Nuclear DNA but not mtDNA controls tumor phenotypes in mouse cells

Disruption of the mouse Shmt2 gene confers embryonic anaemia via foetal liver-specific metabolomic disorders

In a previous study, we proposed that age-related mitochondrial respiration defects observed in elderly subjects are partially due to age-associated downregulation of nuclear-encoded genes, including serine hydroxymethyltransferase 2 (SHMT2), which is involved in mitochondrial one-carbon (1C) metabolism. This assertion is supported by evidence that the disruption of mouse Shmt2 induces mitochondrial respiration defects in mouse embryonic fibroblasts generated from Shmt2-knockout E13.5 embryos experiencing anaemia and lethality. Here, we elucidated the potential mechanisms by which the disruption of this gene induces mitochondrial respiration defects and embryonic anaemia using Shmt2-knockout E13.5 embryos. The livers but not the brains of Shmt2-knockout E13.5 embryos presented mitochondrial respiration defects and growth retardation. Metabolomic profiling revealed that Shmt2 deficiency induced foetal liver-specific downregulation of 1C-metabolic pathways that create taurine and nucleotides required for mitochondrial respiratory function and cell division, respectively, resulting in the manifestation of mitochondrial respiration defects and growth retardation. Given that foetal livers function to produce erythroblasts in mouse embryos, growth retardation in foetal livers directly induced depletion of erythroblasts. By contrast, mitochondrial respiration defects in foetal livers also induced depletion of erythroblasts as a consequence of the inhibition of erythroblast differentiation, resulting in the manifestation of anaemia in Shmt2-knockout E13.5 embryos.

Mice deficient in the Shmt2 gene have mitochondrial respiration defects and are embryonic lethal

Accumulation of somatic mutations in mitochondrial DNA (mtDNA) has been proposed to be responsible for human aging and age-associated mitochondrial respiration defects. However, our previous findings suggested an alternative hypothesis of human aging—that epigenetic changes but not mutations regulate age-associated mitochondrial respiration defects, and that epigenetic downregulation of nuclear-coded genes responsible for mitochondrial translation [e.g., glycine C-acetyltransferase (GCAT), serine hydroxymethyltransferase 2 (SHMT2)] is related to age-associated respiration defects. To examine our hypothesis, here we generated mice deficient in Gcat or Shmt2 and investigated whether they have respiration defects and premature aging phenotypes. Gcat-deficient mice showed no macroscopic abnormalities including premature aging phenotypes for up to 9 months after birth. In contrast, Shmt2-deficient mice showed embryonic lethality after 13.5 days post coitum (dpc), and fibroblasts obtained from 12.5-dpc Shmt2-deficient embryos had respiration defects and retardation of cell growth. Because Shmt2 substantially controls production of N-formylmethionine-tRNA (fMet-tRNA) in mitochondria, its suppression would reduce mitochondrial translation, resulting in expression of the respiration defects in fibroblasts from Shmt2-deficient embryos. These findings support our hypothesis that age-associated respiration defects in fibroblasts of elderly humans are caused not by mtDNA mutations but by epigenetic regulation of nuclear genes including SHMT2.

The complete mitochondrial genome of western Mediterranean mouse, Mus spretus (Rodentia: Muridae)

The western Mediterranean mouse (Mus spretus) is a wide-spread and well-studied small mammal species in Europe. In this study, we report the complete mitochondrial genome sequence of this species for the first time. Data analysis shows that this mitogenome is entirely 16,286 bp in length and has a conservative genomic organization and gene order as most other mice. The overall nucleotide base composition is 34.1% of A, 28.6% of T, 24.6% C, and 12.7% G, with a strong A + T bias of 62.7%. All the genes are encoded on H-strand, except for the ND6 subunit gene and 8 tRNA genes, which are distributed on the L-strand. Totally 13 protein-coding genes initiate with ATN/GTG start codon and terminate with the typical stop codon (TAA/TAG) or a single T (T- -). Most of the transfer RNA genes could fold into the typical clover-leaf structure except for tRNA(Leu) and tRNA(Ser), whose dihydrouridine (DHU) arm are lost. The complete mitochondrial genome sequence reported here will be useful for population genetic and phylogenetic studies in mice.

Cancer as a mitochondrial metabolic disease

Cancer is widely considered a genetic disease involving nuclear mutations in oncogenes and tumor suppressor genes. This view persists despite the numerous inconsistencies associated with the somatic mutation theory. In contrast to the somatic mutation theory, emerging evidence suggests that cancer is a mitochondrial metabolic disease, according to the original theory of Otto Warburg. The findings are reviewed from nuclear cytoplasm transfer experiments that relate to the origin of cancer. The evidence from these experiments is difficult to reconcile with the somatic mutation theory, but is consistent with the notion that cancer is primarily a mitochondrial metabolic disease.

A specific nuclear DNA background is required for high frequency lymphoma development in transmitochondrial mice with G13997A mtDNA

We previously found that mouse mitochondrial DNA (mtDNA) with a G13997A mutation (G13997A mtDNA) controls not only the transformation of cultured lung carcinoma cells from poorly metastatic into highly metastatic cells, but also the transformation of lymphocytes into lymphomas in living C57BL/6 (B6) mice. Because the nuclear genetic background of the B6 strain makes the strain prone to develop lymphomas, here we examined whether G13997A mtDNA independently induces lymphoma development even in mice with the nuclear genetic background of the A/J strain, which is not prone to develop lymphomas. Our results showed that the B6 nuclear genetic background is required for frequent lymphoma development in mice with G13997A mtDNA. Moreover, G13997A mtDNA in mice did not enhance the malignant transformation of lung adenomas into adenocarcinomas or that of hepatocellular carcinomas from poorly metastatic into highly metastatic carcinomas. Therefore, G13997A mtDNA enhances the frequency of lymphoma development under the abnormalities in the B6 nuclear genome, and does not independently control tumor development and tumor progression.

Administration of an antioxidant prevents lymphoma development in transmitochondrial mice overproducing reactive oxygen species

Because of the difficulty to exclude possible involvement of nuclear DNA mutations, it has been a controversial issue whether pathogenic mutations in mitochondrial DNA (mtDNA) and the resultant respiration defects are involved in tumor development. To address this issue, our previous study generated transmitochondrial mice (mito-mice-ND6¹³⁹⁹⁷), which possess the nuclear and mtDNA backgrounds derived from C57BL/6J (B6) strain mice except that they carry B6 mtDNA with a G13997A mutation in the mt-Nd6 gene. Because aged mito-mice-ND6¹³⁹⁹⁷ simultaneously showed overproduction of reactive oxygen species (ROS) in bone marrow cells and high frequency of lymphoma development, current study examined the effects of administrating a ROS scavenger on the frequency of lymphoma development. We used N-acetylcysteine (NAC) as a ROS scavenger, and showed that NAC administration prevented lymphoma development. Moreover, its administration induced longevity in mito-mice-ND6¹³⁹⁹⁷. The gene expression profiles in bone marrow cells indicated the upregulation of the Fasl gene, which can be suppressed by NAC administration. Given that natural-killer (NK) cells mediate the apoptosis of various tumor cells via enhanced expression of genes encoding apoptotic ligands including Fasl gene, its overexpression would reflect the frequent lymphoma development in bone marrow cells. These observations suggest that continuous administration of an antioxidant would be an effective therapeutics to prevent lymphoma development enhanced by ROS overproduction.

The identification of mitochondrial DNA variants in glioblastoma multiforme

BACKGROUND

Mitochondrial DNA (mtDNA) encodes key proteins of the electron transfer chain (ETC), which produces ATP through oxidative phosphorylation (OXPHOS) and is essential for cells to perform specialised functions. Tumor-initiating cells use aerobic glycolysis, a combination of glycolysis and low levels of OXPHOS, to promote rapid cell proliferation and tumor growth. Glioblastoma multiforme (GBM) is an aggressively malignant brain tumor and mitochondria have been proposed to play a vital role in GBM tumorigenesis.

RESULTS

Using next generation sequencing and high resolution melt analysis, we identified a large number of mtDNA variants within coding and non-coding regions of GBM cell lines and predicted their disease-causing potential through in silico modeling. The frequency of variants was greatest in the D-loop and origin of light strand replication in non-coding regions. ND6 was the most susceptible coding gene to mutation whilst ND4 had the highest frequency of mutation. Both genes encode subunits of complex I of the ETC. These variants were not detected in unaffected brain samples and many have not been previously reported. Depletion of HSR-GBM1 cells to varying degrees of their mtDNA followed by transplantation into immunedeficient mice resulted in the repopulation of the same variants during tumorigenesis. Likewise, de novo variants identified in other GBM cell lines were also incorporated. Nevertheless, ND4 and ND6 were still the most affected genes. We confirmed the presence of these variants in high grade gliomas.

CONCLUSIONS

These novel variants contribute to GBM by rendering the ETC. partially dysfunctional. This restricts metabolism to anaerobic glycolysis and promotes cell proliferation.

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.

Specific mitochondrial DNA mutation in mice regulates diabetes and lymphoma development

It has been hypothesized that respiration defects caused by accumulation of pathogenic mitochondrial DNA (mtDNA) mutations and the resultant overproduction of reactive oxygen species (ROS) or lactates are responsible for aging and age-associated disorders, including diabetes and tumor development. However, there is no direct evidence to prove the involvement of mtDNA mutations in these processes, because it is difficult to exclude the possible involvement of nuclear DNA mutations. Our previous studies resolved this issue by using an mtDNA exchange technology and showed that a G13997A mtDNA mutation found in mouse tumor cells induces metastasis via ROS overproduction. Here, using transmitochondrial mice (mito-mice), which we had generated previously by introducing G13997A mtDNA from mouse tumor cells into mouse embryonic stem cells, we provide convincing evidence supporting part of the abovementioned hypothesis by showing that G13997A mtDNA regulates diabetes development, lymphoma formation, and metastasis--but not aging--in this model.

The complete mitochondrial genome of the striped field mouse, Apodemus agrarius (Rodentia, Murinae) from Korea

The striped field mouse, Apodemus agrarius, is the most common rodents in temperate forests of the Korean peninsula. We have determined the complete mitochondrial genome (JN629047) of A. agrarius from Korea. Apodemus agrarius mitogenome is 16,263 bp in length and has a base composition of 33.9% A, 29.9% T, 23.6% C, and 12.5% G. The total length of the 13 protein-coding genes is 11,398 bp long.

Application of 16s rDNA and cytochrome b ribosomal markers in studies of lineage and fish populations structure of aquatic species

The most economically important form of aquaculture is fish farming, which is an industry that accounts for an ever increasing share of world fishery production. Molecular markers can be used to enhance the productivity of the aquaculture and fish industries to meet the increasing demand. Molecular markers can be identified via a DNA test regardless of the developmental stage, age or environmental challenges experienced by the organism. The application of 16s and cytochrome b markers has enabled rapid progress in investigations of genetic variability and inbreeding, parentage assignments, species and strain identification and the construction of high resolution genetic linkage maps for aquaculture fisheries. In this review, the advantages of principles and potential power tools of 16s and cytochrome b markers are discussed. Main findings in term of trend, aspects and debates on the reviewed issue made from the model of aquatic species for the benefit of aquaculture genomics and aquaculture genetics research are discussed. The concepts in this review are illustrated with various research examples and results that relate theory to reality and provide a strong review of the current status of these biotechnology topics.

The complete mitochondrial genome of the Korean field mouse Apodemus peninsulae (Rodentia, Murinae) from Korea

We have determined the complete mitochondrial genome of a subspecies of the Korean field mouse, Apodemus peninsulae peninsulae, from South Korea. The total length of the A. p. peninsulae genome is 16,268 bp with a base composition of 34.5% A, 30.7% T, 22.6% C, and 12.2% G.

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.

Mediating effect of ROS on mtDNA damage and low ATP content induced by arsenic trioxide in mouse oocytes

Mitochondria provide most of the adenosine triphosphates (ATP) necessary for the maturation of oocytes. Various environmental toxicants would lead damage to mitochondrial DNA (mtDNA) and hence interfere with oocytes development. In the current study, the effect of arsenic trioxide (As2O3) on the common 3867 bp deletion and the copy number of mtDNA in mitochondria of mouse oocytes in vivo or in vitro, as well as the molecular pathway leading to the damage were investigated. PCR strategy was used to detect the damage of mtDNA. Reactive oxygen species (ROS) and ATP content in oocytes were checked to determine the influence of As2O3 on oxidative stress and activity of mitochondria. The results showed that As2O3 could obviously decrease the copy number of mtDNA and cause severe 3867 bp deletion in mitochondria together with elevated ROS level, while ATP content was decreased. Co-treatment with N-Acetyl-Cysteine (NAC) efficiently eliminated ROS induced by As2O3, lessened the mtDNA damage and enhanced ATP content in mouse oocytes both in vivo and in vitro. Taken together, the present study revealed that As2O3 could cause severe mtDNA damage and decrease ATP content by inducing excessive ROS, and this damage would then probably restrain the further development of mouse oocytes.

Somatic mutations of mitochondrial DNA in aging and cancer progression

Mitochondria are intracellular organelles responsible for generating ATP through respiration and oxidative phosphorylation (OXPHOS), producing reactive oxygen species, and initiating and executing apoptosis. Mitochondrial dysfunction has been observed to be an important hallmark of aging and cancer. Because mitochondrial DNA (mtDNA) is important in maintaining functionally competent organelles, accumulation of mtDNA mutations can affect energy production, oxidative stress, and cell survival, which may contribute to aging and/or carcinogenesis. This review outlines a variety of somatic mtDNA mutations identified in aging tissues and human cancers, as well as recent advances in understanding the causal role of mtDNA mutations in the aging process and cancer progression. Mitochondrial dysfunction elicited by somatic mutations in mtDNA could induce apoptosis in aging cells and some cancer cells with severe mtDNA mutations. In addition, it could activate mitochondria-to-nucleus retrograde signaling to modulate the expression of nuclear genes involved in a metabolic shift from OXPHOS to glycolysis, facilitate cells to adapt to altered environments and develop resistance to chemotherapeutic agents, or promote metastatic properties of cancer cells. These findings suggest that accumulation of somatic mtDNA mutations is not only an important contributor to human aging but also plays a critical role in cancer progression.

A novel function of mtDNA: its involvement in metastasis

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 transmitochondrial 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). 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.

Mitochondrial DNA instability and metabolic shift in human cancers

A shift in glucose metabolism from oxidative phosphorylation to glycolysis is one of the biochemical hallmarks of tumor cells. Mitochondrial defects have been proposed to play an important role in the initiation and/or progression of various types of cancer. In the past decade, a wide spectrum of mutations and depletion of mtDNA have been identified in human cancers. Moreover, it has been demonstrated that activation of oncogenes or mutation of tumor suppressor genes, such as p53, can lead to the upregulation of glycolytic enzymes or inhibition of the biogenesis or assembly of respiratory enzyme complexes such as cytochrome c oxidase. These findings may explain, at least in part, the well documented phenomena of elevated glucose uptake and mitochondrial defects in cancers. In this article, we review the somatic mtDNA alterations with clinicopathological correlations in human cancers, and their potential roles in tumorigenesis, cancer progression, and metastasis. The signaling pathways involved in the shift from aerobic metabolism to glycolysis in human cancers are also discussed.

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.

Generation of mtDNA-exchanged cybrids for determination of the effects of mtDNA mutations on tumor phenotypes

It has been proposed that mutations of mitochondrial DNA (mtDNA) and resultant mitochondrial dysfunction induce various phenotypes, such as mitochondrial diseases, aging, and tumorigenesis. However, it is difficult to conclude whether mtDNA mutations are truly responsible for these phenotypes due to the regulation of the mitochondrial functions by both mtDNA and nuclear DNA. The mtDNA-exchange techniques are very effective to exclude the influence of nuclear DNA mutations on expression of these phenotypes. Using these techniques, we recently showed that specific mtDNA mutations can regulate tumor cell metastasis. In this chapter, we describe the methods to establish the mtDNA-exchanged cell lines (cybrids). Applying this technique will reveal how mtDNA mutations are related to various biological phenomena.

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.

Mouse inter-subspecific consomic strains for genetic dissection of quantitative complex traits

Consomic strains, also known as chromosome substitution strains, are powerful tools for assigning polygenes that control quantitative complex traits to specific chromosomes. Here, we report generation of a full set of mouse consomic strains, in which each chromosome of the common laboratory strain C57BL/6J (B6) is replaced by its counterpart from the inbred strain MSM/Ms, which is derived from Japanese wild mouse, Mus musculus molossinus. The genome sequence of MSM/Ms is divergent from that of B6, whose genome is predominantly derived from Western European wild mouse, Mus musculus domesticus. MSM/Ms exhibits a number of quantitative complex traits markedly different from those of B6. We systematically determined phenotypes of these inter-subspecific consomic strains, focusing on complex traits related to reproduction, growth, and energy metabolism. We successfully detected more than 200 statistically significant QTLs affecting 26 traits. Furthermore, phenotyping of the consomic strains revealed that the measured values for quantitative complex traits often far exceed the range between B6 host and MSM/Ms donor strains; this may result from segregation of alleles or nonadditive interactions among multiple genes derived from the two mouse subspecies (that is, epistasis). Taken together, the results suggest that the inter-subspecific consomic strains will be very useful for identification of latent genetic components underlying quantitative complex traits.

Depletion of mitochondrial DNA by ethidium bromide treatment inhibits the proliferation and tumorigenesis of T47D human breast cancer cells

In order to investigate the role of mitochondrial DNA (mtDNA) in human breast cancer cell proliferation and apoptosis, a mtDNA-deficient cell line, T47D rho(0), was generated following a long-term exposure to ethidium bromide (EtBr). T47D rho(0) cells showed a marked decrease in mitochondrial membrane potential (DeltaPsi(m)). However, the apoptosis rate of T47D rho(0) cells was the same as that of their parental cells, suggesting that the change in DeltaPsi(m) was insufficient to induce cell death. Electromicroscopy revealed a profound alteration of mitochondrial morphology, which was consistent with the loss of mtDNA and the decrease in DeltaPsi(m). Disruption of mtDNA resulted in a slower proliferation rate in tissue culture and a reduction in anchorage-independent growth. An in vivo assay revealed a severe impairment of tumorigenicity in T47D rho(0) cells, indicating the biological relevance of in vitro studies. Taken together, our results suggest that the integrity of mtDNA plays a critical role in human breast cancer cell proliferation and tumorigenesis.

mtDNA phylogeny and evolution of laboratory mouse strains

Inbred mouse strains have been maintained for more than 100 years, and they are thought to be a mixture of four different mouse subspecies. Although genealogies have been established, female inbred mouse phylogenies remain unexplored. By a phylogenetic analysis of newly generated complete mitochondrial DNA sequence data in 16 strains, we show here that all common inbred strains descend from the same Mus musculus domesticus female wild ancestor, and suggest that they present a different mitochondrial evolutionary process than their wild relatives with a faster accumulation of replacement substitutions. Our data complement forthcoming results on resequencing of a group of priority strains, and they follow recent efforts of the Mouse Phenome Project to collect and make publicly available information on various strains.

Mitochondrial DNA Deletions Serve as Biomarkers of Aging in the Skin, but Are Typically Absent in Nonmelanoma Skin Cancers

The potential role of mitochondrial DNA (mtDNA) deletions in nonmelanoma skin cancer (NMSC) and in cutaneous photoaging was explored using a genetic approach. Tumors and photodamaged tumor-free "margin" skin were obtained from NMSC patients undergoing excision and the mtDNA from these specimens was screened for the presence of deletions using long extension PCR. mtDNA deletions were abundant in margin tissue specimens from older patients and their number correlated with the patient age. There was a statistically significant difference between the number of mtDNA deletions in tumors and margins. Fewer deletions were detected in the tumors than the margins and the tumors often had no deletions, implying a potential selection for full-length mtDNA or perhaps a protective role for mtDNA deletions in the process of tumorigenesis. The observed mtDNA deletions from skin were often unreported (19 of 21 deletions), but typically shared structural features with mtDNA deletions reported in other tissues. Some mtDNA deletions were detected from the skin of multiple individuals, including 3,715 and 6,278-base pair (bp) deletions, whose frequencies approached that of the previously well-characterized 4977-bp "common" deletion. These data support the use of mtDNA mutations as biomarkers of photoaging in the skin.

Absence of pathogenic mitochondrial DNA mutations in mouse brain tumors

BACKGROUND

Somatic mutations in the mitochondrial genome occur in numerous tumor types including brain tumors. These mutations are generally found in the hypervariable regions I and II of the displacement loop and unlikely alter mitochondrial function. Two hypervariable regions of mononucleotide repeats occur in the mouse mitochondrial genome, i.e., the origin of replication of the light strand (OL) and the Arg tRNA.

METHODS

In this study we examined the entire mitochondrial genome in a series of chemically induced brain tumors in the C57BL/6J strain and spontaneous brain tumors in the VM mouse strain. The tumor mtDNA was compared to that of mtDNA in brain mitochondrial populations from the corresponding syngeneic mouse host strain.

RESULTS

Direct sequencing revealed a few homoplasmic base pair insertions, deletions, and substitutions in the tumor cells mainly in regions of mononucleotide repeats. A heteroplasmic mutation in the 16srRNA gene was detected in a spontaneous metastatic VM brain tumor.

CONCLUSION

None of the mutations were considered pathogenic, indicating that mtDNA somatic mutations do not likely contribute to the initiation or progression of these diverse mouse brain tumors.