Genetic Polymorphism Of XPD Gene And Exposure To Environmental Risk Factors On The Risk Of Lung Cancer In Non-smoking Females

IntroductionIn recent yeas, lung cancer has become the leading cause of cancer - related deaths for both men and women in China. The incidence and mortality rate of lung cancer in urban populations have reached the number one among malignant tumors. Smoking is the main risk factor for lung cancer;however, other factors have also been implicated. Cigarette smoking cannot fully explain the epidemio-logic characteristics of lung cancer in Chinese women, who smoke rarely but. have lung cancer relatively often. So it's really significant to explore the risk factors for lung cancer in Chinese non - smoking females in order to control the confounding effect of smoke.Exposure to environmental risk factors play important roles in the development of lung cancer, but in the same environment people differ in their susceptibility to cancer. Some of these differences are ascribed to the concept that genetic susceptibility modifies the effects of environmental exposures. About the genetic susceptibility to lung cancer, studies focus on polymorphisms in genes involved in carcinogen metabolism and DNA repair genes.XPD also named excision repair cross - complimentary group 2 ( ERCC2) is one of the important DNA repair proteins. The 54.3 - kb XPD gene has 23 ex-ons, is located in chromosome 19ql3. 2 - 13. 3 and codes for an evolutionarily conserved helicase, a subunit of TF II H complex that is essential for transcription and nucleotide excision repair ( NER). Mutations in XPD may reduce the function of TF IIH complex, resulting in a defect in NER and transcription. The common polymorphisms of XPD gene include G23592A (Asp312Asn) of exon10 and A35931C ( Lys751Gln) of exon 23 , which do cause amino acid changes. The Lys751 Gin polymorphism is about 50 bases upstream from the poly ( A ) signal and therefore may alter XPD protein function. Although the exact mechanisms how XPD polymorphisms affect cancer risk at the molecular level remain to be unraveled, the published studies imply these polymorphisms may prevent the protein from interacting with p44, another subunit of TF II H complex and decrease helicase activity, resulting in a defect in NER and deficient DNA repair capacity that may be responsible for increased susceptibility of cancer.The earlier studies have reported the relationship of XPD polymorphism at codon 751 and cancer risk or carcinogen - DNA adducts. The polymorphisms of genes coding for DNA repair molecules have been proposed as candidate cancer- susceptibility factors. The effect of the polymorphism of XPD on lung cancer susceptibility in non - smoking females has not been reported so far. In this study, we describe a case - control study of lung cancer in non - smoking female population in Shenyang to test the hypothesis that genetic polymorphism of XPD gene contributes to host susceptibility to lung cancer and explore the interaction of genetic polymorphism and exposure to environmental risk factors in the development of lung cancer.Subjects and MethodsIn this case - control study, the case group consisted of 105 diagnosed patients ( between January 2004 and October 2005 ) with histologically confirmed lung carcinoma in Liaoning Cancer 'Hospital or 202 Hospital, China. The control group comprised 105 healthy volunteers. They were obtained from community centers. Data on age, demographic characters, exposure to risk factors and dietary habit were derived from questionnaires. After informed consent was obtained, each person donated 5 ml of blood in heparinized tubes and stored at- 80T! for biomarker testing.Polymerase chain reaction (PCR) followed by enzymatic digestion was used for the genotyping of the XPD Lys751Gln. An amount of lml whole blood was employed for DNA extraction. PCR was performed in a total reaction volume of25ul. Thermal cycling conditions were as follows;initial denaturation step at 94^C for 2 minutes,30 cycles of PCR consisting of 94^C for 30s,59^ for 30s and 72T1 for 1 min,followed by a final extension step at 12°Q, for 10 min. The 436 bp PCR product was digested with Pst I . Two - side x test was used to compare the distribution of the genotypes and risk factors between cases and controls. Unconditional Logistic regression analysis was performed to calculate the odds ratios (OR) with 95% confidence intervals (CI) for estimating the association between certain genotypes and lung cancer and exploring the interaction of genetic polymorphism and exposure to environmental risk factors. All of the statistical analyses were performed with SPSS (v 12.0) .ResultsAll of the subjects enrolled in this study were non - smoking females in Shenyang. There were no significant demographic differences (age, economic level and education) between cases and controls. Among the 105 lung cancer cases, 45(42.86%) were classified as adenocarcinama^43 (40.95%) as squamous cell carcinoma (SCC) and 17 (16. 19% ) as other carcinoma. There was a significant difference between the frequencies of XPD751 genotypes in cancer cases and controls ( P < 0. 05 ). The risk of lurtg cancer was higher in those with the Lys/Gln + Gin/Gin combined genotype than in those with the Lys/Lys genotype (adjusted OR was 2. 80). The frequencies of XPD751 Gin al-lele were 6. 19% in controls and 13. 81% in cases. The risks of lung adenocar-cinoma and lung SCC were higher in those with the Lys/Gln + Gin/Gin combined genotype than in those with the Lys/Lys genotype and adjusted OR were 3. 82 and 2. 92, respectively. We analyzed the environmental risk factors for lung cancer in non — smoking females. The results showed that exposure to cooking fumes and coal fumes are the risk factors for lung cancer ( OR were 2. 82 and 1.76) and lung adenocarcinoma ( OR were 4. 08 and 3. 20). Exposure to coal fumes was associated with the increasing risk of lung SCC (OR =2. 80). Furthermore, a multiplicative interaction between the variant XPD 751 Gin allele and exposure to environmental risk factors on the risk of lung cancer was ob-served. Individuals with both risk genotype and exposure to cooking fumes or coal fumes have a higher elevated risk of cancer than those with only one of them.DiscussionThis study firstly showed that the variant allele of XPD751 polymorphism was risk factor for lung cancer in the Chinese non - smoking female population. We also found statistically significant interactions between the polymorphism of XPD gene and exposure to environmental risk factors.The XPD gene product is an ATP - dependent DNA helicase, component of the basal transcription factor TF II H complex. It is involved in transcription, NER and in the p53 - dependent apoptic pathway. The association between the XPD genetic polymorphism and lung cancer risk is biologically plausible, because the XPD protein functions as a helicase in the NER that is responsible for the repair of many DNA lesions induced by genotoxic agents present in the environment and plays a role in activating apoptosis through the interaction between p53 and TF H H to remove damaged cells. The XPD Lys751Gln polymorphism has been associated with higher levels of chromatid aberrations. The XPD 751 Gin allele was reported to be associated with higher DNA adduct levels in non — smokers. Some studies indicate that XPD751 Lys/Gln or Gin/Gin will induce deficient NER ability. Spitz et al. point out that variant 751 Gin genotype was associated with less optimal DNA repair capacity.. These results suggest that XPD751polymorphism results in a defect in NER and deficient DRC that may be responsible for increased susceptibility of cancer. This is consistent with our result.This is the first report exploring the relationship between the polymorphism of XPD gene and lung cancer in non - smoking females. Our results show that the individual carrying Lys/Gln or Gin/Gin genotype was at an increased risk for lung cancer, lung adenocarcinoma and lung SCC as compared with those carrying Lys/Lys genotype ( OR were 2. 80, 3. 82 and 2.92, adjusted for age and exposure to environmental risk factors). It suggests that the polymorphism of XPD751 site plays an important role in the development of female lung cancer, which is in agreement with other previous studies.This is the first report showing the gene - environment interaction between XPD polymorphism and exposure to cooking fumes or coal fumes on lung cancer risk in non - smoking females. The mechanism of how environmental risk factors changes the DRC posed by each genotype of the DNA repair genetic polymorphism is unknown. One possible explanation may be that Polycyclic Aromatic Hydrocarbons (PAHs) in the cooking or coal fumes can form PAH - DNA adducts by covalently binding with DNA and then irreversibly result in DNA mutations. These damages are mainly repaired by NER. The polymorphism of XPD gene affects the function of XPD protein, resulting in a defect in NER and increasing risk of lung cancer. Another mechanism maybe associated with the function of p53. Cooking fumes and coal fumes contain a myriad of chemical carcinogens, which are related to a specific mutational spectrum in the p53 gene, inhibiting combination of p53 and XPD, so affecting the function of NER. Functional changes in repair capacity due to inheritance of certain polymorphisms could increase the chance that adducts produced from carcinogens result in p53 mutations.ConclusionsThe above findings indicate the Lys751Gln polymorphism in the XPD gene is associated with the risk of lung cancer in non - smoking females.Individuals with both XPD751Gln allele genotype and exposure to cooking fumes or coal fumes have a higher elevated risk of lung cancer than those with only one of them in non - smoking females.