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Cancer Epidemiology Biomarkers & Prevention Vol. 14, 1626-1632, July 2005
© 2005 American Association for Cancer Research

Joint Effects between UDP-Glucuronosyltransferase 1A7 Genotype and Dietary Carcinogen Exposure on Risk of Colon Cancer

Lesley M. Butler1, Yannick Duguay2, Robert C. Millikan3, Rashmi Sinha5, Jean-François Gagné2, Robert S. Sandler3,4 and Chantal Guillemette2

1 Department of Public Health Sciences, Division of Epidemiology, University of California, Davis, California; 2 Pharmacogenomics Laboratory, Oncology and Molecular Endocrinology Research Center, Centre Hospitalier de l'Université Laval and Faculty of Pharmacy Laval University, Québec, Canada; Departments of 3 Epidemiology and 4 Medicine; University of North Carolina, Chapel Hill, North Carolina; and 5 Division of Cancer Epidemiology and Genetics, National Cancer Institute, Rockville, Maryland

Requests for reprints: Chantal Guillemette, Canada Research Chair in Pharmacogenomics, Pharmacogenomics Laboratory, CHUQ Research Center, 2705 Boulevard Laurier, Quebec, Canada, G1V 4G2. E-mail: Chantal.Guillemette{at}crchul.ulaval.ca


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The UDP-glucuronosyltransferase 1A7 (UGT1A7) gene is polymorphic and encodes an enzyme involved in the detoxification of heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH). Consumption of pan-fried and well-done meat are surrogates for HCA and PAH exposure and are possibly associated with colon cancer. We have evaluated whether UGT1A7 allelic variations are associated with colon cancer and whether UGT1A7 genotype modified associations among meat intake, exposure to HCAs and PAHs, and colon cancer in a population-based case-control study of African Americans (197 cases and 202 controls) and whites (203 cases and 210 controls). As part of a 150-item food frequency questionnaire, meat intake was assessed by cooking method and doneness and used to estimate individual HCA and PAH exposure. UGT1A7 alleles (UGT1A7*1, UGT1A7*2, UGT1A7*3, and UGT1A7*4) were measured and genotypes were categorized into predicted activity groups (high: *1/*1, *1/*2, *2/*2; intermediate: *1/*3, *1/*4, *2/*3; low: *3/*3, *3/*4, *4/*4). There was no association with UGT1A7 low versus high/intermediate genotype [odds ratio (OR), 1.1; 95% confidence interval (95% CI), 0.7-1.8], regardless of race. Greater than additive joint effects were observed for UGT1A7 low genotype and HCA-related factors. For example, equal to or greater than the median daily intake of the HCA, 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and having UGT1A7 low genotype was positively associated with colon cancer (OR, 2.4; 95% CI, 1.2-4.8), compared with less than the median daily intake and UGT1A7 high/intermediate genotypes. These data suggest that the associations among cooked meat–derived compound exposure, and colon cancer are modified by the UGT1A7 genotype.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Heterocyclic amines (HCA) and polycyclic aromatic hydrocarbons (PAH) are known mutagens and possible human carcinogens (1-3) formed in meat while it is cooked and are most concentrated on the meat surface. The optimal conditions for HCA formation include high-temperature cooking such as pan-frying and grilling (4-6). Meat that is cooked above a heat source, by methods such as grilling or barbecuing, contain the highest levels of PAHs because the meat is exposed to smoke formed from the pyrolysis of fatty juices that drip down onto the heat source (7). Consumption of pan-fried, grilled, or barbecued well-done meat are surrogates for HCA and PAH exposure and may be positively associated with colon cancer (8).

UDP-glucuronosyltransferases (UGT) are a family of enzymes that catalyze the glucuronidation of both endogenous compounds, such as bilirubin and steroid hormones, as well as exogenous compounds, such as environmental carcinogens and dietary constituents (9). Glucuronidation is a primary route of detoxification of the HCA, 2-amino-1-methyl-6-phenyl-imidazo[4,5-b]pyridine (PhIP) and its carcinogenic intermediate 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP; refs. 10-12). Results from a human exposure study identified N2-OH-PhIP-N2-glucuronide as the predominant urinary metabolite (13), suggesting that a large proportion of ingested PhIP is converted into N-OH-PhIP and subsequently conjugated with glucuronic acid by UGTs (14, 15). In contrast, the extent of the in vivo role of the glucuronidation pathway for other HCAs, such as 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) remains to be shown but most likely involved UGT1A enzyme family (16). In vitro evidence is present for the glucuronidation of the PAH, benzo(a)pyrene, and its hydroxylated derivatives, resulting in the detoxification of these compounds (17, 18).

The polymorphic isozyme UGT1A7 has been specifically implicated in the glucuronidation of HCAs (19, 20) and benzo(a)pyrene (21, 22). The UGT1A7*3 and UGT1A7*4 alleles have shown a lower catalytic activity towards a number of substrates including PhIP and the 3-hydroxy-benzo(a)pyrene, 7-hydroxy-benzo(a)pyrene, and 9-hydroxy-benzo(a)pyrene derivatives, suggesting that these variants confer a slow glucuronidation phenotype (19, 21). In addition, localization of glucuronidation throughout the digestive tract provides strong etiologic evidence for its role in HCA and benzo(a)pyrene-mediated carcinogenesis in the gut (23, 24).

Few epidemiologic studies have examined UGT1A7 alleles in relation to cancer. The presence of the UGT1A7*3 allele was reported to have an almost 3-fold association [odds ratio (OR), 2.8; 95% confidence interval (95% CI), 1.6-4.7] with colorectal cancer compared with the wild-type UGT1A7*1 allele (19). However, this case-control study was relatively small and hospital-based. Given the biological plausibility for the role of UGTs in the etiology of colon cancer, a larger epidemiologic study conducted in a population-based sample with relevant UGT substrate exposure information is needed to more fully explore whether UGT1A7 polymorphisms are associated with colon cancer.

We recently reported modest positive associations with increasing intake for well-done and pan-fried red meat, and the HCA, DiMeIQx from the North Carolina Colon Cancer Study, a population-based, case-control study of African Americans and whites (8). In the analyses presented here, we have evaluated the hypothesis that lower UGT1A7 predicted activity genotypes are associated with an increased risk of colon cancer and that lower UGT1A7 activity results in greater susceptibility to dietary sources of HCA and benzo(a)pyrene exposure on risk of colon cancer.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Study Population
Cases and controls of the North Carolina Colon Cancer Study were selected from 33 counties in North Carolina and frequency matched to cases by race, age, and sex (25). Cases were selected through a rapid ascertainment system (26) and were eligible if they were between 40 and 80 years of age at first primary diagnosis of invasive adenocarcinoma of the colon and diagnosed between October 1, 1996 and June 30, 2000. Controls were randomly selected from North Carolina Division of Motor Vehicle lists if they were under 65 years of age, or from the Center for Medicare and Medicaid Services list if they were ≥65 years. Completed interviews were obtained from 701 African Americans (274 cases and 427 controls) and 957 whites (346 cases and 611 controls). Of those who were eligible, 84% of cases and 62% of controls were interviewed. The study was approved by the Institutional Review Board at the University of North Carolina School of Medicine and by equivalent committees at the collaborating hospitals.

Exposure Assessment
Questionnaires were administered in person in the participants' homes by specially trained registered nurses. The questionnaire collected information on lifestyle factors, such as physical activity and tobacco use; medical, family, and work histories; and use of over-the-counter medications. A 150-item food frequency questionnaire was used to measure usual dietary intake over the year before diagnosis for cases, or year before date of selection for controls (27). The questionnaire was modified to assess individual exposure to dietary carcinogens based on a meat cooking and doneness module developed by Sinha et al. (28). Details regarding the collection of dietary history and specifically HCA and PAH exposure have been previously documented (8). In brief, questions were added to assess 14 meat and fish items (i.e., hamburgers/cheeseburgers, beef steaks, pork chops/ham steaks, bacon, sausage, hot dogs, fried chicken, chicken/turkey, and fried fish/shellfish/fish sandwich) for frequency of intake, portion size (i.e., small, medium, or large), and cooking method. Color photographs were shown of each meat type (i.e., hamburger, steak, pork chop, bacon, and chicken/turkey) to facilitate reporting of cooking doneness. Meat intake frequency data, cooking method, and level of doneness were used to estimate values of three HCAs (MeIQx, PhIP, and DiMeIQx) and benzo(a)pyrene, using an exposure index that has been previously described in detail (28, 29).

Genotyping
Of the individuals with completed questionnaire data, 88% (93% of cases and 85% of controls) also agreed to provide a blood sample for DNA analyses. The 399 African Americans (197 cases and 202 controls) and 413 whites (203 cases and 210 controls) included for this analysis were based on all of the DNA samples available at the time this project was initiated and were selected in order of enrollment in the study.

We assessed whether there were differences between individuals who provided a blood sample and those who did not. Cases and controls who did not provide blood samples were more likely to be female (P < 0.01) and White (P < 0.01). There were no other significant differences (e.g., by age, education level, income, family history of colorectal cancer, smoking status, or total meat intake). In addition, there were no appreciable differences in the associations among meat intake, HCA exposure, and colon cancer, among cases and controls who provided blood samples, and the associations calculated among all cases and controls.

Genomic DNA was extracted from whole blood specimens using the PureGene DNA isolation kit (Gentra Systems, Inc., Minneapolis, MN). To determine UGT1A7 genotype at position 129/131 and 208, two PCR-based assays were used to measure four alleles: *1 (Asn129Arg131Trp208), *2 (Lys129Lys131Trp208), *3 (Lys129Lys131Arg208), and *4 (Asn129Arg131Arg208). The Taqman assay was used to discriminate the polymorphisms at codons 129 and 131 (Applied Biosystems, Branchburg, NJ), as previously described (30). RFLP methods were used to discriminate the polymorphism at codon 208, as previously described (31).

The following quality control measures were employed. First, positive and negative controls were included in each PCR and Taqman experiment. Homozygote wild-type, heterozygote, and homozygote variants for each UGT1A7 genotype from genomic DNA samples of known (via direct DNA sequencing) UGT1A7 genotype were included. Second, repeated assays were conducted on five randomly selected samples from each experiment. There was 100% agreement. Third, five additional randomly selected samples were confirmed by direct DNA sequencing. Fourth, laboratory personnel were blinded to the case status of the samples.

Statistical Analysis
UGT1A7 allele and genotype frequencies were calculated among African Americans and whites, cases and controls, separately. A {chi}2 test was used to assess differences in allele frequencies between cases and controls. UGT1A7 genotypes were categorized into the following imputed activity groups, based on the reduced HCA and PAH detoxification activity associated with the UGT1A7*3 and UGT1A7*4 alleles, as previously reported (19, 21): high (*1/*1, *1/*2, *2/*2), intermediate (*1/*3, *1/*4, *2/*3), and low (*3/*3, *3/*4, *4/*4). Observed UGT1A7 genotype frequencies among controls were compared with expected genotype frequencies, calculated based on observed allele frequencies under the assumption of Hardy-Weinberg equilibrium (32). The Pearson {chi}2 statistics (with degrees of freedom = number of alleles – 1) was used to test whether the expected number of individuals was significantly different from the observed number of individuals with each genotype, stratified by race.

All meat (by type, cooking method, and doneness preference), HCA (MeIQx, DiMeIQx, and PhIP), and benzo(a)pyrene variables were derived from food frequency questionnaire responses. The HCA and benzo(a)pyrene variables were derived by multiplying grams of meat intake (stratified by type, doneness, and method) by the compound concentration (ng/d) measured in that meat type. These variables were dichotomized, based on the median value (e.g., < and ≥ median) among controls. These variables have been previously described in detail (8).

For continuous covariates, tertile cut points were determined based on the distributions among all controls. These covariates included intake of fruits, vegetables, dietary fiber, total fat, dietary folate, and total energy; physical activity, height, weight, and body mass index (kg/m2). Fat intake was adjusted for total caloric intake using the residual method (33). Alcohol consumption was low in this population and therefore categorized as ever/never drank wine, beer, or liquor in the past year. Previously reported findings in this study population included an inverse association for dietary fiber (34) and no association with folate (35) or alcohol consumption (34) with risk of colon cancer.

Adjusted ORs and 95% CIs for colon cancer were calculated from unconditional logistic regression models (36). PROC LOGISTIC of the software package SAS (version 8.1; SAS Institute, Cary, NC) was used with the option in the MODEL statement to incorporate offsets, which takes into account the selection probabilities by age, race, and sex (25). Multivariable gene effects models included the following variables to adjust for potential confounding: race (African American and white), 5-year age groups (≤45, 46-50, ..., ≥76 years), and sex. Multivariable joint effect models included the previously mentioned variables for race, age, and sex, in addition to dietary fiber, total fat, and total energy intake. Potential confounding was assessed by calculating the percent change observed in the ORs for various meat intake variables. The covariates included in the multivariable models resulted in a ≥10% difference in the ORs when added individually to the model. Covariates that were assessed but did not fulfill the criteria for confounding were mean daily folate intake (>276.6 and ≤276.6 µg), smoking (ever, never; current, former, never; current, formerly smoked for ≥36 years, formerly smoked for <36 years, never), mean body mass index (>28.5 and ≤28.5 kg/m2), and alcohol intake in past year (ever, never beer, wine, or liquor).

Potential interaction or joint effects for UGT1A7 genotype and meat-related exposures on risk of colon cancer were evaluated overall and separately among African Americans and whites. Interaction on the multiplicative scale was evaluated by the fit of an interaction term in the model, where P < 0.10 for the likelihood ratio test was interpreted as a statistically significant finding. Indicator variables were created to estimate the joint effects between dietary exposures and UGT1A7, where individuals with the lowest hypothesized associations, less than the median daily intake, and combined UGT1A7 high/intermediate genotypes, comprised the common reference group (OR00). These ORs were used to assess the expected joint effects for either additive (OR10 + OR 01 – OR00 > OR11) or multiplicative interaction (OR10 x OR01 > OR11), where OR10 was for high intake and UGT1A7 high/intermediate genotype, OR01 was for low intake and UGT1A7 low genotype, and OR11 was for their combined effects. Interaction contrast ratios (ICR) and 95% CIs were used to assess the magnitude and precision of the departure from additive joint effects, where ICR = (OR11 – OR10 – OR01 + 1; refs. 37, 38). For interpretation, an ICR > 0 implies joint effects are greater than additive (synergy), an ICR < 0 implies joint effects are less than additive (antagonism), and an ICR = 0 implies no departure from additivity.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Characteristics of the study population are presented stratified by race in Table 1. In general, distribution of demographic and dietary characteristics was similar to what was previously reported for all North Carolina Colon Cancer Study participants (8). A summary of the differences observed among controls by race include a greater percentage of African Americans that had a lower level of education compared with whites, and there was a slightly greater percentage of ever smokers among whites than among African Americans (results not shown). For dietary factors, controls consumed less energy, similar fiber, and less fat compared with cases, regardless of race. The trend of greater total and red meat consumption among cases compared with controls was more apparent in African Americans than in whites. Median levels of intake were greater for all meat-related compounds among cases compared with controls, except for benzo(a)pyrene in whites. Among controls, MeIQx and PhIP were higher among African Americans, and benzo(a)pyrene was higher among whites. A detailed description of meat-related intake correlations in the entire North Carolina Colon Cancer Study population has been previously described (8). Briefly, among controls regardless of race, the strongest correlations for DiMeIQx, MeIQx, PhIP, and mutagenicity were with well/very well done red meat, and the strongest correlation for benzo(a)pyrene was with grilled/barbecued red meat (8).


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  		Table 1. Demographic characteristics and daily dietary intake among colon cancer cases and population-based controls: The North Carolina Colon Cancer Study

 
Genotype and allele frequencies for UGT1A7 among African Americans and whites are presented in Table 2. The UGT1A7*3 allele and the *3/*3 genotype were less common among African-American controls compared with white controls. There were no differences in allele frequencies between cases and controls, regardless of race. The *2/*3 genotype frequency was slightly lower among white cases than controls, but genotype differences were not statistically significant. UGT1A7 genotype frequencies were in Hardy-Weinberg equilibrium for African-American controls (P = 0.571) and white controls (P = 0.113).


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  		Table 2. UGT1A7 allele and genotype frequencies: The North Carolina Colon Cancer Study

 
When UGT1A7 genotypes were grouped based on predicted activity assessed in vitro (21), we observed similar frequencies of the low-activity alleles (*3/*3, *3/*4, and *4/*4) among cases and controls for both African Americans (0.06 cases and 0.05 controls) and whites (0.17 cases and 0.17 controls). There was no association with colon cancer for UGT1A7 low versus UGT1A7 high or combined high/intermediate genotypes, overall or when assessed separately among African Americans or whites (Table 3).


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  		Table 3. ORs for UGT1A7 genotype and colon cancer among all participants, African Americans and Whites: The North Carolina Colon Cancer Study

 
Estimated joint effects between UGT1A7 genotype and meat intake and cooked meat-derived HCA and benzo(a)pyrene exposure in relation to colon cancer are presented in Table 4, where the common reference group is individuals with less than the median dietary intake and UGT1A7 high/intermediate genotype. UGT1A7 modified the associations of several meat-related and colon cancer associations, although depending on the variable, the direction of interaction suggested either synergy or antagonism. For example, greater than additive joint effects were observed for UGT and well/very well done red meat and DiMeIQx, where ICRs were >0.5. For these factors, a moderate positive association was observed among individuals with the highest intake and UGT1A7 low genotype, whereas null to weak associations were observed for meat/compound intake or genotype alone. Departure from interaction on the multiplicative scale was not statistically significant for UGT and well/very well done red meat (likelihood ratio test, P = 0.707) or DiMeIQx (likelihood ratio test, P = 0.342) on risk of colon cancer. Similar results were observed for UGT genotype and pan-fried meat. For PhIP, the ICRs were close to zero. Less than additive effects were observed for UGT1A7 and red meat, grilled/barbecued red meat, and benzo(a)pyrene, where the ORs for genotype alone were greater than those for the joint effects and all three ICRs were <–0.5. The joint effects for UGT and meat-related factors differed by race only for DiMeIQx. However, because these ORs were imprecise, only results combining African Americans and whites are presented.


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  		Table 4. Main effects of meat-related intake and joint effects of meat-related intake and UGT1A7 genotype on the association with colon cancer: The North Carolina Colon Cancer Study

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Individual effects of UGT1A7 genotype and joint effects with meat-related intake on the risk of colon cancer were estimated using data from a population-based, case-control study of African Americans and whites in North Carolina. No association was observed for UGT1A7 genotype, comparing low to combined high and intermediate predicted activity groups, among African Americans or whites. Greater than additive joint effects were present for UGT1A7 and well/very well done red meat and the HCA, DiMeIQx.

Our result for the individual effect of UGT1A7 genotype does not support the previously reported positive association for the UGT1A7*3 allele and colorectal cancer (19). However, the previous finding from a study by Strassburg et al. (19) may be a result of treatment-related selection bias. Irinotecan-based chemotherapy is a first-line treatment of advanced metastatic colorectal cancer, but toxicity in the form of severe neutropenia and diarrhea impede its use. Polymorphisms in the UGT1A7 are thought to be responsible in part for increased toxicity (31), because of increased biotransformation of 7-ethyl-10-hydroxycamptothecin (SN-38), the pharmacologically active metabolite of irinotecan (39). It is possible that the prevalent cases available for participation in the previous study were more likely to have UGT1A7 low-activity alleles, because they were more likely to have received more aggressive chemotherapy and survive long enough to be recruited. Genotype was not likely related to participant enrollment in the North Carolina Colon Cancer Study, because colon cancer cases were recruited within 6 months of diagnosis using a rapid ascertainment protocol (26).

We reported a slightly lower frequency of the *1 wild-type allele among White controls than the range (0.32-0.42) previously reported in German (19) and U.S. control groups (21, 30, 31) from either blood donor (21, 30) or clinic-based (19) populations. UGT1A7*1 allele frequency among African-American controls was almost identical to the 0.38 frequency previously reported (30). Prevalence of the UGT1A7 low-activity alleles among our population-based control group were similar to previous reports for the *4 allele among African Americans (30) and whites (19, 21) and for the *3 among African Americans (30). However, we observed a greater frequency for the *3 allele among whites compared with German (19) and U.S. control groups (21, 30). These previous studies were not population based, which may account for the differences seen in allelic prevalence.

We found contrasting patterns of interaction between types of meat and meat-related compound intake and UGT1A7 genotype on risk of colon cancer. Greater than additive joint effects were present for UGT1A7 and well/very well-done red meat and the HCA, DiMeIQx. Unexpectedly, there was a suggestion of antagonism between UGT1A7 and grilled/barbecued red meat and the PAH, benzo(a)pyrene. The joint effects between meat intake or cooked meat–derived compound exposure and UGT polymorphisms have not been previously reported, although we had hypothesized that genotypes thought to confer less activity would increase the susceptibility for colon cancer among those with increased intake of HCAs and benzo(a)pyrene, and their surrogates, such as increased intake of well-done meat, or pan-fried meat. The similar effects for well/very well-done red meat and DiMeiQx and for grilled/barbecued red meat and benzo(a)pyrene are consistent with our previously reported finding for stronger correlations between individual HCAs and well-done meat, and for benzo(a)pyrene and grilled/barbecued red meat, than for other combinations (8). However, the contrasting direction of interaction remains difficult to explain, but it may reflect the variable degree of involvement of the glucuronidation pathway and more precisely, the UGT1A7 protein, in the in vivo metabolism of HCAs their N-hydroxylated metabolites and benzo(a)pyrene derivatives (21–23, 40, 41). For example, recent in vitro metabolic investigations showed that UGT1A7 plays a lesser role in the metabolism of N-OH-PhIP (42), that what was previously proposed (20). Instead, UGT1A1 may be the primary UGT involved in N-OH-PhIP glucuronidation (43). Another possible explanation for the contrasting joint effects is that the UGTs are differentially inducible by other dietary compounds, such as flavonoids (44, 45), which may effect expression of the individual isozymes.

Among individual HCAs, statistically joint effects were only present for DiMeIQx and UGT1A7. This was somewhat unexpected, because PhIP has the highest levels in cooked meat (5, 6, 46), and N-OH-PhIP has been documented as a good substrate for UGT1A7 (20). Despite overall levels, there is some evidence for greater carcinogen potential with DiMeIQx than with PhIP or MeIQx (47); however, both PhIP and MeIQx but not DiMeIQx have been documented as possible human carcinogens (2, 3). The possibility that the observed statistically significant joint effect for DiMeIQx and UGT1A7 genotype may be due to chance given our small sample size, or due to multiple comparisons must be considered.

A limitation in our study was the retrospective assessment of diet. Cases may recall usual diet differently than controls, because of the effect of disease on dietary habits, resulting in biased ORs towards or away from the null (48). Bias due to disease-related changes in diet were of small concern, because total meat, red meat, and DiMeIQx levels did not significantly differ by stage of disease (results not shown), suggesting that if there were changes in diet following diagnosis, they were minimal.

Misclassification of gene effects may be an issue in these data, because genotype data was used to make assumptions about metabolic activity. We used the same categorization strategy of previous epidemiologic studies (30) for easier comparison. In addition, previous studies of UGT alleles and glucuronidation of benzo(a)pyrene derivatives and PhIP have indicated that the *3 and *4 alleles have the lowest activity (20, 21).

Another source of misclassification is that the effects observed may be independent of the UGT1A7 genotype and linked to polymorphisms in other members of the UGT1A subfamily. Functional polymorphisms in UGT1A enzymes expressed in the liver and extrahepatic tissues that are involved in the in vitro metabolism of HCAs and benzo(a)pyrene have also been reported for UGT1A1 (42), UGT1A4 (49), UGT1A6 (50), UGT1A8 (51), UGT1A9 (31, 52), and UGT1A10 (53). The entire UGT1 family is derived from a single gene locus (UGT1) which is composed of 17 exons (54). To synthesize the final protein, only one of 13 different exon 1 sequences on the locus is associated with four downstream exons, common to all UGT1A isoforms. It is expected that significant linkage disequilibrium exists between the UGT1A7 polymorphisms and variants of other UGT1A genes relevant to the in vivo detoxification of HCAs and benzo(a)pyrene, because of the genomic structure of the UGT1A genes. For example, it has been shown that the variation at codon 208 of the UGT1A7 gene is linked to the UGT1A1*28 promoter variation (55), and this allele was also recently shown to modulate the glucuronidation of both N-OH-PhIP and benzo(a)pyrene derivatives in human livers (43, 56).6 Therefore, it is possible that UGT1A genes other than UGT1A7 are more biologically relevant in the metabolism of HCAs, such as UGT1A1.

Our findings do not support an increased risk of colon cancer due to polymorphisms in UGT1A7. However, our data do support possible joint effects between UGT1A7 and dietary HCAs and benzo(a)pyrene on risk of colon cancer in a population-based case-control study. Our data also suggest that the metabolic genetic effects, such as those between UGT1A7 and benzo(a)pyrene, may be more relevant at lower exposures, because at high levels the exposure is likely to saturate the enzyme activity and diminish the differences between "high" and "low" glucuronidation (57, 58). To the best of our knowledge, this is the first study to estimate joint effects between a UGT isozyme and dietary carcinogen exposure for colon cancer. In addition, the physiologic importance of UGT1A7 in colon tissue is unclear because expression of this isozyme has not been confirmed to be present in the colon in all studies (24, 59). The diversity of catalytic activity and substrate binding affinity of a number of additional UGT proteins involved in colon carcinogen detoxification and their expression found in colon tissue (10, 11, 59), may overshadow the effects of UGT1A7, especially in the colon. It may prove useful to further characterize individual isozymes for their etiologic relevance with colon cancer and as possible modifiers of the associations between dietary and environmental carcinogens, such as HCAs and PAHs.


   Acknowledgments
 
We thank Michael Malfatti for his careful review and helpful comments during the preparation of this article.


   Footnotes
 
Grant support: Center for Environmental Health and Susceptibility grant NIEHS P30-ES10126 (R.C. Millikan), Lineberger Comprehensive Cancer Center core grant P30-CA16086 (R.C. Millikan), NIH grants RO1-CA 66635 and P30 DK 34987 (R.S. Sandler), Canadian Institutes of Health Research grant 177282 (C. Guillemette), the Canada Research Chair Program (C. Guillemette), Canadian Institutes of Health Research studentship award (Y. Duguay), and Canada's Research-Based Pharmaceutical Companies (Y. Duguay).

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.

Note: L.M. Butler and Y. Duguay contributed equally to this article.

6 H. Girard et al. UDP-Glucuronosyltransferase 1A1 polymorphisms are important determinants of dietary carcinogen detoxification, in press. Back

Received 9/16/04; revised 4/22/05; accepted 4/27/05.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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