The regulation of mitochondrial DNA copy number in glioblastoma cells

A Dickinson,J Donoghue,K Y Yeung,M J Baker,T G Johns,R Dw Kelly,J C St John,M Mckenzie

doi:10.1038/cdd.2013.115

Abstract

As stem cells undergo differentiation, mitochondrial DNA (mtDNA) copy number is strictly regulated in order that specialized cells can generate appropriate levels of adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS) to undertake their specific functions. It is not understood whether tumor-initiating cells regulate their mtDNA in a similar manner or whether mtDNA is essential for tumorigenesis. We show that human neural stem cells (hNSCs) increased their mtDNA content during differentiation in a process that was mediated by a synergistic relationship between the nuclear and mitochondrial genomes and results in increased respiratory capacity. Differentiating multipotent glioblastoma cells failed to match the expansion in mtDNA copy number, patterns of gene expression and increased respiratory capacity observed in hNSCs. Partial depletion of glioblastoma cell mtDNA rescued mtDNA replication events and enhanced cell differentiation. However, prolonged depletion resulted in impaired mtDNA replication, reduced proliferation and induced the expression of early developmental and pro-survival markers including POU class 5 homeobox 1 (OCT4) and sonic hedgehog (SHH). The transfer of glioblastoma cells depleted to varying degrees of their mtDNA content into immunocompromised mice resulted in tumors requiring significantly longer to form compared with non-depleted cells. The number of tumors formed and the time to tumor formation was relative to the degree of mtDNA depletion. The tumors derived from mtDNA depleted glioblastoma cells recovered their mtDNA copy number as part of the tumor formation process. These outcomes demonstrate the importance of mtDNA to the initiation and maintenance of tumorigenesis in glioblastoma multiforme.

Highlights

We show that human neural stem cells increased their mitochondrial DNA (mtDNA) content during differentiation in a process that was mediated by a synergistic relationship between the nuclear and mitochondrial genomes and results in increased respiratory capacity
The circular, double-stranded human mitochondrial genome is 16 569 bp in size and encodes 13 subunits of the electron transfer chain (ETC),[1] which is the major generator of cellular adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS).[2]
MtDNA replication is initiated by mitochondrial transcription factor A (TFAM),[4] which generates the primer used by the catalytic subunit of the mtDNA-specific DNA polymerase, polymerase g A (POLGA), to copy mtDNA

Summary

Introduction

The circular, double-stranded human mitochondrial genome (mitochondrial DNA, mtDNA) is 16 569 bp in size and encodes 13 subunits of the electron transfer chain (ETC),[1] which is the major generator of cellular adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS).[2]. MtDNA replication is initiated by mitochondrial transcription factor A (TFAM),[4] which generates the primer used by the catalytic subunit of the mtDNA-specific DNA polymerase, polymerase g A (POLGA), to copy mtDNA. Tumors utilize aerobic glycolysis even under normoxic conditions, which normally promotes OXPHOS.[9] This promotes self-renewal and the highly proliferative nature of tumor cells,[10] enabling them to generate sufficient energy and pools of metabolic intermediates.

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