Chin-San Liu

A prospective study of mitochondrial DNA copy number and risk of non-Hodgkin lymphoma

Mitochondrial DNA (mtDNA) copy number is increased in patients with chronic lymphocytic leukemia (CLL), in Burkitt lymphoma and Epstein-Barr virus-transformed lymphoblastoid cell lines, and in T cells activated via the T-cell receptor. We hypothesized that having a higher mtDNA copy number in peripheral white blood cell DNA from healthy subjects would be associated with future risk of non-Hodgkin lymphoma (NHL). We analyzed mtDNA copy number in 104 incident male NHL cases and 104 matched controls within the prospective Alpha-Tocopherol, Beta-Carotene (ATBC) Cancer Prevention cohort. There was a dose-response relationship between tertiles of mtDNA copy number and risk of NHL (odds ratio [OR], 95% confidence interval [CI]: 1.0; 1.4 [0.7-2.8]; and 2.4 [1.0-5.5], respectively; P(trend) = .046). The effect was most pronounced for the CLL/small lymphocytic lymphoma (SLL) subtype (OR: 1.0; 3.2 [0.7-15.7]; 14.1 [1.9-103.2]; P(trend) = .009). These results suggest that mtDNA copy number could be associated with the risk of NHL, particularly CLL/SLL.

Mitochondrial DNA content and lung cancer risk in Xuan Wei, China

Smoky coal contains polycyclic aromatic hydrocarbons (PAHs) and has been strongly implicated in etiology of lung cancer in Xuan Wei, China. While PAHs form bulky adducts in nuclear DNA, they have a 40-90-fold greater affinity for mitochondrial DNA (mtDNA). mtDNA content may increase to compensate for mtDNA damage. We conducted a population-based case-control study of lung cancer in Xuan Wei, China hypothesizing that mtDNA content is positively associated with lung cancer risk. Cases (n=122) and controls (n=121) were individually matched on age (+/-2 years), sex, village of residence, and current fuel type. Lifetime smoky coal use and potential confounders were determined with questionnaires. mtDNA was extracted from sputum and mtDNA content was determined with quantitative PCR. ORs and 95% CIs were calculated with unconditional logistic regression. mtDNA content >157 copies per cell was associated with lung cancer risk (OR=1.8; 95% CI=1.0-3.2) compared with those with <or=157 copies. In summary, mtDNA content was positively associated with lung cancer risk. Furthermore, mtDNA content was more strongly associated with lung cancer risk among older individuals. However, due to the small sample size, additional studies are needed to evaluate this potential association.

Mitochondrial DNA Copy Number and Pancreatic Cancer in the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study

Diabetes, obesity, and cigarette smoke, consistent risk factors for pancreatic cancer, are sources of oxidative stress in humans that could cause mitochondrial DNA (mtDNA) damage and increase mtDNA copy number. To test whether higher mtDNA copy number is associated with increased incident pancreatic cancer, we conducted a nested case–control study in the Alpha-Tocopherol Beta Carotene Cancer Prevention (ATBC) Study cohort of male smokers, aged 50 to 69 years at baseline. Between 1992 and 2004, 203 incident cases of pancreatic adenocarcinoma occurred (follow-up: 12 years) among participants, with whole blood samples used for mtDNA extraction. For these cases and 656 controls, we calculated ORs and 95% CIs using unconditional logistic regression, adjusting for age, smoking, and diabetes history. All statistical tests were two sided. Higher mtDNA copy number was significantly associated with increased pancreatic cancer risk (highest vs. lowest mtDNA copy number quintile, OR = 1.64, 95% CI = 1.01–2.67, continuous OR = 1.14, 95% CI 1.06–1.23), particularly for cases diagnosed during the first 7 years of follow-up (OR = 2.14, 95% CI = 1.16–3.96, Ptrend = 0.01, continuous OR = 1.21, 95% CI = 1.10–1.33), but not for cases occurring during follow-up of 7 years or greater (OR = 1.14, 95% CI = 0.53–2.45, continuous OR = 1.05, 95% CI = 0.93–1.18). Our results support the hypothesis that mtDNA copy number is associated with pancreatic cancer and could possibly serve as a biomarker for pancreatic cancer development. Cancer Prev Res; 4(11); 1912–9. ©2011 AACR.

Association between mitochondrial DNA copy number, blood cell counts, and occupational benzene exposure.

Benzene is a recognized hematotoxicant and carcinogen that produces genotoxic damage. Benzene metabolites can produce reactive oxidative species. Mitochondrial DNA (mtDNA) copy number may be increased in response to oxidative stress to compensate for damaged mitochondria. We carried out a cross‐sectional study of 40 benzene‐exposed workers and 40 controls to evaluate the association between benzene exposure and mtDNA copy number. Copy number of mtDNA in leukocyte DNA was determined by real‐time PCR. Compared with controls, the copy number of mtDNA increased by 4% and by 15% in workers exposed to ≤10 ppm (n = 20) and >10 ppm (n = 20) benzene, respectively. After adjusting for recent infection, the factor that was significantly correlated with mtDNA, the increase of mtDNA was statistically significant in the high exposed group (P = 0.016) with a significant linear trend (P = 0.024). To our best knowledge, this is the first report that benzene exposure was associated with increased mitochondria DNA copy number. Benzene exposure may induce mtDNA amplification, possibly in response to oxidative stress caused by benzene. The finding needs to be replicated by other studies. Environ. Mol. Mutagen., 2008.

Personal exposure to fine particulate matter and benzo[a]pyrene from indoor air pollution and leukocyte mitochondrial DNA copy number in rural China

Households in Xuanwei and Fuyuan, China, possess hazardous levels of fine particulate matter with an aerodynamic diameter <2.5 microns (PM2.5) and polycyclic aromatic hydrocarbons (PAHs) from coal combustion. Previous studies found that increased exposure to PM2.5 and benzo[a]pyrene (BaP; a PAH) were associated with decreased mitochondrial DNA copy number (mtDNAcn), a marker of oxidative stress. We further evaluated these associations in a cross-sectional study of 148 healthy non-smoking women from Xuanwei and Fuyuan. Personal exposure to PM2.5 and BaP was measured using portable devices. MtDNAcn was measured using qPCR amplification of leukocyte DNA that was collected after air measurements. Linear regression models were used to estimate the associations between personal exposure to PM2.5 and BaP, and mtDNAcn adjusted for age, body mass index (BMI) and fuel type. We found inverse associations between exposure to PM2.5 and BaP, and mtDNAcn. Each incremental log-μg/m3 increase in PM2.5 was associated with a significant decrease in mtDNAcn of -10.3 copies per cell [95% confidence interval (95% CI): -18.6, -2.0; P = 0.02]. Additionally, each log-ng/m3 increase in BaP was associated with a significant decrease in mtDNAcn of -5.4 copies per cell (95% CI: -9.9, -0.8, P = 0.02). Age, BMI, fuel type and coal mine type were not significantly associated with mtDNAcn. Exposure to PM2.5 and BaP may alter mitochondrial dynamics in non-smoking Chinese women. MtDNAcn may be a potential mediator of indoor air pollution on chronic disease development.

A prospective study of mitochondrial DNA copy number and the risk of prostate cancer

PurposeEvidence suggests that mitochondrial DNA (mtDNA) copy number increases in response to DNA damage. Increased mtDNA copy number has been observed in prostate cancer (PCa) cells, suggesting a role in PCa development, but this association has not yet been investigated prospectively.MethodsWe conducted a nested case–control study (793 cases and 790 controls) of men randomized to the screening arm of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO) to evaluate the association between pre-diagnosis mtDNA copy number, measured in peripheral blood leukocytes, and the risk of PCa. We used logistic regression to estimate odds ratios (OR) and 95% confidence intervals (CI) and polytomous logistic regression to analyze differences in associations by non-aggressive (Stage I/II AND Gleason grade < 8) or aggressive (Stage III/IV OR Gleason grade ≥ 8) PCa.ResultsAlthough mtDNA copy number was not significantly associated with PCa risk overall (OR 1.23, 95% CI 0.97–1.55, p = 0.089), increasing mtDNA copy number was associated with an increased risk of non-aggressive PCa (OR 1.29, 95% CI 1.01–1.65, p = 0.044) compared to controls. No association was observed with aggressive PCa (OR 1.02, 95% CI 0.64–1.63, p = 0.933). Higher mtDNA copy number was also associated with increased PSA levels among controls (p = 0.014).ConclusionsThese results suggest that alterations in mtDNA copy number may reflect disruption of the normal prostate glandular architecture seen in early-stage disease, as opposed to reflecting the large number of tumor cells seen with advanced PCa.

Abstract 4461: A prospective study of blood mitochondrial DNA copy number and risk of renal cell carcinoma

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Background: Mitochondrial DNA (mtDNA) is vulnerable to mutations, and the number of copies of mtDNA per cell may increase to compensate for DNA damage. Recent studies have evaluated blood leukocyte mtDNA copy number and risk of various types of cancer, including renal cell carcinoma (RCC). Although alterations in mtDNA copy number have been associated with cancer risk, the direction of this association has been inconsistent between studies with prospective and retrospective blood collection. One case-control study found that low mtDNA copy number was associated with an increased risk of RCC; however, this association has not been investigated prospectively. Methods: We conducted a nested case-control study of RCC risk in relation to mtDNA copy number in the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial. A total of 230 cases and 469 controls matched on age, sex, race, date of blood collection and specimen type were included. Measurements of mtDNA copy number were performed in triplicate using a fluorescence-based QPCR assay. Assay results were highly reproducible, with a coefficient of variation of 5.3% for replicate QC samples from a single individual and an intraclass correlation coefficient of 0.69 for duplicate samples collected an average of 3.6 years apart from 45 controls. To evaluate risk of RCC in relation to mtDNA copy number, we calculated odds ratios (OR) and 95% confidence intervals (CI) using unconditional logistic regression models adjusted for all matching variables as well as history of hypertension, body mass index, smoking history, and family history of kidney cancer. Subjects were assigned to quartiles of mtDNA copy number based on the distribution among controls. Results: Median mtDNA copy number was higher for cases than for controls (131 vs. 121; P=0.001, Wilcoxon rank-sum test). Relative to subjects in the lowest quartile of mtDNA copy number, the ORs for subjects in the 2nd, 3rd, and 4th quartiles were 1.0 (95% CI 0.6-1.6), 1.5 (95% 0.9-2.3), and 1.9 (95% CI 1.2-3.1), respectively (Ptrend = 0.003). The association between mtDNA copy number and RCC was stronger among men (highest vs. lowest quartile, OR 2.4, 95% CI 1.3-4.4) than among women (OR 1.3, 95% CI 0.5-3.2; Pint = 0.24). Results were similar in conditional logistic regression analyses of matched sets, and when we restricted to subjects with mtDNA from buffy coat specimens and to cases diagnosed more than two years after sample collection. Conclusions: Results of this study, to our knowledge the first prospective investigation of mtDNA copy number and RCC risk, suggest that high mtDNA copy number is associated with an increased risk of RCC, particularly among men. Although our findings were inconsistent with prior case-control evidence, most prospective studies of other cancers (e.g., lung, pancreas, non-Hodgkin lymphoma) have reported positive associations. Replication of these findings in other prospective cohorts is needed. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4461. doi:1538-7445.AM2012-4461

Constitutive Mitochondrial DNA Copy Number in Peripheral Blood of Melanoma Families with and without CDKN2A Mutations

Quantitative changes in mitochondrial DNA (mtDNA) have been associated with the risk of a number of human cancers; however, the relationship between constitutive mtDNA copy number in blood and the risk of familial cutaneous malignant melanoma (CMM) has not been reported. We measured mtDNA copy number using quantitative PCR in blood-derived DNA from 136 CMM cases and 302 controls in 53 melanoma-prone families (23 segregating CDKN2A germline mutations). MtDNA copy number did not vary by age, sex, pigmentation characteristics, or CMM status. However, germline CDKN2A mutation carriers had significantly higher mean mtDNA copy number compared to non-carriers, particularly among CMM cases (geometric mean mtDNA copy number of 144 and 111 for carrier versus non-carrier, respectively; P= 0.02). When adjusting for age, sex, and familial correlation, having increasing mtDNA copy number was significantly associated with CDKN2A mutation status among CMM cases (OR=1.47, Ptrend=0.024). In particular, individuals with specific CDKN2A mutations with the potential to inactivate or reduce the level of the p16-INK4 reactive oxygen species (ROS) protective function had significantly increased mtDNA copy number levels (P=0.035). Future research in prospective studies is required to validate these findings and to further investigate mtDNA copy number in both blood and melanoma tissues in relation to CMM risk and CDKN2A mutation status.