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International Agency for Research on Cancer, F-69372 Lyon, France [N. M., A-M. C-R., S. L., M. L., P. B.]; Institut Municipal dInvestigació Mèdica, E-08003 Barcelona, Spain [N. M.]; Institute of Environmental Medicine, Karolinska Institute, S-17177 Stockholm, Sweden [F. N.]; Bremen Institute for Prevention Research, D-28199 Bremen, Germany [W.A.]; Institute of Public Health, R-76256 Bucharest, Romania [V. C.]; Institute of Carcinogenesis, Cancer Research Center, 115478 Moscow, Russia [A. Mu.]; INSERM U157, F-94805 Villejuif, France [S. B.]; Pneumological Hospital, 60-568 Poznan, Poland [H. B-G.]; GSF Institute for Epidemiology, D-85758 Munich, Germany [I. B-H.]; Veneto Cancer Registry, Department of Oncology, University of Padua, I-35128 Padua, Italy [L. S.]; Federal University of Rio Grande do Sul, 96100 Pelotas, RG, Brazil [A. Me.]; School of Public Health, University of California, Berkeley, California 94720-7360 [S. L.]; and Uppsala University, S-75123 Uppsala, Sweden [M. L.]
| Abstract |
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| Introduction |
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Lung cancer in nonsmokers has also been associated with dietary factors (OR range, 0.40.7 for consumption of vegetables; Refs. 57 ), indoor exposure to fumes from cooking oils (OR range, 1.43.8; Refs. 79 ), coal or wood combustion (OR range, 1.32.5; Refs. 7, 9 ), occupation (OR range, 1.53.0; Ref. 10 ), prior lung diseases (OR range, 1.55.9; Refs. 6, 7, and 9 ), and family history of lung cancer (OR, 2.3; Ref. 8 ).
Because the excess risk from exposure to the above-mentioned factors is
relatively low, it is important to identify individuals that might be
more susceptible to lung cancer development. Individual variability in
activating and detoxifying carcinogens may explain varying
susceptibilities of developing lung cancer. The equilibrium between
phase I (activating) and phase II (detoxifying) enzymes may contribute
to this variability. Phase II enzymes are involved in detoxification of
mutagens, carcinogens, and other substances (11, 12)
. GST
family members (mu and theta) are among the most studied phase II
enzymes regarding cancer susceptibility (13)
. There are
well-defined genetic polymorphisms in the expression of GSTM1 and GSTT1
enzymes. The GSTM1*2 (null) genotype, inherited in a
dominant fashion, determines the absence of the enzyme in
approximately 4555% of Caucasians (12, 14)
. Lack of
GSTM1 enzyme has been associated with susceptibility to lung cancer
among smokers (12, 13)
. GSTT1*2 homozygosity is
associated with the absence of the GSTT1 enzyme. GSTT1 null
prevalence in Caucasians is
20%, but its role as a susceptibility
factor for lung cancer is not well established (12, 13)
.
The study of the effect of GST polymorphism in nonsmokers provides the opportunity to assess the role of genetic susceptibility factors in lung cancer among individuals exposed to a low level of carcinogens. We conducted an international collaborative study to investigate the role of metabolic polymorphism of GST enzymes in lung carcinogenesis among nonsmokers and their interaction with environmental risk factors.
| Materials and Methods |
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Information on demographics and environmental exposures was obtained through a personal interview using a standard questionnaire. A screening questionnaire allowed us to distinguish between smokers (subjects who smoked >400 cigarettes in their lifetime), occasional smokers (smokers of up to 400 cigarettes in their lifetime), and nonsmokers (pure never smokers). Occasional smokers were considered together with nonsmokers on the basis that estimates for the risk of lung cancer were below one, and results did not change when they were included in the analyses. We collected information on exposure to ETS from several sources: parents, spouses, other cohabitants at the same house during adulthood life, workplace, and vehicles. We considered in particular exposure to ETS from the spouse and at the workplace because in previous studies these were the main predictors of urinary cotinine (15) and showed the strongest association with lung cancer risk in a previous IARC case-control study of nonsmokers (4) . A weighted duration of ETS exposure was calculated as hours of exposure/day and times the years of exposure from the spouse, and as hours/day, times the days/week of exposure at the workplace. To reduce misclassification, which has been shown to act mainly at low levels of exposure (16) , subjects were then categorized in three levels: never exposed and exposed below or above the 75th percentile of the distribution among controls. Furthermore, exposure to known and suspected occupational lung carcinogens was considered on the basis of a job exposure matrix (17) . Residential history was categorized as living more than 75% lifetime in a rural setting (cities <50,000 inhabitants) or in an urban setting (cities >50,000 inhabitants) or mixed. Use of wood or coal for cooking or heating was considered as source of indoor air pollution: duration of exposure was categorized in two groups according to the median of years of exposure to both sources of combustion among controls. Hospital records were reviewed to collect information on histological type of tumors.
Samples of peripheral blood (30 ml) were collected before any treatment in most patients. Plasma, red, and WBCs were separated for all individuals except for some from Sweden, from whom only whole blood was available. Samples were frozen in each center and stored at -80°C.
DNA was extracted from either whole blood or WBCs, using proteinase K
(10 mg/ml) digestion at 55°C for 30 min. The presence of
GSTM1 and GSTT1 genes was first tested using a
multiplex PCR. To amplify both genes at the same time, primers M1E4 and
M1E5, corresponding to exons 4 and 5 of the GSTM1 gene
(18, 19)
, and primers T11 and T12, corresponding to 3'
noncoding region of the GSTT1 gene (19, 20)
,
were used. In addition to specific GSTs oligonucleotides, a
pair of primers of ß-globin (G3 and G4) was included as an internal
amplification control (21)
. Ten µl of crude DNA extract
(
0.2 µg DNA) were added to a final volume of 50 µl containing
10x PCR buffer [50 mM KCl, 10
mM Tris-HCl (pH 8.3), and 1
mM MgCl2], 5
mM deoxynucleoside triphosphate mixture, 10
µM of each primer, 1 unit of Taq DNA polymerase
(Perkin-Elmer Cetus, Norwalk, CT), and sterile water. A PCR program of
30 cycles (94°C denaturation for 1 min, 52°C annealing for 1 min,
and 72°C extension for 1 min) was conducted using a Perkin-Elmer 9600
thermal cycler (Perkin-Elmer Cetus), preceded by a 2.5-min denaturation
phase at 94°C and followed by an extension step of 72°C for 9 min.
Ten µl of PCR product were analyzed by 3% agarose gel
electrophoresis (NuSieve GTG), and gels were stained with ethidium
bromide (20 µg/ml). An UV transilluminator was used to evaluate the
PCR product content. Individuals carrying the GSTM1 and
GSTT1 genes were identified by the presence of 273- and
480-bp DNA fragments, respectively (Fig. 1A
). Eighty-six subjects from Sweden were reanalyzed in a
different laboratory (22)
, and results were concordant in
all but one. The case was not excluded from the study because identical
results were obtained after a reanalysis of this sample.
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Statistical analyses were conducted using STATA software (STATA
Corp., College Station, TX). Comparison of qualitative variables
was performed using Pearsons
2 test or
Fishers exact test when 20% or more of the cells had expected counts
of less than five. Students t test and Mann-Whitney tests
were used to assess the different distribution of a normal and
nonnormal continuous variable, respectively (25)
. Age,
gender, and center adjusted ORs of lung cancer and 95% CIs were
calculated by unconditional logistic regression models
(26)
. The association of GSTs polymorphism to
lung cancer was stratified by histological type, gender, age, and
educational level. The role of GST polymorphism as a
modifier of the effect of environmental exposures on lung cancer risk
was analyzed both by stratification and by including the interaction
term for each exposure-genotype combination in the logistic regression
model. Results were considered significant at the two-sided
P of 0.05 level.
| Results |
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Table 1
shows the main demographic characteristics, as well as information on
environmental exposures of cases and controls. Eighty-six % of cases
and 72% of controls were women. Controls were younger than cases (mean
age, 59 versus 64 years; P = 0.004). The
proportion of subjects exposed to ETS from a spouse was higher among
cases than among controls (P = 0.04). In addition, a
higher proportion of cases were occasional smokers (P =
0.1), lived in a rural setting (P = 0.004), and were
exposed to indoor pollution from wood combustion (P =
0.001) than controls.
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To improve the statistical power of the study and to eliminate the potential control selection bias, a case-case analysis was conducted to assess the modifier effect of GST genotypes on the association of selected exposures with lung cancer. As expected, no important differences from the case-control approach were observed. Occupational exposures was the only variable in which an interaction with GSTM1 genotype was observed (OR, 2.5; 95% CI, 0.78.8; P = 0.156). Although nonsignificant, possible because of the small number of subjects, occupational exposures deserve more attention in future studies.
| Discussion |
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Our results show that, as in smokers (12) , GSTM1*2 genotype is associated with an increased risk for lung cancer in nonsmokers. The overall excess risk for this genotype is 50%, although it varies when subjects are stratified according to exposure. The factors that showed the greatest change in GSTM1-associated risk were indoor wood combustion, area of residence, and occupational exposure to carcinogens. However, when lung cancer risk because of indoor pollution from wood combustion was considered, taking into account the area of residence, the estimates became nonsignificant, although they still show an increase of risk: OR for all subjects, 1.7 (95% CI, 0.64.6), for GSTM1*1 and GSTM1*2 individuals 1.8 (95% CI, 0.47.6) and 3.0 (95% CI, 0.615), respectively. No confounding effect was observed when indoor pollution from coal combustion was modeled. ORs for residential history retained their former values in both models. Regarding this latter risk factor, we found a significantly decreased in risk of lung cancer among GSTM1*2 patients who did live in urban areas in comparison to those who lived in rural areas. Furthermore and in agreement with the case-control results, a case-case design conducted within the series of cases of the present study showed that both occupational exposures (OR, 2.5; 95% CI, 0.78.8) and a residential history in a urban area (OR, 0.4; 95% CI, 0.11.3) presented interesting patterns of interaction with GSTM1 genotype. The study Nielsen et al. (31) conducted in nonsmoker healthy residents of rural and urban areas found no effect of GSTM1*2 genotype on DNA or protein adduct levels. Possible explanations of this discrepancy could be differences among centers in control selection criteria applied; in frequency of metabolic polymorphisms that could interact with GSTM1 null genotype (29) ; and in prevalence of additional environment factors, such as vitamins, that could also modify the interaction of GSTM1 and the study variables on lung cancer risk (32) . Subjects exposed to passive smoking and occasional smokers did not show an excess risk. Nyberg et al. (22) have recently published the only study on GSTM1 polymorphism and lung cancer in a nonsmoker series, and they found no association with the null genotype. In addition to random variation, the authors pointed out methodological issues to explain contradictory results among studies.
From our results, GSTM1*2 genotype conferred a higher risk for squamous and small cell carcinomas than for adenocarcinoma. This finding agrees with our knowledge of the role of GSTs enzymes in detoxification of tobacco-related carcinogens because adenocarcinoma type is less associated with tobacco consumption, and consistent results regarding squamous and small cell histology have been shown from studies conducted in smokers (12) .
In accordance with Bennett et al. (33) , we found that GSTT1*2 genotype did not present an increased risk for lung cancer. Our a priori hypothesis postulated that null GSTT1 subjects (GSTT1*2 genotype) would be at higher risk of cancer because they have a reduced ability to conjugate carcinogens to excretable hydrophilic metabolites (12) . Previous epidemiological studies on the role of GSTT1*2 genotype in lung carcinogenesis have not demonstrated a consistent increase of lung cancer risk in smokers (24, 34, 35) . Rebbeck (12) points to the fact that some GSTT1 metabolites could act as tissue-specific mutagens. Regarding other tobacco-related neoplasms, two studies (36, 37) have found a statistically significant increased risk for bladder cancer only among nonsmokers with GSTT1 null genotype. In studies of other tumors, no consistent results on the role of GSTT1 enzyme in cancer susceptibility have been reported (12, 38) . This result is further substantiated by the study conducted by Rojas et al. (39) in which they did not observe differences in benzo[a]pyrene diol epoxide-DNA adduct level between GSTT1 null and active genotypes in polycyclic aromatic hydrocarbon-exposed workers.
According to the hypothesis mentioned above, subjects with the GSTM1*2-GSTT1*2 haplotype, 14% of lung cancer cases in our series, would show the highest risk for lung cancer. Nevertheless, our results do not support such a contention. In agreement with our findings, To-Figueras et al. (24) did not find an increased lung cancer risk among smokers with this haplotype. On the other hand, in our data the GSTM1*2-GSTT1*1 haplotype had an overall 80% excess risk for lung cancer, although this difference did not reach statistical significance.
The following limitations should be considered in interpreting our results. The statistical power of the study is low, mainly for assessing the modifying role of GSTs on the effect of some environmental risk factors on lung cancer (40) . Furthermore, false-positive results attributable to multiple comparisons cannot be discarded, although we present only data derived from a prespecified hypothesis. Controls were selected from two different sources, i.e., hospital and population. GST null genotypes could be associated with illnesses among hospital controls or some characteristics, gender, age, and tobacco habits, in the general population (41) . We observed different estimates for GSTM1*2 genotype when we stratified the analysis according to the controls origin, population (sex and age adjusted OR, 2.36; 95% CI, 1.065.28; P = 0.036) versus hospital (adjusted OR, 1.13; 95% CI, 0.542.39; P = 0.746). Unfortunately, an insufficient number of subjects was available to stratify the analysis by this factor. Scarce bibliography is available on population heterogeneity of GST polymorphisms among European Caucasians (20) . However, the possibility that, if present, this fact could bias our results was checked by assessing the differences among GSTM1*2 and GSTT1*2 prevalences among controls from population-based and hospital-based centers. No significant variability was observed; nevertheless, results were all adjusted for center. Finally, recall information on past exposure to ETS could be subject to exposure misclassification, resulting in bias (42) . Because misclassification is more likely to be present at low doses of ETS (16) , we addressed this potential problem by categorizing the ETS variables in three groups: never exposed, and subjects exposed to less and more than 75% percentile of the dose-index variable computed for each passive smoking source. In addition, the questionnaire used in this study allows the collection of detailed information on ETS exposure and has previously been applied and validated in a larger case-control study (4, 22) .
Our study suggests that the effect of GSTM1 polymorphism in nonsmokers is similar to that found in smokers. It does not seem to interact with ETS exposure, although we cannot exclude that it does in selected groups of individuals in association with exposure to other specific environmental carcinogens. Because of the complexity of lung cancer etiology, it is unlikely that a single polymorphism, either GSTM1*2 or GSTT1*2, could explain most cancer susceptibility. The joint analysis of several metabolic gene polymorphisms implied in carcinogen activation and detoxification may provide new clues on lung carcinogenesis in nonsmokers.
| Acknowledgments |
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| Footnotes |
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1 This study was partially supported by a grant
from the Environment Programme of the European Commission, DG-XII
(Contract No. EV5V-CT94-0555). N. M. was partly supported by a Special
Training Award from the International Agency for Research on Cancer and
by a fellowship from the Direcció General de Recerca, Generalitat
de Catalunya (CIRIT 1996BEA/300015). ![]()
2 To whom requests for reprints should be
addressed, at Institut Municipal dInvestigació Mèdica,
Carrer del Dr. Aiguader 80, E-08003, Barcelona, Spain. Phone:
(3493)-221-1009; Fax: (3493)-221-3237; E-mail: nuria{at}imim.es ![]()
3 The abbreviations used are: ETS, environmental
tobacco smoke; GST, glutathione S-transferase; GSTM1*2,
GST mu null; GSTT1*2, GST theta null; IARC, International Agency for
Research on Cancer; ASA, allele-specific amplification; OR, odds ratio;
CI, confidence interval. ![]()
Received 10/11/99; revised 5/17/00; accepted 5/24/00.
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