Polymorphic mutations in mouse mitochondrial DNA regulate a tumor phenotype

Mitochondrial DNA lineages determine tumor progression through T cell reactive oxygen signaling

Mitochondrial DNA (mtDNA) is highly polymorphic, and host mtDNA variation has been associated with altered cancer severity. To determine the basis of this mtDNA-cancer association, we analyzed conplastic mice with the C57BL/6J (B6) nucleus but two naturally occurring mtDNA lineages, mtDNAB6 and mtDNANZB, where mtDNANZB mitochondria generate more oxidative phosphorylation (OXPHOS)-derived reactive oxygen species (mROS). In a cardiac transplant model, mtDNAB6 Foxp3+ T regulatory (Treg) cells supported long-term allograft survival, whereas mtDNANZB Treg cells failed to suppress host T effector (Teff) cells, leading to acute rejection. When challenged with melanoma or colon cancer cells, the mtDNANZB mice exhibited strikingly impaired tumor growth while mtDNAB6 mice showed Treg-dependent inhibition of Teff cells and allowed rapid tumor growth. Transcriptional analysis showed that activation of mtDNANZB Teff cells increased mitochondrial gene expression while activation of mtDNANZB Treg cells impaired mitochondrial gene expression and resulted in mtDNANZB Treg cell exhaustion. Induction of the mitochondrially targeted catalytic antioxidant, mCAT, in hematopoietic cells normalized mtDNANZB Treg function in both transplant and tumor models, indicating a key role for mROS in promoting Treg dysfunction. Anti-PD-L1 therapy did not modulate these effects, indicating that modulation of host mitochondrial function provides an independent approach for enhancing tumor cell destruction.

Relationship of single nucleotide polymorphisms and haplotype interaction of mitochondrial unfolded protein response pathway genes with head and neck cancer

Aim: In this study, we evaluated the effect of selected polymorphisms of mitochondrial unfolded protein response (UPR^mt) pathway in 500 head and neck cancer (HNC) patients and 500 healthy controls from Pakistan. Materials & methods: The experiments were conducted using tetra-ARMS PCR followed by DNA sequencing. Results: Multivariate analysis showed that AA genotype of rs3782116 showed fivefold, GG genotype of rs6598072 approximately twofold and CC genotype of rs4946936 and TT genotype of rs12212067 showed twofold increased risk of HNC. Furthermore, haplotype analysis showed that certain haplotypes of UPR^mt pathway single nucleotide polymorphisms have significant association with increased HNC risk. Conclusion: These results show that genetic aberrations in UPR^mt pathway genes have association with increased HNC risk and can be an indicator of advance clinical outcome especially invasion and metastasis.

Mitochondrial polymorphisms contribute to aging phenotypes in MNX mouse models

Many inbred strains of mice develop spontaneous tumors as they age. Recent awareness of the impacts of mitochondrial DNA (mtDNA) on cancer and aging has inspired developing a mitochondrial-nuclear exchange (MNX) mouse model in which nuclear DNA is paired with mitochondrial genomes from other strains of mouse. MNX mice exhibit mtDNA influences on tumorigenicity and metastasis upon mating with transgenic mice. However, we also wanted to investigate spontaneous tumor phenotypes as MNX mice age. Utilizing FVB/NJ, C57BL/6J, C3H/HeN, and BALB/cJ wild-type inbred strains, previously documented phenotypes were observed as expected in MNX mice with the same nuclear background. However, aging nuclear matched MNX mice exhibited decreased occurrence of mammary tumors in C3H/HeN mice containing C57BL/6J mitochondria compared to wild-type C3H/HeN mice. Although aging tumor phenotypes appear to be driven by nuclear genes, evidence suggesting that some differences are modified by the mitochondrial genome is presented.

Cytoplasmic transfer of heritable elements other than mtDNA from SAMP1 mice into mouse tumor cells suppresses their ability to form tumors in C57BL6 mice

In a previous study, we generated transmitochondrial P29mtSAMP1 cybrids, which had nuclear DNA from the C57BL6 (referred to as B6) mouse strain-derived P29 tumor cells and mitochondrial DNA (mtDNA) exogenously-transferred from the allogeneic strain SAMP1. Because P29mtSAMP1 cybrids did not form tumors in syngeneic B6 mice, we proposed that allogeneic SAMP1 mtDNA suppressed tumor formation of P29mtSAMP1 cybrids. To test this hypothesis, current study generated P29mt(sp)B6 cybrids carrying all genomes (nuclear DNA and mtDNA) from syngeneic B6 mice by eliminating SAMP1 mtDNA from P29mtSAMP1 cybrids and reintroducing B6 mtDNA. However, the P29mt(sp)B6 cybrids did not form tumors in B6 mice, even though they had no SAMP1 mtDNA, suggesting that SAMP1 mtDNA is not involved in tumor suppression. Then, we examined another possibility of whether SAMP1 mtDNA fragments potentially integrated into the nuclear DNA of P29mtSAMP1 cybrids are responsible for tumor suppression. We generated P29^H(sp)B6 cybrids by eliminating nuclear DNA from P29mt(sp)B6 cybrids and reintroducing nuclear DNA with no integrated SAMP1 mtDNA fragment from mtDNA-less P29 cells resistant to hygromycin in selection medium containing hygromycin. However, the P29^H(sp)B6 cybrids did not form tumors in B6 mice, even though they carried neither SAMP1 mtDNA nor nuclear DNA with integrated SAMP1 mtDNA fragments. Moreover, overproduction of reactive oxygen species (ROS) and bacterial infection were not involved in tumor suppression. These observations suggest that tumor suppression was caused not by mtDNA with polymorphic mutations or infection of cytozoic bacteria but by hypothetical heritable cytoplasmic elements other than mtDNA from SAMP1 mice.

From discovery of the CHOP axis and targeting ClpP to the identification of additional axes of the UPRmt driven by the estrogen receptor and SIRT3

The mitochondrial UPR (UPR^mt) is rapidly gaining attention. While most studies on the UPR^mt have focused on its role in aging, emerging studies suggest an important role of the UPR^mt in cancer. Further, several of the players of the UPR^mt in mammalian cells have well reported roles in the maintenance of the organelle. The goal of this review is to emphasize aspects of the UPR^mt that have been overlooked in the current literature, describe the role of specific players of the UPR^mt in the biology of the mitochondria and highlight the intriguing possibility that targeting the UPR^mt in cancer may be already within reach.

mtDNA, Metastasis, and the Mitochondrial Unfolded Protein Response (UPRmt)

While several studies have confirmed a link between mitochondrial DNA (mtDNA) mutations and cancer cell metastasis, much debate remains regarding the nature of the alternations in mtDNA leading to this effect. Meanwhile, the mitochondrial unfolded protein response (UPR^mt) has gained much attention in recent years, with most studies of this pathway focusing on its role in aging. However, the UPR^mt has also been studied in the context of cancer. More recent work suggests that rather than a single mutation or alternation, specific combinatorial mtDNA landscapes able to activate the UPR^mt may be those that are selected by metastatic cells, while mtDNA landscapes unable to activate the UPR^mt do not. This review aims at offering an overview of the confusing literature on mtDNA mutations and metastasis and the more recent work on the UPR^mt in this setting.

Selected mitochondrial DNA landscapes activate the SIRT3 axis of the UPRmt to promote metastasis

By causing mitochondrial DNA (mtDNA) mutations and oxidation of mitochondrial proteins, reactive oxygen species (ROS) leads to perturbations in mitochondrial proteostasis. Several studies have linked mtDNA mutations to metastasis of cancer cells but the nature of the mtDNA species involved remains unclear. Our data suggests that no common mtDNA mutation identifies metastatic cells; rather the metastatic potential of several ROS-generating mutations is largely determined by their mtDNA genomic landscapes, which can act either as an enhancer or repressor of metastasis. However, mtDNA landscapes of all metastatic cells are characterized by activation of the SIRT/FOXO/SOD2 axis of the mitochondrial unfolded protein response (UPR^mt). The UPR^mt promotes a complex transcription program ultimately increasing mitochondrial integrity and fitness in response to oxidative proteotoxic stress. Using SOD2 as a surrogate marker of the UPR^mt, we found that in primary breast cancers, SOD2 is significantly increased in metastatic lesions. We propose that the ability of selected mtDNA species to activate the UPR^mt is a process that is exploited by cancer cells to maintain mitochondrial fitness and facilitate metastasis.

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.