Coordinated regulation of mitochondrial topoisomerase IB with mitochondrial nuclear encoded genes and MYC.

Mitochondrial DNA (mtDNA) is entirely dependent on nuclear genes for its transcription and replication. One of these genes is TOP1MT, which encodes the mitochondrial DNA topoisomerase IB, involved in mtDNA relaxation. To elucidate TOP1MT regulation, we performed genome-wide profiling across the 60-cell line panel (the NCI-60) of the National Cancer Institute Developmental Therapeutics Program. We show that TOP1MT mRNA expression varies widely across these cell lines with the highest levels in leukemia (HL-60, K-562) and melanoma (SK-MEL-28), intermediate levels in breast (MDA-MB-231), ovarian (OVCAR) and colon (HCT-116, HCT-15, KM-12), and lowest levels in renal (ACHN, A498), prostate (PC-3, DU-145) and central nervous system cell lines (SF-539, SF-268, SF-295). Genome-wide analyses show that TOP1MT expression is significantly correlated with the other mitochondrial nuclear-encoded genes including the mitochondrial nucleoid genes, and demonstrate an overall co-regulation of the mitochondrial nuclear-encoded genes. We also find very high correlation between the expression of TOP1MT and the proto-oncogene MYC (c-myc). TOP1MT contains E-boxes (c-myc binding sites) and TOP1MT transcription follows MYC up- and down-regulation by MYC promoter activation and siRNA against MYC. Our finding implicates MYC as a novel regulator of TOP1MT and confirms its role as a master regulator of MNEGs and mitochondrial nucleoids.