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A genetic variant in the promoter of APE1 gene (− 656 T > G) is associated with breast cancer risk and progression in a Chinese population
GENE-38953; No. of pages: 4; 4C: Gene xxx (2013) xxx–xxx
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Gene journal homepage: www.elsevier.com/locate/gene
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A genetic variant in the promoter of APE1 gene (− 656 T N G) is associated with breast cancer risk and progression in a Chinese population
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Methods paper
Huafeng Kang a,⁎,1, Zhijun Dai a,1, Xiaobin Ma a,⁎,1, Li Ma b, Yaofeng Jin b, Xiaoxu Liu a, Xijing Wang a a b
Department of Oncology, the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University, Xi'an, China Department of Pathology, the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University, Xi'an, China
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Background/aims: APE1 is an important DNA repair protein in the base excision repair pathway. Genetic variations in APE1 have been suggested to influence individuals' susceptibility to human malignancies. The present study was aimed to investigate the associations between two functional polymorphisms in APE1 (−656 TNG and 1349 TNG) and breast cancer risk. Methods: We genotyped the two polymorphisms in a case-control study of 500 breast cancer patients and 799 age-matched cancer-free controls using the TaqMan method. Unconditional logistic regression adjusted for potential confounding factors was used to assess the associations. Results: We found that the variant genotypes of the −656 TNG were significantly associated with decreased breast cancer risk, compared with the wild genotype [TG/GG vs. TT: adjusted odds ratio (OR) = 0.71, 95% confidence interval (CI) = 0.56–0.91], and the protective effect of this polymorphism was more predominant among the subgroups of younger subjects (b52 years) (OR = 0.65, 95% CI = 0.46–0.92). Besides, we found that the variant genotypes were associated with less frequent lymph node metastasis (P = 0.020, OR = 0.64, 95% CI = 0.44–0.94). We did not observe any significant association between the 1349 TNG polymorphism and breast cancer risk. Conclusion: Our results suggest that the APE1 −656 TNG but not the 1349 TNG polymorphism may influence the susceptibility and progression of breast cancer in the Chinese population. Large population-based prospective studies are required to validate these findings. © 2013 Elsevier B.V. All rights reserved.
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Article history: Accepted 14 August 2013 Available online xxxx
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Keywords: APE1 Polymorphism Breast cancer Susceptibility
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1. Introduction
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Breast cancer is the most common malignancy affecting women worldwide and its incidence rate is increasing in both developed and developing countries in recent years (Jemal et al., 2011). The etiology of breast cancer has not been completely identified yet, but is thought to be multifactorial, with both environmental and genetic factors (Lichtenstein et al., 2000). Human DNA repair systems play an important role in protecting the genome from DNA damage caused by endogenous and environmental agents (Wood et al., 2005). There are five major DNA repair pathways: direct repair, base excision repair (BER), nucleotide excision repair, mismatch repair, and double-strand break repair; among these repair systems, BER pathway is responsible for repairing small lesions such
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Abbreviations: SNP, single nucleotide polymorphism; OR, odds ratio; CI, confidence interval; BER, base excision repair; APE1, AP endonuclease 1. ⁎ Corresponding authors. Tel.: +86 29 8767 9226; fax: +86 29 8767 9282. E-mail addresses: [email protected] (H. Kang), [email protected] (X. Ma). 1 These authors contributed equally to this work.
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as oxidative damage, alkylation, or methylation (Hoeijmakers, 2001; Wilson and Bohr, 2007). AP endonuclease 1 (APE1), also known as APE, APEX, HAP1, or REF-1, is an important DNA repair protein in the BER pathway which acts as a 3′-phosphodiesterase to initiate repair of DNA single strand breaks producing either directly by reactive oxygen species or in directly through the enzymatic removal of damaged bases (Barzilay and Hickson, 1995). It also functions as a redox agent maintaining transcription factors involved in cancer promotion and progression in an active reduced state and is considered as a promising tool for anticancer therapy (Tell et al., 2010). Genetic polymorphisms in DNA repair genes have been shown to influence the susceptibility to various carcinomas (Goode et al., 2002; Smith et al., 2003; Thompson et al., 2012). Up to data, a number of epidemiological studies have suggested that genetic polymorphisms in APE1 were associated with the risk of cancer (Cao et al., 2011; Gu et al., 2009). The human APE1 gene is located on chromosome 14q11.2–q12 and consists of five exons spanning 2.21 kb (Xi et al., 2004). A total of 18 single nucleotide polymorphisms (SNP) in APE1 have been identified, but the most extensively studied polymorphisms are the 1349 TNG (rs1130409) in the fifth exon and the −656 TNG (rs1760944) in the promoter region. Gu et al. have conducted a meta-analysis on the association between the 1349 TNG polymorphism and cancer risk and suggested the
0378-1119/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gene.2013.08.052
Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
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2. Materials and methods
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2.1. Ethics statement
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The study was approved by the Second Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China. At recruitment, written informed consent was obtained from all participants involved in this study.
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2.4. Statistical analyses
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SNP allele frequencies in the controls were tested against departure from the Hardy-Weinberg equilibrium (HWE) using a goodness-of-fit χ2-test before analysis. Differences in the distributions of categorical variables such as smoking and drinking status and the frequencies of genotypes of the −656 TNG and 1349 TNG polymorphisms between the cases and controls were evaluated using the χ2-test, and continuous variables such as age using the Student's t-test. The associations between the genotypes of the APE1 −656 TNG and 1349 TNG polymorphisms and risk of breast cancer and patients' clinical characteristics were estimated by computing odds ratios (ORs) and 95% confidence intervals (CIs) from unconditional logistic regression analysis with the adjustment for age and age at menarche. P b 0.05 was considered statistically significant, and all statistical tests were two sided. All of the statistical analyses were performed with the software SAS 9.1.3 (SAS Institute, Cary, NC, USA).
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polymorphism is a low-penetrance risk factor for cancer development (Gu et al., 2009). However, in a latter meta-analysis with a larger sample size performed by Zhou et al., they concluded that the APE1 −656 TNG polymorphism has a possible protective effect on cancer risk whereas the 1349 TNG polymorphism did not contribute to the development of cancer (Zhou et al., 2011). There are also several studies that have investigated the association between the APE1 1349 TNG polymorphism and breast cancer risk; and the results from these are inconclusive. Recently, Wu et al. conducted a meta-analysis including 5 studies on this issue and suggested the APE1 1349 TNG polymorphism was not associated with breast cancer risk (Wu et al., 2012); however, only one of the studies was conducted in Asian population (Sangrajrang et al., 2008). As to the −656 TNG polymorphism, there is still a lack of study on its association with breast cancer risk. Herein, in the present study, we investigated the association of the two APE1 SNPs (−656 TNG and 1349 TNG) with breast cancer risk in a case-control study in a Chinese population.
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3. Results
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Our study consisted of 500 breast cancer patients which were consecutively recruited between Jan 2010 and September 2012 at the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University. The Cases were recruited without the restriction of age. All of the patients were pathologically confirmed, sporadic breast cancer. Those patients that received chemotherapy or radiotherapy before surgery or had other type of cancer were excluded from the present study. For comparison, 799 cancer-free controls were recruited from subjects who were seeking health care in the outpatient departments at the hospital and were frequency-matched to the cases on age (±5 years). Before recruitment, a standard questionnaire was administered through faceto-face interviews by trained interviewers to obtain information on demographic data and related factors. The clinical information of the patients group and the demographic characteristics of both the cases and controls were present in Table 1.
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2.3. Genotyping
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Genomic DNA was isolated and purified from leucocytes of peripheral blood by proteinase K digestion and phenol/chloroform extraction. Genotyping of the two APE1 polymorphisms was performed using the predesigned TaqMan probe assay (Applied Biosystems, Foster City, CA, USA). The sequences of primer and probe for the two SNPs are available on request. The reaction mixture of 10 μL contained 20 ng genomic DNA, 3.5 μL of 2×TaqMan Genotyping Master Mix, 0.25 μL of the primers and probes mix and 6.25 μL of double distilled water. The amplification was performed under the following conditions: 50 ºC for 2 min, 95 ºC for 10 min followed by 45 cycles of 95 ºC for 15 s, and 60 ºC for 1 min. Amplifications were performed in the 384-well ABI 7900HT Real Time PCR System (Applied Biosystems), following the manufacturer's instructions. After the completion of the amplification, the fluorescence intensity in each well of the plate was read and analyzed with SDS 2.4 automated software. Four blank controls were included in each plate to ensure accuracy of the genotyping. About 10% of the samples were randomly selected for repeated assays, and the results were in agreement with the results of the first assays.
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Allele and genotype distributions of the APE1 −656 TNG and 1349 TNG polymorphisms are presented in Table 2. The observed genotype frequencies of the two SNPs in the controls were in agreement with the HWE (P = 0.294 for the −656 TNG polymorphism and P = 0.199 for the 1349 TNG polymorphism). We found that the distributions of the APE1 −656 TNG genotypes were significantly different between cases and controls (P = 0.012). Comparing with subjects with the − 656 TT genotype, individuals with the variant GG genotype had a significant reduced breast cancer risk (adjusted OR = 0.63, 95% CI = 0.45– 0.88). Furthermore, in a recessive model, the −656 TG/GG genotypes were also significantly associated with a decreased risk for breast cancer (OR = 0.71, 95% CI = 0.56–0.91). However, we did not observe significant associations between the 1349 TNG polymorphism and breast cancer risk in any comparison, as shown in Table 2.
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Table 1 Distributions of select variables in breast cancer cases and cancer-free controls.
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3.1. Association between APE1 polymorphisms and risk of breast cancer
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Control (N = 799), %
Pa
Age at diagnosis or recruitment (year) Mean ± SD 52.0 ± 11.0 b52 239 51.4 ≥52 226 48.6
50.8 ± 13.2 437 54.7 362 45.3
0.110 0.257
Age at menarche (year) Mean ± SD b14 ≥14
14.89 ± 1.76 183 22.9 616 77.1
0.002 0.012
Variables
Tumor size Less than 2 cm 2 to 5 cm More than 5 cm LN involvement Negative Positive ER Negative Positive PR Negative Positive HER-2 Negative Positive a
Case (N = 465), %
14.55 ± 1.84 136 29.3 329 70.7 126 310 29
27.1 66.7 6.2
251 214
54.0 46.0
162 303
34.8 65.2
209 256
44.9 55.1
309 156
66.4 33.6
T-test or two-sided χ2-test.
Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
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t1:3 t1:4 t1:5 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21 t1:22 t1:23 t1:24 t1:25 t1:26 t1:27 t1:28 t1:29
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−656 TNG TT TG GG TG+GG T G 1349 TNG TT TG GG TG+GG T G
t2:20 t2:21
a
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%
OR (95% CI)b
%
180 207 78 285 567 363
38.7 44.5 16.8 61.3 61.0 39.0
248 381 170 551 877 721
31.0 47.7 21.3 69.0 54.9 45.1
165 211 89 300 541 389
35.5 45.4 19.1 64.5 58.2 41.8
276 372 151 523 924 674
34.5 46.6 18.9 65.5 57.8 42.2
0.012
0.917
1.00 (Reference) 0.95 (0.73–1.22) 0.95 (0.69–0.32) 0.95 (0.75–0.131) 1.00 (Reference) 0.99 (0.84–1.16)
0.735 0.864
Two-sided χ2 test for the distributions of genotype and allele frequencies. Adjusted for age and age at menarche.
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3.2. Stratified analysis of the APE1 −656 TNG polymorphism and risk of breast cancer
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We then evaluated the effect of the APE1 −656 TNG polymorphism on breast cancer stratified by age. As shown in Table 3, the protect effect of the APE1 −656 TG+GG genotypes was more pronounced in younger subjects (P = 0.015, OR = 0.65, 95% CI = 0.46–0.92) rather than old subjects (P = 0.153, OR = 0.80, 95% CI = 0.56–1.15), suggesting the younger individuals could benefit more from carrying the genotype. The same analyses were also performed for the APE1 1349 TNG polymorphism; however, no positive result was observed (data not shown).
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t3:1 t3:2 t3:3 t3:4
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3.3. Association between the APE1 polymorphisms and clinical parameters of breast cancer patients
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To determine whether the APE1 polymorphisms have effect on the clinical features of breast cancer patients, we then analyzed the association between the APE1 polymorphisms and a series of clinicopathologic parameters, including tumor size, clinical stage, lymph node metastasis, and the statuses of ER, PR and Her-2. As shown in Table 4, we found that the frequency of the variant genotypes (TG/GG) was significantly lower in the patients with lymph node involvement (P = 0.020, OR = 0.64, 95% CI = 0.44–0.94). No other significant association between the APE1 polymorphisms and the clinical features was found in the present study.
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Table 3 Stratification analyses on age between APE1 −656 TNG genotypes and risk of −656 TNG breast cancer. The −656 TNG genotypes
t3:5 t3:6 t3:7 t3:8 t3:9 t3:10 t3:11
Age b52 TT TG+GG Age ≥52 TT TG+GG
t3:12 t3:13
a b
Pa
OR (95% CI)b
Case (N = 465)
Control (N = 799)
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%
N
%
92 147
38.5 61.5
128 309
29.3 70.7
0.015
1.00 (Reference) 0.65 (0.46–0.92)
88 138
38.9 61.1
120 242
33.2 66.9
0.153
1.00 (Reference) 0.80 (0.56–1.15)
Two-sided χ2 test for the distributions of genotype frequencies. Adjusted for age and age at menarche.
Pa
OR (95% CI) b
TG/GG (N, %)
47 (62.7) 381 (33.5)
79 (37.3) 757 (66.5)
0.390
1.00 (Reference) 1.18 (0.80–1.73)
85 (33.9) 95 (44.4)
166 (66.1) 119 (55.6)
0.020
1.00 (Reference) 0.64 (0.44–0.94)
63 (38.9) 117 (38.6)
99 (61.1) 186 (61.4)
0.954
1.00 (Reference) 1.11 (0.99–1.24)
84 (40.2) 96 (37.5)
125 (59.8) 160 (62.5)
0.553
1.00 (Reference) 1.13 (0.78–1.65)
0.258
1.00 (Reference) 0.79 (0.53–1.18)
114 (36.9) 66 (42.3)
195 (63.1) 90 (57.7)
t4:4 t4:5
TT (N, %)
Two-sided χ2 test for the distributions of genotype and allele frequencies. Adjusted for tumor size, lymph node involvement, ER, PR and HER-2 status.
4. Discussion
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165 166
168 169
Tumor size Less than 2 cm More than 2 cm LN involvement Negative Positive ER Negative Positive PR Negative Positive HER-2 Negative Positive
1.00 (Reference) 0.75 (0.58–0.97) 0.63 (0.45–0.88) 0.71 (0.56–0.91) 1.00 (Reference) 0.78 (0.66–0.92)
0.005 0.003
APE1 −656 TNG genotypes
Variables
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Pa
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t2:6 t2:7 t2:8 t2:9 t2:10 t2:11 t2:12 t2:13 t2:14 t2:15 t2:16 t2:17 t2:18 t2:19
Control (N = 799)
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Case (N = 465)
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Polymorphisms
In the present study, we found a significant association between the APE1 −656 TNG polymorphism and breast cancer risk. Compared with individuals with TT genotype, those with GG or GG/TG genotypes had a significantly decreased breast cancer risk of 0.63 and 0.71, respectively; and the younger subjects could benefit more from carrying the genotypes. Besides, we also found that patients with breast cancer carrying the GG/TG genotypes had a significantly less frequency of lymph node involvement, suggesting the variant genotypes of this polymorphism may play a protective role during the progression of breast cancer. To the best of our knowledge, this is the first study to address the role of the APE1 −656 TNG polymorphism in the development and progression of breast cancer. Apurinic/apyrimidinic (AP) sites are common mutagenic and cytotoxic DNA lesions that are caused by the loss of normal bases (Lindahl, 1995). APE1 has been considered as the major AP endonuclease that is involved in the repair of AP sites (Izumi et al., 2000). Aberrant expression of APE1 gene may lead to defects in repairing these lesions and confer individuals' susceptibility to the cancer. For instance, there are studies that suggested polymorphisms in APE1 may regulate its expression and influence individual's susceptibility to cancer (Lo et al., 2009; Lu et al., 2009). Epidemiological studies have now linked genetic polymorphisms in APE1 with the risk various types of cancer including lung cancer, breast cancer, colorectal cancer and bladder cancer (Gu et al., 2009). The most widely studied polymorphisms in APE1 is the 1349 TNG which locates in the fifth exon of the gene. Functional studies have demonstrated that APE1 harboring the 1349G allele had altered endonuclease and DNA-binding activity, reduced ability to communicate with other BER proteins and decreased capacity to repair DNA oxidative damage (Au et al., 2003; Hadi et al., 2000). Gu et al. reported in their meta-analysis that the APE1 1349 TNG polymorphism was a lowpenetrance risk factor for cancer development (Gu et al., 2009); however, in a recently conducted meta-analysis which excluded four studies that did not conform to HWE and included four additional studies, Zhou et al. reported that the APE1 1349 TNG polymorphism did not contribute to genetic susceptibility to cancer (Zhou et al., 2011). For breast cancer, a meta-analysis including five studies also denied a significant association between the APE1 1349 TNG polymorphism and breast cancer risk (Wu et al., 2012). Our results were consistent with these studies, which, taken together with the above results, suggest that the APE1 1349 TNG polymorphism is not likely to be a susceptibility factor for breast cancer.
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Table 4 t4:1 The associations between The APE1 −656 TNG polymorphism and clinical characteristics t4:2 of breast cancer patients. t4:3
Table 2 Genotype and allele frequencies of the APE1 polymorphisms among the cases and controls and the associations with risk the breast cancer.
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t2:1 t2:2 t2:3
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Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
t4:6 t4:7 t4:8 t4:9 t4:10 t4:11 t4:12 t4:13 t4:14 t4:15 t4:16 t4:17 t4:18 t4:19 t4:20 t4:21 t4:22
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In conclusion, our case-control study indicates that the APE1 − 656 TNG but not 1349 TNG polymorphism has a significant influence on the occurrence and progression of breast cancer in the Chinese population. To our knowledge, this is the first report regarding the role of the APE1 −656 TNG polymorphism in breast cancer. Although the association appeared to be statistically significant in our population, the initial findings should be independently verified by other large independent population-base studies.
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Au, W.W., Salama, S.A., Sierra-Torres, C.H., 2003. Functional characterization of polymorphisms in DNA repair genes using cytogenetic challenge assays. Environ. Health Perspect. 111 (15), 1843–1850 (doi:sc271_5_1835 [pii]). Barzilay, G., Hickson, I.D., 1995. Structure and function of apurinic/apyrimidinic endonucleases. Bioessays 17 (8), 713–719. http://dx.doi.org/10.1002/bies.950170808. Cao, Q., et al., 2011. Genetic polymorphisms in APE1 are associated with renal cell carcinoma risk in a Chinese population. Mol. Carcinog. 50 (11), 863–870. http:// dx.doi.org/10.1002/mc.20791. Goode, E.L., Ulrich, C.M., Potter, J.D., 2002. Polymorphisms in DNA repair genes and associations with cancer risk. Cancer Epidemiol. Biomarkers Prev. 11 (12), 1513–1530. Gu, D., Wang, M., Zhang, Z., Chen, J., 2009. The DNA repair gene APE1 T1349G polymorphism and cancer risk: a meta-analysis of 27 case-control studies. Mutagenesis 24 (6), 507–512 (doi:gep036 [pii] http://dx.doi.org/10.1093/mutage/gep036). Hadi, M.Z., Coleman, M.A., Fidelis, K., Mohrenweiser, H.W., Wilson III, D.M., 2000. Functional characterization of Ape1 variants identified in the human population. Nucleic Acids Res. 28 (20), 3871–3879. Hoeijmakers, J.H., 2001. Genome maintenance mechanisms for preventing cancer. Nature 411 (6835), 366–374. http://dx.doi.org/10.1038/35077232 (35077232 [pii]). Izumi, T., et al., 2000. Requirement for human AP endonuclease 1 for repair of 3′-blocking damage at DNA single-strand breaks induced by reactive oxygen species. Carcinogenesis 21 (7), 1329–1334. Jemal, A., Bray, F., Center, M.M., Ferlay, J., Ward, E., Forman, D., 2011. Global cancer statistics. CA Cancer J. Clin. 61 (2), 69–90 (doi:caac.20107 [pii] http://dx.doi.org/10.3322/ caac.20107). Lichtenstein, P., et al., 2000. Environmental and heritable factors in the causation of cancer–analyses of cohorts of twins from Sweden, Denmark, and Finland. N. Engl. J. Med. 343 (2), 78–85. http://dx.doi.org/10.1056/NEJM200007133430201. Lindahl, T., 1995. Recognition and processing of damaged DNA. J. Cell Sci. Suppl. 19, 73–77. Lo, Y.L., et al., 2009. A polymorphism in the APE1 gene promoter is associated with lung cancer risk. Cancer Epidemiol. Biomarkers Prev. 18 (1), 223–229 (doi:18/1/223 [pii] http://dx.doi.org/10.1158/1055-9965.EPI-08-0749). Lu, J., et al., 2009. Functional characterization of a promoter polymorphism in APE1/Ref-1 that contributes to reduced lung cancer susceptibility. FASEB J. 23 (10), 3459–3469 (doi:fj.09-136549 [pii] http://dx.doi.org/10.1096/fj.09-136549). Sangrajrang, S., et al., 2008. Polymorphisms in three base excision repair genes and breast cancer risk in Thai women. Breast Cancer Res. Treat. 111 (2), 279–288. http:// dx.doi.org/10.1007/s10549-007-9773-7. Smith, T.R., et al., 2003. DNA-repair genetic polymorphisms and breast cancer risk. Cancer Epidemiol. Biomarkers Prev. 12 (11 Pt 1), 1200–1204. Tell, G., Fantini, D., Quadrifoglio, F., 2010. Understanding different functions of mammalian AP endonuclease (APE1) as a promising tool for cancer treatment. Cell. Mol. Life Sci. 67 (21), 3589–3608. http://dx.doi.org/10.1007/s00018-010-0486-4. Thompson, E.R., et al., 2012. Exome sequencing identifies rare deleterious mutations in DNA repair genes FANCC and BLM as potential breast cancer susceptibility alleles. PLoS Genet. 8 (9), e1002894. http://dx.doi.org/10.1371/journal.pgen.1002894 (PGENETICS-D-12-00361 [pii]). Wilson III, D.M., Bohr, V.A., 2007. The mechanics of base excision repair, and its relationship to aging and disease. DNA Repair (Amst) 6 (4), 544–559. Wood, R.D., Mitchell, M., Lindahl, T., 2005. Human DNA repair genes, 2005. Mutat. Res. 577 (1–2), 275–283 (doi:S0027-5107(05)00163-6 [pii] http://dx.doi.org/10.1016/j. mrfmmm.2005.03.007). Wu, B., Liu, H.L., Zhang, S., Dong, X.R., Wu, G., 2012. Lack of an association between two BER gene polymorphisms and breast cancer risk: a meta-analysis. PLoS One 7 (12), e50857. http://dx.doi.org/10.1371/journal.pone.0050857 (PONE-D-1215192 [pii]). Xi, T., Jones, I.M., Mohrenweiser, H.W., 2004. Many amino acid substitution variants identified in DNA repair genes during human population screenings are predicted to impact protein function. Genomics 83 (6), 970–979. http://dx.doi.org/10.1016/ j.ygeno.2003.12.016 (S0888754304000229 [pii]). Zhou, B., et al., 2011. The association of APE1–656 TNG and 1349 TNG polymorphisms and cancer risk: a meta-analysis based on 37 case-control studies. BMC Cancer 11, 521 (doi:1471-2407-11-521 [pii] http://dx.doi.org/10.1186/1471-2407-11-521).
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The APE1 −656 TNG is another widely studied polymorphism which was suggested to influence the gene expression. In lung cancer, Lo et al. found that the variant genotypes were associated with a significantly decreased lung cancer risk; in further functional studies, they demonstrated that the −656 G allele had a significantly higher transcriptional activity than that of the APE1 −656 T allele, which indicated that the “higher production” genotype for APE1 may offer protection against the development of lung cancer (Lo et al., 2009). In the meta-analysis conducted by Zhou et al., they reported that the −656 TNG polymorphism has a possible protective effect on cancer risk particularly among Asian populations. However, up to date, there still lack of study investigating the role of −656 TNG polymorphism in breast cancer risk. In the present study, we found that the −656 TNG polymorphism was significantly associated with breast cancer risk. Comparing with individuals with −656TT genotype, those carrying the −656TG/GG genotypes had a 0.71-fold decreased breast cancer risk. Accumulation of DNA lesions also involved in the development of breast cancer. Considering the aforementioned view that the −656G allele may be harboring a “higher production” of APE1, it is biologically plausible that the APE1 − 656TG/GG genotypes are associated with a decreased breast cancer risk. In the subgroup analysis, we also found that individuals younger than 52 years of age could benefit more from carrying the APE1 − 656TG/GG genotypes. As it is known, more DNA lesions will occur as aging; thus the small effect of the genotype may be overwhelmed by the accumulation of more DNA lesions in elders. In addition, we also observed that the APE1 −656TG/GG genotypes was associated with less frequent lymph node involvement of breast cancer, but the results should be interpreted cautiously since there is the possibility that the association may due to a late stage at diagnosis. Nevertheless, if confirmed by additional studies, this polymorphism may help to accurately predict the clinical course of breast cancer. When interpreting our results, a limitation should be concerned. Since our study was hospital-based design, we could not rule out the possible of selection bias of subjects that may have been associated with a particular genotype. However, the genotype distributions of APE1 polymorphisms in our study population all conformed to HWE that suggested the selection bias in terms of genotype distribution would not be substantial.
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A genetic variant in the promoter of APE1 gene (− 656 T N G) is associated with breast cancer risk and progression in a Chinese population
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Methods paper
Huafeng Kang a,⁎,1, Zhijun Dai a,1, Xiaobin Ma a,⁎,1, Li Ma b, Yaofeng Jin b, Xiaoxu Liu a, Xijing Wang a a b
Department of Oncology, the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University, Xi'an, China Department of Pathology, the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University, Xi'an, China
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Background/aims: APE1 is an important DNA repair protein in the base excision repair pathway. Genetic variations in APE1 have been suggested to influence individuals' susceptibility to human malignancies. The present study was aimed to investigate the associations between two functional polymorphisms in APE1 (−656 TNG and 1349 TNG) and breast cancer risk. Methods: We genotyped the two polymorphisms in a case-control study of 500 breast cancer patients and 799 age-matched cancer-free controls using the TaqMan method. Unconditional logistic regression adjusted for potential confounding factors was used to assess the associations. Results: We found that the variant genotypes of the −656 TNG were significantly associated with decreased breast cancer risk, compared with the wild genotype [TG/GG vs. TT: adjusted odds ratio (OR) = 0.71, 95% confidence interval (CI) = 0.56–0.91], and the protective effect of this polymorphism was more predominant among the subgroups of younger subjects (b52 years) (OR = 0.65, 95% CI = 0.46–0.92). Besides, we found that the variant genotypes were associated with less frequent lymph node metastasis (P = 0.020, OR = 0.64, 95% CI = 0.44–0.94). We did not observe any significant association between the 1349 TNG polymorphism and breast cancer risk. Conclusion: Our results suggest that the APE1 −656 TNG but not the 1349 TNG polymorphism may influence the susceptibility and progression of breast cancer in the Chinese population. Large population-based prospective studies are required to validate these findings. © 2013 Elsevier B.V. All rights reserved.
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Article history: Accepted 14 August 2013 Available online xxxx
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Keywords: APE1 Polymorphism Breast cancer Susceptibility
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1. Introduction
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Breast cancer is the most common malignancy affecting women worldwide and its incidence rate is increasing in both developed and developing countries in recent years (Jemal et al., 2011). The etiology of breast cancer has not been completely identified yet, but is thought to be multifactorial, with both environmental and genetic factors (Lichtenstein et al., 2000). Human DNA repair systems play an important role in protecting the genome from DNA damage caused by endogenous and environmental agents (Wood et al., 2005). There are five major DNA repair pathways: direct repair, base excision repair (BER), nucleotide excision repair, mismatch repair, and double-strand break repair; among these repair systems, BER pathway is responsible for repairing small lesions such
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Abbreviations: SNP, single nucleotide polymorphism; OR, odds ratio; CI, confidence interval; BER, base excision repair; APE1, AP endonuclease 1. ⁎ Corresponding authors. Tel.: +86 29 8767 9226; fax: +86 29 8767 9282. E-mail addresses: [email protected] (H. Kang), [email protected] (X. Ma). 1 These authors contributed equally to this work.
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as oxidative damage, alkylation, or methylation (Hoeijmakers, 2001; Wilson and Bohr, 2007). AP endonuclease 1 (APE1), also known as APE, APEX, HAP1, or REF-1, is an important DNA repair protein in the BER pathway which acts as a 3′-phosphodiesterase to initiate repair of DNA single strand breaks producing either directly by reactive oxygen species or in directly through the enzymatic removal of damaged bases (Barzilay and Hickson, 1995). It also functions as a redox agent maintaining transcription factors involved in cancer promotion and progression in an active reduced state and is considered as a promising tool for anticancer therapy (Tell et al., 2010). Genetic polymorphisms in DNA repair genes have been shown to influence the susceptibility to various carcinomas (Goode et al., 2002; Smith et al., 2003; Thompson et al., 2012). Up to data, a number of epidemiological studies have suggested that genetic polymorphisms in APE1 were associated with the risk of cancer (Cao et al., 2011; Gu et al., 2009). The human APE1 gene is located on chromosome 14q11.2–q12 and consists of five exons spanning 2.21 kb (Xi et al., 2004). A total of 18 single nucleotide polymorphisms (SNP) in APE1 have been identified, but the most extensively studied polymorphisms are the 1349 TNG (rs1130409) in the fifth exon and the −656 TNG (rs1760944) in the promoter region. Gu et al. have conducted a meta-analysis on the association between the 1349 TNG polymorphism and cancer risk and suggested the
0378-1119/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gene.2013.08.052
Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
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2. Materials and methods
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2.1. Ethics statement
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The study was approved by the Second Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China. At recruitment, written informed consent was obtained from all participants involved in this study.
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2.4. Statistical analyses
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SNP allele frequencies in the controls were tested against departure from the Hardy-Weinberg equilibrium (HWE) using a goodness-of-fit χ2-test before analysis. Differences in the distributions of categorical variables such as smoking and drinking status and the frequencies of genotypes of the −656 TNG and 1349 TNG polymorphisms between the cases and controls were evaluated using the χ2-test, and continuous variables such as age using the Student's t-test. The associations between the genotypes of the APE1 −656 TNG and 1349 TNG polymorphisms and risk of breast cancer and patients' clinical characteristics were estimated by computing odds ratios (ORs) and 95% confidence intervals (CIs) from unconditional logistic regression analysis with the adjustment for age and age at menarche. P b 0.05 was considered statistically significant, and all statistical tests were two sided. All of the statistical analyses were performed with the software SAS 9.1.3 (SAS Institute, Cary, NC, USA).
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polymorphism is a low-penetrance risk factor for cancer development (Gu et al., 2009). However, in a latter meta-analysis with a larger sample size performed by Zhou et al., they concluded that the APE1 −656 TNG polymorphism has a possible protective effect on cancer risk whereas the 1349 TNG polymorphism did not contribute to the development of cancer (Zhou et al., 2011). There are also several studies that have investigated the association between the APE1 1349 TNG polymorphism and breast cancer risk; and the results from these are inconclusive. Recently, Wu et al. conducted a meta-analysis including 5 studies on this issue and suggested the APE1 1349 TNG polymorphism was not associated with breast cancer risk (Wu et al., 2012); however, only one of the studies was conducted in Asian population (Sangrajrang et al., 2008). As to the −656 TNG polymorphism, there is still a lack of study on its association with breast cancer risk. Herein, in the present study, we investigated the association of the two APE1 SNPs (−656 TNG and 1349 TNG) with breast cancer risk in a case-control study in a Chinese population.
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Our study consisted of 500 breast cancer patients which were consecutively recruited between Jan 2010 and September 2012 at the Second Affiliated Hospital, Medical School of Xi'an Jiaotong University. The Cases were recruited without the restriction of age. All of the patients were pathologically confirmed, sporadic breast cancer. Those patients that received chemotherapy or radiotherapy before surgery or had other type of cancer were excluded from the present study. For comparison, 799 cancer-free controls were recruited from subjects who were seeking health care in the outpatient departments at the hospital and were frequency-matched to the cases on age (±5 years). Before recruitment, a standard questionnaire was administered through faceto-face interviews by trained interviewers to obtain information on demographic data and related factors. The clinical information of the patients group and the demographic characteristics of both the cases and controls were present in Table 1.
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2.3. Genotyping
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Genomic DNA was isolated and purified from leucocytes of peripheral blood by proteinase K digestion and phenol/chloroform extraction. Genotyping of the two APE1 polymorphisms was performed using the predesigned TaqMan probe assay (Applied Biosystems, Foster City, CA, USA). The sequences of primer and probe for the two SNPs are available on request. The reaction mixture of 10 μL contained 20 ng genomic DNA, 3.5 μL of 2×TaqMan Genotyping Master Mix, 0.25 μL of the primers and probes mix and 6.25 μL of double distilled water. The amplification was performed under the following conditions: 50 ºC for 2 min, 95 ºC for 10 min followed by 45 cycles of 95 ºC for 15 s, and 60 ºC for 1 min. Amplifications were performed in the 384-well ABI 7900HT Real Time PCR System (Applied Biosystems), following the manufacturer's instructions. After the completion of the amplification, the fluorescence intensity in each well of the plate was read and analyzed with SDS 2.4 automated software. Four blank controls were included in each plate to ensure accuracy of the genotyping. About 10% of the samples were randomly selected for repeated assays, and the results were in agreement with the results of the first assays.
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Allele and genotype distributions of the APE1 −656 TNG and 1349 TNG polymorphisms are presented in Table 2. The observed genotype frequencies of the two SNPs in the controls were in agreement with the HWE (P = 0.294 for the −656 TNG polymorphism and P = 0.199 for the 1349 TNG polymorphism). We found that the distributions of the APE1 −656 TNG genotypes were significantly different between cases and controls (P = 0.012). Comparing with subjects with the − 656 TT genotype, individuals with the variant GG genotype had a significant reduced breast cancer risk (adjusted OR = 0.63, 95% CI = 0.45– 0.88). Furthermore, in a recessive model, the −656 TG/GG genotypes were also significantly associated with a decreased risk for breast cancer (OR = 0.71, 95% CI = 0.56–0.91). However, we did not observe significant associations between the 1349 TNG polymorphism and breast cancer risk in any comparison, as shown in Table 2.
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Table 1 Distributions of select variables in breast cancer cases and cancer-free controls.
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3.1. Association between APE1 polymorphisms and risk of breast cancer
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Control (N = 799), %
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Age at diagnosis or recruitment (year) Mean ± SD 52.0 ± 11.0 b52 239 51.4 ≥52 226 48.6
50.8 ± 13.2 437 54.7 362 45.3
0.110 0.257
Age at menarche (year) Mean ± SD b14 ≥14
14.89 ± 1.76 183 22.9 616 77.1
0.002 0.012
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Tumor size Less than 2 cm 2 to 5 cm More than 5 cm LN involvement Negative Positive ER Negative Positive PR Negative Positive HER-2 Negative Positive a
Case (N = 465), %
14.55 ± 1.84 136 29.3 329 70.7 126 310 29
27.1 66.7 6.2
251 214
54.0 46.0
162 303
34.8 65.2
209 256
44.9 55.1
309 156
66.4 33.6
T-test or two-sided χ2-test.
Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
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t1:3 t1:4 t1:5 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17 t1:18 t1:19 t1:20 t1:21 t1:22 t1:23 t1:24 t1:25 t1:26 t1:27 t1:28 t1:29
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−656 TNG TT TG GG TG+GG T G 1349 TNG TT TG GG TG+GG T G
t2:20 t2:21
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OR (95% CI)b
%
180 207 78 285 567 363
38.7 44.5 16.8 61.3 61.0 39.0
248 381 170 551 877 721
31.0 47.7 21.3 69.0 54.9 45.1
165 211 89 300 541 389
35.5 45.4 19.1 64.5 58.2 41.8
276 372 151 523 924 674
34.5 46.6 18.9 65.5 57.8 42.2
0.012
0.917
1.00 (Reference) 0.95 (0.73–1.22) 0.95 (0.69–0.32) 0.95 (0.75–0.131) 1.00 (Reference) 0.99 (0.84–1.16)
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Two-sided χ2 test for the distributions of genotype and allele frequencies. Adjusted for age and age at menarche.
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3.2. Stratified analysis of the APE1 −656 TNG polymorphism and risk of breast cancer
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We then evaluated the effect of the APE1 −656 TNG polymorphism on breast cancer stratified by age. As shown in Table 3, the protect effect of the APE1 −656 TG+GG genotypes was more pronounced in younger subjects (P = 0.015, OR = 0.65, 95% CI = 0.46–0.92) rather than old subjects (P = 0.153, OR = 0.80, 95% CI = 0.56–1.15), suggesting the younger individuals could benefit more from carrying the genotype. The same analyses were also performed for the APE1 1349 TNG polymorphism; however, no positive result was observed (data not shown).
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3.3. Association between the APE1 polymorphisms and clinical parameters of breast cancer patients
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To determine whether the APE1 polymorphisms have effect on the clinical features of breast cancer patients, we then analyzed the association between the APE1 polymorphisms and a series of clinicopathologic parameters, including tumor size, clinical stage, lymph node metastasis, and the statuses of ER, PR and Her-2. As shown in Table 4, we found that the frequency of the variant genotypes (TG/GG) was significantly lower in the patients with lymph node involvement (P = 0.020, OR = 0.64, 95% CI = 0.44–0.94). No other significant association between the APE1 polymorphisms and the clinical features was found in the present study.
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Table 3 Stratification analyses on age between APE1 −656 TNG genotypes and risk of −656 TNG breast cancer. The −656 TNG genotypes
t3:5 t3:6 t3:7 t3:8 t3:9 t3:10 t3:11
Age b52 TT TG+GG Age ≥52 TT TG+GG
t3:12 t3:13
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OR (95% CI)b
Case (N = 465)
Control (N = 799)
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92 147
38.5 61.5
128 309
29.3 70.7
0.015
1.00 (Reference) 0.65 (0.46–0.92)
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38.9 61.1
120 242
33.2 66.9
0.153
1.00 (Reference) 0.80 (0.56–1.15)
Two-sided χ2 test for the distributions of genotype frequencies. Adjusted for age and age at menarche.
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TG/GG (N, %)
47 (62.7) 381 (33.5)
79 (37.3) 757 (66.5)
0.390
1.00 (Reference) 1.18 (0.80–1.73)
85 (33.9) 95 (44.4)
166 (66.1) 119 (55.6)
0.020
1.00 (Reference) 0.64 (0.44–0.94)
63 (38.9) 117 (38.6)
99 (61.1) 186 (61.4)
0.954
1.00 (Reference) 1.11 (0.99–1.24)
84 (40.2) 96 (37.5)
125 (59.8) 160 (62.5)
0.553
1.00 (Reference) 1.13 (0.78–1.65)
0.258
1.00 (Reference) 0.79 (0.53–1.18)
114 (36.9) 66 (42.3)
195 (63.1) 90 (57.7)
t4:4 t4:5
TT (N, %)
Two-sided χ2 test for the distributions of genotype and allele frequencies. Adjusted for tumor size, lymph node involvement, ER, PR and HER-2 status.
4. Discussion
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Tumor size Less than 2 cm More than 2 cm LN involvement Negative Positive ER Negative Positive PR Negative Positive HER-2 Negative Positive
1.00 (Reference) 0.75 (0.58–0.97) 0.63 (0.45–0.88) 0.71 (0.56–0.91) 1.00 (Reference) 0.78 (0.66–0.92)
0.005 0.003
APE1 −656 TNG genotypes
Variables
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t2:6 t2:7 t2:8 t2:9 t2:10 t2:11 t2:12 t2:13 t2:14 t2:15 t2:16 t2:17 t2:18 t2:19
Control (N = 799)
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Case (N = 465)
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In the present study, we found a significant association between the APE1 −656 TNG polymorphism and breast cancer risk. Compared with individuals with TT genotype, those with GG or GG/TG genotypes had a significantly decreased breast cancer risk of 0.63 and 0.71, respectively; and the younger subjects could benefit more from carrying the genotypes. Besides, we also found that patients with breast cancer carrying the GG/TG genotypes had a significantly less frequency of lymph node involvement, suggesting the variant genotypes of this polymorphism may play a protective role during the progression of breast cancer. To the best of our knowledge, this is the first study to address the role of the APE1 −656 TNG polymorphism in the development and progression of breast cancer. Apurinic/apyrimidinic (AP) sites are common mutagenic and cytotoxic DNA lesions that are caused by the loss of normal bases (Lindahl, 1995). APE1 has been considered as the major AP endonuclease that is involved in the repair of AP sites (Izumi et al., 2000). Aberrant expression of APE1 gene may lead to defects in repairing these lesions and confer individuals' susceptibility to the cancer. For instance, there are studies that suggested polymorphisms in APE1 may regulate its expression and influence individual's susceptibility to cancer (Lo et al., 2009; Lu et al., 2009). Epidemiological studies have now linked genetic polymorphisms in APE1 with the risk various types of cancer including lung cancer, breast cancer, colorectal cancer and bladder cancer (Gu et al., 2009). The most widely studied polymorphisms in APE1 is the 1349 TNG which locates in the fifth exon of the gene. Functional studies have demonstrated that APE1 harboring the 1349G allele had altered endonuclease and DNA-binding activity, reduced ability to communicate with other BER proteins and decreased capacity to repair DNA oxidative damage (Au et al., 2003; Hadi et al., 2000). Gu et al. reported in their meta-analysis that the APE1 1349 TNG polymorphism was a lowpenetrance risk factor for cancer development (Gu et al., 2009); however, in a recently conducted meta-analysis which excluded four studies that did not conform to HWE and included four additional studies, Zhou et al. reported that the APE1 1349 TNG polymorphism did not contribute to genetic susceptibility to cancer (Zhou et al., 2011). For breast cancer, a meta-analysis including five studies also denied a significant association between the APE1 1349 TNG polymorphism and breast cancer risk (Wu et al., 2012). Our results were consistent with these studies, which, taken together with the above results, suggest that the APE1 1349 TNG polymorphism is not likely to be a susceptibility factor for breast cancer.
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Table 4 t4:1 The associations between The APE1 −656 TNG polymorphism and clinical characteristics t4:2 of breast cancer patients. t4:3
Table 2 Genotype and allele frequencies of the APE1 polymorphisms among the cases and controls and the associations with risk the breast cancer.
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Please cite this article as: Kang, H., et al., A genetic variant in the promoter of APE1 gene (−656 TNG) is associated with breast cancer risk and progression in a Chinese p..., Gene (2013), http://dx.doi.org/10.1016/j.gene.2013.08.052
t4:6 t4:7 t4:8 t4:9 t4:10 t4:11 t4:12 t4:13 t4:14 t4:15 t4:16 t4:17 t4:18 t4:19 t4:20 t4:21 t4:22
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In conclusion, our case-control study indicates that the APE1 − 656 TNG but not 1349 TNG polymorphism has a significant influence on the occurrence and progression of breast cancer in the Chinese population. To our knowledge, this is the first report regarding the role of the APE1 −656 TNG polymorphism in breast cancer. Although the association appeared to be statistically significant in our population, the initial findings should be independently verified by other large independent population-base studies.
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Au, W.W., Salama, S.A., Sierra-Torres, C.H., 2003. Functional characterization of polymorphisms in DNA repair genes using cytogenetic challenge assays. Environ. Health Perspect. 111 (15), 1843–1850 (doi:sc271_5_1835 [pii]). Barzilay, G., Hickson, I.D., 1995. Structure and function of apurinic/apyrimidinic endonucleases. Bioessays 17 (8), 713–719. http://dx.doi.org/10.1002/bies.950170808. Cao, Q., et al., 2011. Genetic polymorphisms in APE1 are associated with renal cell carcinoma risk in a Chinese population. Mol. Carcinog. 50 (11), 863–870. http:// dx.doi.org/10.1002/mc.20791. Goode, E.L., Ulrich, C.M., Potter, J.D., 2002. Polymorphisms in DNA repair genes and associations with cancer risk. Cancer Epidemiol. Biomarkers Prev. 11 (12), 1513–1530. Gu, D., Wang, M., Zhang, Z., Chen, J., 2009. The DNA repair gene APE1 T1349G polymorphism and cancer risk: a meta-analysis of 27 case-control studies. Mutagenesis 24 (6), 507–512 (doi:gep036 [pii] http://dx.doi.org/10.1093/mutage/gep036). Hadi, M.Z., Coleman, M.A., Fidelis, K., Mohrenweiser, H.W., Wilson III, D.M., 2000. Functional characterization of Ape1 variants identified in the human population. Nucleic Acids Res. 28 (20), 3871–3879. Hoeijmakers, J.H., 2001. Genome maintenance mechanisms for preventing cancer. Nature 411 (6835), 366–374. http://dx.doi.org/10.1038/35077232 (35077232 [pii]). Izumi, T., et al., 2000. Requirement for human AP endonuclease 1 for repair of 3′-blocking damage at DNA single-strand breaks induced by reactive oxygen species. Carcinogenesis 21 (7), 1329–1334. Jemal, A., Bray, F., Center, M.M., Ferlay, J., Ward, E., Forman, D., 2011. Global cancer statistics. CA Cancer J. Clin. 61 (2), 69–90 (doi:caac.20107 [pii] http://dx.doi.org/10.3322/ caac.20107). Lichtenstein, P., et al., 2000. Environmental and heritable factors in the causation of cancer–analyses of cohorts of twins from Sweden, Denmark, and Finland. N. Engl. J. Med. 343 (2), 78–85. http://dx.doi.org/10.1056/NEJM200007133430201. Lindahl, T., 1995. Recognition and processing of damaged DNA. J. Cell Sci. Suppl. 19, 73–77. Lo, Y.L., et al., 2009. A polymorphism in the APE1 gene promoter is associated with lung cancer risk. Cancer Epidemiol. Biomarkers Prev. 18 (1), 223–229 (doi:18/1/223 [pii] http://dx.doi.org/10.1158/1055-9965.EPI-08-0749). Lu, J., et al., 2009. Functional characterization of a promoter polymorphism in APE1/Ref-1 that contributes to reduced lung cancer susceptibility. FASEB J. 23 (10), 3459–3469 (doi:fj.09-136549 [pii] http://dx.doi.org/10.1096/fj.09-136549). Sangrajrang, S., et al., 2008. Polymorphisms in three base excision repair genes and breast cancer risk in Thai women. Breast Cancer Res. Treat. 111 (2), 279–288. http:// dx.doi.org/10.1007/s10549-007-9773-7. Smith, T.R., et al., 2003. DNA-repair genetic polymorphisms and breast cancer risk. Cancer Epidemiol. Biomarkers Prev. 12 (11 Pt 1), 1200–1204. Tell, G., Fantini, D., Quadrifoglio, F., 2010. Understanding different functions of mammalian AP endonuclease (APE1) as a promising tool for cancer treatment. Cell. Mol. Life Sci. 67 (21), 3589–3608. http://dx.doi.org/10.1007/s00018-010-0486-4. Thompson, E.R., et al., 2012. Exome sequencing identifies rare deleterious mutations in DNA repair genes FANCC and BLM as potential breast cancer susceptibility alleles. PLoS Genet. 8 (9), e1002894. http://dx.doi.org/10.1371/journal.pgen.1002894 (PGENETICS-D-12-00361 [pii]). Wilson III, D.M., Bohr, V.A., 2007. The mechanics of base excision repair, and its relationship to aging and disease. DNA Repair (Amst) 6 (4), 544–559. Wood, R.D., Mitchell, M., Lindahl, T., 2005. Human DNA repair genes, 2005. Mutat. Res. 577 (1–2), 275–283 (doi:S0027-5107(05)00163-6 [pii] http://dx.doi.org/10.1016/j. mrfmmm.2005.03.007). Wu, B., Liu, H.L., Zhang, S., Dong, X.R., Wu, G., 2012. Lack of an association between two BER gene polymorphisms and breast cancer risk: a meta-analysis. PLoS One 7 (12), e50857. http://dx.doi.org/10.1371/journal.pone.0050857 (PONE-D-1215192 [pii]). Xi, T., Jones, I.M., Mohrenweiser, H.W., 2004. Many amino acid substitution variants identified in DNA repair genes during human population screenings are predicted to impact protein function. Genomics 83 (6), 970–979. http://dx.doi.org/10.1016/ j.ygeno.2003.12.016 (S0888754304000229 [pii]). Zhou, B., et al., 2011. The association of APE1–656 TNG and 1349 TNG polymorphisms and cancer risk: a meta-analysis based on 37 case-control studies. BMC Cancer 11, 521 (doi:1471-2407-11-521 [pii] http://dx.doi.org/10.1186/1471-2407-11-521).
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The APE1 −656 TNG is another widely studied polymorphism which was suggested to influence the gene expression. In lung cancer, Lo et al. found that the variant genotypes were associated with a significantly decreased lung cancer risk; in further functional studies, they demonstrated that the −656 G allele had a significantly higher transcriptional activity than that of the APE1 −656 T allele, which indicated that the “higher production” genotype for APE1 may offer protection against the development of lung cancer (Lo et al., 2009). In the meta-analysis conducted by Zhou et al., they reported that the −656 TNG polymorphism has a possible protective effect on cancer risk particularly among Asian populations. However, up to date, there still lack of study investigating the role of −656 TNG polymorphism in breast cancer risk. In the present study, we found that the −656 TNG polymorphism was significantly associated with breast cancer risk. Comparing with individuals with −656TT genotype, those carrying the −656TG/GG genotypes had a 0.71-fold decreased breast cancer risk. Accumulation of DNA lesions also involved in the development of breast cancer. Considering the aforementioned view that the −656G allele may be harboring a “higher production” of APE1, it is biologically plausible that the APE1 − 656TG/GG genotypes are associated with a decreased breast cancer risk. In the subgroup analysis, we also found that individuals younger than 52 years of age could benefit more from carrying the APE1 − 656TG/GG genotypes. As it is known, more DNA lesions will occur as aging; thus the small effect of the genotype may be overwhelmed by the accumulation of more DNA lesions in elders. In addition, we also observed that the APE1 −656TG/GG genotypes was associated with less frequent lymph node involvement of breast cancer, but the results should be interpreted cautiously since there is the possibility that the association may due to a late stage at diagnosis. Nevertheless, if confirmed by additional studies, this polymorphism may help to accurately predict the clinical course of breast cancer. When interpreting our results, a limitation should be concerned. Since our study was hospital-based design, we could not rule out the possible of selection bias of subjects that may have been associated with a particular genotype. However, the genotype distributions of APE1 polymorphisms in our study population all conformed to HWE that suggested the selection bias in terms of genotype distribution would not be substantial.
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