Background: Different cell types possess different copies of mtDNA to support their specific requirements for cellular metabolism. Cell-specific mtDNA copy numbers are established through cell-specific mtDNA replication during cell differentiation. However, cancer cells are trapped in a "pseudo-differentiated" state as they fail to expand mtDNA copy number. Global DNA methylation can regulate this process, as induced DNA demethylation promotes differentiation of cancer cells and expansion of mtDNA copy number.
Results: To determine the role that mtDNA methylation plays in regulating mtDNA replication during tumorigenesis, we have characterized the patterns of mtDNA methylation using glioblastoma and osteosarcoma tumor models that have different combinations of mtDNA genotypes and copy number against common nuclear genome backgrounds at different stages of tumor progression. To ensure the reliability of the findings, we have applied a robust experimental pipeline including three approaches, namely whole-mtDNA bisulfite-sequencing with mtDNA-genotype-specific analysis, pyrosequencing, and methylated immunoprecipitation against 5mC and 5hmC. We have determined genotype-specific methylation profiles, which were modulated through tumor progression. Moreover, a strong influence from the nuclear genome was also observed on mtDNA methylation patterns using the same mtDNA genotype under different nuclear genomes. Furthermore, the numbers of mtDNA copy in tumor-initiating cells affected mtDNA methylation levels in late-stage tumors.
Conclusions: Our findings highlight the influences that the nuclear and mitochondrial genomes have in setting mtDNA methylation patterns to regulate mtDNA copy number in tumorigenesis. They have important implications for assessing global DNA methylation patterns in tumorigenesis and the availability of mtDNA template for mtDNA replication.
Keywords: DNA methylation; Glioblastoma multiforme; Mitochondrial DNA; Osteosarcoma; mtDNA copy number; mtDNA haplotype.