Relationship between XPG codon 1104 polymorphism and risk of primary lung cancer

Hyo-Sung Jeon1, Kyung Mee Kim2, Sun Ha Park1, Su Yeon Lee1, Jin Eun Choi1, Ga Young Lee2, Sin Kam3, Rang Woon Park2, In-San Kim2, Chang Ho Kim4, Tae Hoon Jung4 and Jae Yong Park1,2,4,5

1 Cancer Research Institute, 2 Department of Biochemistry and 3 Department of Preventive Medicine, School of Medicine, Kyungpook National University, Dong In 2 Ga 101, Daegu, 700-422, Korea and 4 Department of Internal Medicine, Kyungpook National University Hospital, Samduk 2ga 50, Daegu, 700-412, Korea


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
DNA repair plays a critical role in protecting the genome from insults of cancer-causing agents, such as those found in tobacco smoke. Therefore, reduced DNA repair capacity can increase the susceptibility to smoking-related cancers. Recently, several polymorphisms have been identified in the xeroderma pigmentosum group G (XPG) gene, and it is possible that these polymorphisms may affect the DNA repair capacity, thereby modulating cancer susceptibility. We investigated the relationship between the His1104Asp polymorphism in the XPG gene and the risk of lung cancer. The study population consisted of 310 lung cancer patients and 311 healthy controls who were frequency (1:1) matched based on age and sex. The Asp/Asp genotype was more frequent in the controls (28.9%) than in the cases (18.7%) and associated with a significantly decreased risk of lung cancer [adjusted odds ratio (OR) = 0.54, 95% confidence interval (CI) = 0.37–0.80] when the combined His/His and His/Asp genotype was used as the reference. The protective effect of the Asp/Asp genotype against lung cancer was statistically significant in the older subjects (adjusted OR = 0.51, 95% CI = 0.37–0.80), males (adjusted OR = 0.54, 95% CI = 0.35–0.83), and lighter smokers (adjusted OR = 0.48, 95% CI = 0.25–0.94) in a stratification analysis. When the lung cancers were analyzed by histologic type, the Asp/Asp genotype was associated with a significantly decreased risk of squamous cell carcinoma (adjusted OR = 0.55, 95% CI = 0.34–0.88) and small cell lung cancer (adjusted OR = 0.44, 95% CI = 0.20–0.97), but non-significant decreased risk of adenocarcinoma (adjusted OR = 0.64, 95% CI = 0.36–1.12). These results suggest that the XPG codon 1104 polymorphism contributes to genetic susceptibility to lung cancer.

Abbreviations: CI, confidence interval; NER, nucleotide excision repair; OR, odds ratio; XPG, xeroderma pigmentosum group G


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Lung cancer has been considered as a disease determined solely by exposure to environmental carcinogens. However, there is a growing realization that genetic constitution is of importance in determining an individual's susceptibility to lung cancer (1,2). This genetic susceptibility may result from inherited polymorphisms in genes involved in carcinogen metabolism and repair of DNA damage (3,4).

The complex system of DNA repair enzymes plays a vital role in protecting the genome from the insults of cancer- causing agents, such as those found in tobacco smoke (5,6). In humans, more than 70 genes are involved in the five major DNA repair pathways: nucleotide excision repair (NER), base excision repair (BER), mismatch repair, homologous recombinational repair and non-homologous end joining (6,7). BER removes small base adducts produced by oxidation, methylation and radiation, while NER targets UV light-induced photoadducts and DNA adducts induced by tobacco carcinogens, such as benzo[a]pyrene diol epoxide (69).

Molecular epidemiologic studies have shown considerable inter-individual variations in the DNA repair capacity (DRC) in the general population. Individuals with a suboptimal DRC are at an increased risk of smoking-related cancers, such as lung cancer and squamous cell carcinoma of the head and neck (12,13). The variation in the DRC may be the result of functional polymorphisms in the DNA repair gene.

Based on the hypothesis that sequence variations in the DNA repair genes may have an effect on the DRC, thereby modulating the susceptibility to lung cancer, we studied previously the contribution of polymorphisms in the DNA repair genes X-ray repair cross-complementing group 1 (XRCC1), xeroderma pigmentosum group D (XPD) and XPA, to the risk of lung cancer in the Korean population (1416). We noted that the XRCC1 Arg399Gln and XPA A-23G polymorphisms were associated with the risk for lung cancer (14,16). Xeroderma pigmentosum group G (XPG) is one of seven genetic complementation groups encoding for the proteins involved in the NER pathway (1719). XPG, a protein of 1186 amino acids, functions as a structure-specific endonuclease that cleaves the damaged DNA strand on the 3' side, and is also required non-enzymatically for a subsequent 5' incision by the XPF/excision repair cross-complementing group 1 (ERCC1) heterodimer during NER in human cells (1921). Three polymorphic variants in the coding sequence of the XPG gene (Arg1053Gly, Arg1080Gly and His1104Asp) have been identified (22,23). While the functional effects of these polymorphisms are still unknown, it is possible that these polymorphisms may have an effect on host capacity for removing bulky adducts caused by cigarette smoke and thus modulate the susceptibility to lung cancer. In the present study, we evaluated the association between the XPG codon 1104 polymorphism (C3507G, in exon 15) and the risk of lung cancer. Among the three known coding polymorphisms in the XPG gene, the codon 1104 polymorphism was chosen, as the codon 1053 and 1080 polymorphisms were not detected in the preliminary study (consisting of 50 cases and 50 controls).


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Study population
The case-control study included 310 lung cancer patients and 311 healthy controls. Eligible cases included all patients newly diagnosed with primary lung cancer between January 1999 and February 2000 at Kyungpook National University Hospital, Daegu, Korea. There was no age, sex, histologic or stage restrictions, but patients with prior history of cancers were excluded. The cases were 154 (49.7%) squamous cell carcinomas, 100 (32.2%) adenocarcinomas, 51 (16.5%) small cell carcinomas and 5 (1.6%) large cell carcinomas. The demographics and clinical characteristics of the cases were compatible with those of a nationwide lung cancer survey conducted by the Korean Academy of Tuberculosis and Respiratory disease in 1998 (24). Controls were randomly selected from a pool of healthy volunteers who visited the general health check-up center at Kyungpook National University Hospital during the same period. A total of 3047 (1576 males and 1471 females) of 5780 healthy subjects agreed to this study (participation rate, 52.7%). Compared with refused subjects, participated subjects were similar in the distribution of sex (% of male, 52.7% versus 51.7%; P = 0.24) and age (52.4 ± 11.4 versus 52.2 ± 11.5; P = 0.48). From 3047 healthy volunteers, we selected 311 control subjects frequency matched to the cases based on sex and age (±5 years). All cases and controls were residents of Daegu City or the surrounding regions. A detailed questionnaire was completed for each case and control by a trained interviewer. The questionnaire included information on the average number of cigarettes smoked daily and number of years the subject had been smoking. For ex-smokers, the time elapsed since quitting was recorded.

XPG genotyping
Genomic DNA was extracted from peripheral blood lymphocytes by proteinase K digestion and phenol–chloroform extraction. The XPG codon 1104 genotypes were determined by a PCR–RFLP assay. The PCR primers for the codon 1104 polymorphism (GenBank accession no. NM_000123) were 5'-GACCTGCCTCTCAGAATCATC-3' (bases 3467–3486 of XPG) and 5'-CCTCGCACGTCTTAGTTTCC-3' (bases 3737–3718 of XPG), which generated a 271 bp fragment. The PCR reaction was performed in a 20 µl reaction volume containing 200 ng of genomic DNA, 10 pmol of each primer, 0.2 mM deoxynucleotide triphosphate, 1x PCR buffer [75 mM Tris-HCl (pH 9.0), 15 mM amonium sulfate, and 0.1 µg/ml BSA], 2.5 mM MgCl2, and 1 U Taq polymerase (Takrara Shuzo Co., Otsu, Shiga, Japan). The PCR program was initiated by a 5-min denaturation step at 94°C followed by 35 cycles of 94°C for 30 s, 62°C for 30 s, 72°C for 30 s, and a final elongation step of 72°C for 10 min. The PCR products were digested for 4 h with 10 U of NlaIII (New England BioLabs Inc., Beverly, MA, USA) at 37°C. The digestion product was then resolved on 6% polyacrylamide gel. The His/His genotype was determined by the presence of two bands at 227 and 44 bp, the Asp/Asp genotype by the presence of an uncut 271 bp band (absence of NlaIII cutting site), and the heterozygous His/Asp genotype by the presence of three bands at 271, 227 and 44 bp. Genotyping was successful for all subjects. The genotyping analysis was repeated twice for all subjects, and selected PCR-amplified DNA samples (n = 2, respectively, for His/His, His/Asp and Asp/Asp genotypes) were examined by DNA sequencing to confirm the genotyping results.

Statistical analyses
An individual who had smoked at least once a day for >1 year of their life was regarded as a smoker. A former smoker was defined as one who had stopped smoking at least 1 year before diagnosis in the case of patients and 1 year before the study began in the case of controls. The cumulative cigarette dose (pack-years) was calculated using the following formula: pack-years = [(pack per day) x (years smoked)]. The cases and controls were compared using Student's t-test for continuous variables and {chi}2 test for categorial variables. When multiple comparisons are made, significant associations may arise by chance. To avoid such errors, the corrected P-values (Pc-values) were also calculated for multiple testing using Bonferroni's inequality method. Hardy–Weinberg equilibrium was tested by a goodness-of-fit {chi}2 test to compare the observed genotype frequencies with the expected genotype frequencies among the cases and controls. The odds ratios (ORs) and 95% confidence intervals (CIs) were obtained using an unconditional logistic regression analysis. Crude ORs and ORs adjusted for age, sex and pack-years were calculated. To analyze the association between the genotype and the risk of lung cancer after stratification into age (<=60 years/>60 years), sex, and pack-years (median pack-years, <=39 pack-years/>39 pack-years), multiple logistic regression analyses were performed. All analyses were performed using Statistical Analysis Software for Windows, version 6.12 (SAS Institute, Cary, NC).


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The details of the cases and controls enrolled in this study are shown in Table I. There were no significant differences in the mean age and sex distribution between the cases and the controls, suggesting that the matching based on these two variables was adequate. The cases showed a higher prevalence of current smokers compared with the controls (P < 0.01). The pack-years in the smokers was significantly higher in cases than in controls (41.3 ± 21.9 versus 34.6 ± 17.8 pack-years, P < 0.01). These differences were controlled in the later multivariate analyses.


View this table:
[in this window]
[in a new window]
 
Table I. Characteristics of the study population

 
The distributions of the XPG codon 1104 genotypes (His/His, His/Asp and Asp/Asp) among the cases and controls are shown in Table II. The distribution of genotypes among controls was in Hardy–Weinberg equilibrium. The frequencies of the His/His, His/Asp and Asp/Asp genotypes among cases (28.4, 52.9 and 18.7%, respectively) were significantly different from those among controls (28.6, 42.4 and 28.9%, respectively; P = 0.006). The His/Asp genotype was more frequent in cases than in controls, whereas the Asp/Asp genotype was less frequent in cases than in controls (P = 0.001, Pc = 0.002). These findings suggest that the Asp/Asp genotype may be a protective genotype for lung cancer. When the combined His/His and His/Asp genotype was used as the reference group, the Asp/Asp genotype was associated with a significantly decreased risk of lung cancer (adjusted OR = 0.54, 95% CI = 0.37–0.80).


View this table:
[in this window]
[in a new window]
 
Table II. Distributions of the XPG codon 1104 genotypes among cases and controls

 
The association between the XPG codon 1104 polymorphism and the risk of lung cancer was further examined after stratifying for potential confounding variables, such as age, sex and pack-years. The risk estimates for the Asp/Asp genotype are presented in Table III. In all subgroups, the Asp/Asp genotype was less frequent in the cases than in the controls. The protective effect of the Asp/Asp genotype against lung cancer was statistically significant in the older subjects (adjusted OR = 0.51, 95% CI = 0.31–0.83), males (adjusted OR = 0.54, 95% CI = 0.35–0.83), and lighter smokers (adjusted OR = 0.48, 95% CI = 0.25–0.94).


View this table:
[in this window]
[in a new window]
 
Table III. Stratification analysis of the XPG codon 1104 genotype frequencies in cases and controls

 
The contribution of the XPG codon 1104 polymorphism in each histological subcategory is shown in Table IV. When the combined His/His and His/Asp genotype was used as the reference group, the Asp/Asp genotype was associated with a significantly decreased risk of squamous cell carcinoma (adjusted OR = 0.55, 95% CI = 0.34–0.88) and small cell carcinoma (adjusted OR = 0.44, 95% CI = 0.20–0.97), but non-significant decreased risk of adenocarcinoma (adjusted OR = 0.64, 95% CI = 0.36–1.12).


View this table:
[in this window]
[in a new window]
 
Table IV. Lung cancer risk related to the XPG codon 1104 genotypes according to histologic types

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
This is the first case-control study of XPG polymorphisms in relation to lung cancer. In the current study, the XPG codon 1104 Asp/Asp genotype was associated with a significantly decreased risk of lung cancer. This finding suggests that the XPG His1104Asp polymorphism may contribute to an inherited genetic susceptibility to lung cancer.

In this study, the frequency of the XPG 1104Asp allele among the healthy controls was higher than among the lung cancer patients (0.50 and 0.45, respectively). The Asp/Asp genotype was associated with a significantly decreased risk of lung cancer compared with the combined His/His and His/Asp genotype. The association between XPG polymorphisms and the risk of lung cancer is biologically plausible as the XPG protein plays an important role in the NER mechanism that is responsible for the repair of DNA lesions induced by environmental genotoxic agents, such as tobacco carcinogens. A normal NER function requires interaction between many DNA repair gene products including XPG (1721). The His1104Asp polymorphism is located in the XPG C-terminus, which is required for interactions with the XPB, XPD, p62 and p44 subunits of transcription factor IIH in the incision complex of NER (10,20). The C-terminal extremities are conserved in character rather than sequence, being very rich in basic amino acids (2628). Therefore, an amino acid change from basic His to acidic Asp could alter the binding to other proteins in the incision complex and the NER capacity. Another possible explanation for the association between the XPG codon 1104 polymorphism and lung cancer is that this polymorphism may be in a linkage disequilibrium with either another polymorphism, thereby altering the function of XPG, or with an adjacent susceptibility gene.

Genetic susceptibility to lung cancer may depend on the level of carcinogen exposure (2931). Therefore, the association between tobacco smoke exposure and the distribution of the XPG genotypes was examined in smokers [as the number of former smokers among the cases was small (n = 19), the former smokers were combined with the current smoker as one group]. When the subjects were dichotomized into two groups based on the median pack-years of smoking, the protective effect of the Asp/Asp genotype against lung cancer was statistically significant in the group with low level of exposure, yet not significant in the group of heavier smokers. This finding is biologically plausible, as genetic differences in risk may be smaller at high doses of carcinogens when environmental influences may overpower any genetic predisposition (2426). However, it is possible that such a finding was attributable to chance due to the relatively small numbers in the subgroups. Thus, additional studies with more subjects are needed to confirm this finding.

When the lung cancers were analyzed by histologic type, it was found that the protective effect of the XPG Asp/Asp genotype was significant for squamous cell carcinoma and small cell lung cancer, but not for adenocarcinoma. Various lines of evidence suggest that the histological type of lung cancer may be determined by the particular initiating agent to which an individual is exposed (3234). Considering the strong relationship of polyaromatic hydrocarbons in tobacco smoke to squamous cell carcinoma and small cell lung cancer, this finding may have been due to the fact that XPG plays an important role in the repair of the DNA damage induced by polyaromatic hydrocarbons in tobacco smoke. However, the lack of statistical significance for adenocarcinoma might be due to the small number of adenocarcinoma cases. A large study is therefore warranted to confirm this finding.

One must consider potential biases that might influence the results of case-control studies: primarily selection bias and information bias (35,36). First, there may be a selection bias. Given that most lung cancer patients are treated at university hospitals in Korea, the demographics and clinical characteristics of the cancer patients in the current study were compatible with those of a nationwide lung cancer survey (24). Furthermore, as all the lung cancer patients diagnosed at a national university hospital were included in this study, it is reasonable to assume that the case group represents the lung cancer cases in our community. Secondly, another selection bias may derive from controls that did not participate in this study. However, because the age and sex distribution of non-participating controls were similar to those of the participating controls in the current study, a non-participant bias is unlikely. Thirdly, the disease status may have been misclassified. All the cancer cases in this study were pathologically confirmed and the controls were proved by a health examination. Therefore, this type of bias is also unlikely. Fourthly, the exposure may have been misclassified because of a differential recall between the cases and the controls during the interview. However, the cases and controls were all interviewed using the same instrument and the questionnaires rechecked by randomly re-interviewing 10% of the subjects, which generated similar results. Therefore, a recall bias is also unlikely.

In conclusion, we found that the XPG His1104Asp polymorphism was associated with the risk of lung cancer. It is possible that our findings, particularly from the stratified analyses, are attributable to chance due to the relatively small numbers in the subgroups. Therefore, the functional relevance of this XPG polymorphism and its role in cancer susceptibility remain to be determined in larger epidemiological studies.


    Notes
 
5 To whom correspondence should be addressed Email: jaeyong{at}kyungpook.ac.kr Back


    Acknowledgments
 
This study was supported in part by the KOSEF through the Biomolecular Engineering Center at Kyungpook National University.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

  1. Sellers,T.A., Bailey-Wilson,J.E., Elston,R.C., Wilson,A.F., Elston,G.Z., Ooi,W.L. and Rothschild,H. (1990) Evidence for mendelian inheritance in the pathgenesis of lung cancer. J. Natl Cancer Inst., 82, 1272–1279.[Abstract]
  2. Shields,P.G. and Harris,C.C. (2000) Cancer risk and low-penetrance susceptibility genes in gene–environment interactions. J. Clin. Oncol., 18, 2309–2315.[Abstract/Free Full Text]
  3. Hecht,S.S. (1999) Tobacco smoke carcinogens and lung cancer. J. Natl Cancer Inst., 91, 1194–1210.[Abstract/Free Full Text]
  4. Mohrenweiser,H.W. and Jones,I.M. (1998) Variation in DNA repair is a factor in cancer susceptibility: a paradism for the promises and perils of individuals and population risk estimation? Mutat. Res., 400, 15–24.[ISI][Medline]
  5. Hoeijmakers,J.H.J. (2001) Genome maintenance mechanisms for preventing cancer. Nature, 411, 366–374.[CrossRef][ISI][Medline]
  6. Wood,R.C., Mitchell,M., Sgouros,J. and Lindahl,T. (2001) Human DNA repair genes. Science, 291, 1284–1288.[Abstract/Free Full Text]
  7. Bernstein,C., Bernstein,H., Payne,C.M. and Garewal,H. (2002) DNA repair/pro-apoptotic dual-role proteins in five major DNA repair pathways: fail-safe protection against carcinogenesis. Mutat. Res., 511, 145–178.[ISI][Medline]
  8. Yu,Z., Chen,J., Ford,B.N., Brackley,M.E. and Glickman,B.W. (1999) Human DNA repair systems: an overview. Environ. Mol. Mutagen., 33, 3–20.[CrossRef][ISI][Medline]
  9. Thompson,L.H. and West,M.G. (2000) XRCC1 keeps DNA from getting stranded. Mutat. Res., 459, 1–18.[ISI][Medline]
  10. Wakasugi,M. and Sancar,A. (1999) Order of assembly of human DNA repair excision nuclease. J. Biol. Chem., 274, 18759–18768.[Abstract/Free Full Text]
  11. Batty,D.P. and Wood,R.D. (2000) Damage recognition in nucleotide excision repair of DNA. Gene, 241, 193–204.[CrossRef][ISI][Medline]
  12. Wei,Q., Cheng,L., Hong,W.K. and Spitz,M.R. (1996) Reduced DNA repair capacity in lung cancer patients. Cancer Res., 56, 4103–4107.[Abstract]
  13. Cheng,L., Eicher,S.A., Guo,Z., Hong,W.K., Spitz,M.R. and Wei,Q. (1998) Reduced DNA capacity in head and neck cancer patients. Cancer Epidemiol. Biomarkers Prev., 7, 465–468.[Abstract]
  14. Park,J.Y., Lee,S.Y., Jeon,H.-S. et al. (2002) Polymorphism of the DNA repair gene XRCC1 and risk of primary lung cancer. Cancer Epidemiol. Biomarkers Prev., 11, 23–27.[Abstract/Free Full Text]
  15. Park,J.Y., Lee,S.Y., Jeon,H.-S. et al. (2002) Lys751Gln polymorphism in the DNA repair gene XPD and risk of primary lung cancer. Lung Cancer, 36, 15–16.[CrossRef][ISI][Medline]
  16. Park,J.Y., Park,S.H., Choi,J.E. et al. (2002) Polymorphisms of the DNA repair gene XPA and risk of primary lung cancer. Cancer Epidemiol. Biomarkers Prev., 11, 993–997.[Abstract/Free Full Text]
  17. Mudgett,J.S. and MacInnes,M.A. (1990) Isolation of the functional human excision repair gene ERCC5 by intercosmid recombination. Genomics, 8, 623–633.[ISI][Medline]
  18. Shiromi,T., Harada,Y., Saito,T., Shiomi,N., Okuno,Y. and Yamaizumi,M. (1994) An ERCC5 gene with homology to yeast RAD2 is involved in group G xeroderma pigmentosum. Mutat. Res., 314, 167–175.[ISI][Medline]
  19. O'Donovan,A., Davies,A.A., Moggs,J.G., West,S.C. and Wood,R.D. (1994) XPG endonuclease makes the 3' incision in human DNA nucleotide excision repair. Nature, 371, 432–435.[CrossRef][ISI][Medline]
  20. Wakasugi,M., Reardon,J.T. and Sancar,A. (1997) The non-catalytic function of XPG protein during dual incision in human nucleotide excision repair. J. Biol. Chem., 272, 16030–16034.[Abstract/Free Full Text]
  21. Araujo,S.J. and Wood,R.D. (1999) Protein complex in nucleotide excision repair. Mutat. Res., 435, 23–33.[ISI][Medline]
  22. Nouspikel,T. and Clarkson,S.G. (1994) Mutations that disable the DNA repair gene XPG in a xeroderma pigmentosum group G patient. Hum. Mol. Genet., 3, 963–967.[Abstract]
  23. Emmert,S., Schneider,T.D., Khan,S.G. and Kraemer,K.H. (2001) The human XPG gene: gene architecture, alternative splicing and single nucleotide polymorphisms. Nucleic Acids Res., 29, 1443–1452.[Abstract/Free Full Text]
  24. Lee,C.-T., Kang,K.H., Koh,Y., Chang,J., Chung,H.S., Park,S.K., Yoo,K.-Y. and Song,J.S. (2000) Characteristics of lung cancer in Korea, 1997. Lung Cancer, 30, 15–22.[CrossRef][ISI][Medline]
  25. Iyer,N., Reagan,M.S., Wu,K.-J., Canagarajah,B. and Friedberg,E.C. (1996) Interactions involving the human RNA polymerase II transcription/nucleotide excision repair complex TFIIH, the nucleotide excision repair protein XPG, and Cockayne Syndrome Group B (CSB) protein. Biochemistry, 35, 2157–2167.[CrossRef][ISI][Medline]
  26. Madura,K. and Prakash,S. (1986) Nucleotide sequence, transcript mapping, and regulation of the RAD2 gene of Saccharomyces cerevisiae. J. Bacteriol., 166, 914–923.[ISI][Medline]
  27. Scherly,D., Nouspikel,T., Corlet,J., Ucla,C., Bairoch,A. and Clarkson,S.G. (1993) Complementation of the DNA repair defect in xeroderma pigmentosum group G cells by a human cDNA related to yeast RAD2. Nature, 363, 182–185.[CrossRef][ISI][Medline]
  28. MacInnes,M.K., Dickson,J.A., Hernandez,R.R. et al. (1993) Human ERCC5 cDNA-cosmid complementation for excision repair and bipartite amino acid domains conserved with RAD proteins of Saccharomyces cerevisiae and Schzosaccharomyces pombe. Mol. Cell. Biol., 13, 6393–6402.[Abstract]
  29. Vineis,P. (1997) Molecular epidemiology: low-dose carcinogens and genetic susceptiblity. Int. J. Cancer, 71, 1–3.[ISI][Medline]
  30. Nakachi,K., Iami,K., Hayashi,S., Watanabe,J. and Kawajiri,K. (1991) Genetic susceptibility to squamous cell carcinoma of the lung in relation to cigarette smoking dose. Cancer Res., 51, 5177–5180.[Abstract]
  31. Xu,L.L., Wain,J.C., Miller,D.P., Thurston,S.W., Su,L., Lynch,T.J. and Christiani,D.C. (2001) The NAD(P)H:quinone oxidoreductase 1 gene polymorphism and lung cancer: differential susceptibility based on smoking behavior. Cancer Epidemiol. Biomarkers Prev., 10, 303–309.[Abstract/Free Full Text]
  32. Deutsch-Wenzel,R., Brune,H., Grimmer,G., Dettbarn,G. and Misfeld,J. (1983) Experimental studies in rat lungs on the carcinogenicity and dose-response relationships of eight frequently occuring environmental polyaromatic hydrocarbons. J. Natl Cancer Inst., 71, 539–544.[ISI][Medline]
  33. Hoffman,D., Rivenson,A., Murphy,S.E., Chung,F.-L., Amin,S. and Hecht,S.S. (1993) Cigarette smoking and adenocarcinoma of the lung: the relevance of nicotine-derived nitrosamines. J. Smoking Relat. Disord., 4, 165–190.
  34. Smith,C.J., Livingston,S.D. and Doolittle,D.J. (1997) An international literature survey of ‘IARC group I carcinogens’ reported mainstream cigarette smoke. Food Chem. Toxicol., 35, 1107–1130.[CrossRef][ISI][Medline]
  35. Garcia-Closas,M., Rothman,N. and Lubin,J. (1999) Misclassification in case-control studies of gene--environment interactions: assessment of bias and sample size. Cancer Epidemiol. Biomarkers Prev., 8, 1043–1050.[Abstract/Free Full Text]
  36. Grimes,D.A. and Schulz,K.F. (2002) Bias and causal associations in observational research. Lancet, 359, 248–252.[CrossRef][ISI][Medline]
Received March 28, 2003; revised June 19, 2003; accepted June 25, 2003.