POLG and glioblastoma: This is supported by experiments using DNA demethylation agents, such as 5-azacytidine [29] and vitamin C [30], where modulation of DNA methylation at exon 2 of POLG increased mtDNA copy number in HSR-GBM1 cells derived from a glioblastoma multiforme (GBM) tumour [16, 18, 31].