m.6267G>A: a recurrent mutation in the human mitochondrial DNA that reduces cytochrome c oxidase activity and is associated with tumors

Mitochondrial metabolism as a dynamic regulatory hub to malignant transformation and anti-cancer drug resistance

Glycolysis is the fundamental cellular process that permits cancer cells to convert energy and grow anaerobically. Recent developments in molecular biology have made it evident that mitochondrial respiration is critical to tumor growth and treatment response. As the principal organelle of cellular energy conversion, mitochondria can rapidly alter cellular metabolic processes, thereby fueling malignancies and contributing to treatment resistance. This review emphasizes the significance of mitochondrial biogenesis, turnover, DNA copy number, and mutations in bioenergetic system regulation. Tumorigenesis requires an intricate cascade of metabolic pathways that includes rewiring of the tricarboxylic acid (TCA) cycle, electron transport chain and oxidative phosphorylation, supply of intermediate metabolites of the TCA cycle through amino acids, and the interaction between mitochondria and lipid metabolism. Cancer recurrence or resistance to therapy often results from the cooperation of several cellular defense mechanisms, most of which are connected to mitochondria. Many clinical trials are underway to assess the effectiveness of inhibiting mitochondrial respiration as a potential cancer therapeutic. We aim to summarize innovative strategies and therapeutic targets by conducting a comprehensive review of recent studies on the relationship between mitochondrial metabolism, tumor development and therapeutic resistance.

Recent Advances in Targeted Drug Delivery Strategy for Enhancing Oncotherapy

Targeted drug delivery is a precise and effective strategy in oncotherapy that can accurately deliver drugs to tumor cells or tissues to enhance their therapeutic effect and, meanwhile, weaken their undesirable side effects on normal cells or tissues. In this research field, a large number of researchers have achieved significant breakthroughs and advances in oncotherapy. Typically, nanocarriers as a promising drug delivery strategy can effectively deliver drugs to the tumor site through enhanced permeability and retention (EPR) effect-mediated passive targeting and various types of receptor-mediated active targeting, respectively. Herein, we review recent targeted drug delivery strategies and technologies for enhancing oncotherapy. In addition, we also review two mainstream drug delivery strategies, passive and active targeting, based on various nanocarriers for enhancing tumor therapy. Meanwhile, a comparison and combination of passive and active targeting are also carried out. Furthermore, we discuss the associated challenges of passive and active targeted drug delivery strategies and the prospects for further study.

Oncogenic metabolic reprogramming in breast cancer: focus on signaling pathways and mitochondrial genes

Oncogenic metabolic reprogramming impacts the abundance of key metabolites that regulate signaling and epigenetics. Metabolic vulnerability in the cancer cell is evident from the Warburg effect. The research on metabolism in the progression and survival of breast cancer (BC) is under focus. Oncogenic signal activation and loss of tumor suppressor are important regulators of tumor cell metabolism. Several intrinsic and extrinsic factors contribute to metabolic reprogramming. The molecular mechanisms underpinning metabolic reprogramming in BC are extensive and only partially defined. Various signaling pathways involved in the metabolism play a significant role in the modulation of BC. Notably, PI3K/AKT/mTOR pathway, lactate-ERK/STAT3 signaling, loss of the tumor suppressor Ras, Myc, oxidative stress, activation of the cellular hypoxic response and acidosis contribute to different metabolic reprogramming phenotypes linked to enhanced glycolysis. The alterations in mitochondrial genes have also been elaborated upon along with their functional implications. The outcome of these active research areas might contribute to the development of novel therapeutic interventions and the remodeling of known drugs.

Mitochondrial haplotype modulates genome expression and mitochondrial structure/function in cardiomyocytes following volume overload

Mitochondrial DNA (mtDNA) haplotype regulates mitochondrial structure/function and reactive oxygen species in aortocaval fistula (ACF) in mice. Here, we unravel the mitochondrial haplotype effects on cardiomyocyte mitochondrial ultrastructure and transcriptome response to ACF in vivo. Phenotypic responses and quantitative transmission electron microscopy (TEM) and RNA sequence at 3 days were determined after sham surgery or ACF in vivo in cardiomyocytes from wild-type (WT) C57BL/6J (C57n:C57mt) and C3H/HeN (C3Hn:C3Hmt) and mitochondrial nuclear exchange mice (C57n:C3Hmt or C3Hn:C57mt). Quantitative TEM of cardiomyocyte mitochondria C3HWT hearts have more electron-dense compact mitochondrial cristae compared with C57WT. In response to ACF, mitochondrial area and cristae integrity are normal in C3HWT; however, there is mitochondrial swelling, cristae lysis, and disorganization in both C57WT and MNX hearts. Tissue analysis shows that C3HWT hearts have increased autophagy, antioxidant, and glucose fatty acid oxidation-related genes compared with C57WT. Comparative transcriptomic analysis of cardiomyocytes from ACF was dependent upon mtDNA haplotype. C57mtDNA haplotype was associated with increased inflammatory/protein synthesis pathways and downregulation of bioenergetic pathways, whereas C3HmtDNA showed upregulation of autophagy genes. In conclusion, ACF in vivo shows a protective response of C3Hmt haplotype that is in large part driven by mitochondrial nuclear genome interaction.NEW & NOTEWORTHY The results of this study support the effects of mtDNA haplotype on nuclear gene expression in cardiomyocytes. Currently, there is no acceptable therapy for volume overload due to mitral regurgitation. The findings of this study could suggest that mtDNA haplotype activates different pathways after ACF warrants further investigations on human population of heart disease from different ancestry backgrounds.

A Mutation in Mouse MT-ATP6 Gene Induces Respiration Defects and Opposed Effects on the Cell Tumorigenic Phenotype

As the last step of the OXPHOS system, mitochondrial ATP synthase (or complex V) is responsible for ATP production by using the generated proton gradient, but also has an impact on other important functions linked to this system. Mutations either in complex V structural subunits, especially in mtDNA-encoded ATP6 gene, or in its assembly factors, are the molecular cause of a wide variety of human diseases, most of them classified as neurodegenerative disorders. The role of ATP synthase alterations in cancer development or metastasis has also been postulated. In this work, we reported the generation and characterization of the first mt-Atp6 pathological mutation in mouse cells, an m.8414A>G transition that promotes an amino acid change from Asn to Ser at a highly conserved residue of the protein (p.N163S), located near the path followed by protons from the intermembrane space to the mitochondrial matrix. The phenotypic consequences of the p.N163S change reproduce the effects of MT-ATP6 mutations in human diseases, such as dependence on glycolysis, defective OXPHOS activity, ATP synthesis impairment, increased ROS generation or mitochondrial membrane potential alteration. These observations demonstrate that this mutant cell line could be of great interest for the generation of mouse models with the aim of studying human diseases caused by alterations in ATP synthase. On the other hand, mutant cells showed lower migration capacity, higher expression of MHC-I and slightly lower levels of HIF-1α, indicating a possible reduction of their tumorigenic potential. These results could suggest a protective role of ATP synthase inhibition against tumor transformation that could open the door to new therapeutic strategies in those cancer types relying on OXPHOS metabolism.

Differential Epigenetic Status and Responses to Stressors between Retinal Cybrids Cells with African versus European Mitochondrial DNA: Insights into Disease Susceptibilities

Mitochondrial (mt) DNA can be classified into haplogroups, which represent populations with different geographic origins. Individuals of maternal African backgrounds (L haplogroup) are more prone to develop specific diseases compared those with maternal European-H haplogroups. Using a cybrid model, effects of amyloid-β (Amyβ), sub-lethal ultraviolet (UV) radiation, and 5-Aza-2'-deoxycytidine (5-aza-dC), a methylation inhibitor, were investigated. Amyβ treatment decreased cell metabolism and increased levels of reactive oxygen species in European-H and African-L cybrids, but lower mitochondrial membrane potential (ΔΨM) was found only in African-L cybrids. Sub-lethal UV radiation induced higher expression levels of CFH, EFEMP1, BBC3, and BCL2L13 in European-H cybrids compared to African-L cybrids. With respect to epigenetic status, the African-L cybrids had (a) 4.7-fold higher total global methylation levels (p = 0.005); (b) lower expression patterns for DNMT3B; and (c) elevated levels for HIST1H3F. The European-H and African-L cybrids showed different transcription levels for CFH, EFEMP1, CXCL1, CXCL8, USP25, and VEGF after treatment with 5-aza-dC. In conclusion, compared to European-H haplogroup cybrids, the African-L cybrids have different (i) responses to exogenous stressors (Amyβ and UV radiation), (ii) epigenetic status, and (iii) modulation profiles of methylation-mediated downstream complement, inflammation, and angiogenesis genes, commonly associated with various human diseases.

Mitochondrial mutations and mitoepigenetics: Focus on regulation of oxidative stress-induced responses in breast cancers

Epigenetic regulation of mitochondrial DNA (mtDNA) is an emerging and fast-developing field of research. Compared to regulation of nucler DNA, mechanisms of mtDNA epigenetic regulation (mitoepigenetics) remain less investigated. However, mitochondrial signaling directs various vital intracellular processes including aerobic respiration, apoptosis, cell proliferation and survival, nucleic acid synthesis, and oxidative stress. The later process and associated mismanagement of reactive oxygen species (ROS) cascade were associated with cancer progression. It has been demonstrated that cancer cells contain ROS/oxidative stress-mediated defects in mtDNA repair system and mitochondrial nucleoid protection. Furthermore, mtDNA is vulnerable to damage caused by somatic mutations, resulting in the dysfunction of the mitochondrial respiratory chain and energy production, which fosters further generation of ROS and promotes oncogenicity. Mitochondrial proteins are encoded by the collective mitochondrial genome that comprises both nuclear and mitochondrial genomes coupled by crosstalk. Recent reports determined the defects in the collective mitochondrial genome that are conducive to breast cancer initiation and progression. Mutational damage to mtDNA, as well as its overproliferation and deletions, were reported to alter the nuclear epigenetic landscape. Unbalanced mitoepigenetics and adverse regulation of oxidative phosphorylation (OXPHOS) can efficiently facilitate cancer cell survival. Accordingly, several mitochondria-targeting therapeutic agents (biguanides, OXPHOS inhibitors, vitamin-E analogues, and antibiotic bedaquiline) were suggested for future clinical trials in breast cancer patients. However, crosstalk mechanisms between altered mitoepigenetics and cancer-associated mtDNA mutations remain largely unclear. Hence, mtDNA mutations and epigenetic modifications could be considered as potential molecular markers for early diagnosis and targeted therapy of breast cancer. This review discusses the role of mitoepigenetic regulation in cancer cells and potential employment of mtDNA modifications as novel anti-cancer targets.

Drp1 mediates high glucose-induced mitochondrial dysfunction and epithelial-mesenchymal transition in endometrial cancer cells

This study aims to clarify the role and molecular mechanism of dynamin-related protein 1 (Drp1)-mediated mitochondrial homeostasis in high glucose (HG)-induced endometrial cancer (EC). Normal endometrium and tumor tissues of EC patients with normal and HG levels were collected, and Drp1 and p-Drp1 expression levels were detected by immunohistochemistry. Human EC cells were cultured with different glucose concentrations, and Drp1 and p-Drp1 expression levels were evaluated by Western blotting. Cell models of control and siDrp1 groups under normal and HG conditions were established, and subsequent functional experiments were conducted. Histology and in vitro experiments showed that the HG environment increased Drp1 activation, which could lead to mitochondrial dysfunction. Moreover, the imbalance of mitochondrial homeostasis mediated by Drp1 resulted in cell dysfunction, including altered glucose metabolism and increased epithelial-mesenchymal transition (EMT), migration and invasion. All these changes caused by HG could be partially alleviated by Drp1 knockdown. This study revealed that Drp1 was involved in the progression of EC associated with HG, and Drp1 might be a new potential therapeutic target for EC patients with diabetes.

Role of ectopically expressed mtDNA encoded cytochrome c oxidase subunit I (MT-COI) in tumorigenesis

Somatic mutations within mitochondrial DNA (mtDNA) encoded cytochrome c oxidase subunit I (MT-CO1 or MT-COI) are frequent in various cancer types. In addition, perturbation from orchestrated expression of mitochondrial DNA encoded genes is also associated with complex disorders, including cancer. Since codon bias and the mitochondrial translation system restricts functional characterization of over-expressed wild type or mutant mitochondrial DNA encoded genes, the codon optimization and artificial synthesis of entire MT-CO1 allowed us to over-express the wild type and one of its deleterious mutants into the mitochondria of the transfected cells. Ectopically expressed MT-CO1 was observed to efficiently express and localized to mitochondria but showed high level of aggregation under denaturing condition. Over-expression of wild type or mutant variant of MT-CO1 promoted anchorage dependent and independent proliferation potential in in-vitro experiments and introduced the cancer cell metabolic phenotype of high glucose uptake and lactate release. Reactive oxygen species generated in cells over-expressing MT-CO1 variants acted as key effectors mediating differential expression of apoptosis and DNA damage pathway related genes. High ROS generated also down-regulated the expression of global regulators of gene expression, DNMT3A and DNMT3B. The down-regulated expression of DNMTs co-related with differential methylation of the CpG islands in the promoter region of a select set of studied genes, in a manner to promote pro-cancerous phenotype. Apart from assigning the mechanistic role to the MT-CO1 variants and their perturbed expression in cancer development, the present study provides novel insights into the functional role of somatic mutations within MT-CO1 promoting cancer phenotype.

Association between mitochondrial genetic variation and breast cancer risk: The Multiethnic Cohort

Background

The mitochondrial genome encodes for thirty-seven proteins, among them thirteen are essential for the oxidative phosphorylation (OXPHOS) system. Inherited variation in mitochondrial genes may influence cancer development through changes in mitochondrial proteins, altering the OXPHOS process and promoting the production of reactive oxidative species.

Methods

To investigate the association between mitochondrial genetic variation and breast cancer risk, we tested 314 mitochondrial SNPs (mtSNPs), capturing four complexes of the mitochondrial OXPHOS pathway and mtSNP groupings for rRNA and tRNA, in 2,723 breast cancer cases and 3,260 controls from the Multiethnic Cohort Study.

Results

We examined the collective set of 314 mtSNPs as well as subsets of mtSNPs grouped by mitochondrial OXPHOS pathway, complexes, and genes, using the sequence kernel association test and adjusting for age, sex, and principal components of global ancestry. We also tested haplogroup associations using unconditional logistic regression and adjusting for the same covariates. Stratified analyses were conducted by self-reported maternal race/ethnicity. No significant mitochondrial OXPHOS pathway, gene, and haplogroup associations were observed in African Americans, Asian Americans, Latinos, and Native Hawaiians. In European Americans, a global test of all genetic variants of the mitochondrial genome identified an association with breast cancer risk (P = 0.017, q = 0.102). In mtSNP-subset analysis, the gene MT-CO2 (P = 0.001, q = 0.09) in Complex IV (cytochrome c oxidase) and MT-ND2 (P = 0.004, q = 0.19) in Complex I (NADH dehydrogenase (ubiquinone)) were significantly associated with breast cancer risk.

Conclusions

In summary, our findings suggest that collective mitochondrial genetic variation and particularly in the MT-CO2 and MT-ND2 may play a role in breast cancer risk among European Americans. Further replication is warranted in larger populations and future studies should evaluate the contribution of mitochondrial proteins encoded by both the nuclear and mitochondrial genomes to breast cancer risk.

Transcriptomic characterisation of the optimised rat model of Walker 256 breast cancer cell-induced bone pain

In patients with breast cancer, metastases of cancer cells to the axial skeleton may cause excruciating pain, particularly in the advanced stages. The current drug treatments available to alleviate this debilitating pain condition often lack efficacy and/or produce undesirable side effects. Preclinical animal models of cancer-induced bone pain are key to studying the mechanisms that cause this pain and for the success of drug discovery programs. In a previous study conducted in our laboratory, we validated and characterised the rat model of Walker 256 cell-induced bone pain, which displayed several key resemblances to the human pain condition. However, gene level changes that occur in the pathophysiology of cancer-induced bone pain in this preclinical model are unknown. Hence, in this study, we performed the transcriptomic characterisation of the Walker 256 cell line cultured in vitro to predict the molecular genetic profile of this cell line. We also performed transcriptomic characterisation of the Walker 256 cell-induced bone pain model in rats using the lumbar spinal cord and lumbar dorsal root ganglia tissues. Here we show that the Walker 256 cell line resembles the basal-B molecular subtype of human breast cancer cell lines. We also identify several genes that may underpin the progression of pain hypersensitivities in this condition, however, this needs further confirmatory studies. These transcriptomic insights have the potential to direct future studies aimed at identifying various mechanisms underpinning pain hypersensitivities in this model that may also assist in discovery of novel pain therapeutics for breast cancer-induced bone pain.

Mitochondrial genome and functional defects in osteosarcoma are associated with their aggressive phenotype

Osteosarcoma (OSA) is an aggressive mesenchymal tumor of the bone that affects children and occurs spontaneously in dogs. Human and canine OSA share similar clinical, biological and genetic features, which make dogs an excellent comparative model to investigate the etiology and pathogenesis of OSA. Mitochondrial (mt) defects have been reported in many different cancers including OSA, although it is not known whether these defects contribute to OSA progression and metastasis. Taking a comparative approach using canine OSA cell lines and tumor tissues we investigated the effects of mtDNA content and dysfunction on OSA biology. OSA tumor tissues had low mtDNA contents compared to the matched non-tumor tissues. We observed mitochondrial heterogeneity among the OSA cell lines and the most invasive cells expressing increased levels of OSA metastasis genes contained the highest amount of mitochondrial defects (reduced mtDNA copies, mt respiration, and expression of electron transport chain proteins). While mitochondria maintain a filamentous network in healthy cells, the mitochondrial morphology in OSA cells were mostly "donut shaped", typical of "stressed" mitochondria. Moreover the expression levels of mitochondrial retrograde signaling proteins Akt1, IGF1R, hnRNPA2 and NFkB correlated with the invasiveness of the OSA cells. Furthermore, we demonstrate the causal role of mitochondrial defects in inducing the invasive phenotype by Ethidium Bromide induced-mtDNA depletion in OSA cells. Our data suggest that defects in mitochondrial genome and function are prevalent in OSA and that lower mtDNA content is associated with higher tumor cell invasiveness. We propose that mt defects in OSA might serve as a prognostic biomarker and a target for therapeutic intervention in OSA patients.

Overview of mitochondrial germline variants and mutations in human disease: Focus on breast cancer (Review)

High lactate production in cells during growth under oxygen-rich conditions (aerobic glycolysis) is a hallmark of tumor cells, indicating the role of mitochondrial function in tumorigenesis. In fact, enhanced mitochondrial biogenesis and impaired quality control are frequently observed in cancer cells. Mitochondrial DNA (mtDNA) encodes 13 subunits of oxidative phosphorylation (OXPHOS), is present in thousands of copies per cell, and has a very high mutation rate. Mutations in mtDNA and nuclear DNA (nDNA) genes encoding proteins that are important players in mitochondrial biogenesis and function are involved in oncogenic processes. A wide range of germline mtDNA polymorphisms, as well as tumor mtDNA somatic mutations have been identified in diverse cancer types. Approximately 72% of supposed tumor-specific somatic mtDNA mutations reported, have also been found as polymorphisms in the general population. The ATPase 6 and NADH dehydrogenase subunit genes of mtDNA are the most commonly mutated genes in breast cancer (BC). Furthermore, nuclear genes playing a role in mitochondrial biogenesis and function, such as peroxisome proliferators-activated receptor gamma coactivator-1 (PGC-1), fumarate hydratase (FH) and succinate dehydrogenase (SDH) are frequently mutated in cancer. In this review, we provide an overview of the mitochondrial germline variants and mutations in cancer, with particular focus on those found in BC.

Mitochondrial DNA Integrity Is Maintained by APE1 in Carcinogen-Induced Colorectal Cancer

Changes in mitochondrial DNA (mtDNA) integrity have been reported in many cancers; however, the contribution of mtDNA integrity to tumorigenesis is not well understood. We used a transgenic mouse model that is haploinsufficient for the apurinic/apyrimidinic endonuclease 1 (Apex1^+/-) gene, which encodes the base excision repair (BER) enzyme APE1, to determine its role in protecting mtDNA from the effects of azoxymethane (AOM), a carcinogen used to induce colorectal cancer. Repair kinetics of AOM-induced mtDNA damage was evaluated using qPCR after a single AOM dose and a significant induction in mtDNA lesions in colonic crypts from both wild-type (WT) and Apex1^+/-animals were observed. However, Apex1^+/- mice had slower repair kinetics in addition to decreased mtDNA abundance. Tumors were also induced using multiple AOM doses, and both WT and Apex1^+/-animals exhibited significant loss in mtDNA abundance. Surprisingly, no major differences in mtDNA lesions were observed in tumors from WT and Apex1^+/- animals, whereas a significant increase in nuclear DNA lesions was detected in tumors from Apex1^+/- mice. Finally, tumors from Apex1^+/- mice displayed an increased proliferative index and histologic abnormalities. Taken together, these results demonstrate that APE1 is important for preventing changes in mtDNA integrity during AOM-induced colorectal cancer.Implications: AOM, a colorectal cancer carcinogen, generates damage to the mitochondrial genome, and the BER enzyme APE1 is required to maintain its integrity. Mol Cancer Res; 15(7); 831-41. ©2017 AACR.

Mitochondrial dysfunction and mitochondrial dynamics-The cancer connection

Mitochondrial dysfunction is a hallmark of many diseases. The retrograde signaling initiated by dysfunctional mitochondria can bring about global changes in gene expression that alters cell morphology and function. Typically, this is attributed to disruption of important mitochondrial functions, such as ATP production, integration of metabolism, calcium homeostasis and regulation of apoptosis. Recent studies showed that in addition to these factors, mitochondrial dynamics might play an important role in stress signaling. Normal mitochondria are highly dynamic organelles whose size, shape and network are controlled by cell physiology. Defective mitochondrial dynamics play important roles in human diseases. Mitochondrial DNA defects and defective mitochondrial function have been reported in many cancers. Recent studies show that increased mitochondrial fission is a pro-tumorigenic phenotype. In this paper, we have explored the current understanding of the role of mitochondrial dynamics in pathologies. We present new data on mitochondrial dynamics and dysfunction to illustrate a causal link between mitochondrial DNA defects, excessive fission, mitochondrial retrograde signaling and cancer progression. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Decoding Warburg's hypothesis: tumor-related mutations in the mitochondrial respiratory chain

Otto Warburg observed that cancer cells derived their energy from aerobic glycolysis by converting glucose to lactate. This mechanism is in opposition to the higher energy requirements of cancer cells because oxidative phosphorylation (OxPhos) produces more ATP from glucose. Warburg hypothesized that this phenomenon occurs due to the malfunction of mitochondria in cancer cells. The rediscovery of Warburg's hypothesis coincided with the discovery of mitochondrial tumor suppressor genes that may conform to Warburg's hypothesis along with the demonstrated negative impact of HIF-1 on PDH activity and the activation of HIF-1 by oncogenic signals such as activated AKT. This work summarizes the alterations in mitochondrial respiratory chain proteins that have been identified and their involvement in cancer. Also discussed is the fact that most of the mitochondrial mutations have been found in homoplasmy, indicating a positive selection during tumor evolution, thereby supporting their causal role.

Correlational study on mitochondrial DNA mutations as potential risk factors in breast cancer

The presented study performed an mtDNA genome-wide association analysis to screen the peripheral blood of breast cancer patients for high-risk germline mutations. Unlike previous studies, which have used breast tissue in analyzing somatic mutations, we looked for germline mutations in our study, since they are better predictors of breast cancer in high-risk groups, facilitate early, non-invasive diagnoses of breast cancer and may provide a broader spectrum of therapeutic options. The data comprised 22 samples of healthy group and 83 samples from breast cancer patients. The sequencing data showed 170 mtDNA mutations in the healthy group and 393 mtDNA mutations in the disease group. Of these, 283 mtDNA mutations (88 in the healthy group and 232 in the disease group) had never been reported in the literature. Moreover, correlation analysis indicated there was a significant difference in 32 mtDNA mutations. According to our relative risk analysis of these 32 mtDNA mutations, 27 of the total had odds ratio values (ORs) of less than 1, meaning that these mutations have a potentially protective role to play in breast cancer. The remaining 5 mtDNA mutations, RNR2-2463 indelA, COX1-6296 C>A, COX1-6298 indelT, ATP6-8860 A>G, and ND5-13327 indelA, whose ORs were 8.050, 4.464, 4.464, 5.254 and 4.853, respectively, were regarded as risk factors of increased breast cancer. The five mutations identified here may serve as novel indicators of breast cancer and may have future therapeutic applications. In addition, the use of peripheral blood samples was procedurally simple and could be applied as a non-invasive diagnostic technique.

Enhanced tumorigenicity by mitochondrial DNA mild mutations

To understand how mitochondria are involved in malignant transformation we have generated a collection of transmitochondrial cybrid cell lines on the same nuclear background (143B) but with mutant mitochondrial DNA (mtDNA) variants with different degrees of pathogenicity. These include the severe mutation in the tRNA^Lys gene, m.8363G>A, and the three milder yet prevalent Leber's hereditary optic neuropathy (LHON) mutations in the MT-ND1 (m.3460G>A), MT-ND4 (m.11778G>A) and MT-ND6 (m.14484T>C) mitochondrial genes. We found that 143B ρ⁰ cells devoid of mtDNA and cybrids harboring wild type mtDNA or that causing severe mitochondrial dysfunction do not produce tumors when injected in nude mice. By contrast cybrids containing mild mutant mtDNAs exhibit different tumorigenic capacities, depending on OXPHOS dysfunction.

The differences in tumorigenicity correlate with an enhanced resistance to apoptosis and high levels of NOX expression. However, the final capacity of the different cybrid cell lines to generate tumors is most likely a consequence of a complex array of pro-oncogenic and anti-oncogenic factors associated with mitochondrial dysfunction.

Our results demonstrate the essential role of mtDNA in tumorigenesis and explain the numerous and varied mtDNA mutations found in human tumors, most of which give rise to mild mitochondrial dysfunction.

Disruption of cytochrome c oxidase function induces the Warburg effect and metabolic reprogramming

Defects in mitochondrial oxidative phosphorylation complexes, altered bioenergetics and metabolic shift are often seen in cancers. Here we show a role for the dysfunction of electron transport chain component, cytochrome c oxidase (CcO) in cancer progression. We show that genetic silencing of the CcO complex by shRNA expression and loss of CcO activity in multiple cell types from the mouse and human sources resulted in metabolic shift to glycolysis, loss of anchorage dependent growth and acquired invasive phenotypes. Disruption of CcO complex caused loss of transmembrane potential and induction of Ca²⁺/Calcineurin-mediated retrograde signaling. Propagation of this signaling, includes activation of PI3-kinase, IGF1R and Akt, Ca²⁺ sensitive transcription factors and also, TGFβ1, MMP16, periostin that are involved in oncogenic progression. Whole genome expression analysis showed up regulation of genes involved in cell signaling, extracellular matrix interactions, cell morphogenesis, cell motility and migration. The transcription profiles reveal extensive similarity to retrograde signaling initiated by partial mtDNA depletion, though distinct differences are observed in signaling induced by CcO dysfunction. The possible CcO dysfunction as a biomarker for cancer progression was supported by data showing that esophageal tumors from human patients show reduced CcO subunits IVi1 and Vb in regions that were previously shown to be hypoxic core of the tumors. Our results show that mitochondrial electron transport chain defect initiates a retrograde signaling. These results suggest that a defect in CcO complex can potentially induce tumor progression.

5-aminolevulinic acid, a precursor of heme, reduces both fasting and postprandial glucose levels in mildly hyperglycemic subjects

OBJECTIVE

The aim of this study was to evaluate the combined effects of 5-aminolevulinic acid phosphate (ALA-P) and iron on the glycemic index in mildly hyperglycemic adults.

METHODS

This double-blind, randomized placebo-controlled trial comprised 212 subjects (ages 35-70 y, fasting plasma glucose 105-125 mg/dL or hemoglobin (Hb)A1c 6.1%-7.1%). These participants were randomly assigned to four groups receiving either one of three doses of ALA-P and iron as sodium ferrous citrate (5 mg and 0.6 mg, 5 mg and 1.8 mg, or 15 mg and 1.8 mg, respectively) or a placebo, administered orally once a day over a 12-wk period.

RESULTS

Fifteen mg ALA-P plus 1.8 mg iron decreased the fasting plasma glucose level (2.32 mg/dL, 95% confidence interval [CI], 0.24-4.42, P = 0.029), serum glycoalbumin (0.22%, 95% CI, 0.02-0.42; P = 0.031), and 2h-oral glucose tolerance test levels (14.2 mg/dL, 95% CI, 1.8-26.6; P = 0.025) more than the placebo. However, the levels of HbA1c, fasting insulin, serum 1,5-anhydro-d-glucitol, and Homeostasis Model of Assessment-Insulin Resistance showed no appreciable changes. The participant numbers with impaired glucose tolerance and impaired fasting glucose decreased in the highest dosage group of ALA-P plus iron compared with the placebo group.

CONCLUSION

An oral intake of ALA would be a novel approach to prevent type 2 diabetes mellitus.

Mitochondrial DNA polymorphism in genes encoding ND1, COI and CYTB in canine malignant cancers

The aim of the study was to identify DNA changes in mitochondrial gene fragments: NADH dehydrogenase subunit 1 (ND1), cytochrome c oxidase subunit I (COI) and cytochrome b (CYTB) in tumor tissue, normal tissue and blood, and to define their association with the tumor type in dogs. Molecular analysis included 144 tests in total. A functional effect of the non-synonymous protein coding SNP was predicted. The presence of polymorphisms in all tested gene fragments in individual tissues of dogs was observed. Heteroplasmic changes were found in ND1 and CYTB in epithelioma glandulae sebacei and in CYTB in lymphoma centroblasticum. The results of in silico analysis show the impact of these alleles (COI: 507, ND1: 450, 216, CYTB: 748) on the functioning of proteins and thus their potential role in carcinogenesis. The possible harmful effects of changes in polypeptides in positions T193N, V98M, V118M and H196P were evaluated. It seems that polymorphisms occurring in cells can have a negative impact on functioning of proteins. This promotes disorders of the energy level in cells.

Length variation in the mouse mitochondrial tRNA(Arg) DHU loop size promotes oxidative phosphorylation functional differences

The efficiency of the cellular oxidative phosphorylation system was recently shown to be modulated by common mitochondrial tRNA(A) (rg) haplotypes. The molecular mechanism by which some mt-Tr haplotypes induce these functional differences remains undetermined. Common polymorphisms in mouse mt-Tr genes affect the size of the dihydrouridine loop in the mature tRNA, producing loops of between five and seven nucleotides, the largest being a rare variant among mammals. Here, we analyzed a new mt-Tr variant identified in C3H mice, and found that it is mitochondrial tRNA loop size, but not the specific sequence, that is responsible for the observed differences in cellular respiration. We further found that the sensitivity of mitochondrial protein synthesis to specific inhibitors is dependent on the mt-Tr gene haplotype, and confirmed that the differences in oxidative phosphorylation performance are masked by a reactive oxygen species-induced compensatory increase in mitochondrial biogenesis.

The role of mitochondrial DNA damage and repair in the resistance of BCR/ABL-expressing cells to tyrosine kinase inhibitors

Chronic myeloid leukemia (CML) is a hematological malignancy that arises from the transformation of stem hematopoietic cells by the fusion oncogene BCR/ABL and subsequent clonal expansion of BCR/ABL-positive progenitor leukemic cells. The BCR/ABL protein displays a constitutively increased tyrosine kinase activity that alters many regulatory pathways, leading to uncontrolled growth, impaired differentiation and increased resistance to apoptosis featured by leukemic cells. Current CML therapy is based on tyrosine kinase inhibitors (TKIs), primarily imatinib, which induce apoptosis in leukemic cells. However, some patients show primary resistance to TKIs while others develop it in the course of therapy. In both cases, resistance may be underlined by perturbations in apoptotic signaling in leukemic cells. As mitochondria may play an important role in such signaling, alteration in mitochondrial metabolism may change resistance to pro-apoptotic action of TKIs in BCR/ABL-positive cells. Because BCR/ABL may induce reactive oxygen species and unfaithful DNA repair, it may affect the stability of mitochondrial DNA, influencing mitochondrial apoptotic signaling and in this way change the sensitivity of CML cells to TKIs. Moreover, cancer cells, including BCR/ABL-positive cells, show an increased level of glucose metabolism, resulting from the shift from oxidative phosphorylation to glycolysis to supply ATP for extensive proliferation. Enhanced level of glycolysis may be associated with TKI resistance and requires change in the expression of several genes regulated mostly by hypoxia-inducible factor-1α, HIF-1α. Such regulation may be associated with the impaired mitochondrial respiratory system in CML cells. In summary, mitochondria and mitochondria-associated molecules and pathways may be attractive targets to overcome TKI resistance in CML.

Supercomplex Assembly Determines Electron Flux in the Mitochondrial Electron Transport Chain

The textbook description of mitochondrial respiratory complexes (RCs) views them as free-moving entities linked by the mobile carriers coenzyme Q (CoQ) and cytochrome c (cyt c). This model (known as the fluid model) is challenged by the proposal that all RCs except complex II can associate in supercomplexes (SCs). The proposed SCs are the respirasome (complexes I, III, and IV), complexes I and III, and complexes III and IV. The role of SCs is unclear, and their existence is debated. By genetic modulation of interactions between complexes I and III and III and IV, we show that these associations define dedicated CoQ and cyt c pools and that SC assembly is dynamic and organizes electron flux to optimize the use of available substrates.

Regulation of metastasis; mitochondrial DNA mutations have appeared on stage

It has been controversial whether mtDNA mutations are responsible for tumorigenesis and for the process to develop metastases. To clarify this issue, we established trans-mitochondrial cybrids with mtDNA exchanged between mouse tumor cells that possess high and low metastatic potential. The results revealed that the G13997A mutation in the ND6 gene of mtDNA from highly metastatic tumor cells reversibly controlled development of metastases by overproduction of reactive oxygen species (ROS). The transmitochondrial model mice possessing G13997A mtDNA showed symptoms of impaired glucose tolerability, suggesting that ROS generated mtDNA mutations can regulate not only metastatic potential, but also age-associated disorders such as diabetes. We also identified other mtDNA mutations that affect metastatic potential but the mechanisms are independent of ROS production. The mtDNA-mediated reversible control of metastasis and age-associated disorders are novel functions of mtDNA, and suggests that ROS scavengers may be therapeutically effective to suppress these phenotypes.

Thermodynamic stability explains the differential evolutionary dynamics of cytochrome b and COX I in mammals

By using a combination of evolutionary and structural data from 231 species, we have addressed the relationship between evolution and structural features of cytochrome b and COX I, two mtDNA-encoded proteins. The interior of cytochrome b, in contrast to that of COX I, exhibits a remarkable tolerance to changes. The higher evolvability of cytochrome b contrasts with the lower rate of synonymous substitutions of its gene when compared to that of COX I, suggesting that the latter is subjected to a stronger purifying selection. We present evidences that the stability effect of mutations (ΔΔG) may be behind these differential behaviour.

Somatic mitochondrial DNA mutations in human cancers

Mitochondria are ubiquitous organelles in eukaryotic cells principally responsible for regulating cellular energy metabolism, free radical production, and the execution of apoptotic pathways. Abnormal oxidative phosphorylation (OXPHOS) and aerobic metabolism as a result of mitochondrial dysfunction have long been hypothesized to be involved in tumorigenesis. In the past decades, numerous somatic mutations in both the coding and control regions of mitochondrial DNA (mtDNA) have been extensively examined in a broad range of primary human cancers, underscoring that accumulation of mtDNA alterations may be a critical factor in eliciting persistent mitochondrial defects and consequently contributing to cancer initiation and progression. However, the roles of these mtDNA mutations in the carcinogenic process remain largely unknown. This review outlines a wide variety of somatic mtDNA mutations identified in common human malignancies and highlights recent advances in understanding the causal roles of mtDNA variations in neoplastic transformation and tumor progression. In addition, it briefly illustrates how mtDNA alterations activate mitochondria-to-nucleus retrograde signaling so as to modulate the expression of relevant nuclear genes or induce epigenetic changes and promote malignant phenotypes in cancer cells. The present state of our knowledge regarding how mutational changes in the mitochondrial genome could be used as a diagnostic biomarker for early detection of cancer and as a potential target in the development of new therapeutic approaches is also discussed. These findings strongly indicate that mtDNA mutations exert a crucial role in the pathogenic mechanisms of tumor development, but continued investigations are definitely required to further elucidate the functional significance of specific mtDNA mutations in the etiology of human cancers.

Mitochondrial tRNA valine as a recurrent target for mutations involved in mitochondrial cardiomyopathies

The aim of this study was to identify the genetic defect in two patients having cardiac dysfunction accompanied by neurological symptoms, and in one case MRI evidence of cortical and cerebellar atrophy with hyperintensities in the basal ganglia. Muscle biopsies from each patient revealed single and combined mitochondrial respiratory chain deficiency. The complete mtDNA sequencing of both patients revealed two transitions in the mitochondrial tRNA(Val) gene (MT-TV) (m.1628C>T in Patient 1, and m.1644G>A in Patient 2). The functional and molecular analyses reported here suggest that the MT-TV gene should be routinely considered in the diagnosis of mitochondrial cardiomyopathies.

Mitochondrial DNA mutations regulate metastasis of human breast cancer cells

Mutations in mitochondrial DNA (mtDNA) might contribute to expression of the tumor phenotypes, such as metastatic potential, as well as to aging phenotypes and to clinical phenotypes of mitochondrial diseases by induction of mitochondrial respiration defects and the resultant overproduction of reactive oxygen species (ROS). To test whether mtDNA mutations mediate metastatic pathways in highly metastatic human tumor cells, we used human breast carcinoma MDA-MB-231 cells, which simultaneously expressed a highly metastatic potential, mitochondrial respiration defects, and ROS overproduction. Since mitochondrial respiratory function is controlled by both mtDNA and nuclear DNA, it is possible that nuclear DNA mutations contribute to the mitochondrial respiration defects and the highly metastatic potential found in MDA-MB-231 cells. To examine this possibility, we carried out mtDNA replacement of MDA-MB-231 cells by normal human mtDNA. For the complete mtDNA replacement, first we isolated mtDNA-less (ρ⁰) MDA-MB-231 cells, and then introduced normal human mtDNA into the ρ⁰ MDA-MB-231 cells, and isolated trans-mitochondrial cells (cybrids) carrying nuclear DNA from MDA-MB-231 cells and mtDNA from a normal subject. The normal mtDNA transfer simultaneously induced restoration of mitochondrial respiratory function and suppression of the highly metastatic potential expressed in MDA-MB-231 cells, but did not suppress ROS overproduction. These observations suggest that mitochondrial respiration defects observed in MDA-MB-231 cells are caused by mutations in mtDNA but not in nuclear DNA, and are responsible for expression of the high metastatic potential without using ROS-mediated pathways. Thus, human tumor cells possess an mtDNA-mediated metastatic pathway that is required for expression of the highly metastatic potential in the absence of ROS production.

Evolution meets disease: penetrance and functional epistasis of mitochondrial tRNA mutations

About half of the mitochondrial DNA (mtDNA) mutations causing diseases in humans occur in tRNA genes. Particularly intriguing are those pathogenic tRNA mutations than can reach homoplasmy and yet show very different penetrance among patients. These mutations are scarce and, in addition to their obvious interest for understanding human pathology, they can be excellent experimental examples to model evolution and fixation of mitochondrial tRNA mutations. To date, the only source of this type of mutations is human patients. We report here the generation and characterization of the first mitochondrial tRNA pathological mutation in mouse cells, an m.3739G>A transition in the mitochondrial mt-Ti gene. This mutation recapitulates the molecular hallmarks of a disease-causing mutation described in humans, an m.4290T>C transition affecting also the human mt-Ti gene. We could determine that the pathogenic molecular mechanism, induced by both the mouse and the human mutations, is a high frequency of abnormal folding of the tRNA(Ile) that cannot be charged with isoleucine. We demonstrate that the cells harboring the mouse or human mutant tRNA have exacerbated mitochondrial biogenesis triggered by an increase in mitochondrial ROS production as a compensatory response. We propose that both the nature of the pathogenic mechanism combined with the existence of a compensatory mechanism can explain the penetrance pattern of this mutation. This particular behavior can allow a scenario for the evolution of mitochondrial tRNAs in which the fixation of two alleles that are individually deleterious can proceed in two steps and not require the simultaneous mutation of both.

Functional effects of mutations in cytochrome c oxidase related to prostate cancer

A number of missense mutations in subunit I of cytochrome c oxidase (CytcO) have previously been linked to prostate cancer (Petros et al., 2005). To investigate the effects of these mutations at the molecular level, in the present study we prepared four different structural variants of the bacterial Rhodobacter sphaeroides CytcO (cytochrome aa(3)), each carrying one amino-acid residue replacement corresponding to the following substitutions identified in the above-mentioned study: Asn11Ser, Ala122Thr, Ala341Ser and Val380Ile (residues Asn25, Ser168, Ala384 and Val423 in the R. sphaeroides oxidase). This bacterial CytcO displays essentially the same structural and functional characteristics as those of the mitochondrial counterpart. We investigated the overall activity, proton pumping and internal electron- and proton-transfer reactions in the structural variants. The results show that the turnover activities of the mutant CytcOs were reduced by at most a factor of two. All variants pumped protons, but in Ser168Thr, Ala384Ser and Val423Ile we observed slight internal proton leaks. In all structural variants the internal electron equilibrium was slightly shifted away from the catalytic site at high pH (10), resulting in a slower observed ferryl to oxidized transition. Even though the effects of the mutations were relatively modest, the results suggest that they destabilize the proton-gating machinery. Such effects could be manifested in the presence of a transmembrane electrochemical gradient resulting in less efficient energy conservation. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.

Maternally inherited susceptibility to cancer

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.

Five entry points of the mitochondrially encoded subunits in mammalian complex I assembly

Complex I (CI) is the largest enzyme of the mammalian mitochondrial respiratory chain. The biogenesis of the complex is a very complex process due to its large size and number of subunits (45 subunits). The situation is further complicated due to the fact that its subunits have a double genomic origin, as seven of them are encoded by the mitochondrial DNA. Understanding of the assembly process and characterization of the involved factors has advanced very much in the last years. However, until now, a key part of the process, that is, how and at which step the mitochondrially encoded CI subunits (ND subunits) are incorporated in the CI assembly process, was not known. Analyses of several mouse cell lines mutated for three ND subunits allowed us to determine the importance of each one for complex assembly/stability and that there are five different steps within the assembly pathway in which some mitochondrially encoded CI subunit is incorporated.

A pathogenic mutation in cytochrome c oxidase results in impaired proton pumping while retaining O(2)-reduction activity

In this work we have investigated the effect of a pathogenic mitochondrial DNA mutation found in human colon cells, at a functional-molecular level. The mutation results in the amino-acid substitution Tyr19His in subunit I of the human CytcO and it is associated with respiratory deficiency. It was introduced into Rhodobacter sphaeroides, which carries a cytochrome c oxidase (cytochrome aa(3)) that serves as a model of the mitochondrial counterpart. The residue is situated in the middle of a pathway that is used to transfer substrate protons as well as protons that are pumped across the membrane. The Tyr33His (equivalent residue in the bacterial CytcO) structural variant of the enzyme was purified and its function was investigated. The results show that in the structurally altered CytcO the activity decreased due to slowed proton transfer; proton transfer from an internal proton donor, the highly-conserved Glu286, to the catalytic site was slowed by a factor of approximately 5, while reprotonation of the Glu from solution was slowed by a factor of approximately 40. In addition, in the structural variant proton pumping was completely impaired. These results are explained in terms of introduction of a barrier for proton transfer through the D pathway and changes in the coordination of water molecules surrounding the Glu286 residue. The study offers an explanation, at the molecular level, to the link between a specific amino-acid substitution and a pathogenic phenotype identified in human colon cells.

Implications of mitochondrial DNA mutations and mitochondrial dysfunction in tumorigenesis

Alterations in oxidative phosphorylation resulting from mitochondrial dysfunction have long been hypothesized to be involved in tumorigenesis. Mitochondria have recently been shown to play an important role in regulating both programmed cell death and cell proliferation. Furthermore, mitochondrial DNA (mtDNA) mutations have been found in various cancer cells. However, the role of these mtDNA mutations in tumorigenesis remains largely unknown. This review focuses on basic mitochondrial genetics, mtDNA mutations and consequential mitochondrial dysfunction associated with cancer. The potential molecular mechanisms, mediating the pathogenesis from mtDNA mutations and mitochondrial dysfunction to tumorigenesis are also discussed.

Mutations in the mitochondrial genome confer resistance of cancer cells to anticancer drugs

The majority of cancer cells harbor homoplasmic somatic mutations in the mitochondrial genome. We show here that mutations in mitochondrial DNA (mtDNA) are responsible for anticancer drug tolerance. We constructed several trans-mitochondrial hybrids (cybrids) with mtDNA derived from human pancreas cancer cell lines CFPAC-1 and CAPAN-2 as well as from healthy individuals. These cybrids contained the different mitochondrial genomes with the common nuclear background. We compared the mutant and wild-type cybrids for resistance against an apoptosis-inducing reagent and anticancer drugs by exposing the cybrids to staurosporine, 5-fluorouracil, and cisplatin in vitro, and found that all mutant cybrids were more resistant to the apoptosis-inducing and anticancer drugs than wild-type cybrids. Next, we transplanted mutant and wild-type cybrids into nude mice to generate tumors. Tumors derived from mutant cybrids were more resistant than those from wild-type cybrids in suppressing tumor growth and inducing massive apoptosis when 5-fluorouracil and cisplatin were administered. To confirm the tolerance of mutant cybrids to anticancer drugs, we transplanted a mixture of mutant and wild-type cybrids at a 1:1 ratio into nude mice and examined the effect by the drugs on the drift of the ratio of mutant and wild-type mtDNA. The mutant mtDNA showed better survival, indicating that mutant cybrids were more resistant to the anticancer drugs. Thus, we propose that mutations in the mitochondrial genome are potential targets for prognosis in the administration of anticancer drugs to cancer patients.

A mitochondrial DNA mutation linked to colon cancer results in proton leaks in cytochrome c oxidase

An increasing number of cancer types have been found to be linked to specific mutations in the mitochondrial DNA, which result in specific structural changes of the respiratory enzyme complexes. In this study, we have investigated the effect of 2 such mutations identified in colon cancer patients, leading to the amino acid substitutions Ser458Pro and Gly125Asp in subunit I of cytochrome c oxidase (complex IV) [Greaves et al. (2006) Proc Natl Acad Sci USA 103:714-719]. We introduced these mutations in Rhodobacter sphaeroides, which carries an oxidase that serves as a model of the mitochondrial counterpart. The lack of expression of the former variant indicates that the amino acid substitution results in severely altered overall structure of the enzyme. The latter mutation (Gly171Asp in the bacterial oxidase) resulted in a structurally intact enzyme, but with reduced activity (approximately 30%), mainly due to slowed reduction of the redox site heme a. Furthermore, even though the Gly171Asp CytcO pumps protons, an intrinsic proton leak was identified, which would lead to a decreased overall energy-conversion efficiency of the respiratory chain, and would also perturb transport processes such as protein, ion, and metabolite trafficking. Furthermore, the specific leak may act to alter the balance between the electrical and chemical components of the proton electrochemical gradient.

Trading mtDNA uncovers its role in metastasis

It has been controversial for many years of whether mtDNA mutations are involved in phenotypes related to cancer due to the difficulty in excluding possible involvement of nuclear DNA mutations in these phenotypes. We addressed this issue by complete trading of mtDNAs between tumor cells expressing different metastatic phenotypes. Resultant trans-mitochondrial cybrids share the same nuclear background, but possess mtDNA from tumor cells expressing different metastatic phenotypes, and thus can be used to uncover the role of mtDNA in these phenotypes. The results showed that mtDNA controls development of metastasis in tumor cells, while tumor development is controlled by nuclear genome.

Reversible regulation of metastasis by ROS-generating mtDNA mutations

It has been controversial whether mtDNA mutations are responsible for oncogenic transformation (normal cells to develop tumors), and for malignant progression (tumor cells to develop metastases). To clarify this issue, we created trans-mitochondrial cybrids with mtDNA exchanged between mouse tumor cells that express different metastatic phenotypes. The G13997A mutation in the ND6 gene of mtDNA from high metastatic tumor cells reversibly controlled development of metastases by overproduction of reactive oxygen species (ROS), but did not control development of tumors. The mtDNA-mediated reversible control of metastasis reveals a novel function of mtDNA, and suggests that ROS scavengers may be therapeutically effective in suppressing metastasis.

ROS-generating mitochondrial DNA mutations can regulate tumor cell metastasis

Mutations in mitochondrial DNA (mtDNA) occur at high frequency in human tumors, but whether these mutations alter tumor cell behavior has been unclear. We used cytoplasmic hybrid (cybrid) technology to replace the endogenous mtDNA in a mouse tumor cell line that was poorly metastatic with mtDNA from a cell line that was highly metastatic, and vice versa. Using assays of metastasis in mice, we found that the recipient tumor cells acquired the metastatic potential of the transferred mtDNA. The mtDNA conferring high metastatic potential contained G13997A and 13885insC mutations in the gene encoding NADH (reduced form of nicotinamide adenine dinucleotide) dehydrogenase subunit 6 (ND6). These mutations produced a deficiency in respiratory complex I activity and were associated with overproduction of reactive oxygen species (ROS). Pretreatment of the highly metastatic tumor cells with ROS scavengers suppressed their metastatic potential in mice. These results indicate that mtDNA mutations can contribute to tumor progression by enhancing the metastatic potential of tumor cells.

Functional genetic analysis of the mammalian mitochondrial DNA encoded peptides: a mutagenesis approach

Animal mitochondria are refractory to transformation. This fact has hampered the study of the oxidative phosphorylation system biogenesis by genetic manipulation of the mitochondrial DNA (mtDNA). In humans, a larger variety of mutants have been obtained from patients with mitochondrial diseases, but still we lack a great portion of the range of potential mutants and there is a major obstacle: Animal models cannot be derived from human mtDNA mutants. Until now the only source of mtDNA mutants in mouse was restricted to some drug-resistant-specific cell lines in which a given mtDNA mutation provided growth advantage in the presence of the inhibitor for a specific complex. To overcome these limitations, the authors have developed a protocol that allows the systematic generation of cells harboring mutations in their mtDNA affecting all types of mitochondrial genes. Chemical mutagenesis followed by mtDNA copy number reduction and the use of large-scale negative selection in duplicate cultures, are the key steps of the strategy used.

Somatic mutations of mitochondrial genome in early stage breast cancer

The complete mitochondrial genomes of the primary cancerous, matched paracancerous normal and distant normal tissues from 10 early-stage breast cancer patients were analyzed in this study, with special attempt (i) to investigate whether the reported high frequency of mitochondrial DNA (mtDNA) somatic mutations in breast cancer could be repeated under a stringent data quality control, and (ii) to characterize the spectrum of mtDNA somatic mutations in Chinese breast cancer patients and evaluate their potential significance in early cancer diagnosis. Two heteroplasmic somatic transitions (T2275C and A8601G) were identified in our samples. The transition A8601G was present in the primary cancerous and paracancerous normal tissues from patient no. 3. Transition T2275C was found in the primary cancerous tissue but not in other normal tissues from patient no. 6; this transition has been reported in the colonic crypts and is located at a highly conserved site in the 16S rRNA gene. Subsequent cloning sequencing confirmed the absence of both mutations in the distant normal tissues from the 2 patients. The overall rate of somatic mutations in our patients was much lower than those of previous studies of breast cancer. Our results gave support to the recent claim that the high frequency of mtDNA somatic mutations in cancer studies is overestimated. Based on the mtDNA mutation pattern in early stage breast cancer observed in this study, we cautioned the enthusiasm and efforts to look for somatic mutations that were of diagnostic value in cancer early detection.

Mitochondrial DNA and its mutations: novel fields in a new era

Az utóbbi két évtizedet tartják a klinikai mitokondriális DNS-kutatás aranykorának. Folyamatosan bővül a patológiás variánsok száma, amelyek betegséggel társulnak, illetve bővül az ismeretanyag azokról az entitásokról, melyek hátterében a mitokondriális DNS kóros elváltozásai állnak. A cirkuláris mitokondriális DNS öröklődése eltér a Mendel-féle szabályoktól, anyai öröklésmenetet mutat; számos vonatkozásban eltérő sajátosságokkal rendelkezik a nukleáris DNS-hez viszonyítva. A molekuláris biológiai módszerek terjedésével egyre több kórkép ismerhető fel, noha a diagnosztika manapság is komoly kihívást jelent. Napjainkban a mitokondriális medicina számos orvosi szubspecialitáshoz kapcsolódóan jelentős előrelépéseket mutatott; így körvonalazódott a mitokondriális gasztroenterológia, endokrinológia, otológia, oftalmológia, nefrológia, hematológia, onkológia, reproduktív medicina és pszichiátria, mintegy az adott szubspecialitás mitokondriális DNS-sel kapcsolatos, többé-kevésbé részleges önállósodással megjelenő territóriuma. A jelen összefoglaló közlemény a mitokondriális medicina rövid, általános összefoglalása mellett e fejezetekre próbál rátekintést nyújtani.

Application of mitochondrial genome information in cancer epidemiology

Two genomes, nuclear and mitochondrial, exist in humans although information contained in the mitochondrial genome has not been fully utilized in cancer epidemiology. Over the last few years, a variety of approaches have been developed to improve results of conventional cancer screening by detecting molecular markers in different populations. Mitochondrial DNA alterations (mutations, deletions and instability) are emerging as new molecular markers for detecting a variety of cancers in tissue samples and biofluids which can be included in population screening studies. Since mitochondrial genome is small (16.6 kb) and high-throughput assays have been developed for sequencing whole mitochondrial genome, it can be adopted by most of the laboratories conducting epidemiological studies. Applications of mitochondrial DNA markers to identify high risk populations and future challenges are discussed in this article.

Differences in reactive oxygen species production explain the phenotypes associated with common mouse mitochondrial DNA variants

Common mitochondrial DNA (mtDNA) haplotypes in humans and mice have been associated with various phenotypes, including learning performance and disease penetrance. Notably, no influence of mtDNA haplotype in cell respiration has been demonstrated. Here, using cell lines carrying four different common mouse mtDNA haplotypes in an identical nuclear background, we show that the similar level of respiration among the cell lines is only apparent and is a consequence of compensatory mechanisms triggered by different production of reactive oxygen species. We observe that the respiration capacity per molecule of mtDNA in cells with the NIH3T3 or NZB mtDNA is lower than in those with the C57BL/6J, CBA/J or BALB/cJ mtDNA. In addition, we have determined the genetic element underlying these differences. Our data provide insight into the molecular basis of the complex phenotypes associated with common mtDNA variants and anticipate a relevant contribution of mtDNA single nucleotide polymorphisms to phenotypic variability in humans.