Mitochondrial DNA instability in human cancers

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.

Whole mitochondrial genome sequencing highlights mitochondrial impact in gastric cancer

Mitochondria are organelles that perform major roles in cellular operation. Thus, alterations in mitochondrial genome (mtGenome) may lead to mitochondrial dysfunction and cellular deregulation, influencing carcinogenesis. Gastric cancer (GC) is one of the most incident and mortal types of cancer in Brazil, particularly in the Amazon region. Here, we sequenced and compared the whole mtGenome extracted from FFPE tissue samples of GC patients (tumor and internal control – IC) and cancer-free individuals (external control – EC) from this region. We found 3-fold more variants and up to 9-fold more heteroplasmic regions in tumor when compared to paired IC samples. Moreover, tumor presented more heteroplasmic variants when compared to EC, while IC and EC showed no significant difference when compared to each other. Tumor also presented substantially more variants in the following regions: MT-RNR1, MT-ND5, MT-ND4, MT-ND2, MT-DLOOP1 and MT-CO1. In addition, our haplogroup results indicate an association of Native American ancestry (particularly haplogroup C) to gastric cancer development. To the best of our knowledge, this is the first study to sequence the whole mtGenome from FFPE samples and to apply mtGenome analysis in association to GC in Brazil.

Recent advancements in the study of breast cancer exosomes as mediators of intratumoral communication

Breast cancer is a heterogeneous disease, with a morbidity rate of 27.8% and a mortality rate of 15% among women population worldwide. Understanding how this cancer develops and the mechanisms behind tumor progression and chemoresistance is of utmost importance. Exosomes mediate communication in a population of heterogeneous tumoral cells. They have a cargo composed of oncogenes and oncomiRs which change the transcriptomic scenario of their targeted cells and activate numerous tumor-promoting signaling pathways. Exosomes secreted by breast cancer cells lead to enhanced cell proliferation, replicative immortality, angiogenesis, invasion, migration, and chemoresistance. Studying exosomes from this perspective offers more in depth understanding of breast malignancy and may aid in the future development of early diagnostic, prognostic and therapeutic options. We present the latest findings in this area and offer practical solutions which may further stimulate the much-needed research of exosome in breast cancer.

Role of Mitochondrial DNA Copy Number Alteration in Human Renal Cell Carcinoma

We investigated the role of mitochondrial DNA (mtDNA) copy number alteration in human renal cell carcinoma (RCC). The mtDNA copy numbers of paired cancer and non-cancer parts from five resected RCC kidneys after radical nephrectomy were determined by quantitative polymerase chain reaction (Q-PCR). An RCC cell line, 786-O, was infected by lentiviral particles to knock down mitochondrial transcriptional factor A (TFAM). Null target (NT) and TFAM-knockdown (TFAM-KD) represented the control and knockdown 786-O clones, respectively. Protein or mRNA expression levels of TFAM; mtDNA-encoded NADH dehydrogenase subunit 1 (ND1), ND6 and cytochrome c oxidase subunit 2 (COX-2); nuclear DNA (nDNA)-encoded succinate dehydrogenase subunit A (SDHA); v-akt murine thymoma viral oncogene homolog 1 gene (AKT)-encoded AKT and v-myc myelocytomatosis viral oncogene homolog gene (c-MYC)-encoded MYC; glycolytic enzymes including hexokinase II (HK-II), glucose 6-phosphate isomerase (GPI), phosphofructokinase (PFK), and lactate dehydrogenase subunit A (LDHA); and hypoxia-inducible factors the HIF-1α and HIF-2α, pyruvate dehydrogenase kinase 1 (PDK1), and pyruvate dehydrogenase E1 component α subunit (PDHA1) were analyzed by Western blot or Q-PCR. Bioenergetic parameters of cellular metabolism, basal mitochondrial oxygen consumption rate (mOCR_B) and basal extracellular acidification rate (ECAR_B), were measured by a Seahorse XF^e-24 analyzer. Cell invasiveness was evaluated by a trans-well migration assay and vimentin expression. Doxorubicin was used as a chemotherapeutic agent. The results showed a decrease of mtDNA copy numbers in resected RCC tissues (p = 0.043). The TFAM-KD clone expressed lower mtDNA copy number (p = 0.034), lower mRNA levels of TFAM (p = 0.008), ND1 (p = 0.007), and ND6 (p = 0.017), and lower protein levels of TFAM and COX-2 than did the NT clone. By contrast, the protein levels of HIF-2α, HK-II, PFK, LDHA, AKT, MYC and vimentin; trans-well migration activity (p = 0.007); and drug resistance to doxorubicin (p = 0.008) of the TFAM-KD clone were significantly higher than those of the NT clone. Bioenergetically, the TFAM-KD clone expressed lower mOCR_B (p = 0.009) but higher ECAR_B (p = 0.037) than did the NT clone. We conclude that a reduction of mtDNA copy number and decrease of respiratory function of mitochondria in RCC might be compensated for by an increase of enzymes and factors that are involved in the upregulation of glycolysis to confer RCC more invasive and a drug-resistant phenotype in vitro.

Identification of sequence polymorphisms in the D-Loop region of mitochondrial DNA as a risk factor for colon cancer

The accumulation of single nucleotide polymorphisms (SNPs) in the displacement loop (D-Loop) of mitochondrial DNA (mtDNA) has been identified for their association with cancer risk in a number of cancers. We investigated the colon cancer risk profile of D-Loop SNPs in a case-control study. The frequent alleles of nucleotides 73G/A, 146T/C, 195T/C, 324C/G, 16261C/T, and 16304T/C as well as the minor allele of 309C/C insert were significantly associated with an increased risk for colon cancer. In conclusion, SNPs in the mtDNA D-Loop were found to be valuable markers for colon cancer risk evaluation.

Mitochondrial DNA haplogroup N is associated good outcome of gastric cancer

Accumulation of mutations and single nucleotide polymorphisms (SNPs) in the displacement loop (D-loop) of mitochondrial DNA (mtDNA) has been identified for their association with cancer risk and disease outcome in a variety of cancers. We have identified cancer risk-associated D-loop SNPs in gastric cancer patients. In this study, we evaluated the predictive value of these SNPs for cancer outcome. Two SNP sites of nucleotides 489C/T and 523-524AC/del were identified for statistically significant prediction of postoperative survival in gastric cancer by univariate analysis with log-rank test. In addition, the mitochondrial DNA haplogroup N (489T) contributed to the good survival of gastric cancer patients compared with the mitochondrial DNA haplogroup M (489C) genotype (relative risk, 1.753; 95 %CI, 1.005-3.060; p = 0.048) by multivariate analysis with COX hazards model. In conclusion, analysis of genetic polymorphisms in the mitochondrial D-loop can help identify subgroups of patients who are at a high risk of a poor disease outcome.