MC, female, is the third child of a nonconsanguineous Portuguese couple, born after an uneventful pregnancy and delivery. A positive family history of ornithine transcarbamylase deficiency, associated with the IVS8+1 G>A mutation in the ornithine transcarbamylase gene, prompted prenatal diagnosis with identification of the same mutation in the proband. During an episode of Klebsiella pneumoniae sepsis at 1.5 months of age, lactic acidosis and moderate hyperammonemia were noticed. After a short asymptomatic period, progressive neurologic symptoms, with normal ammonemia, persistent hyperlactacidemia, and typical lesions in brain computed tomography (CT) scan led to a diagnosis of Leigh syndrome. Mitochondrial respiratory chain complex V was reduced in the liver. The mtDNA 8993T>G mutation was identified in the liver, muscle, and blood (82%-87% heteroplasmy). She died at 6 months of age. This case represents a benign phenotype of ornithine transcarbamylase deficiency, associated with a severe mitochondrial respiratory chain disorder due to an mtDNA pathogenic mutation.

Mitochondrial dysfunction promotes cell migration via reactive oxygen species-enhanced β5-integrin expression in human gastric cancer SC-M1 cells

BACKGROUND

Mitochondrial dysfunction has been shown to promote cancer cell migration. However, molecular mechanism by which mitochondrial dysfunction enhances gastric cancer (GC) cell migration remains unclear.

METHODS

Mitochondria specific inhibitors, oligomycin and antimycin A, were used to induce mitochondrial dysfunction and to enhance cell migration of human gastric cancer SC-M1 cells. Antioxidant N-acetylcysteine (NAC) was used for evaluating the effect of reactive oxygen species (ROS). Protein expressions of epithelial-to-mesenchymal transition (EMT) markers and the cell-extracellular matrix (ECM) adhesion molecules, the integrin family, were analyzed. A migratory subpopulation of SC-M1 cells (SC-M1-3rd) was selected using a transwell assay for examining the association of mitochondrial bioenergetic function, intracellular ROS content and β5-integrin expression. Clinicopathologic characteristics of β5-integrin expression were analyzed in GC specimens by immunohistochemical staining.

RESULTS

Treatments with mitochondrial inhibitors elevated mitochondria-generated ROS and cell migration of SC-M1 cells. The protein expression of β5-integrin and cell surface expression of αvβ5-integrin were upregulated, and which were suppressed by NAC. Pretreatments with NAC and anti-αvβ5-integrin neutralizing antibody respectively prevented the mitochondrial dysfunction-induced cell migration. The selected migratory SC-M1-3rd cells showed impaired mitochondrial function, higher mitochondria-generated ROS, and increased β5-integrin expression. The migration ability was also repressed by anti-αvβ5-integrin neutralizing antibody. In clinical specimens, GCs with higher β5-integrin protein expression had more aggressive behavior. In conclusion, mitochondrial dysfunction may lead to GC progression by enhancing migration through mitochondria-generated ROS mediated β5-integrin expression.

GENERAL SIGNIFICANCE

These results support the role of mitochondrial dysfunction in GC progression.

Effect of mtDNA point mutations on cellular bioenergetics

This overview discusses the results of research on the effects of most frequent mtDNA point mutations on cellular bioenergetics. Thirteen proteins coded by mtDNA are crucial for oxidative phosphorylation, 11 of them constitute key components of the respiratory chain complexes I, III and IV and 2 of mitochondrial ATP synthase. Moreover, pathogenic point mutations in mitochondrial tRNAs and rRNAs generate abnormal synthesis of the mtDNA coded proteins. Thus, pathogenic point mutations in mtDNA usually disturb the level of key parameter of the oxidative phosphorylation, i.e. the electric potential on the inner mitochondrial membrane (Δψ), and in a consequence calcium signalling and mitochondrial dynamics in the cell. Mitochondrial generation of reactive oxygen species is also modified in the mutated cells. The results obtained with cultured cells and describing biochemical consequences of mtDNA point mutations are full of contradictions. Still they help elucidate the biochemical basis of pathologies and provide a valuable tool for finding remedies in the future. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Hyperoxia fully protects mitochondria of explanted livers

Liver ischemia-reperfusion injury is still an open problem in many clinical circumstances, including surgery and transplantation. This study investigates how mitochondrial structure, mass and oxidative phosphorylation change and may be preserved during a brief period of ischemia followed by a long period of reperfusion, an experimental model that mimics the condition to which a liver is exposed during transplantation. Livers were explanted from rats and exposed for 24 h to three different oxygen availability conditions at 4 °C. Mitochondrial mass, respiration, oxidative phosphorylation (OXPHOS), and levels of OXPHOS complexes were all significantly altered in livers stored under the currently used preservation condition of normoxia. Remarkably, liver perfusion with hyperoxic solutions fully preserved mitochondrial morphology and function, suggesting that perfusion of the graft with hyperoxic solution should be considered in human transplantation.

Cardiolipin content in mitochondria from cultured skin fibroblasts harboring mutations in the mitochondrial ATP6 gene

The role of phospholipids in normal assembly and organization of the membrane proteins has been well documented. Cardiolipin, a unique tetra-acyl phospholipid localized in the inner mitochondrial membrane, is implicated in the stability of many inner-membrane protein complexes. Loss of cardiolipin content, alterations in its acyl chain composition and/or cardiolipin peroxidation have been associated with dysfunction in multiple tissues in a variety of pathological conditions. The aim of this study was to analyze the phospholipid composition of the mitochondrial membrane in the four most frequent mutations in the ATP6 gene: L156R, L217R, L156P and L217P but, more importantly, to investigate the possible changes in the cardiolipin profile. Mitochondrial membranes from fibroblasts with mutations at codon 217 of the ATP6 gene, showed a different cardiolipin content compared to controls. Conversely, results similar to controls were obtained for mutations at codon 156. These findings may be attributed to differences in the biosynthesis and remodeling of cardiolipin at the level of the inner mitochondrial transmembrane related to some mutations of the ATP6 gene.

A yeast-based assay identifies drugs active against human mitochondrial disorders

Due to the lack of relevant animal models, development of effective treatments for human mitochondrial diseases has been limited. Here we establish a rapid, yeast-based assay to screen for drugs active against human inherited mitochondrial diseases affecting ATP synthase, in particular NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome. This method is based on the conservation of mitochondrial function from yeast to human, on the unique ability of yeast to survive without production of ATP by oxidative phosphorylation, and on the amenability of the yeast mitochondrial genome to site-directed mutagenesis. Our method identifies chlorhexidine by screening a chemical library and oleate through a candidate approach. We show that these molecules rescue a number of phenotypes resulting from mutations affecting ATP synthase in yeast. These compounds are also active on human cybrid cells derived from NARP patients. These results validate our method as an effective high-throughput screening approach to identify drugs active in the treatment of human ATP synthase disorders and suggest that this type of method could be applied to other mitochondrial diseases.

Oxidative phosphorylation in cancer cells

Evidence suggests that mitochondrial metabolism may play a key role in controlling cancer cells life and proliferation. Recent evidence also indicates how the altered contribution of these organelles to metabolism and the resistance of cancer mitochondria against apoptosis-associated permeabilization are closely related. The hallmarks of cancer growth, increased glycolysis and lactate production in tumours, have raised attention due to recent observations suggesting a wide spectrum of oxidative phosphorylation deficit and decreased availability of ATP associated with malignancies and tumour cell expansion. More specifically, alteration in signal transduction pathways directly affects mitochondrial proteins playing critical roles in controlling the membrane potential as UCP2 and components of both MPTP and oxphos complexes, or in controlling cells life and death as the Bcl-2 proteins family. Moreover, since mitochondrial bioenergetics and dynamics, are also involved in processes of cells life and death, proper regulation of these mitochondrial functions is crucial for tumours to grow. Therefore a better understanding of the key pathophysiological differences between mitochondria in cancer cells and in their non-cancer surrounding tissue is crucial to the finding of tools interfering with these peculiar tumour mitochondrial functions and will disclose novel approaches for the prevention and treatment of malignant diseases. Here, we review the peculiarity of tumour mitochondrial bioenergetics and the mode it is linked to the cell metabolism, providing a short overview of the evidence accumulated so far, but highlighting the more recent advances.

Oxidative stress in the denervated muscle

Following experimental hind limb denervation in rats, this study demonstrates that oxidative stress occurs and advances an hypothesis about its origin. In fact: (i) ROS are formed; (ii) membrane lipids are oxidized; (iii) oxidized ion channels and pumps may lead to increased [Ca(2+)](i); all the above mentioned events increase with denervation time. In the denervated muscle, (iv) mRNA abundance of cytoprotective and anti-oxidant proteins (Hsp70, Hsp27, Sod1, Catalase, Gpx1, Gpx4, Gstm1), as well as (v) SOD1 enzymatic activity and HSP70i protein increase; (vi) an unbalance in mitochondrial OXPHOS enzymes occurs, presumably leading to excess mitochondrial ROS production; (vii) increased cPLA2alpha expression (mRNA) and activation (increased [Ca(2+)](i)) may lead to increased hydroperoxides release. Since anti-oxidant defences appear inadequate to counterbalance increased ROS production with increased denervation time, an anti-oxidant therapeutic strategy seems to be advisable in the many medical conditions where the nerve-muscle connection is impaired.

Energy metabolism of the visual system

The visual system is one of the most energetically demanding systems in the brain. The currency of energy is ATP, which is generated most efficiently from oxidative metabolism in the mitochondria. ATP supports multiple neuronal functions. Foremost is repolarization of the membrane potential after depolarization. Neuronal activity, ATP generation, blood flow, oxygen consumption, glucose utilization, and mitochondrial oxidative metabolism are all interrelated. In the retina, phototransduction, neurotransmitter utilization, and protein/organelle transport are energy-dependent, yet repolarization-after-depolarization consumes the bulk of the energy. Repolarization in photoreceptor inner segments maintains the dark current. Repolarization by all neurons along the visual pathway following depolarizing excitatory glutamatergic neurotransmission preserves cellular integrity and permits reactivation. The higher metabolic activity in the magno- versus the parvo-cellular pathway, the ON- versus the OFF-pathway in some (and the reverse in other) species, and in specialized functional representations in the visual cortex all reflect a greater emphasis on the processing of specific visual attributes. Neuronal activity and energy metabolism are tightly coupled processes at the cellular and even at the molecular levels. Deficiencies in energy metabolism, such as in diabetes, mitochondrial DNA mutation, mitochondrial protein malfunction, and oxidative stress can lead to retinopathy, visual deficits, neuronal degeneration, and eventual blindness.

Hypoxia and mitochondrial oxidative metabolism

It is now clear that mitochondrial defects are associated with a large variety of clinical phenotypes. This is the result of the mitochondria's central role in energy production, reactive oxygen species homeostasis, and cell death. These processes are interdependent and may occur under various stressing conditions, among which low oxygen levels (hypoxia) are certainly prominent. Cells exposed to hypoxia respond acutely with endogenous metabolites and proteins promptly regulating metabolic pathways, but if low oxygen levels are prolonged, cells activate adapting mechanisms, the master switch being the hypoxia-inducible factor 1 (HIF-1). Activation of this factor is strictly bound to the mitochondrial function, which in turn is related with the oxygen level. Therefore in hypoxia, mitochondria act as [O2] sensors, convey signals to HIF-1 directly or indirectly, and contribute to the cell redox potential, ion homeostasis, and energy production. Although over the last two decades cellular responses to low oxygen tension have been studied extensively, mechanisms underlying these functions are still indefinite. Here we review current knowledge of the mitochondrial role in hypoxia, focusing mainly on their role in cellular energy and reactive oxygen species homeostasis in relation with HIF-1 stabilization. In addition, we address the involvement of HIF-1 and the inhibitor protein of F1F0 ATPase in the hypoxia-induced mitochondrial autophagy.

Reactive oxygen species, oxidative stress, and cell death correlate with level of CoQ10 deficiency

Coenzyme Q(10) (CoQ(10)) is essential for electron transport in the mitochondrial respiratory chain and antioxidant defense. The relative importance of respiratory chain defects, ROS production, and apoptosis in the pathogenesis of CoQ(10) deficiency is unknown. We determined previously that severe CoQ(10) deficiency in cultured skin fibroblasts harboring COQ2 and PDSS2 mutations produces divergent alterations of bioenergetics and oxidative stress. Here, to better understand the pathogenesis of CoQ(10) deficiency, we have characterized the effects of varying severities of CoQ(10) deficiency on ROS production and mitochondrial bioenergetics in cells harboring genetic defects of CoQ(10) biosynthesis. Levels of CoQ(10) seem to correlate with ROS production; 10-15% and >60% residual CoQ(10) are not associated with significant ROS production, whereas 30-50% residual CoQ(10) is accompanied by increased ROS production and cell death. Our results confirm that varying degrees of CoQ(10) deficiency cause variable defects of ATP synthesis and oxidative stress. These findings may lead to more rational therapeutic strategies for CoQ(10) deficiency.

Oxidative stress and ATPase6 mutation is associated with primary ovarian insufficiency

PURPOSE

Primary ovarian insufficiency (POI) is a heterogeneous, multifactorial disorder. Though genetic anomalies, infections, autoimmune disorder and hormonal imbalance are few of the causes of POI, in the majority of patients (50-60%) no etiology has been identified. Mitochondrial bioenergetics and biogenesis play an important role in oocyte and embryo development, whereas mtDNA integrity and content are essential for the normal development of oocytes. ATPase6 helps to maintain the mt genome integrity, and mutations in ATPase6 are associated with overproduction of reactive oxygen species (ROS) in a variety of diseases; however, its role in POI has not been evaluated. Therefore, we planned to evaluate the potential role of ATPase6 gene mutations and associated oxidative stress in idiopathic cases of POI.

METHODS

This pilot study included: 20 cases of POI with FSH level of >40 mIU/ml; 4 cases of occult ovarian insufficiency (occult OI) with irregular menses and mean FSH levels of 16.4 mIU/ml; and 20 age-matched healthy female controls (FSH 2-5 mIU/ml). ROS levels in blood plasma were measured by luminol-dependent chemiluminescence assay and the ROS values were expressed as relative light unit per minute (RLU/min). mtDNA ATPase6 gene was amplified and sequenced from the blood lymphocyte DNA.

RESULTS

Of all, 50% patients showed nucleotide changes in the ATPase6 gene, as compared to 10% in controls, and the majority of these mutations were non-synonymous. ATPase6 mt.8684 C>T and mt.9094 C>T were found to be significantly (P < 0.005) higher in cases as compared to controls. ROS levels were found to be significantly (P < 0.005) higher in POI and occult OI patients compared to controls and nucleotide changes were found to positively correlate with ROS levels. Moreover, ROS production was found to positively correlate (r = 0.7038, P < 0.001) with FSH levels of the patients (POI and OI) compared to controls.

CONCLUSIONS

This pilot study clearly demonstrates for the first time ATPase6 gene nucleotide alterations and elevated ROS levels in idiopathic cases of POI. Therefore, it may be possible that OS associated with ATPase6 gene mutation may be causal in idiopathic cases of premature OI. However, larger studies with inclusion of more cases of both POI and occult OI are required to strongly establish the correlation between oxidative stress and mitochondrial nucleotide alterations in the pathogenesis of POI. Such cases with OS-induced POI may benefit immensely by early diagnosis and prompt antioxidant administration.

Mitochondrial Complex I decrease is responsible for bioenergetic dysfunction in K-ras transformed cells

Many cancer cells are characterized by high rate of glycolysis and reduced rate of aerobic respiration, whose mechanism is still elusive. Here we investigate the down-regulation of oxidative phosphorylation (OXPHOS) in K-ras transformed mouse fibroblasts as compared to a control counterpart. Transcriptional analysis showed different expression levels of several OXPHOS nuclear genes in the two cell lines. In particular, during the exponential growth phase most genes encoding proteins of Complex I were expressed at lower levels in transformed cells. Consistently, a significant decrease of Complex I content was found in transformed cells. Moreover, analysis of NAD-dependent respiration and ATP synthesis indicated a strong decrease of Complex I activity in the mitochondria from neoplastic cells, that was confirmed by direct assay of the enzyme redox activity. At variance, succinate-dependent respiration and ATP synthesis were not significantly affected. Taken together, our results provide the new insight that the reduction of respiration observed in K-ras transformed cells is specifically due to a Complex I activity decrease.

Mitochondrial DNA background modifies the bioenergetics of NARP/MILS ATP6 mutant cells

Mutations in the mitochondrial DNA (mtDNA) encoded subunit 6 of ATPase (ATP6) are associated with variable disease expression, ranging from adult onset neuropathy, ataxia and retinitis pigmentosa (NARP) to fatal childhood maternally inherited Leigh's syndrome (MILS). Phenotypical variations have largely been attributed to mtDNA heteroplasmy. However, there is often a discrepancy between the levels of mutant mtDNA and disease severity. Therefore, the correlation among genetic defect, bioenergetic impairment and clinical outcome in NARP/MILS remains to be elucidated. We investigated the bioenergetics of cybrids from five patients carrying different ATP6 mutations: three harboring the T8993G, one with the T8993C and one with the T9176G mutation. The bioenergetic defects varied dramatically, not only among different ATP6 mutants, but also among lines carrying the same T8993G mutation. Mutants with the most severe ATP synthesis impairment showed defective respiration and disassembly of respiratory chain complexes. This indicates that respiratory chain defects modulate the bioenergetic impairment in NARP/MILS cells. Sequencing of the entire mtDNA from the different mutant cell lines identified variations in structural genes, resulting in amino acid changes that destabilize the respiratory chain. Taken together, these results indicate that the mtDNA background plays an important role in modulating the biochemical defects and clinical outcome in NARP/MILS.

A suggested role for mitochondria in Noonan syndrome

Noonan syndrome (NS) is an autosomal dominant disorder, and a main feature is congenital heart malformation. About 50% of cases are caused by gain-of-function mutations in the tyrosine phosphatase SHP2/PTPN11, a downstream regulator of ERK/MAPK. Recently it was reported that SHP2 also localizes to the mitochondrial intercristae/intermembrane space (IMS), but the role of SHP2 in mitochondria is unclear. The mitochondrial oxidative phosphorylation (OxPhos) system provides the vast majority of cellular energy and produces reactive oxygen species (ROS). Changes in ROS may interfere with organ development such as that observed in NS patients. Several phosphorylation sites have been found in OxPhos components including cytochrome c oxidase (CcO) and cytochrome c (Cytc), and we hypothesized that OxPhos complexes may be direct or indirect targets of SHP2. We analyzed mitochondrial function using mouse fibroblasts from wild-types, SHP2 knockdowns, and D61G SHP2 mutants leading to constitutively active SHP2, as found in NS patients. Levels of OxPhos complexes were similar except for CcO and Cytc, which were 37% and 28% reduced in the D61G cells. However, CcO activity was significantly increased, as we also found for two lymphoblast cell lines from NS patients with two independent mutations in PTPN11. D61G cells showed lower mitochondrial membrane potential and 30% lower ATP content compared to controls. ROS were significantly increased; aconitase activity, a marker for ROS-induced damage, was decreased; and catalase activity was increased in D61G cells. We propose that decreased energy levels and/or increased ROS may explain, at least in part, some of the clinical features in NS that overlap with children with mitochondrial disorders.

Mitochondrial DNA mutations and human disease

Mitochondrial disorders are a group of clinically heterogeneous diseases, commonly defined by a lack of cellular energy due to oxidative phosphorylation (OXPHOS) defects. Since the identification of the first human pathological mitochondrial DNA (mtDNA) mutations in 1988, significant efforts have been spent in cataloguing the vast array of causative genetic defects of these disorders. Currently, more than 250 pathogenic mtDNA mutations have been identified. An ever-increasing number of nuclear DNA mutations are also being reported as the majority of proteins involved in mitochondrial metabolism and maintenance are nuclear-encoded. Understanding the phenotypic diversity and elucidating the molecular mechanisms at the basis of these diseases has however proved challenging. Progress has been hampered by the peculiar features of mitochondrial genetics, an inability to manipulate the mitochondrial genome, and difficulties in obtaining suitable models of disease. In this review, we will first outline the unique features of mitochondrial genetics before detailing the diseases and their genetic causes, focusing specifically on primary mtDNA genetic defects. The functional consequences of mtDNA mutations that have been characterised to date will also be discussed, along with current and potential future diagnostic and therapeutic advances.

A study of mitochondrial DNA D-loop mutations and p53 status in nonmelanoma skin cancer

BACKGROUND

Mitochondrial DNA (mtDNA) displacement-loop (D-loop) mutations have previously demonstrated potential as smoking-induced biomarkers in oral squamous cell carcinoma (SCC). Additionally, they have been observed in SCC and basal cell carcinoma of nonmelanoma skin cancer (NMSC). However, they have not been examined in the SCC precursor lesions, Bowen disease or actinic keratosis.

OBJECTIVES

Here, we present a novel study of mtDNA D-loop mutations in these two precursors, a rare keratoacanthoma and NMSC (all tumours not related to smoking).

METHODS

We used a polymerase chain reaction and direct sequencing approach. Furthermore, as the tumour suppressor protein p53 has been reported as having a novel role in maintaining mitochondrial genetic stability, we assessed p53 status using immunohistochemistry, evaluating potential association with the presence of mtDNA mutations.

RESULTS

Of 36 tumours, nine (25%) exhibited mutations in the D-loop. In total, 13 base substitutions were observed across all patients: seven (53.8%) were A : T to G : C; two (15.4%) were G : C to T : A; two (15.4%) were G : C to A : T and two (15.4%) were G : C to C : G. Four of the 13 (30.8%) base substitutions were observed at nucleotide 146. We observed abnormal p53 accumulation in over half of the samples analysed (55.5%), suggesting it to be a major part of the carcinogenic process of NMSC; however; there was no association between p53 positivity and the presence of mtDNA mutations (P = 0.47).

CONCLUSIONS

It is unlikely that alteration in p53 status is a contributing factor to mtDNA mutagenesis.

Biochemical consequences in yeast of the human mitochondrial DNA 8993T>C mutation in the ATPase6 gene found in NARP/MILS patients

We have created and analyzed the properties of a yeast model of the human mitochondrial DNA T8993C mutation that has been associated with maternally-inherited Leigh syndrome and/or with neurogenic muscle weakness, ataxia and retinitis pigmentosa. This mutation changes a highly conserved leucine to proline in the Atp6p subunit of the ATP synthase, at position 156 in the human protein, position 183 in yeast. In vitro the yeast T8993C mitochondria showed a 40-50% decrease in the rate of ATP synthesis. The ATP-driven translocation of protons across the inner mitochondrial membrane was normal in the mutant and fully sensitive to oligomycin, an inhibitor of the ATP synthase proton channel. However under conditions of maximal ATP hydrolytic activity, using non-osmotically protected mitochondria, the mutant ATPase activity was poorly inhibited by oligomycin (by 40% versus 85% in wild type cells). These anomalies were attributed by BN-PAGE and mitochondrial protein synthesis analyses to a less efficient incorporation of Atp6p within the ATP synthase. Interestingly, the cytochrome c oxidase content was selectively decreased by 40-50% in T8993C yeast, apparently due to a reduced synthesis of its mitochondrially encoded Cox1p subunit. This observation further supports the existence of a control of cytochrome c oxidase expression by the ATP synthase in yeast mitochondria. Despite the ATPase deficiency, growth of the atp6-L183P mutant on respiratory substrates and the efficiency of oxidative phosphorylation were similar to that of wild type, indicating that the mutation did not affect the proton permeability of the mitochondrial inner membrane.

A heteroplasmic, not homoplasmic, mitochondrial DNA mutation promotes tumorigenesis via alteration in reactive oxygen species generation and apoptosis

Mitochondrial alteration has been long proposed to play a major role in tumorigenesis. Recently, mitochondrial DNA (mtDNA) mutations have been found in a variety of cancer cells. In this study, we examined the contribution of mtDNA mutation and mitochondrial dysfunction in tumorigenesis first using human cell lines carrying a frame-shift at NADH dehydrogenase (respiratory complex I) subunit 5 gene (ND5); the same homoplasmic mutation was also identified in a human colorectal cancer cell line earlier. With increasing mutant ND5 mtDNA content, respiratory function including oxygen consumption and ATP generation through oxidative phosphorylation declined progressively, while lactate production and dependence on glucose increased. Interestingly, the reactive oxygen species (ROS) levels and apoptosis exhibited antagonistic pleiotropy associated with mitochondrial defects. Furthermore, the anchorage-dependence phenotype and tumor-forming capacity of cells carrying wild-type and mutant mtDNA were tested by growth assay in soft agar and subcutaneous implantation of the cells in nude mice. Surprisingly, the cell line carrying the heteroplasmic ND5 mtDNA mutation showed significantly enhanced tumor growth, while cells with homoplasmic form of the same mutation inhibited tumor formation. Similar results were obtained from the analysis of a series of mouse cell lines carrying a nonsense mutation at ND5 gene. Our results indicate that the mtDNA mutations might play an important role in the early stage of cancer development, possibly through alteration of ROS generation and apoptosis.

Leigh syndrome: clinical and neuroimaging follow-up

Leigh syndrome, caused by dysfunction in mitochondrial energy metabolism, is an inherited, heterogeneous, and progressive neurodegenerative disorder of infancy and childhood. From 1983 to August 2006, 14 cases diagnosed with Leigh syndrome were studied in terms of characteristic neuroimaging findings and abnormal mitochondrial configurations under electron microscopy, as well as molecular analysis. Of the 14 cases, 11 presented clinical features before age 1 (79%). All 14 presented with variable symptoms of central nervous system involvement. The three most common symptoms were developmental delay (12/14; 86%), seizures (11/14; 79%), and altered consciousness (8/14; 57%). Extra-central nervous system manifestations were observed in 10 of the 14 cases, the most common symptoms being failure to thrive (5/14; 36%), pericardial effusion and dilated cardiomyopathy (3/14; 21%), and liver function impairment (3/14; 21%). In all 14 cases, neuroimaging revealed abnormal findings over the basal ganglion, brainstem, or both. The putamen was the most common lesion site in the basal ganglia (11/12; 92%). Cranial magnetic resonance imaging was used for follow-up in 6 cases because of changes in clinical features; in all 6 cases the imaging revealed evolution in the brain. Cranial magnetic resonance spectroscopy was performed in 3 cases and in 2 of them revealed lactate peaks during deterioration of the disease course. The prognosis for Leigh syndrome was poor during long-term follow-up. Seven cases were early fatalities, before 1 year and 6 months of age. Follow-up cranial magnetic resonance imaging together with magnetic resonance spectroscopy in cases with clinical evolution is helpful for monitoring this disease.

Pathogenesis and treatment of mitochondrial disorders

In the past 50 years, our understanding of the biochemical and molecular causes of mitochondrial diseases, defined restrictively as disorders due to defects of the mitochondrial respiratory chain (RC), has made great strides. Mitochondrial diseases can be due to mutations in mitochondrial DNA (mtDNA) or in nuclear DNA (nDNA) and each group can be subdivided into more specific classes. Thus, mtDNA-related disorders can result from mutations in genes affecting protein synthesis in toto or mutations in protein-coding genes. Mendelian mitochondrial disorders can be attributed to mutations in genes that (i) encode subunits of the RC ("direct hits"); (ii) encode assembly proteins or RC complexes ("indirect hits"); (iii) encode factors needed for mtDNA maintenance, replication, or translation (intergenomic signaling); (iv) encode components of the mitochondrial protein import machinery; (v) control the synthesis and composition of mitochondrial membrane phospholipids; and (vi) encode proteins involved in mitochondrial dynamics.In contrast to this wealth of knowledge about etiology, our understanding of pathogenic mechanism is very limited. We discuss pathogenic factors that can influence clinical expression, especially ATP shortage and reactive oxygen radicals (ROS) excess. Therapeutic options are limited and fall into three modalities: (i) symptomatic interventions, which are palliative but crucial for day-to-day management; (ii) radical approaches aimed at correcting the biochemical or molecular error, which are interesting but still largely experimental; and (iii) pharmacological means of interfering with the pathogenic cascade of events (e.g. boosting ATP production or scavenging ROS), which are inconsistently and incompletely effective, but can be safe and helpful.

A novel deletion in the GTPase domain of OPA1 causes defects in mitochondrial morphology and distribution, but not in function

Autosomal dominant optic atrophy (ADOA), the commonest cause of inherited optic atrophy, is caused by mutations in the ubiquitously expressed gene optic atrophy 1 (OPA1), involved in fusion and biogenesis of the inner membrane of mitochondria. Bioenergetic failure, mitochondrial network abnormalities and increased apoptosis have all been proposed as possible causal factors. However, their relative contribution to pathogenesis as well as the prominent susceptibility of the retinal ganglion cell (RGC) in this disease remains uncertain. Here we identify a novel deletion of OPA1 gene in the GTPase domain in three patients affected by ADOA. Muscle biopsy of the patients showed neurogenic atrophy and abnormal morphology and distribution of mitochondria. Confocal microscopy revealed increased mitochondrial fragmentation in fibroblasts as well as in myotubes, where mitochondria were also unevenly distributed, with clustered organelles alternating with areas where mitochondria were sparse. These abnormalities were not associated with altered bioenergetics or increased susceptibility to pro-apoptotic stimuli. Therefore, changes in mitochondrial shape and distribution can be independent of other reported effects of OPA1 mutations, and therefore may be the primary cause of the disease. The arrangement of mitochondria in RGCs, which degenerate in ADOA, may be exquisitely sensitive to disturbance, and this may lead to bioenergetic crisis and/or induction of apoptosis. Our results highlight the importance of mitochondrial dynamics in the disease per se, and point to the loss of the fine positioning of mitochondria in the axons of RGCs as a possible explanation for their predominant degeneration in ADOA.

Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches

In mammals, the majority of cellular ATP is produced by the mitochondrial F1F(O)-ATP synthase through an elaborate catalytic mechanism. While most subunits of this enzymatic complex are encoded by the nuclear genome, a few essential components are encoded in the mitochondrial genome. The biogenesis of this multi-subunit enzyme is a sophisticated multi-step process that is regulated on levels of transcription, translation and assembly. Defects that result in diminished abundance or functional impairment of the F1F(O)-ATP synthase can cause a variety of severe neuromuscular disorders. Underlying mutations have been identified in both the nuclear and the mitochondrial DNA. The pathogenic mechanisms are only partially understood. Currently, the therapeutic options are extremely limited. Alternative methods of treatment have however been proposed, but still encounter several technical difficulties. The application of novel scientific approaches promises to deepen our understanding of the molecular mechanisms of the ATP synthase, unravel novel therapeutic pathways and improve the unfortunate situation of the patients suffering from such diseases.

The study of the pathogenic mechanism of mitochondrial diseases provides information on basic bioenergetics

Mitochondrial F(1)F(0)-ATPase was studied in lymphocytes from patients with neuropathy, ataxia, and retinitis pigmentosa (NARP), caused by a mutation at leu-156 in the ATPase 6 subunit. The mutation giving the milder phenotype (Leu156Pro) suffered a 30% reduction in proton flux, and a similar loss in ATP synthetic activity. The more severe mutation (Leu156Arg) also suffered a 30% reduction in proton flux, but ATP synthesis was virtually abolished. Oligomycin sensitivity of the proton translocation through F(0) was enhanced by both mutations. We conclude that in the Leu156Pro mutation, rotation of the c-ring is slowed but coupling of ATP synthesis to proton flux is maintained, whereas in the Leu156Arg mutation, proton flux appears to be uncoupled. Modelling indicated that, in the Leu156Arg mutation, transmembrane helix III of ATPase 6 is unable to span the membrane, terminating in an intramembrane helix II-helix III loop. We propose that the integrity of transmembrane helix III is essential for the mechanical function of ATPase 6 as a stator element in the ATP synthase, but that it is not relevant for oligomycin inhibition.

Respiratory chain dysfunction and oxidative stress correlate with severity of primary CoQ10 deficiency

Coenzyme Q(10) (CoQ(10)) is essential for electron transport in the mitochondrial respiratory chain and antioxidant defense. Last year, we reported the first mutations in CoQ(10) biosynthetic genes, COQ2, which encodes 4-parahydroxybenzoate: polyprenyl transferase; and PDSS2, which encodes subunit 2 of decaprenyl diphosphate synthase. However, the pathogenic mechanisms of primary CoQ(10) deficiency have not been well characterized. In this study, we investigated the consequence of severe CoQ(10) deficiency on bioenergetics, oxidative stress, and antioxidant defenses in cultured skin fibroblasts harboring COQ2 and PDSS2 mutations. Defects in the first two committed steps of the CoQ(10) biosynthetic pathway produce different biochemical alterations. PDSS2 mutant fibroblasts have 12% CoQ(10) relative to control cells and markedly reduced ATP synthesis, but do not show increased reactive oxygen species (ROS) production, signs of oxidative stress, or increased antioxidant defense markers. In contrast, COQ2 mutant fibroblasts have 30% CoQ(10) with partial defect in ATP synthesis, as well as significantly increased ROS production and oxidation of lipids and proteins. On the basis of a small number of cell lines, our results suggest that primary CoQ(10) deficiencies cause variable defects of ATP synthesis and oxidative stress, which may explain the different clinical features and may lead to more rational therapeutic strategies.