Introduction. Glutathione S-transferases M1 ( GSM1) and T1 (GSTT1) enzymes may play a critical role in lung carcinogenesis. They inactivate reactive carcinogens, e.g. tobacco-derived polycyclic aromatic hydrocarbons, and prevent DNA damage, such as formation of DNA adducts. Copy number variants of GSTM1 and GSTT1 may alter detoxifying capability and modulate the risk of lung cancer. Isothiocyanates, derived from cruciferous vegetables, may protect against lung cancer by inducing GSTM1 and GSTT1 expression and promoting cell apoptosis. To better understand the etiologic factors involved in tobacco-related lung carcinogenesis, we investigated the association between copy number variant alleles of GSTM1 and GSTT1 and lung cancer risk in a population-based sample. In addition, we investigated the role of isothiocyanates in relation to lung cancer risk and explored for possible interaction between GSTs and isothiocyanates.
Methods. A systematic review and quantitative meta-analyses were conducted to evaluate the epidemiological evidence for the association between total cruciferous vegetable consumption and lung cancer risk. To address the two specific aims, a casecontrol study, nested in the CLUE II cohort, with 274 cases and 1089 matched controls were conducted. Study participants provided blood sample, basic demographic information, and dietary data at baseline. Lung cancer cases and cancer-free controls were carefully matched on smoking history. GSTM1 and GSTT1 were genotyped using recently developed PCR assay that quantified copy variant alleles. Three genotypes (+/+, +/0, 0/0) corresponded to 0, 1, and 2 copy number of variant alleles, respectively, for GSTM1 and GSTT1 genes were assessed. Summary measures were created for total consumption of cruciferous vegetables, total vegetables, non-cruciferous vegetables, fruits and combined fruits and vegetables. Consumption of each dietary group was divided into quartiles using the distribution of the controls. Conditional logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs).
Results. Phenotypic differences were observed between the +/+ and the +/0 genotypes of GSTM1 with regards to lung cancer risk. Non-statistical significant higher lung cancer risks were associated with the +/0 (OR: 1.48; 95% CI: 0.65-3.35) and 0/0 (OR: 1.78; 95% CI: 0.80-3.97) genotypes compared to the wild-type +/+ genotype of GSTM1, with the greatest risks among ever smokers who smoked up to a pack of cigarettes per day (OR: 3.25; 95% Cl: 30.94-11.86). The test for trend suggested higher lung cancer risk with copy-number of variant alleles of GSTM1 (p-value for trend: 0.07). The risks associated with variant of GSTT1 were unclear. The results showed no association between variant alleles of GSTT1 and lung cancer risk compared to +/+ genotype. However, positive associations were observed for smokers who smoked more than a pack of cigarettes per day (OR: 2.41, 95% CI: 0.83-6.99; OR: 1.96, 95% CI: 0.61-6.34, respectively).
Comparing highest-versus-lowest quartiles of cruciferous vegetable intake, we found associations, adjusted for various dietary confounders, in the direction of protection against lung cancer risk (ORs ranged: 0.87-0.76), but none were statistically significant. The greatest reduction in risk of lung cancer were observed for current smokers (OR: 0.62; 95% CI: 0.33-1.16, adjusted for combined fruit and vegetable intake) and individuals with 0/0 genotype (OR: 0.52; 95% CI: 0.18-1.51). Meta-analyses of casecontrol and prospective cohort studies showed statistically significant 23% and 17% reductions in lung cancer risk for the highest-versus-lowest category of cruciferous vegetable intake, respectively. The reduction in risk was strongest (59%) among those with GSTM1 null compared to the present genotype.
Conclusions. The dissertation's findings suggest that variant alleles of GSTM1 modulate the risk associated with lung cancer and that smokers with at least one variant allele (+/0 or 0/0 genotype) may be more susceptible to lung cancer risk compared to individuals with two copies of the functional alleles. The measures of association found by this dissertation research were within the magnitude and protective direction as those previously reported from prospective cohort studies. Coupled with the pooled associations observed from our meta-analysis of these studies, the accumulating evidence suggests that higher intake of isothiocyanates may be weakly protective against lung cancer, above and beyond the confounding effects of cigarette smoking. The protection conferred by isothiocyanates may be more pronounced among current smokers and individuals with homozygous alleles of GSTM1.
Future studies with greater statistical precision are needed to provide a more precise characterization of GST genes, isothiocyanates, and their inter-relationship in relation to lung cancer risk before definitive conclusions can be drawn. Clarification of these findings will have significant public health impact. Genetic testing can be used to identify high-risk individuals for more targeted intervention and counseling against cigarette smoking. Higher intake of cruciferous vegetables could be recommended for individuals who are smokers and carriers of variant alleles of GSTM1 to reduce the harmful effects of cigarette smoking and consequently lung cancer burden.