Janeric Seidegård

Using the International Project on Genetic Susceptibility to Environmental Carcinogens (GSEC) database containing information on over 15,000 control (noncancer) subjects, the allele and genotype frequencies for many of the more commonly studied metabolic genes (CYP1A1, CYP2E1, CYP2D6, GSTM1, GSTT1, NAT2, GSTP, and EPHX) in the human population were determined. Major and significant differences in these frequencies were observed between Caucasians (n = 12,525), Asians (n = 2,136), and Africans and African Americans (n = 996), and some, but much less, heterogeneity was observed within Caucasian populations from different countries. No differences in allele frequencies were seen by age, sex, or type of controls (hospital patients versus population controls). No examples of linkage disequilibrium between the different loci were detected based on comparison of observed and expected frequencies for combinations of specific alleles.

Glutathione transferase (GT; EC 2.5.1.18) mRNA levels were measured in human liver samples by using mouse and human cDNA clones that encode class-mu and class-alpha GT. Although all the RNA samples examined contained class-alpha GT mRNA, class-mu GT mRNA was found only in individuals whose peripheral leukocytes expressed GT activity on the substrate trans-stilbene oxide. The mouse class-mu cDNA clone was used to identify a human class-mu GT cDNA clone, lambda GTH411. The amino acid sequence of the GT encoded by lambda GTH411 is identical with the 23 residues determined for the human liver GT-mu isoenzyme and shares 76-81% identity with mouse and rat class-mu GT isoenzymes. The mouse and human class-mu GT cDNA inserts hybridize with multiple BamHI and EcoRI restriction fragments in the human genome. One of these hybridizing fragments is missing in the DNA of individuals who lack GT activity on trans-stilbene oxide. Hybridizations with nonoverlapping subfragments of lambda GTH411 suggest that there are at least three class-mu genes in the human genome. One of these genes appears to be deleted in individuals lacking GT activity on trans-stilbene oxide.

Leukocyte isozyme(s) of glutathione transferase (GT-tSBO) has been shown to be dominantly inherited. The frequency of the phenotypes of this isozyme in bronchial carcinoma and control patients matched for age and smoking history is reported here. Control smokers showed an increased likelihood of having GT-tSBO (59%) compared with lung cancer patients (35%). The lack of GT-tSBO was related to the extent of smoking by the lung cancer patients but not to the pathology of the lung tumor. It was concluded that the gene expressing this isozyme(s) may be a host genetic determinant of susceptibility to lung cancer in smokers.

Glutathione transferase are divided into three classes: Alpha, Mu and Pi. Isoenzyme(s) from one of these classes, class Mu, is dominantly inherited and can be determined by activity measurements directed towards the substrate trans-stilbene oxide. The frequency of this phenotype has been measured in patients with bronchial carcinoma and in control subjects matched for age and smoking history. After combining an earlier study from our laboratory (Carcinogenesis, 7, 751-753, 1986) with the additional material presented here (control smokers, n = 114, lung cancers, n = 125) non-cancer smokers had an increased number of subjects who expressed class Mu isoenzymes (58.3% of total n = 192) compared with lung cancer patients (36.6% of total n = 191; P less than 0.0001). The pathology of lung tumors related to the lack of class Mu isoenzymes which occurred most frequently in patients with adenocarcinomas. It is concluded that the gene expressing class Mu isoenzymes may be a host determinant of genetic susceptibility to lung cancer among smokers.

Susceptibility to lung cancer may in part be attributable to inter-individual variability in metabolic activation or detoxification of tobacco carcinogens. The glutathione S-transferase M1 (GSTM1) genetic polymorphism has been extensively studied in this context; two recent meta-analyses of case-control studies suggested an association between GSTM1 deletion and lung cancer. At least 15 studies have been published after these overviews. We undertook a new meta-analysis to summarize the results of 43 published case-control studies including >18 000 individuals. A slight excess of risk of lung cancer for individuals with the GSTM1 null genotype was found (odds ratio (OR) = 1.17, 95% confidence interval (CI) 1.07-1.27). No evidence of publication bias was found (P = 0.4), however, it is not easy to estimate the extent of such bias and we cannot rule out some degree of publication bias in our results. A pooled analysis of the original data of about 9500 subjects involved in 21 case-control studies from the International Collaborative Study on Genetic Susceptibility to Environmental Carcinogens (GSEC) data set was performed to assess the role of GSTM1 genotype as a modifier of the effect of smoking on lung cancer risk with adequate power. Analyses revealed no evidence of increased risk of lung cancer among carriers of the GSTM1 null genotype (age-, gender- and center-adjusted OR = 1.08, 95% CI 0.98-1.18) and no evidence of interaction between GSTM1 genotype and either smoking status or cumulative tobacco consumption.