Life Sciences 89 (2011) 65–71

Minireview

Generation, function and diagnostic value of mitochondrial DNA copy number alterations

in human cancers

미토콘드리아 실험실김환영

2012.05.14

Contents

• Introduction

• Quantitative mtDNA alterations in cancer

• Roles of mtDNA content turnovers in cancer onset and progression

- Clinicopathological correlation between altered mtDNA levels and cancer

- Implications of mtDNA copy number changes in conferring oncogenic properties

• MtDNA copy number variations as a novel cancer biomarker

• Conclusions

Introduction

• The content of mtDNA is precisely modulated according to cellular physiological conditions and may undergo significant changes under diverse internal or external microen-vironments, such as hypoxia and steroid hormone stimulation.

• Human mtDNA is a 16569 bp, maternally inherited, closed circular double-stranded molecule encoding 13 core polypeptide subunits of the respiratory chain apparatus, two rRNAs and a set of 22 tRNAs required for mitochondrial protein synthesis.

•In addition to the coding sequences, mtDNA contains a unique 1124 bp noncoding frag-ment at nucleotide positions 16024-576, designated as the displacement (D)-loop, which serves as a major regulatory site responsible for controlling mtDNA replication and tran-scription.

•By virtue of the absence of protective histones, limited DNA repair capacity, lack of in-trons and its close physical proximity to high levels of endogenous reactive oxygen species (ROS) in the mitochondrial inner membrane, mtDNA is extremely prone to oxida-tive or other genotoxic damages and thus acquire mutations at a much higher rate (10- to 200-fold) than nDNA (Liu and Demple, 2010).

Introduction

Mitochondrial functional defects have long been hypothesized to contribute to the devel-opment and probably progression of cancer.Given the central role of mtDNA in maintaining normal mitochondrial function, consider-able efforts have been devoted over the past two decades to better define the potential im-plication of mtDNA copy number aberrations in the carcinogenic process. Besides a heavy load of somatic mutations including point mutations, large-scale deletions and insertions accumulated in both the coding and control regions of mtDNA, quantitative changes in mtDNA have been frequently identified in various types of solid tumors as well as in hematologic malignancies, such as leukemias and lymphoma.

Introduction

This review attempts to highlight recent advances in our understanding of the causal roles of quantitative mtDNA variations in neoplastic transformation and tumor progres-sion.

The potential clinical application of altered mtDNA levels as a novel predictive and diagnostic biomarker is also ad-dressed.

Objects of this re-view

The possible implications of mtDNA content in human malignancies may at least have some degree of specificity for particular tumor type or primary site.

Quantitative mtDNA alterations in cancer

Review contents

The increase of mitochondrial biogenesis and mtDNA copy number has been demonstrated to be associated with enhanced oxidative stress in aging human tissues and was also seen in cultured human lung fibroblast cells in a dose- and time-dependent manner when treated with various oxidative stress-inducing reagents. These data indicate that the accretion in mitochondrial mass and mtDNA levels may represent a resultant state of overall response of human cells to endogeous or exogenous oxidative stress. Specifically, increased mtDNA replication may act as a well orchestrated inherent feedback mechanism that compensates for metabolic defects in mitochondria carrying mutated mtDNA and impaired respiratory system. Since aged cells and malignant cells share many common properties, the above scenario may also happen during tumor onset or progression and contribute to increased mtDNA quantities in certain cancers.

1. Reduced mtDNA amount in HCC, invasive breast cancer and EWS => D310 homopolymeric C-stretch (C-tract) in the D-loop region. Considering that these sequences are involved in the formation of a RNA/DNA hybrid neces-sary for priming mtDNA synthesis, enrichment of D-loop alterations at these particular segments might modify the binding affinities of some nDNA encoded inducers and/or modulators of mtDNA transcription, compromise the strength of mtDNA-protein complex, influence the rate of mtDNA transcription/replication and result in a lowered mtDNA amount in tumor cell.

2. Descending mtDNA copy number also feasibly stems from a deficiency in the p53-mediated signaling pathway. The p53 tumor suppressor not only maintains mitochondrial genetic stability in response to certain ROS-induced mtDNA damages through its ability to translocate into mitochondria and interact with mtDNA polymerase γ(POLG), but also physically stimulates mitochondrial biogenesis as a mito-checkpoint protein

3. mtDNA quantities were drastically decreased when mutant proofreading-deficient POLG was ectopically ex-pressed in breast cancer cells, supporting an idea that reduced mtDNA amount in breast tumors may come forth owing to inefficient PLOG activities. Furthermore, altered mRNA expression of other vital nDNA-encoded trans-acting factors associated with mitochondrial biogenesis and mtDNA maintenance, such as mitochondrial single strand DNA binding protein (mtSSB) and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), was found to be correlated with or accountable for descending mtDNA copies in HCC. Therefore, it is highly conceivable that mtDNA copy number reduction is ascribed to a combined overall effect of both nDNA and mtDNA mutations.

Roles of mtDNA content turnovers in cancer onset and progression

• HCC, gastric carcinoma, NSCLC patients bearing lower mtDNA quantitative levels displayed remarkably poorer disease-free survival (DFS) and overall survival as compared to individuals with normal mtDNA levels. that decreased relative value of mtDNA copy number was intimately linked to the advancement of tumor progres-sion.

• In children with ALL, non-Hodgkin lymphoma, ovarian cancer elevated mtDNA levels were recently reported to be associated with the positivity of minimal residual and/or persistent disease and disease relapse. The authors infer that this phenomenon probably originates as a feedback mechanism in order to counterbalance a general-ized decline in respiratory function in the process of malignant transformation.

The extent of mtDNA content changes in extracted blood samples has been examined for the ability as a molecular predictor to screen individuals with higher future cancer susceptibility and evaluate malignant progression during the carcinogenic process.

MtDNA copy number variations as a novel cancer biomarker

Summary and Conclusions

Association studies between mtDNA copy number alterations and clinicopathological features or prognosis of some human maligancies further strengthen this perspective.

Monitoring mtDNA quantitative variations could have important clinical implications by creating new tumor biomarkers for the sake of identifying high-risk individuals, screening premalignant lesions, tracing cancer progression and predicting disease recur-rence or prognosis.

Future researches are certainly needed to further pave the way for delineating the pre-cise events causing altered mtDNA levels during cancer development, which is antici-pated to have translational merits by introducing strategies for better therapeutic deci-sions.

Abstract Summary

This review outlines mtDNA content changes present in various types of common human malignancies and briefly describes the possible causes and their potential connections to the carcinogenic process.

The present state of our knowledge regarding how altered mtDNA quantitative levels could be utilized as a diagnostic biomarker for identifying genetically predisposed popula-tion that should undergo intensive screening and early surveillance program is also dis-cussed.

Taken together, these findings strongly indicate that mtDNA copy number alterations may exert a crucial role in the pathogenic mechanisms of tumor development.

Continued insights into the functional significance of altered mtDNA quantities in the eti-ology of human cancers will hopefully help in establishing novel potential targets for anti-tumor drugs and intervention therapies.