PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Cancer Epidemiol Biomarkers Prev. Author manuscript; available in PMC 2010 November 1.
Published in final edited form as:
PMCID: PMC2938741
NIHMSID: NIHMS220583

Telomere Length in Peripheral Leukocyte DNA and Gastric Cancer Risk

Abstract

Telomere length reflects lifetime cumulative oxidative stress from environmental exposures, such as cigarette smoking and chronic inflammation. Shortened telomere length is thought to cause genomic instability and has been associated with several cancers. We examined the association of telomere length in peripheral leukocyte DNA with gastric cancer risk as well as potential confounding factors and risk modifiers for telomere length–related risk. In a population-based study of gastric cancer conducted in a high-risk population in Warsaw, Poland, between 1994 and 1996, we measured relative telomere length in 300 cases and 416 age- and gender-matched controls using quantitative real-time PCR. Among controls, telomeres were significantly shorter in association with aging (P < 0.001), increasing pack-years of cigarette smoking (P = 0.02), decreasing fruit intake (P = 0.04), and Helicobacter pylori positivity (P = 0.03). Gastric cancer cases had significantly shorter telomere length (mean ± SD relative telomere length, 1.25 ± 0.34) than controls (1.34 ± 0.35; P = 0.0008). Gastric cancer risk doubled [odds ratio (OR), 2.04; 95% confidence interval (95% CI), 1.33-3.13] among subjects in the shortest compared with the highest quartile of telomere length (Ptrend < 0.001). Telomere length–associated risks were higher among individuals with the lowest risk profile, those H. pylori–negative (OR, 5.45; 95% CI, 2.10-14.1), non-smokers (OR, 3.07; 95% CI, 1.71-5.51), and individuals with high intake of fruits (OR, 2.43; 95% CI, 1.46-4.05) or vegetables (OR, 2.39; 95% CI, 1.51-3.81). Our results suggest that telomere length in peripheral leukocyte DNA was associated with H. pylori positivity, cigarette smoking, and dietary fruit intake. Shortened telomeres increased gastric cancer risk in this high-risk Polish population.

Introduction

Telomeres consist of repetitive nucleotide sequences and an associated terminal protein complex that help prevent loss of chromosomal integrity (1). Telomere shortening in genomic DNA appears to reflect lifetime cumulative oxidative stress from environmental exposures, such as smoking, poor nutrition, and chronic inflammation (2-6). Short telomeres are associated with cellular senescence and decreased tissue renewal capacity (7, 8). Telomerase-knockout mouse models, in which animals possess critically short telomeres, exhibit increased cancer rates (9-11). Shortened telomere length were found in human epithelial cancers due to the formation of complex nonreciprocal translocations and increased chromosome instability (12-15). Telomere length measured in either blood leukocyte or buccal cell genomic DNA has been associated with increased risks for bladder, lung, head and neck, and renal cancers (5, 16-20).

Helicobacter pylori infection is a known risk factor for gastric cancer and chronic inflammation is closely linked with telomere length shortening (21, 22). Furthermore, shorter telomere length was identified in H. pylori–positive gastric epithelial tissue (23, 24) and tumor tissues (25, 26) compared with normal gastric tissues. We therefore hypothesized that telomere length in blood leukocyte DNA is associated with H. pylori infection and gastric cancer risk. In the present study, we examined the association between telomere length and gastric cancer risk, the associations between the risk factors and telomere length, and their potential effect modification of these factors on telomere length–related gastric cancer risk.

Materials and Methods

Study Population and Design

The study design has been described elsewhere (27). Briefly, a population-based case-control study was conducted in Warsaw, Poland. Residents ages 21 to 79 years, who were newly diagnosed with gastric adenocarcinoma (International Classification of Diseases for Oncology code 151 or International Classification of Diseases for Oncology, Second Edition code C16) between March 1, 1994, and April 30, 1996, were identified by collaborating physicians in all the 22 hospitals of Warsaw. A total of 72 clinics and endoscopic departments within these hospitals and 8 private endoscopic units were covered. In addition, the population-based Cancer Registry files were reviewed regularly to ensure completeness of case ascertainment. Diagnostic information was abstracted in a standardized manner from hospital records of endoscopy and surgical and pathology reports by a collaborating physician or by the study physician, who visited the hospitals once a month. All pathologic slides were reviewed for confirmation of the diagnosis and standardized reclassification using the Lauren (1965) and WHO criteria (28) by two pathologists, one from Poland and the other from the United States. The final decision on classification of borderline cases was made by a senior U.S. pathologist specializing in gastrointestinal tumor pathology. We only included cases with invasive adenocarcinoma.

Controls were randomly selected among Warsaw residents from a computerized registry of all residents in Poland, the Polish Electronic System of Residence Evidency, and frequency-matched to cases by sex and age in 5-year groups. The system is updated monthly, and completeness of registration is estimated to be ~100%. After written consent was obtained, controls and cases or next of kin of deceased cases were interviewed by trained interviewers to elicit information on demographic background, usual diet before 1990, childhood living conditions, family history of cancer, history of selected medical conditions and medication use, lifetime occupational history, and consumption of cigarettes, alcohol, and other beverages.

An ever-smoker was defined as a smoker of at least 1 cigarette smoked per day for ≥6 months. An ever-drinker was defined as a person drinking at least 1 serving of beer (12 oz.), wine (4 oz.), or liquor (1.5 oz.) per month for ≥6 months. Among smokers and drinkers, information was collected on the age when exposure to each product started and stopped and total years and frequency of use. Pack-years of smoking were calculated as the product of packs of cigarettes smoked per day and total years of smoking. Total drink-years were calculated as the product of yearly frequency and total years of alcohol use. The information on dietary intakes was collected as described previously (29). Briefly, usual frequency of intake of 118 food and beverage items was elicited. Nutrient intake was estimated from the weekly consumption of food items, the average portion size, and the nutrient composition of each food item. Total intake of each nutrient then was summed across all food items. The information on cancer treatment was collected by asking if the patients had obtained radiation or chemotherapy before the blood draw.

Of the 515 eligible gastric adenocarcinoma cases identified, 34 (6.6%) refused and 17 (3.3%) were untraceable or unavailable for other reasons. Interviews were obtained in person for 324 (62.9%) cases and with next-of-kin of 140 (27.2%) cases who died or were too ill to participate. Of the 586 potential controls identified, 37 (6.3%) had moved. Of the 549 remaining subjects, 480 (87.4%) agreed to be interviewed. The predominant reason for noninterview was subject refusal (10.9%). Among the 464 gastric cancer cases and 480 controls included in the study, 345 cases and 442 controls agreed to donate a 30-mL blood sample (30-32). Peripheral leukocyte DNAwas obtained from 305 cases and 427 controls (33-35). In the present study, telomere length measurement was successfully conducted in 300 cases and 416 controls.

The study was approved by the institutional review boards of the U.S. National Cancer Institute and The M.Sklodowska-Curie Memorial Cancer Center and Institute of Oncology. Written informed consent was obtained from all participants.

Measurement of Serum Levels of IgG Antibodies to H. pylori and cagA

Serum levels of IgG antibodies to H. pylori and to the cagA protein were determined by antigen-specific ELISA as described previously (36, 37). We defined individuals who tested negative for both serum IgG antibodies to H. pylori and cagA antibody as H. pylori–negative and those who tested positive for either or both markers as H. pylori–positive subjects.

Telomere Length Measurement by Quantitative PCR

Telomere length was measured in blood leukocyte DNA using quantitative real-time PCR as described by Cawthon (38) and modified by McGrath et al. (19). This method measures relative telomere length (RTL) in genomic DNA by determining the ratio of telomere repeat copy number (T) to single copy gene (S) copy number (T/S ratio) in individual samples relative to a reference pooled DNA. The reference pooled DNA was created using DNA from 60 subjects who were randomly selected from the 416 control subjects of the current study (400 ng from each sample) and used to generate a fresh standard curve ranging from 0.25 to 8 ng/μL in every T and S PCR run. The T (telomere) PCR mix was iQ SYBR Green Supermix (Bio-Rad) 1×, tel 1b 100 nmol/L, tel 2b 900 nmol/L, DMSO 1%, and EDTA 1×. The S (human β-globin) PCR mix was iQ SYBR Green Supermix (Bio-Rad) 1×, hbg1 300 nmol/L, hbg2 700 nmol/L, DMSO 1%, DTT 2.5 mmol/L, and EDTA 1×. After addition of Escherichia coli DNA (Sigma-Aldrich), DNA samples were heated at 96°C for 10 min and then cooled to room temperature. All PCRs were done on a DNA Engine thermal cycler Chromo4 (Bio-Rad). DNA (15 ng) was used in each PCR (final volume 20 μL). The thermal cycling profile for both amplicons began with 95°C incubation for 3 min. The T PCR included 25 cycles of 95°C for 15 s and annealing/extension at 54°C for 49 s. The S PCR included 35 cycles of 95°C for 15 s, annealing at 58°C for 1 s, and extension at 72°C for 15 s. At the end of each reaction, a melting curve was used for both T and S PCRs. All samples were run in duplicate and the mean of two measurements was used in the statistical analyses. The interbatch variability (coefficient of variation) in the present study was 8.1%.

Statistical Analyses

Graphical inspection of RTL distribution separately in cases and controls showed no departures from the normal distribution. RTL data normality was confirmed using the Shapiro-Wilk test. Linear regression models were used to evaluate differences in RTL (continuous-dependent variable in the models) among controls in relation to age at blood draw, gender, H. pylori infection status, family history of cancer, and other oxidative stress-related factors, including smoking, alcohol drinking, and fruit and vegetable consumption. Mean RTL for each of the variables evaluated was estimated using the post-estimation command adjust in Stata 10.0 from models including age, gender, smoking status, and pack-years as independent variables. Unconditional logistic regression was used to estimate odds ratios (OR) for gastric cancer and corresponding 95% confidence intervals (95% CI). Quartile cut-points were based on distributions among controls. All models were adjusted for age, gender, smoking status (nonsmokers, former smokers, and current smokers), and pack-years of smoking (nonsmokers, 0.1-30, and >30). Further adjustment by other potential confounding variables, including education, body mass index, alcohol drinking, caloric intake, intake of fruits, vegetables, sausages, red meats, or preserved vegetables, family history of gastric cancer or other cancers, and H. pylori status, did not alter the risk estimates. Therefore, these variables were not included in the final model. All tests were two-sided and α < 0.05 was considered significant.

Results

Among 345 cases and 442 controls who donated a blood sample, RTL results were available in 300 cases (87.0%) and 416 controls (94.1%). Cases and controls had similar age and sex distributions (Table 1), as they were frequency-matched. When compared with the controls, gastric cancer cases tended to have lower levels of education (P = 0.004) and reported having more first-degree relatives diagnosed with gastric cancer (P < 0.001). Cases also smoked more (P = 0.002), drank more alcohol (P = 0.008), and consumed fewer vegetables (P = 0.02).

Table 1
Characteristics of study subjects

As expected, telomere length shortened significantly in controls with increasing age at blood collection (Ptrend < 0.001). RTL decreased from 1.43 among controls ages ≤61 to 1.28 years among those ages ≥70 years (Table 2). H. pylori–positive controls had significantly shorter telomere length (RTL, 1.32; 95% CI, 1.29-1.36) the H. pylori–negative controls (RTL, 1.42; 95% CI, 1.34-1.51; P = 0.03). Telomere length tended to decrease with increasing pack-years of cigarette smoking (P = 0.02) and decreasing frequencies of fruit intake (P = 0.04). RTL was marginally associated with alcohol consumption (P = 0.06) and level of education (P = 0.05) but not with body mass index, family history of cancer, smoking status, and vegetable intake.

Table 2
Relation of gastric cancer risk factors to telomere length among controls

Gastric cancer cases had significantly shorter telomeres (mean ± SD RTL, 1.25 ± 0.34) than controls (1.34 ± 0.35; P = 0.0008; Table 1). Analyses of RTL in quartiles, based on the distribution in the controls, showed that the risk of gastric cancer was doubled (OR, 2.04; 95% CI, 1.33-3.13) among cases in the lowest quartile of RTL (RTL ≤ 1.13) when compared with those in the highest quartile (RTL > 1.53; Table 3). The risks among those in the second (RTL, 1.14-1.30) and third (RTL, 1.31-1.53) quartiles of RTL were comparable with those in the highest quartile. These three groups therefore were combined into a group with “long” telomere length for the stratified analyses.

Table 3
Gastric cancer risk and telomere length

When stratified by known or potential risk factors, the association between short telomere length and gastric cancer risk tended to be stronger among men and older persons, although the interaction did not reach statistical significance (Table 4). The magnitude of association with short telomere length tended to be stronger among persons with the lowest risk profile, H. pylori–negative individuals (OR, 5.45; 95% CI, 2.10-14.1), nonsmokers (OR, 3.07; 95% CI, 1.71-5.51), and persons with high intake of fruits (OR, 2.43; 95% CI, 1.46-4.05) or vegetables (OR, 2.39; 95% CI, 1.51-3.81). In contrast, the short telomere length– associated gastric cancer risks were 1.78 (95% CI, 1.25-2.54) for H. pylori–positive subjects, 1.40 (95% CI, 0.79-2.45) among current smokers, and 1.73 (95% CI, 0.80-3.76) and 2.00 (95% CI, 0.73-5.50) for those who rarely consumed fruits and vegetables, respectively.

Table 4
Gastric cancer risk and telomere length by known or potential risk factors

Telomere length did not vary significantly by Lauren pathologic classification (67.7% intestinal type, 16.3% diffuse, and 16.0% indeterminate or unknown; P = 0.86) or tumor subsite of origin (11.7% cardia, 72.7% distal, and 15.7% indeterminate or unknown; P = 0.35; data not shown). In addition, excluding patients who received chemotherapy before the blood draw (n = 42) produced no meaningful difference in results, with no changes in statistical significance.

Discussion

In the present population-based investigation, we showed that short telomeres are related to an increased risk of gastric cancer. Among our control population, shortened telomeres were related to several recognized gastric cancer risk factors, including older age, smoking, low fruit intake, and H. pylori positivity, suggesting possible etiologic pathways.

Our observation that telomere length in peripheral leukocyte DNA declined with increasing age is consistent with previous reports (39-43). However, the extent of telomere shortening may vary considerably among individuals within age groups, suggesting that environmental and lifestyle factors could play critical roles in the rate of telomere attrition. Because of high guanine content in specific telomere sequences, telomeres are remarkably sensitive to damage by oxidative stress (44) and telomeric DNA is deficient in the repair of single-strand breaks induced by oxidative DNA damage (3, 45, 46). H. pylori–positive gastric mucosa has been shown to have shorter telomere length than H. pylori–negative mucosa (23, 24). We found that telomere length in leukocyte DNA was also significantly shortened in H. pylori–positive persons compared with those without the organism. This observation provides evidence that carriage of gastric H. pylori, like other persistent microbes (5, 6, 47, 48), can contribute to the progressive shortening of telomeres in blood leukocyte genomic DNA. Infection-induced chronic inflammation increases the turnover of peripheral leukocytes, leading to a greater loss of leukocyte telomere repeats over time. H. pylori infection may also facilitate telomere shortening process by increasing cumulative oxidative stress (3, 4) that has been proposed as a potential mechanism for H. pylori infection-related gastric cancer (49).

Cigarette smoke contains a mixture of compounds that generate reactive oxygen species in hosts (50). In our control population, we found that persons with >30 pack-years of smoking had shorter telomere length than nonsmokers or subjects who smoked less. This finding supports previous observations that telomere length is related to cumulative lifetime cigarette smoke exposure in a dose-related manner (51). Others have shown that age-adjusted telomere length was 5 bp shorter per pack-year of smoking (4).

Treatment with N-acetylcysteine, an antioxidant, has been shown to decrease the telomere attrition rate (52). Fruits and vegetables are important natural antioxidant sources (53), and their high intake has been associated with reduced risk of gastric cancer (29, 50). Our observation of shortened telomere length with decreasing fruit intake among controls suggests that diets rich in antioxidants may decelerate telomere shortening, thus modifying cancer risk (3). Gastric cancer cases had shorter telomeres than the controls in our study. This observation is in agreement with findings from most studies of cancer, including cancers of the bladder, lung, kidney, and head and neck (5, 17, 19, 20), but not with a recent study reporting longer blood telomeres in breast cancer patients than controls (54). Our results lend further support to the hypothesis that telomere shortening in blood leukocyte DNA is a marker of cancer risk (5, 9, 12, 17, 19). Telomere shortening may increase cancer risk through impairment of cellular functions resulting from chromosome instability (13-15, 55) and cell senescence (9, 56).

In our stratified analyses, we observed a higher risk related to telomere length shortening among individuals without some of the risk factors for gastric cancer (nonsmokers, H. pylori infection negative, frequent fruit consumption, or frequent vegetable consumption) compared with individuals with these risk factors. This phenomenon may be, in part, due to competing risks among high-risk individuals, who might have been exposed to multiple risk factors that involve carcinogenic mechanisms other than telomere length shortening. The plausibility of this phenomenon is supported by the similarity of the RTL among our controls with these risk factors (mean RTL, 1.26, 1.32, 1.26, and 1.29 for controls who were heavy smokers, H. pylori infection positive, rarely consumed fruit, and rarely consumed vegetables, respectively) to that seen in cancer cases (mean RTL, 1.25).

Because this is the first study evaluating blood leukocyte genomic DNA telomere shortening in relation to gastric cancer risk, our findings need to be confirmed in future studies, preferably with prospectively collected genomic DNA samples. Despite the efforts to recruit cases immediately after diagnosis, 30% of eligible cases had died before they could be contacted (27). To the extent that telomere shortening might be related to survival, our results might not be generalizable to patients with advanced gastric cancer. However, we observed no consistent pattern when we stratified the results by tumor grade or tumor stage at diagnosis (data not shown), suggesting that survival bias in our study was likely minimal. The study sample size is limited for stratified analyses. Therefore, caution needs to be exercised in interpreting these results. Another potential limitation was the relatively low rate of blood donation among cases (65.7%). However, we found no significant difference in selected demographic or lifestyle characteristics, including age, gender, education, alcohol drinking, smoking status, fresh vegetables and fruits intake, body mass index and family history of gastric cancer, between cases with and without blood donation data (32, 34). In addition, telomere length in peripheral leukocyte DNA may be different from telomere length in gastric tumor tissue in the same patient because of differences in telomerase activity. Although tumor samples were collected from most patients, the quality and quantity of their DNA were insufficient for any informative genetic analyses (57). In the present study, H. pylori infection is not positively associated with gastric cancer risk. The disappearance of H. pylori after cancer diagnosis may partially explain the observed association, especially in populations with relatively high baseline prevalence (58, 59). The H. pylori prevalence is 84.8% among controls in the present study.

In summary, telomere shortening in blood leukocyte genomic DNA in this high-risk Polish population is associated with gastric H. pylori colonization, cigarette smoking, and fruit intake. Shortened telomeres increased gastric cancer risk.

Acknowledgments

Grant support: Intramural Research Program of the Division of Cancer Epidemiology and Genetics, National Cancer Institute/NIH; Diane Belfer Program for Human Microbial Diversity; and NIH grant R01GM63270.

Footnotes

Disclosure of Potential Conflicts of Interest No potential conflicts of interest were disclosed.

References

1. de Lange T. The protein complex that shapes and safeguards human telomeres. Genes Dev. 2005;19:2100–10. [PubMed]
2. Jennings BJ, Ozanne SE, Hales CN. Nutrition, oxidative damage, telomere shortening, and cellular senescence: individual or connected agents of aging? Mol Genet Metab. 2000;71:32–42. [PubMed]
3. von Zglinicki T. Oxidative stress shortens telomeres. Trends Biochem Sci. 2002;27:339–44. [PubMed]
4. Valdes AM, Andrew T, Gardner JP, et al. Obesity, cigarette smoking, and telomere length in women. Lancet. 2005;366:662–4. [PubMed]
5. Wu X, Amos CI, Zhu Y, et al. Telomere dysfunction: a potential cancer predisposition factor. J Natl Cancer Inst. 2003;95:1211–8. [PubMed]
6. Schonland SO, Lopez C, Widmann T, et al. Premature telomeric loss in rheumatoid arthritis is genetically determined and involves both myeloid and lymphoid cell lineages. Proc Natl Acad Sci U S A. 2003;100:13471–6. [PubMed]
7. Harley CB, Futcher AB, Greider CW. Telomeres shorten during ageing of human fibroblasts. Nature. 1990;345:458–60. [PubMed]
8. Vulliamy T, Marrone A, Dokal I, Mason PJ. Association between aplastic anaemia and mutations in telomerase RNA. Lancet. 2002;359:2168–70. [PubMed]
9. Blasco MA. Telomeres and human disease: ageing, cancer and beyond. Nat Rev Genet. 2005;6:611–22. [PubMed]
10. Chang S. Modeling aging and cancer in the telomerase knockout mouse. Mutat Res. 2005;576:39–53. [PubMed]
11. Hande MP, Samper E, Lansdorp P, Blasco MA. Telomere length dynamics and chromosomal instability in cells derived from telomerase null mice. J Cell Biol. 1999;144:589–601. [PMC free article] [PubMed]
12. Artandi SE, Chang S, Lee SL, et al. Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice. Nature. 2000;406:641–5. [PubMed]
13. Bailey SM, Murnane JP. Telomeres, chromosome instability and cancer. Nucleic Acids Res. 2006;34:2408–17. [PMC free article] [PubMed]
14. Cheung AL, Deng W. Telomere dysfunction, genome instability and cancer. Front Biosci. 2008;13:2075–90. [PubMed]
15. Murnane JP. Telomeres and chromosome instability. DNA Repair (Amst) 2006;5:1082–92. [PubMed]
16. Shao L, Wood CG, Zhang D, et al. Telomere dysfunction in peripheral lymphocytes as a potential predisposition factor for renal cancer. J Urol. 2007;178:1492–6. [PubMed]
17. Broberg K, Bjork J, Paulsson K, Hoglund M, Albin M. Constitutional short telomeres are strong genetic susceptibility markers for bladder cancer. Carcinogenesis. 2005;26:1263–71. [PubMed]
18. Widmann TA, Herrmann M, Taha N, Konig J, Pfreundschuh M. Short telomeres in aggressive non-Hodgkin’s lymphoma as a risk factor in lymphomagenesis. Exp Hematol. 2007;35:939–46. [PubMed]
19. McGrath M, Wong JY, Michaud D, Hunter DJ, De Vivo I. Telomere length, cigarette smoking, and bladder cancer risk in men and women. Cancer Epidemiol Biomarkers Prev. 2007;16:815–9. [PubMed]
20. Jang JS, Choi YY, Lee WK, et al. Telomere length and the risk of lung cancer. Cancer Sci. 2008;99:1385–9. [PubMed]
21. Parsonnet J. Helicobacter pylori and gastric cancer. Gastroenterol Clin North Am. 1993;22:89–104. [PubMed]
22. Peek RM, Jr, Blaser MJ. Helicobacter pylori and gastrointestinal tract adenocarcinomas. Nat Rev Cancer. 2002;2:28–37. [PubMed]
23. Aida J, Izumiyama-Shimomura N, Nakamura K, et al. Telomere length variations in 6 mucosal cell types of gastric tissue observed using a novel quantitative fluorescence in situ hybridization method. Hum Pathol. 2007;38:1192–200. [PubMed]
24. Kuniyasu H, Kitadai Y, Mieno H, Yasui W. Helicobacter pylori infection is closely associated with telomere reduction in gastric mucosa. Oncology. 2003;65:275–82. [PubMed]
25. Maruyama Y, Hanai H, Fujita M, Kaneko E. Telomere length and telomerase activity in carcinogenesis of the stomach. Jpn J Clin Oncol. 1997;27:216–20. [PubMed]
26. Maruyama Y, Hanai H, Kaneko E. Telomere length and telomerase activity in intestinal metaplasia, adenoma and well differentiated adenocarcinoma of the stomach. Nippon Rinsho. 1998;56:1186–9. [PubMed]
27. Chow WH, Swanson CA, Lissowska J, et al. Risk of stomach cancer in relation to consumption of cigarettes, alcohol, tea and coffee in Warsaw, Poland. Int J Cancer. 1999;81:871–6. [PubMed]
28. Watanabe H, Jass JR, Sobin LH. Histological typing of esophageal, and gastric tumors. 2. Berlin: Springer; 1990.
29. Lissowska J, Gail MH, Pee D, et al. Diet and stomach cancer risk in Warsaw, Poland. Nutr Cancer. 2004;48:149–59. [PubMed]
30. Wright ME, Andreotti G, Lissowska J, et al. Genetic variation in sodium-dependent ascorbic acid transporters and risk of gastric cancer in Poland. Eur J Cancer. 2009;45:1824–30. [PMC free article] [PubMed]
31. Zhang FF, Hou L, Terry MB, et al. Genetic polymorphisms in alcohol metabolism, alcohol intake and the risk of stomach cancer in Warsaw, Poland. Int J Cancer. 2007;121:2060–4. [PubMed]
32. Zhang FF, Terry MB, Hou L, et al. Genetic polymorphisms in folate metabolism and the risk of stomach cancer. Cancer Epidemiol Biomarkers Prev. 2007;16:115–21. [PubMed]
33. Freedman ND, Ahn J, Hou L, et al. Polymorphisms in estrogenand androgen-metabolizing genes and the risk of gastric cancer. Carcinogenesis. 2009;30:71–7. [PMC free article] [PubMed]
34. Hou L, El-Omar EM, Chen J, et al. Polymorphisms in Th1-type cell-mediated response genes and risk of gastric cancer. Carcinogenesis. 2007;28:118–23. [PubMed]
35. Hou L, Grillo P, Zhu ZZ, et al. COX1 and COX2 polymorphisms and gastric cancer risk in a Polish population. Anticancer Res. 2007;27:4243–7. [PubMed]
36. Chow WH, Blaser MJ, Blot WJ, et al. An inverse relation between cagA+ strains of Helicobacter pylori infection and risk of esophageal and gastric cardia adenocarcinoma. Cancer Res. 1998;58:588–90. [PubMed]
37. Blaser MJ, Perez-Perez GI, Kleanthous H, et al. Infection with Helicobacter pylori strains possessing cagA is associated with an increased risk of developing adenocarcinoma of the stomach. Cancer Res. 1995;55:2111–5. [PubMed]
38. Cawthon RM. Telomere measurement by quantitative PCR. Nucleic Acids Res. 2002;30:e47. [PMC free article] [PubMed]
39. Epel ES, Blackburn EH, Lin J, et al. Accelerated telomere shortening in response to life stress. Proc Natl Acad Sci U S A. 2004;101:17312–5. [PubMed]
40. Fossel M. Telomerase and the aging cell: implications for human health. JAMA. 1998;279:1732–5. [PubMed]
41. Iwama H, Ohyashiki K, Ohyashiki JH, et al. Telomeric length and telomerase activity vary with age in peripheral blood cells obtained from normal individuals. Hum Genet. 1998;102:397–402. [PubMed]
42. Tsuji A, Ishiko A, Takasaki T, Ikeda N. Estimating age of humans based on telomere shortening. Forensic Sci Int. 2002;126:197–9. [PubMed]
43. Aviv A, Valdes AM, Spector TD. Human telomere biology: pitfalls of moving from the laboratory to epidemiology. Int J Epidemiol. 2006;35:1424–9. [PubMed]
44. Houben JM, Moonen HJ, van Schooten FJ, Hageman GJ. Telomere length assessment: biomarker of chronic oxidative stress? Free Radic Biol Med. 2008;44:235–46. [PubMed]
45. Meeker AK, Hicks JL, Iacobuzio-Donahue CA, et al. Telomere length abnormalities occur early in the initiation of epithelial carcinogenesis. Clin Cancer Res. 2004;10:3317–26. [PubMed]
46. Honda S, Hjelmeland LM, Handa JT. Oxidative stress-induced single-strand breaks in chromosomal telomeres of human retinal pigment epithelial cells in vitro. Invest Ophthalmol Vis Sci. 2001;42:2139–44. [PubMed]
47. Forsyth NR, Evans AP, Shay JW, Wright WE. Developmental differences in the immortalization of lung fibroblasts by telomerase. Aging Cell. 2003;2:235–43. [PubMed]
48. Von Zglinicki T. Replicative senescence and the art of counting. Exp Gerontol. 2003;38:1259–64. [PubMed]
49. Correa P. Does Helicobacter pylori cause gastric cancer via oxidative stress? Biol Chem. 2006;387:361–4. [PubMed]
50. van der Vaart H, Postma DS, Timens W, ten Hacken NH. Acute effects of cigarette smoke on inflammation and oxidative stress: a review. Thorax. 2004;59:713–21. [PMC free article] [PubMed]
51. Morla M, Busquets X, Pons J, Sauleda J, MacNee W, Agusti AG. Telomere shortening in smokers with and without COPD. Eur Respir J. 2006;27:525–8. [PubMed]
52. Voghel G, Thorin-Trescases N, Farhat N, et al. Chronic treatment with N-acetyl-cysteine delays cellular senescence in endothelial cells isolated from a subgroup of atherosclerotic patients. Mech Ageing Dev. 2008;129:261–70. [PubMed]
53. Boots AW, Haenen GR, Bast A. Health effects of quercetin: from antioxidant to nutraceutical. Eur J Pharmacol. 2008;585:325–37. [PubMed]
54. Svenson U, Nordfjall K, Stegmayr B, et al. Breast cancer survival is associated with telomere length in peripheral blood cells. Cancer Res. 2008;68:3618–23. [PubMed]
55. Charames GS, Bapat B. Genomic instability and cancer. Curr Mol Med. 2003;3:589–96. [PubMed]
56. Sastry PS, Parikh P. The earlier age of onset of malignancy in developing world is related to overall infection burden and could be due to the effect on telomere length. Med Hypotheses. 2003;60:573–4. [PubMed]
57. Lan Q, Chow WH, Lissowska J, et al. Glutathione S-transferase genotypes and stomach cancer in a population-based case-control study in Warsaw, Poland. Pharmacogenetics. 2001;11:655–61. [PubMed]
58. Correa P. Helicobacter pylori infection and gastric cancer. Cancer Epidemiol Biomarkers Prev. 2003;12:238–41s. [PubMed]
59. D’Elios MM, Appelmelk BJ, Amedei A, Bergman MP, Del Prete G. Gastric autoimmunity: the role of Helicobacter pylori and molecular mimicry. Trends Mol Med. 2004;10:316–23. [PubMed]