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Fred Hutchinson Cancer Research Center and University of Washington, Seattle, Washington
Requests for reprints: Anneclaire J. De Roos, Fred Hutchinson Cancer Research Center and University of Washington, 1100 Fairview Avenue North, M4-B874, Seattle, WA 98109-1024. Phone: 206-667-7315; Fax: 206-667-4787. E-mail: deroos{at}u.washington.edu
| Abstract |
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| Introduction |
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The potential of exogenous chemical exposures to cause non-Hodgkin's lymphoma is unclear probably because exposures have been difficult to evaluate and quantify over a period relevant to lymphomagenesis. By studying genes involved in biotransformation of these chemicals and their metabolites, associations between non-Hodgkin's lymphoma risk and variants in specific metabolic genes may provide clues about which of the many chemical exposures to pursue in future research. We studied variation in metabolic genes in a population-based case-control study in the United States. We selected several genes known to play a role in metabolizing a broad spectrum of substrates, including pesticides, organochlorines, solvents, and PAHs, such as the phase I cytochrome P450 enzymes (CYP1A1, CYP1B1, CYP2C9, and CYP2E1), the phase II glutathione S-transferases (GSTP1 and GSTM3), and epoxide hydrolase (EPHX1). Additional genes were selected based on involvement in metabolizing a specific substrate of interest, such as PON1 for organophosphate pesticide metabolism, AHR for its involvement in organochlorine metabolism through transcriptional regulation of CYP1A1 and other genes, and NQO1 for benzene and PAHs. Following gene selection, we studied known variants within those genes that were chosen based on their frequencies and functional consequence.
| Materials and Methods |
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65 years old. To parallel the distributions among cases, all controls were stratified by age (in 5-year categories) and gender within each geographic region and by race in Detroit and Los Angeles County, where oversampling of African Americans occurred. In total, 1,057 (44%) of selected potential controls were interviewed, representing 52% of those approached. Major reasons for nonresponse were subject refusal and failure to locate. Written informed consent was obtained from each participant before interview. A computer-assisted personal interview was administered that contained questions about demographic characteristics, lifestyle factors, occupational history, and pesticide use history. All study participants were asked to provide a venous blood or mouthwash buccal cell sample, and these were shipped to the biological repository for processing and storage. Demographic characteristics (age, education, and gender) for individuals who provided blood versus buccal cells and compared with those who provided neither type of sample were equivalent within each study site. In this analysis of metabolic gene variants in relation to non-Hodgkin's lymphoma, we evaluated the 1,172 (89%) cases and 982 (93%) controls who provided biological samples from which DNA was successfully extracted (Table 1
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Laboratory Methods
DNA Extraction. DNA was extracted from blood clots or buffy coats (from 10 mL blood) at BBI Biotech Repository (Gaithersburg, MD) using Puregene Autopure DNA Extraction kits (Gentra Systems, Minneapolis, MN). DNA was extracted from buccal cell samples by phenol chloroform extraction methods (15).
Genotyping. Genotyping methods have been described previously (16). For this study, we selected 15 variants in 11 metabolic genes (Table 2 ) based on expected population prevalence >5% (for homozygous variant and heterozygote combined), with evidence of functional consequence and/or evidence of an association with lymphoma-related risk conditions in animal or human studies. All genotyping was conducted at the National Cancer Institute (NCI) Core Genotyping Facility (Advanced Technology Corp., Gaithersburg, MD) using the Taqman platform. Sequence data and assay conditions can be found at http://snp500cancer.nci.nih.gov. Blood samples were genotyped first, and buccal samples were subsequently analyzed when there was sufficient buccal cell DNA. In all, we had data from blood and buccal samples for seven variants and data from only blood samples for eight variants. Successful genotyping was achieved for 96% to 100% of DNA samples for all variants. Completion rates were generally slightly higher among blood-based samples, significantly so for the EPHX1 H139R variant. A small percentage of samples (2.7% of controls and 1.9% of cases; P > 0.05) showed chromosomal sex status that differed from gender status for the subject; these subjects were excluded from risk analyses of genotype-non-Hodgkin's lymphoma associations.
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Statistical Analyses
For each genotype and non-Hodgkin's lymphoma outcome (all non-Hodgkin's lymphoma, all B-cell non-Hodgkin's lymphoma, and all T-cell non-Hodgkin's lymphoma), we estimated odds ratios (OR) and 95% confidence intervals (95% CI) by logistic regression using the common homozygous genotype as the reference group. We calculated the Ptrend based on a three-level ordinal variable (0, 1, 2) of the common homozygote, heterozygote, and variant homozygote. Genotype-non-Hodgkin's lymphoma associations for mutually exclusive specific B-cell histologic subtypes were modeled using polytomous logistic regression. All genotype risk estimates were adjusted for age (indicator variables for ages <35, 35-44, 45-54, and 55-64, with age
65 as the reference), gender (male or female), and race (indicator variables for Black and White races, with other race as the reference), as these were all study design variables. We evaluated the risk of false-positive findings by estimating values which account for the false discovery rate (FDR, the Benjamini-Hochberg adjustment; ref. 18). The false discovery rate reflects the expected ratio of false-positive findings to the total number of significant findings. FDR values were estimated for the genotype effects (three-level trend test variable or dichotomous genotype) for all NHL, for B- and T-cell lymphomas, and for B-cell histologic subtypes.
We conducted additional analyses of associations with all non-Hodgkin's lymphoma stratified by age (<50 and
50 years), gender (male or female), and race (non-Hispanic Whites or Blacks) to further explore consistency of associations observed in analyses of all cases. Interactions of metabolic gene variants with the smoking status were examined in an exploratory analysis first by evaluating the gene variant effect stratified by smoking status (never, former, or current) and then by estimating the statistical significance of a multiplicative interaction term for the dichotomous gene variant (heterozygotes and variant homozygotes combined) with smoking (one interaction term for current smoking and one interaction term for former smoking, with never smoking as the reference). However, statistical power for examining these interactions was limited, as information on smoking was by design obtained from approximately half of the study participants.
| Results |
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Genes and variants evaluated for this study are listed in Tables 2 and 3 (18). Prevalences of variant genotypes in the control group were similar to previously reported values (http://snp500cancer.nci.nih.gov/snp.cfm; ref. 20). One variant (AHR R554K) was not in Hardy-Weinberg Equilibrium in Black controls; quality-control inquiries showed no discrepancies for this variant.
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The PON1 L55M variant AA genotype was associated with increased risk of non-Hodgkin's lymphoma (OR, 1.36; 95% CI, 0.96-1.95 FDR value = 0.37). This association was observed in non-Hispanic White subjects (OR, 1.44; 95% CI, 1.00-2.08; only one Black control and no Black case subject carried the variant homozygote genotype) and was fairly consistent among age and gender groups examined. The PON L55M variant was associated with follicular non-Hodgkin's lymphoma (heterozygotes OR, 1.31; 95% CI, 0.90-1.92; variant homozygotes OR, 2.12; 95% CI, 1.27-3.52; Ptrend = 0.003 FDR value = 0.20) and T-cell lymphoma (heterozygotes OR, 1.64; 95% CI, 0.81-3.33; variant homozygotes OR, 2.93; 95% CI, 1.21-7.08; Ptrend = 0.02 FDR value = 0.38) but not with other histologic subtypes.
There was no overall association with non-Hodgkin's lymphoma for the other gene variants we examined in AHR, CYP1A1, CYP1A2, EPHX1, GSTM3, GSTP1, NQO1, or PON1 (Q192R) and no trends by numbers of variant alleles. In addition, the analyses of specific histologic subtypes and analyses stratified by age, race, and gender did not reveal any notable associations (Supplementary Tables are available online).
There were no readily interpretable interactions between metabolic gene variants and smoking (Supplementary Table is available online). Significant interactions between former smoking and EPHX1 H139R (P = 0.03) and PON1 Q192R (P = 0.01) did not have clear biologically meaningful interpretation, as the interactions resulted from inverse associations of the variant genotype among former smokers and positive associations among nonsmokers.
| Discussion |
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There have been few previous studies of metabolic gene variants in relation to non-Hodgkin's lymphoma, and several variants were investigated for the first time in our study. Sarmanova et al. conducted a study in Prague, Czech Republic, of 143 non-Hodgkin's lymphoma cases and 455 population-based controls (21, 22). They did not observe any association with the risk of non-Hodgkin's lymphoma for the CYP2E1 1054C>T variant. Similar to our study, they found no overall associations between risk of non-Hodgkin's lymphoma and EPHX1 variants Y113H or H139R or NQO1 P187S, although subgroup associations according to gender and clinicopathologic characteristics were reported. In a hospital-based study of 169 cases and 205 controls in Australia, Kerridge et al. (23) observed significantly increased non-Hodgkin's lymphoma risk associated with the PON1 Q192R GG genotype; this finding was not replicated in our study. Finally, a study of 389 non-Hodgkin's lymphoma cases and 535 controls in Nebraska found no association between GSTP1 I105V variant and non-Hodgkin's lymphoma risk, similar to our findings (24).
No previous study has evaluated variants in CYP1B1 with risk of non-Hodgkin's lymphoma or other lymphohematopoietic cancers. CYP1B1, like other cytochrome P450 enzymes, plays an important role in phase I metabolism by activating chemicals, such as PAHs and dioxins, to create oxidized, reactive intermediates. CYP1B1 also plays an important role in estrogen metabolism by catalyzing the formation of 4-hydroxyestradiol, a carcinogenic metabolite that retains significant estrogenic activity. The V432L G (Val) allele has been associated with higher activity, resulting in increased oxidation of benzo(a)pyrene and formation of 4-hydroxyestradiol than the C (Leu) allele (25, 26). Furthermore, induction of CYP1B1 by PAHs and/or dioxins has been reported (26, 27), and higher induction has been associated with the G versus C allele of the V432L variant (27). We observed increased non-Hodgkin's lymphoma risk associated with the G allele, which fits our hypothesized biological mechanism that increased CYP1B1 activity leading to increased oxidation and formation of toxic intermediates would contribute to increased cancer risk. It is possible that if increased CYP1B1 activity is truly a risk factor for non-Hodgkin's lymphoma, the effect we observed was weak in the context of our general population sample, which likely had relatively low exposures to PAH and other relevant substrates (e.g., as opposed to a highly exposed occupational cohort). At present, we cannot link PAH exposure to the genotype data because PAH exposure modeling is still under way in our study; this is a topic for future analyses.
We observed decreases in non-Hodgkin's lymphoma risk associated with a variant in CYP2E1. CYP2E1 metabolizes and activates solvent carcinogens that also act to induce its expression, including benzene, styrene, carbon tetrachloride, ethylene glycol, and ethanol (20). In an oral clearance study, the 1054C>T (also called RsaI) variant was associated with decreased metabolic activity (29); however, other functional studies have shown higher expression of the variant allele (30). More information on the functional consequence of the variant is necessary to inform the meaning of our observed association with CYP2E1 1054C>T.
PON1, a high-density lipoprotein-associated enzyme that is synthesized in the liver and secreted into plasma, is a phase II enzyme involved in hydrolyzing various phase I intermediates, such as the highly toxic oxon metabolites of several organophosphate pesticides, including chlorpyrifos, diazinon, and parathion (31). The PON1 L55M methionine allele has shown lower paraoxonase activity than the more common allele (32). The modest association we observed was in the hypothesized direction of increased non-Hodgkin's lymphoma risk associated with lower phase II activity that might make a person more susceptible to toxicity from exogenous chemical exposures, including organophosphate insecticides. However, this variant was only associated with follicular B-cell lymphoma and T-cell non-Hodgkin's lymphoma, and there was no a priori reason to suspect specific associations by histologic subtype. The other variant we studied in PON1 was not associated with the risk of non-Hodgkin's lymphoma, and further studies are needed to completely characterize the role of these variants in PON1 function.
The essentially null effects we observed for most of the metabolic gene variants in relation to non-Hodgkin's lymphoma may reflect the context of exposures within the general population represented in our study. For example, the AHR gene may not have great importance in the context of relatively low levels of exposure to dioxins and dioxin-like compounds in the population (33). It is also possible that the variants we selected within candidate metabolic genes did not adequately characterize variability in gene function. Nevertheless, our study adds to the literature about the potential contribution of variation in metabolic genes to non-Hodgkin's lymphoma. These data provide some evidence for CYP1B1, CYP2E1, and PON1 in non-Hodgkin's lymphoma etiology but do not indicate importance of most of the other genes we examined. The modest associations we observed could be false-positive results due to chance variation, as the FDR values for our key findings were relatively large. These results are therefore considered suggestive and require replication in other study populations and in large, pooled analyses.
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 3/13/06; revised 5/16/06; accepted 7/11/06.
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