Mitochondrial haplotype affects histone modification profiles in MNX mice

Abstract

Cancer is a growing problem. In the US, cancer is roughly tied with cardiac disease as the leading cause of death. Intriguingly, compared to the proportion of deaths attributable to primary tumors, roughly 90% of all cancer patient morbidity and mortality are a direct result of metastatic disease perturbing normal organ and system function. Research has shown that the propensity to form both spontaneous tumors and metastatic growths is a function of both the nuclear and mitochondrial genomes present within an organism. Specifically, studies have also shown that, when the nuclear background is held constant and the mitochondrial haplotype is varied, there are marked discrepancies in both tumor latency and the average number of metastatic growths observed. To further study the effects of varying the mitochondrial haplotype on cancer progression and metastasis, a unique in vivo model—known as Mitochondrial-Nuclear Exchange (MNX) mice—has been developed. Experiments using this model system further support the notion that mitochondrial haplotype can contribute significantly to tumor latency and the overall of metastatic growths. As an initial line of inquiry, the question was asked as to whether or not differences in gene expression could be present upon changing the mitochondrial haplotype. Given that clear differences in genome-wide expression profiles between wildtype and MNX mice were indeed observed, the next line of inquiry sought to determine whether or not DNA methylation profiles could be regulating the differences in expression. Likewise, significant differences in DNA methylation patterns were established. As a corollary to these findings, the subsequent line of inquiry sought to elucidate whether or not mitochondrial haplotype could influence other potential epigenetic mechanisms responsible for the differences in gene expression profiles. Specifically, given the known link between metabolism and epigenetics, the logical hypothesis of the experiment was simply that the mitochondrial haplotype could potentially able to directly influence the epigenetic process of post-translationally modifying the N-terminal tails of histones in order to affect chromatin state dynamics in the cell. To address this hypothesis, following rigorous antibody validation, a ChIP-Seq experiment comparing wildtype and MNX mice using antibodies against four known histone modifications (H3K4me1, H3K4me3, H3K27me3, and H3K27ac) was conducted. Post-sequencing analysis indicated a selective difference in the overall profile of differentially-bound, post-translationally modified histones, thus supporting the hypothesis that mitochondrial haplotype does directly contribute to overall histone modification profiles. As a next logical step, future work will involve integrating previously generated RNA-Seq, Methyl-Seq, and ChIP-Seq experiments (in combination with extensive metabolome data) to develop hypotheses as to possible causal mechanisms involved in driving the observed differences in tumor latency and metastatic growth. Furthermore, another direction could involve expanding the ChIP-Seq study to include newly characterized acylation (propionylation, butyrylation, succinylation, malonylation, etc.) post-translational modifications which functionally are thought to overlap with acetylation and may more closely link metabolism to overall chromatin state. The results of this research not only contribute to our understanding of how mitochondria participate in influencing the DNA packaging mechanisms involved in regulating chromatin dynamics, but, as our research is not limited to any single form of cancer, has the potential to benefit to all cancer patients.