Somatic mutations of mitochondrial genome in early stage breast cancer

Exploring Aerobic Energy Metabolism in Breast Cancer: A Mutational Profile of Glycolysis and Oxidative Phosphorylation

Energy metabolism is a fundamental aspect of the aggressiveness and invasiveness of breast cancer (BC), the neoplasm that most affects women worldwide. Nonetheless, the impact of genetic somatic mutations on glycolysis and oxidative phosphorylation (OXPHOS) genes in BC remains unclear. To fill these gaps, the mutational profiles of 205 screened genes related to glycolysis and OXPHOS in 968 individuals with BC from The Cancer Genome Atlas (TCGA) project were performed. We carried out analyses to characterize the mutational profile of BC, assess the clonality of tumors, identify somatic mutation co-occurrence, and predict the pathogenicity of these alterations. In total, 408 mutations in 132 genes related to the glycolysis and OXPHOS pathways were detected. The PGK1, PC, PCK1, HK1, DONSON, GPD1, NDUFS1, and FOXRED1 genes are also associated with the tumorigenesis process in other types of cancer, as are the genes BRCA1, BRCA2, and HMCN1, which had been previously described as oncogenes in BC, with whom the target genes of this work were associated. Seven mutations were identified and highlighted due to the high pathogenicity, which are present in more than one of our results and are documented in the literature as being correlated with other diseases. These mutations are rs267606829 (FOXRED1), COSV53860306 (HK1), rs201634181 (NDUFS1), rs774052186 (DONSON), rs119103242 (PC), rs1436643226 (PC), and rs104894677 (ETFB). They could be further investigated as potential biomarkers for diagnosis, prognosis, and treatment of BC patients.

The Role of Genetic Mutations in Mitochondrial-Driven Cancer Growth in Selected Tumors: Breast and Gynecological Malignancies

There is an increasing understanding of the molecular and cytogenetic background of various tumors that helps us better conceptualize the pathogenesis of specific diseases. Additionally, in many cases, these molecular and cytogenetic alterations have diagnostic, prognostic, and/or therapeutic applications that are heavily used in clinical practice. Given that there is always room for improvement in cancer treatments and in cancer patient management, it is important to discover new therapeutic targets for affected individuals. In this review, we discuss mitochondrial changes in breast and gynecological (endometrial and ovarian) cancers. In addition, we review how the frequently altered genes in these diseases (BRCA1/2, HER2, PTEN, PIK3CA, CTNNB1, RAS, CTNNB1, FGFR, TP53, ARID1A, and TERT) affect the mitochondria, highlighting the possible associated individual therapeutic targets. With this approach, drugs targeting mitochondrial glucose or fatty acid metabolism, reactive oxygen species production, mitochondrial biogenesis, mtDNA transcription, mitophagy, or cell death pathways could provide further tailored treatment.

Mitochondrial Control Region Variants Related to Breast Cancer

Breast cancer has an important incidence in the worldwide female population. Although alterations in the mitochondrial genome probably play an important role in carcinogenesis, the actual evidence is ambiguous and inconclusive. Our purpose was to explore differences in mitochondrial sequences of cases with breast cancer compared with control samples from different origins. We identified 124 mtDNA sequences associated with breast cancer cases, of which 86 were complete and 38 were partial sequences. Of these 86 complete sequences, 52 belonged to patients with a confirmed diagnosis of breast cancer, and 34 sequences were obtained from healthy mammary tissue of the same patients used as controls. From the mtDNA analysis, two polymorphisms with significant statistical differences were found: m.310del (rs869289246) in 34.6% (27/78) of breast cancer cases and 61.7% (21/34) in the controls; and m.315dup (rs369786048) in 60.2% (47/78) of breast cancer cases and 38.2% (13/34) in the controls. In addition, the variant m.16519T>C (rs3937033) was found in 59% of the control sequences and 52% of the breast cancer sequences with a significant statistical difference. Polymorphic changes are evolutionarily related to the haplogroup H of Indo-European and Euro-Asiatic origins; however, they were found in all non-European breast cancers.

Heterologous Inferential Analysis (HIA) and Other Emerging Concepts: In Understanding Mitochondrial Variation In Pathogenesis: There is no More Low-Hanging Fruit

Here we summarize our latest efforts to elucidate the role of mtDNA variants affecting the mitochondrial translation machinery, namely variants mapping to the mt-rRNA and mt-tRNA genes. Evidence is accumulating to suggest that the cellular response to interference with mitochondrial translation is different from that occurring as a result of mutations in genes encoding OXPHOS proteins. As a result, it appears safe to state that a complete view of mitochondrial disease will not be obtained until we understand the effect of mt-rRNA and mt-tRNA variants on mitochondrial protein synthesis. Despite the identification of a large number of potentially pathogenic variants in the mitochondrially encoded rRNA (mt-rRNA) genes, we lack direct methods to firmly establish their pathogenicity. In the absence of such methods, we have devised an indirect approach named heterologous inferential analysis (HIA ) that can be used to make predictions concerning the disruptive potential of a large subset of mt-rRNA variants. We have used HIA to explore the mutational landscape of 12S and 16S mt-rRNA genes. Our HIA studies include a thorough classification of all rare variants reported in the literature as well as others obtained from studies performed in collaboration with physicians. HIA has also been used with non-mammalian mt-rRNA genes to elucidate how mitotypes influence the interaction of the individual and the environment. Regarding mt-tRNA variations, rapidly growing evidence shows that the spectrum of mutations causing mitochondrial disease might differ between the different mitochondrial haplogroups seen in human populations.

Alter between gut bacteria and blood metabolites and the anti-tumor effects of Faecalibacterium prausnitzii in breast cancer

Background

The aim was to evaluate the changes of 16S rDNA sequencing and LC-MS metabolomics in breast cancer and explore the growth inhibition of breast cancer cells by Faecalibacterium prausnitzii.

Results

Total 49 significantly different flora and 26 different metabolites were screened between two groups, and the correlation was calculated. Relative abudance of Firmicutes and Bacteroidetes were decreased, while relative abundance of verrucomicrobla, proteobacteria and actinobacteria was increased in breast cancer group. Differentially expressed metabolites were mainly enriched in pathways such as linoleic acid metabolism, retrograde endocannabinoid signaling, biosynthesis of unsaturated fatty acids, choline metabolism in cancer and arachidonic acid metabolism. Lipid upregulation was found in breast cancer patients, especially phosphorocholine. The abundance of Faecalibacterium was reduced in breast cancer patients, which was negatively correlated with various phosphorylcholines. Moreover, Faecalibacterium prausnitzii, the most well-known species in Faecalibacterium genus, could inhibit the secretion of interleukin-6 (IL-6) and the phosphorylation of Janus kinases 2 (JAK2)/signal transducers and activators of transcription 3 (STAT3) in breast cancer cells. Faecalibacterium prausnitzii also suppressed the proliferation and invasion and promoted the apoptosis of breast cancer cells, while these effects disappeared after adding recombinant human IL-6.

Conclusions

Flora-metabolites combined with the flora-bacteria (such as Faecalibacterium combined with phosphorocholine) might a new detection method for breast cancer. Faecalibacterium may be helpful for prevention of breast cancer. Faecalibacterium prausnitzii suppresses the growth of breast cancer cells through inhibition of IL-6/STAT3 pathway.

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.

Mitogenomic differences between the normal and tumor cells of colorectal cancer patients

So far, a reliable spectrum of mitochondrial DNA mutations in colorectal cancer cells is still unknown, and neither is their significance in carcinogenesis. Indeed, it remains debatable whether mtDNA mutations are "drivers" or "passengers" of colorectal carcinogenesis. Thus, we analyzed 200 mitogenomes from normal and cancer tissues of 100 colorectal cancer patients. Minority variant mutations were detected at the 1% level. We showed that somatic mutations frequently occur in colorectal cancer cells (75%) and are randomly distributed across the mitochondrial genome. Mutational signatures of somatic mitogenome mutations suggest that they might arise through nucleotide deamination due to oxidative stress. The majority of somatic mutations localized within the coding region (in positions not known from the human phylogeny) and was potentially pathogenic to cell metabolism. Further analysis suggested that the relaxation of negative selection in the mitogenomes of colorectal cancer cells may allow accumulation of somatic mutations. Thus, a shift in glucose metabolism from oxidative phosphorylation to glycolysis may create advantageous conditions for accumulation of mtDNA mutations. Considering the fact that the presence of somatic mtDNA mutations was not associated with any clinicopathological features, we suggested that mtDNA somatic mutations are "passengers" rather than the cause of colorectal carcinogenesis.

The role of the mtDNA set point in differentiation, development and tumorigenesis

Mitochondrial DNA replication is critical for maintaining mtDNA copy number to generate sufficient cellular energy that is required for development and for functional cells. In early development, mtDNA copy number is strictly regulated at different stages, and, as a result, the establishment of the mtDNA set point is required for sequential cell lineage commitment. The failure to establish the mtDNA set point results in incomplete differentiation or embryonic arrest. The regulation of mtDNA copy number during differentiation is closely associated with cellular gene expression, especially with the pluripotency network, and DNA methylation profiles. The findings from cancer research highlight the relationship between mitochondrial function, mtDNA copy number and DNA methylation in regulating differentiation. DNA methylation at exon 2 of DNA polymerase gamma subunit A (POLGA) has been shown to be a key factor, which can be modulated to change the mtDNA copy number and cell fate of differentiating and tumour cells. The present review combines multi-disciplinary data from mitochondria, development, epigenetics and tumorigenesis, which could provide novel insights for further research, especially for developmental disorders and cancers.

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.

Mitochondrial D310 instability in Chinese lung cancer patients

To characterize the somatic mutation spectrum of mitochondrial DNA at D310 in Chinese lung cancer patients and evaluate its potential significance in Chinese lung cancer diagnosis, in this study, 237 samples, including lung tumor, adjacent normal tissue and blood samples of 79 lung cancer patients were analyzed. By comparing sequences of D310 between lung cancer tissues, adjacent normal tissue and blood samples, the somatic mutations at D310 were detected in 17.72% (14/79) of Chinese lung cancer patients; this implied that somatic mutations at D310 could be served as valuable biomarker for diagnostic of Chinese lung cancer. Further analyses indicated that deletion and heterogeneity were the predominant characters for somatic mutations detected at D310 of Chinese lung cancer patients.

Mitochondria and familial predisposition to breast cancer

Mitochondrial genome and functional alterations are related to various diseases including cancer. In all cases, the role of these organelles is associated with defects in oxidative energy metabolism and control of tumor-induced oxidative stress. The present study examines the involvement of mitochondrial DNA in cancer and in particular in breast cancer. Furthermore, since mitochondrial DNA is maternally inherited, hereditary breast cancer has been focused on.

Mitochondrial DNA mutations and breast tumorigenesis

Breast cancer is a heterogeneous disease and genetic factors play an important role in its genesis. Although mutations in tumor suppressors and oncogenes encoded by the nuclear genome are known to play a critical role in breast tumorigenesis, the contribution of the mitochondrial genome to this process is unclear. Like the nuclear genome, the mitochondrial genome also encodes proteins critical for mitochondrion functions such as oxidative phosphorylation (OXPHOS), which is known to be defective in cancer including breast cancer. Mitochondrial DNA (mtDNA) is more susceptible to mutations due to limited repair mechanisms compared to nuclear DNA (nDNA). Thus changes in mitochondrial genes could also contribute to the development of breast cancer. In this review we discuss mtDNA mutations that affect OXPHOS. Continuous acquisition of mtDNA mutations and selection of advantageous mutations ultimately leads to generation of cells that propagate uncontrollably to form tumors. Since irreversible damage to OXPHOS leads to a shift in energy metabolism towards enhanced aerobic glycolysis in most cancers, mutations in mtDNA represent an early event during breast tumorigenesis, and thus may serve as potential biomarkers for early detection and prognosis of breast cancer. Because mtDNA mutations lead to defective OXPHOS, development of agents that target OXPHOS will provide specificity for preventative and therapeutic agents against breast cancer with minimal toxicity.

Detecting the somatic mutations spectrum of Chinese lung cancer by analyzing the whole mitochondrial DNA genomes

To detect the somatic mutations and character its spectrum in Chinese lung cancer patients. In this study, we sequenced the whole mitochondrial DNA (mtDNA) genomes for 10 lung cancer patients including the primary cancerous, matched paracancerous normal and distant normal tissues. By analyzing the 30 whole mtDNA genomes, eight somatic mutations were identified from five patients investigated, which were confirmed with the cloning and sequencing of the somatic mutations. Five of the somatic mutations were detected among control region and the rests were found at the coding region. Heterogeneity was the main character of the somatic mutations in Chinese lung cancer patients. Further potential disease-related screening showed that, except the C deletion at position 309 showed AD-weakly associated, most of them were not disease-related. Although the role of aforementioned somatic mutations was unknown, however, considering the relative higher frequency of somatic mutations among the whole mtDNA genomes, it hints that detecting the somatic mutation(s) from the whole mtDNA genomes can serve as a useful tool for the Chinese lung cancer diagnostic to some extent.

Mitochondria as oxidative signaling organelles in T-cell activation: physiological role and pathological implications

Early scientific reports limited the cell biological role of reactive oxygen species (ROS) to the cause of pathological damage. However, extensive research performed over the last decade led to a wide recognition of intracellular oxidative/redox signaling as a crucial mechanism of homeostatic regulation. Amongst different cellular processes known to be influenced by redox signaling, T-cell activation is one of the most established. Numerous studies reported an indispensible role for ROS as modulators of T-cell receptor-induced transcription. Nevertheless, mechanistic details regarding signaling pathways triggered by ROS are far from being delineated. The nature and interplay between enzymatic sources involved in the generation of "oxidative signals" are also a matter of ongoing research. In particular, active participation of the mitochondrial respiratory chain as ROS producer constitutes an intriguing issue with various implications for bioenergetics of activated T cells as well as for T-cell-mediated pathologies. The aim of the current review is to address these interesting concepts.

[Energy metabolism pathway related genes and adaptive evolution of tumor cells]

The proliferation of tumor cells is an extremely energy-consuming process. However, different from normal cells, tumor cells generate energy via glycolysis even under aerobic conditions, which is one of the ten hallmarks of tumor cells. The switch of energy metabolism results in a series of physiological changes in tumor cells, including rapid generation of ATP and abundant biomass for cell proliferation, which form the basis of tumor cells to successfully adapt to their extreme microenvironment (e.g. lack of oxygen). In this review, we will introduce recent progress in studying somatic mutations on the energy metabolism related genes in tumors, with special focus on the potential factors involving in the "switch" and to decipher the genetic adaptive footprint of the "switch" from the angle of molecular evolution.

Mitochondrial DNA mutations in human tumor cells

Mitochondria play significant roles in cellular energy metabolism, free radical generation and apoptosis. The dysfunction of mitochondria is correlated with the origin and progression of tumors; thus, mutations in the mitochondrial genome that affect mitochondrial function may be one of the causal factors of tumorigenesis. Although the role of mitochondrial DNA (mtDNA) mutations in carcinogenesis has been investigated extensively by various approaches, the conclusions remain controversial to date. This review briefly summarizes the recent progress in this field.

The landscape of mitochondrial DNA variation in human colorectal cancer on the background of phylogenetic knowledge

Recently, an increasing number of studies indicate that mutations in mitochondrial genome may contribute to cancer development or metastasis. Hence, it is important to determine whether the mitochondrial DNA might be a good, clinically applicable marker of cancer. This review describes hereditary as well as somatic mutations reported in mitochondrial DNA of colorectal cancer cells. We showed here that the entire mitochondrial genome mutational spectra are different in colorectal cancer and non-tumor cells. We also placed the described mutations on the phylogenetic context, which highlighted the recurrent problem of data quality. Therefore, the most important rules for adequately assessing the quality of mitochondrial DNA sequence analysis in cancer have been summarized. As follows from this review, neither the reliable spectrum of mtDNA somatic mutations nor the association between hereditary mutations and colorectal cancer risk have been resolved. This indicates that only high resolution studies on mtDNA variability, followed by a proper data interpretation employing phylogenetic knowledge may finally verify the utility of mtDNA sequence (if any) in clinical practice.

Is mitochondrial tRNA(phe) variant m.593T>C a synergistically pathogenic mutation in Chinese LHON families with m.11778G>A?

Mitochondrial transfer RNA (mt-tRNA) mutations have been reported to be associated with a variety of diseases. In a previous paper that studied the mtDNA background effect on clinical expression of Leber's hereditary optic neuropathy (LHON) in 182 Chinese families with m.11778G>A, we found a strikingly high frequency (7/182) of m.593T>C in the mitochondrially encoded tRNA phenylalanine (MT-TF) gene in unrelated LHON patients. To determine the potential role of m.593T>C in LHON, we compared the frequency of this variant in 479 LHON patients with m.11778G>A, 843 patients with clinical features of LHON but without the three known primary mutations, and 2374 Han Chinese from the general populations. The frequency of m.593T>C was higher in LHON patients (14/479) than in suspected LHON subjects (12/843) or in general controls (49/2374), but the difference was not statistically significant. The overall penetrance of LHON in families with both m.11778G>A and m.593T>C (44.6%) was also substantially higher than that of families with only m.11778G>A (32.9%) (P = 0.083). Secondary structure prediction of the MT-TF gene with the wild type or m.593T>C showed that this nucleotide change decreases the free energy. Electrophoretic mobility of the MT-TF genes with the wild type or m.593T>C transcribed in vitro further confirmed the change of secondary structure in the presence of this variant. Although our results could suggest a modest synergistic effect of variant m.593T>C on the LHON causing mutation m.11778G>A, the lack of statistical significance probably due to the relatively small sample size analyzed, makes necessary more studies to confirm this effect.

Uncovering the profile of somatic mtDNA mutations in Chinese colorectal cancer patients

In the past decade, a high incidence of somatic mitochondrial DNA (mtDNA) mutations has been observed, mostly based on a fraction of the molecule, in various cancerous tissues; nevertheless, some of them were queried due to problems in data quality. Obviously, without a comprehensive understanding of mtDNA mutational profile in the cancerous tissue of a specific patient, it is unlikely to disclose the genuine relationship between somatic mtDNA mutations and tumorigenesis. To achieve this objective, the most straightforward way is to directly compare the whole mtDNA genome variation among three tissues (namely, cancerous tissue, para-cancerous tissue, and distant normal tissue) from the same patient. Considering the fact that most of the previous studies on the role of mtDNA in colorectal tumor focused merely on the D-loop or partial segment of the molecule, in the current study we have collected three tissues (cancerous, para-cancerous and normal tissues) respectively recruited from 20 patients with colorectal tumor and completely sequenced the mitochondrial genome of each tissue. Our results reveal a relatively lower incidence of somatic mutations in these patients; intriguingly, all somatic mutations are in heteroplasmic status. Surprisingly, the observed somatic mutations are not restricted to cancer tissues, for the para-cancer tissues and distant normal tissues also harbor somatic mtDNA mutations with a lower frequency than cancerous tissues but higher than that observed in the general population. Our results suggest that somatic mtDNA mutations in cancerous tissues could not be simply explained as a consequence of tumorigenesis; meanwhile, the somatic mtDNA mutations in normal tissues might reflect an altered physiological environment in cancer patients.

Detection of somatic mutations and germline polymorphisms in mitochondrial DNA of uterine fibroids patients

To identify the role of mitochondrial DNA (mtDNA) mutations in uterine fibroids patients, genomic DNA isolated from paired myometrium and fibroid tissues was screened for mutations. The present study represents the first investigation to report that 10.4% of uterine fibroids cases had either mtDNA mutations or polymorphisms or both. Among the 14 mitochondrial sequence variants identified, seven are somatic mutations (A3327C, G3352A, G3376A, G3380A, G3421A, T15312G, and C15493G) and the remaining (G3316A, C3342A, C3442T, T10205A, A10188G, A10229C, and A10301T) are gene polymorphisms. Somatic mutations were both homo- and heteroplasmic in nature. Of the seven somatic mutations located in the MTND1 and MTCYB genes, five (71.42%) are nonsynonymous in nature, whereas four (57.14%) of the polymorphisms located in MTND1 and MTND3 genes are found to be nonsynonymous. Sequence variants such as G3380A, G3421A, T15312G, G3376A, and G3316A have been earlier described in different human pathologies, but the remaining are novel ones. Mitochondrial somatic mutations and polymorphisms may predispose women to an earlier onset of degenerative cellular processes, which impair oxidative phosphorylation capacity and thereby promote tumorigenesis in uterine smooth muscle cells. Detection of mtDNA sequence variations in fibroid patients raises the need for larger case-control studies to screen the whole mitochondrial genome and evaluate as a future diagnostic biomarker in fibroid patients.

Rapid identification of mtDNA somatic mutations in gastric cancer tissues based on the mtDNA phylogeny

Mitochondrial DNA (mtDNA) somatic mutations have been identified in nearly all kinds of cancer during the past decade. Normally one need to determine the complete mtDNA sequences from both cancerous and normal tissues of the same patient to score the somatic mutation in cancer. In this study, we intended to explore a strategy to quickly identify somatic mutations in the entire mtDNA genome based on its phylogeny. The principal assumption for this strategy is that somatic mutations, as recently accumulated in cancerous tissue, have younger age and will be located in the terminal branches of mtDNA phylogenetic tree. In contrast, the haplogroup-specific variants, which appear as germline variants and have ancient age, will be located in the basal or intermediate-node branches of the tree, depending on their relative age. When the complete mtDNA sequence of the cancerous tissue is determined and is classified relative to the available mtDNA phylogeny, we only need to screen the variants that are located in the terminal branch in the paracancerous tissue or other normal tissue from the same patient to identify somatic mutations in cancer. We validated this strategy by using paired gastric cancer tissue and paracancerous tissue or blood from 10 Chinese patients (including one with gastric stromal tumor). A total of seven somatic mutations were identified in the cancerous tissues from four patients. Our result suggests that employing mtDNA phylogenetic knowledge facilitates rapid identification of mitochondrial genome somatic mutations in cancer.

Number of somatic mutations in the mitochondrial D-loop region indicates poor prognosis in breast cancer, independent of TP53 mutation

The objective of this study was to investigate whether somatic mutations in the mitochondrial DNA (mtDNA) D-loop region correlate with known prognostic factors, namely, age, tumor size, lymph node status, metastasis, tumor-node-metastasis stage, lymphovascular invasion, and status of the progesterone receptor, estrogen receptor, ERBB2 (alias HER2/neu), and TP53 proteins (as determined by immunohistochemistry) and to investigate their relationship, if any, to TP53 mutations in human breast cancer. Thirty breast tumors without BRCA mutation, along with adjacent nontumorous tissues, were genotyped for the mtDNA D-loop region and for the promoter as well as the coding region of the TP53 gene. Clinicopathological parameters were recorded and assessed. In all, 17 somatic mtDNA D-loop mutations were identified, in 13 of 30 tumor samples (43%); two mutations were novel: 544C>T and 16510A>C. Four TP53 mutations were found in six tumor samples (20%), and two (c.437G>A and c.706T>C) were novel. Only progesterone receptor status correlated with the number of somatic mtDNA D-loop mutations (likelihood chi-square test; P < 0.05). Somatic mutations in the mtDNA D-loop and in TP53 were independent of each other (Fisher's exact test; P > 0.05). These results suggest that the number of somatic mtDNA D-loop mutations may be an indicator of poor prognosis through a mechanism independent of TP53.

Large-scale mtDNA screening reveals a surprising matrilineal complexity in east Asia and its implications to the peopling of the region

In order to achieve a thorough coverage of the basal lineages in the Chinese matrilineal pool, we have sequenced the mitochondrial DNA (mtDNA) control region and partial coding region segments of 6,093 mtDNAs sampled from 84 populations across China. By comparing with the available complete mtDNA sequences, 194 of those mtDNAs could not be firmly assigned into the available haplogroups. Completely sequencing 51 representatives selected from these unclassified mtDNAs identified a number of novel lineages, including five novel basal haplogroups that directly emanate from the Eurasian founder nodes (M and N). No matrilineal contribution from the archaic hominid was observed. Subsequent analyses suggested that these newly identified basal lineages likely represent the genetic relics of modern humans initially peopling East Asia instead of being the results of gene flow from the neighboring regions. The observation that most of the newly recognized mtDNA lineages have already differentiated and show the highest genetic diversity in southern China provided additional evidence in support of the Southern Route peopling hypothesis of East Asians. Specifically, the enrichment of most of the basal lineages in southern China and their rather ancient ages in Late Pleistocene further suggested that this region was likely the genetic reservoir of modern humans after they entered East Asia.

Accumulation of mutations over the entire mitochondrial genome of breast cancer cells obtained by tissue microdissection

The occurrence of heteroplasmy and mixtures is technically challenging for the analysis of mitochondrial DNA. More than that, observed mutations need to be carefully interpreted in the light of the phylogeny as mitochondrial DNA is a uniparental marker reflecting human evolution. Earlier attempts to explain the role of mtDNA in cancerous tissues led to substantial confusion in medical genetics mainly due to the presentation of low sequence data quality and misinterpretation of mutations representing a particular haplogroup background rather than being cancer-specific. The focus of this study is to characterize the extent and level of mutations in breast cancer samples obtained by tissue microdissection by application of an evaluated full mtDNA genome sequencing protocol. We amplified and sequenced the complete mitochondrial genomes of microdissected breast cancer cells of 15 patients and compared the results to those obtained from paired non-cancerous breast tissue derived from the same patients. We observed differences in the heteroplasmic states of substitutions between cancerous and normal cells, one of which was affecting a position that has been previously reported in lung cancer and another one that has been identified in 16 epithelial ovarian tumors, possibly indicating functional relevance. In the coding region, we found full transitions in two cancerous mitochondrial genomes and 12 heteroplasmic substitutions as compared to the non-cancerous breast cells. We identified somatic mutations over the entire mtDNA of human breast cancer cells potentially impairing the mitochondrial OXPHOS system.

The acquisition of an inheritable 50-bp deletion in the human mtDNA control region does not affect the mtDNA copy number in peripheral blood cells

The mitochondrial DNA (mtDNA) control region is believed to play an important biological role in mtDNA replication. Large deletions in this region are rarely found, but when they do occur they might be expected to interfere with the replication of the molecule, thus leading to a reduction of mtDNA copy number. During a survey for mtDNA sequence variations in 5,559 individuals from the general Chinese population and 2,538 individuals with medical disorders, we identified a 50-bp deletion (m.298_347del50) in the mtDNA control region in a member of a healthy Han Chinese family belonging to haplogroup B4c1b2, as suggested by complete mtDNA genome sequencing. This deletion removes the conserved sequence block II (CSBII; region 299-315) and the replication primer location (region 317-321). However, quantification of the mtDNA copy number in this subject showed a value within a range that was observed in 20 healthy subjects without the deletion. The deletion was detected in the hair samples of the maternal relatives of the subject and exhibited variable heteroplasmy. Our current observation, together with a recent report for a benign 154-bp deletion in the mtDNA control region, suggests that the control of mtDNA replication may be more complex than we had thought.

Do alterations in mitochondrial DNA play a role in breast carcinogenesis?

A considerable body of evidence supports a role for oxidative stress in breast carcinogenesis. Due to their role in producing energy via oxidative phosphorylation, the mitochondria are a major source of production of reactive oxygen species, which may damage DNA. The mitochondrial genome may be particularly susceptible to oxidative damage leading to mitochondrial dysfunction. Genetic variants in mtDNA and nuclear DNA may also contribute to mitochondrial dysfunction. In this review, we address the role of alterations in mtDNA in the etiology of breast cancer. Several studies have shown a relatively high frequency of mtDNA mutations in breast tumor tissue in comparison with mutations in normal breast tissue. To date, several studies have examined the association of genetic variants in mtDNA and breast cancer risk. The G10398A mtDNA polymorphism has received the most attention and has been shown to be associated with increased risk in some studies. Other variants have generally been examined in only one or two studies. Genome-wide association studies may help identify new mtDNA variants which modify breast cancer risk. In addition to assessing the main effects of specific variants, gene-gene and gene-environment interactions are likely to explain a greater proportion of the variability in breast cancer risk.

Generation, function, and prognostic utility of somatic mitochondrial DNA mutations in cancer

Exciting new studies are increasingly strengthening the link between mitochondrial mutagenesis and tumor progression. Here we provide a comprehensive review and meta-analysis of studies reporting on mitochondrial DNA mutations in common human cancers. We discuss possible mechanisms by which mitochondrial DNA mutations may influence carcinogenesis, outline important caveats for interpreting the detected mutations--particularly differentiating causality from association--and suggest how new mutational assays may help resolve fundamental controversies in the field and delineate the origin and expansion of neoplastic cell lineages. Finally, we discuss the potential clinical utility of mtDNA mutations for improving the sensitivity of early cancer diagnosis, rapidly detecting cancer recurrence, and predicting the disease outcome.

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.

Mitochondrial copy number and risk of breast cancer: a pilot study

It has been proposed that the copy number of mitochondria DNA (mtDNA) per cell reflects gene-environment interactions between unknown hereditary factors and exposures affecting levels of oxidative stress. However, whether copy number of mtDNA could be a risk predictor of oxidative stress-related human cancers, such as breast cancer, remains to be determined. To explore the role of mtDNA copy number in breast cancer etiology, we analyzed mtDNA copy number in whole blood from 103 patients with breast cancer and 103 matched control subjects and examined in relation to endogenous antioxidants. Case patients with breast cancer had a statistically significantly higher mtDNA copy number than control subjects (median: 1.29 vs. 0.80, P<0.01). High mtDNA copy number (above the median in controls) was associated with a statistically significantly increased risk of breast cancer, compared with low copy number (Odds ratio (OR)=4.67, 95% CI: 2.45-8.92), with a statistically significant dose-response relationship in trend analysis (P<0.01). Moreover, mtDNA copy number was significantly inversely associated with several important endogenous oxidants and antioxidants in blood in either the cases (total glutathione, CuZn-SOD activity and myeloperoxidase (MPO)) or the controls (catalase (CAT) activity). These results suggest the mtDNA copy number could be associated with risk of breast cancer, perhaps through an oxidative stress mechanism.

Sequencing strategy for the whole mitochondrial genome resulting in high quality sequences

Background

It has been demonstrated that a reliable and fail-safe sequencing strategy is mandatory for high-quality analysis of mitochondrial (mt) DNA, as the sequencing and base-calling process is prone to error. Here, we present a high quality, reliable and easy handling manual procedure for the sequencing of full mt genomes that is also appropriate for laboratories where fully automated processes are not available.

Results

We amplified whole mitochondrial genomes as two overlapping PCR-fragments comprising each about 8500 bases in length. We developed a set of 96 primers that can be applied to a (manual) 96 well-based technology, which resulted in at least double strand sequence coverage of the entire coding region (codR).

Conclusion

This elaborated sequencing strategy is straightforward and allows for an unambiguous sequence analysis and interpretation including sometimes challenging phenomena such as point and length heteroplasmy that are relevant for the investigation of forensic and clinical samples.

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.

Environmental mutagens induced transversions but not transitions in regulatory region of mitochondrial DNA

One of the long-term objectives of the research in our laboratory was to determine whether mitochondrial DNA (mtDNA) mutations were generated in cell lines exposed to a variety of known mutagens. Many of these mutagens are known to increase oxidative stress in the cell, and one potential outcome of this would be an increased incidence of point mutations in mtDNA. Recently, there has been some controversy regarding the validity of point mutations in the regulatory region of mtDNA as a predictive or causative marker for carcinogenesis. Studies were undertaken to assess whether nuclear mutagens such as arsenic (As), asbestos, and ultraviolet (UV) and gamma-radiation, induced both heteroplasmic and homoplasmic point mutations in mtDNA. A direct sequencing approach was used to reduce the occurrence of experimental errors and cross-checked all base changes with databases of known polymorphisms. Our results showed that, while base changes did occur, there was no marked difference between the number of changes in treated and untreated cells. Furthermore, in human lymphocyte samples from subjects exposed to As, most of these base changes were previously reported. Interestingly, there was an increase in the number of transversions (purine ( pyrimidine) in smokers from a human population study, but as with the findings in cell culture samples, there was no difference in the total number of base changes. Data suggest that only a change in the number of rare transversions would be indicative of an increase in point mutations in mtDNA after exposure to mutagens.

External contamination in single cell mtDNA analysis

Background

Mitochondrial DNA (mtDNA) variation in single hematopoietic cells, muscle fibers, oocytes, and from tiny amount of tumor tissues and degraded clinical specimens has been reported in many medical publications. External DNA contamination, notoriously difficult to avoid, threatens the integrity of such studies.

Methodology/Principal Findings

Employing a phylogenetic approach, we analyzed the geographic origins of mtDNA sequence anomalies observed during multiple studies of mtDNA sequence variation in a total of 7094 single hematopoietic cells. 40 events with irregular mtDNA patterns were detected: eight instances (from seven different haplotypes) could be traced to laboratory personnel; six cases were caused by sample cross-contamination. The sources of the remaining events could not be identified, and the anomalous sequence variation referred to matrilines from East Asia, Africa, or West Eurasia, respectively. These mtDNA sequence anomalies could be best explained by contamination.

Conclusions

Using the known world mtDNA phylogeny, we could distinguish the geographic origin of the anomalous mtDNA types, providing some useful information regarding the source of contamination. Our data suggest that routine mtDNA sequence analysis of laboratory personnel is insufficient to identify and eliminate all contaminants. A rate of 0.6% of external contamination in this study, while low, is not negligible: Unrecognized contaminants will be mistaken as evidence of remarkable somatic mutations associated with the development of cancer and other diseases. The effective contamination rate can increase by a factor of more than an order of magnitude in some studies that did not institute high standards. Our results are of particular relevance to mtDNA research in medicine, and such an approach should be adopted to maintain and improve quality control in single-cell analyses.