The role of interindividual variation in human carcinogenesis

Colon Cancer: From Epidemiology to Prevention

Colorectal cancer (CRC) is one of the most prevalent cancers affecting humans, with a complex genetic and environmental aetiology. Unlike cancers with known environmental, heritable, or sex-linked causes, sporadic CRC is hard to foresee and has no molecular biomarkers of risk in clinical use. One in twenty CRC cases presents with an established heritable component. The remaining cases are sporadic and associated with partially obscure genetic, epigenetic, regenerative, microbiological, dietary, and lifestyle factors. To tackle this complexity, we should improve the practice of colonoscopy, which is recommended uniformly beyond a certain age, to include an assessment of biomarkers indicative of individual CRC risk. Ideally, such biomarkers will be causal to the disease and potentially modifiable upon dietary or therapeutic interventions. Multi-omics analysis, including transcriptional, epigenetic as well as metagenomic, and metabolomic profiles, are urgently required to provide data for risk analyses. The aim of this article is to provide a perspective on the multifactorial derailment of homeostasis leading to the initiation of CRC, which may be explored via multi-omics and Gut-on-Chip analysis to identify much-needed predictive biomarkers.

Genetic polymorphisms in CYP1A1, CYP1B1 and COMT genes in Greenlandic Inuit and Europeans

Background

The Indigenous Arctic population is of Asian descent, and their genetic background is different from the Caucasian populations. Relatively little is known about the specific genetic polymorphisms in genes involved in the activation and detoxification mechanisms of environmental contaminants in Inuit and its relation to health risk. The Greenlandic Inuit are highly exposed to legacy persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs), and an elucidation of gene–environment interactions in relation to health risks is needed.

Objectives

The aim of this study was to determine and compare the genotype and allele frequencies of the cytochrome P450 CYP1A1 Ile462Val (rs1048943), CYP1B1 Leu432Val (rs1056836) and catechol-O-methyltransferase COMT Val158Met (rs4680) in Greenlandic Inuit (n=254) and Europeans (n=262) and explore the possible relation between the genotypes and serum levels of POPs.

Results

The genotype and allele frequency distributions of the three genetic polymorphisms differed significantly between the Inuit and Europeans. For Inuit, the genotype distribution was more similar to those reported for Asian populations. We observed a significant difference in serum polychlorinated biphenyl (CB-153) and the pesticide 1,1-dichloro-2,2-bis(p-chlorophenyl)-ethylene (p,p′-DDE) levels between Inuit and Europeans, and for Inuit also associations between the POP levels and genotypes for CYP1A1, CYP1B1 and COMT.

Conclusion

Our data provide new information on gene polymorphisms in Greenlandic Inuit that might support evaluation of susceptibility to environmental contaminants and warrant further studies.

CYP2E1 Rsa I/Pst I polymorphism contributes to oral cancer susceptibility: a meta-analysis

Previous data on association between CYP2E1 Rsa I/Pst I polymorphism and oral cancer risk were controversial. To investigate the association between CYP2E1 Rsa I/Pst I polymorphism and oral cancer risk. We performed a meta-analysis to assess the relationship between oral cancer and genotype with English language until June 2010. Twelve published case-control studies of 1259 patients with oral cancer and 2262 controls were acquired. Odds ratios (ORs) with 95% confidence intervals (CIs) were used to assess the strength of the association in codominant and dominant models. Overall, the pooled ORs indicated a significant association between CYP2E1 Rsa I/Pst I polymorphism and oral cancer risk (for c1/c2 vs. c1/c1: OR=1.30, 95% CI=1.04-1.62, Pheterogeneity=0.57; for (c1/c2+c2/c2) vs. c1/c1: OR=1.32, 95% CI=1.07-1.64, Pheterogeneity=0.57, respectively). In subgroup analysis by race, the same significant risks were found among Asian (for c1/c2 vs. c1/c1: OR=1.41, 95% CI=1.05-1.91, Pheterogeneity=0.92; for (c1/c2+c2/c2) vs. c1/c1: OR=1.43, 95% CI=1.08-1.88, Pheterogeneity=0.97, respectively). In conclusion, this meta-analysis demonstrates that CYP2E1 Rsa I/Pst I c2 allele may be a biomarker for oral cancer, especially among Asian populations.

Meta-analysis and pooled analysis of GSTM1 and CYP1A1 polymorphisms and oral and pharyngeal cancers: a HuGE-GSEC review

The association of GSTM1 and CYP1A1 polymorphisms and oral and pharyngeal cancers was assessed through a meta-analysis of published case-control studies and a pooled analysis of both published and unpublished case-control studies from the Genetic Susceptibility to Environmental Carcinogens database (http://www.upci.upmc.edu/research/ccps/ccontrol/index.html ). Thirty publications used in the meta-analysis included a total of 7783 subjects (3177 cases and 4606 controls); 21 datasets, 9397 subjects (3130 cases and 6267 controls) were included in the pooled analysis. The GSTM1 deletion was 2-fold more likely to occur in African American and African cases than controls (odds ratio: 1.7, 95% confidence interval: 0.9-3.3), although this was not observed among whites (odds ratio: 1.0, 95% confidence interval: 0.9-1.1). The meta-analysis and pooled analysis showed a significant association between oral and pharyngeal cancer and the CYP1A1 MspI homozygous variant (meta-ORm2/m2: 1.9, 95% confidence interval: 1.4-2.7; Pooled ORm2m2: 2.0, 95% confidence interval: 1.3-3.1; ORm1m2 or [infi]m2m2: 1.3, 95% confidence interval: 1.1-1.6). The association was present for the CYP1A1 (exon 7) polymorphism (ORVal/Val: 2.2, 95% confidence interval: 1.1-4.5) in ever smokers. A joint effect was observed for GSTM1 homozygous deletion and the CYP1A1 m1m2 variant on cancer risk. Our findings suggest that tobacco use and genetic factors play a significant role in oral and pharyngeal cancer.

CYP2E1 polymorphisms and gene–environment interactions in the risk of upper aerodigestive tract cancers among Indians

Introduction: The CYP2E1 enzyme is responsible for the metabolic activation of several procarcinogens into reactive metabolites that result in carcinogenesis. The genetic polymorphisms that modify these enzymatic activities may be associated with upper aerodigestive tract cancer risk. Methods: This hospital-based study evaluated CYP2E1*1B, CYP2E1*5B and CYP2E1*6 polymorphisms in 408 histopathologically confirmed cases and 220 population-based controls using PCR-RFLP methods. Results: The multivariate logistic regression analyses demonstrated no significant differences between groups for all three polymorphisms when analyzed separately. However, the gene–environment interactions analyses revealed significant interactions among tobacco smokers (11–20 pack years), 20–40 pack years and >40 pack years), regular tobacco chewers and alcoholics carrying CYP2E1*1B mutant genotypes. Similarly, CYP2E1*6 polymorphisms resulted in significant interactions among tobacco smokers (>40 pack years) and regular tobacco chewers on the multiplicative scale. Conclusion: The significant gene–environment interactions observed for CYP2E1*1B and CYP2E1*6 polymorphic genotypes may confer a substantial risk for upper aerodigestive tract cancers among Indians.

Genetic susceptibility in pancreatic ductal adenocarcinoma

BACKGROUND

The strongest risk factors for pancreatic adenocarcinoma are tobacco smoking and increasing age. However, only a few smokers or elderly individuals develop the disease and genetic factors are also likely to be important.

METHODS

The literature on genetic factors modifying susceptibility to cancer was reviewed, with particular regard to the interindividual variation that exists in the development of pancreatic adenocarcinoma.

RESULTS

Tobacco-derived carcinogen-metabolizing enzyme gene variants have been the main area of study in stratifying the risk of sporadic pancreatic cancer. Inconsistent results have emerged from the few molecular epidemiological studies performed.

CONCLUSION

There is great scope for further investigation of critical pathways and unidentified genetic influences may be revealed. This may eventually allow the identification of individuals at high risk who might be targeted for screening.

Chemical carcinogenesis

The use of chemical compounds benefits society in a number of ways. Pesticides, for instance, enable foodstuffs to be produced in sufficient quantities to satisfy the needs of millions of people, a condition that has led to an increase in levels of life expectancy. Yet, at times, these benefits are offset by certain disadvantages, notably the toxic side effects of the chemical compounds used. Exposure to these compounds can have varying effects, ranging from instant death to a gradual process of chemical carcinogenesis. There are three stages involved in chemical carcinogenesis. These are defined as initiation, promotion and progression. Each of these stages is characterised by morphological and biochemical modifications and result from genetic and/or epigenetic alterations. These genetic modifications include: mutations in genes that control cell proliferation, cell death and DNA repair - i.e. mutations in proto-oncogenes and tumour suppressing genes. The epigenetic factors, also considered as being non-genetic in character, can also contribute to carcinogenesis via epigenetic mechanisms which silence gene expression. The control of responses to carcinogenesis through the application of several chemical, biochemical and biological techniques facilitates the identification of those basic mechanisms involved in neoplasic development. Experimental assays with laboratory animals, epidemiological studies and quick tests enable the identification of carcinogenic compounds, the dissection of many aspects of carcinogenesis, and the establishment of effective strategies to prevent the cancer which results from exposure to chemicals.

Glutathione S-transferase polymorphisms and the synergy of alcohol and tobacco in oral, pharyngeal, and laryngeal carcinoma

Investigations of the ability of polymorphisms in the GSTM1, GSTT1, and GSTP1 genes to alter susceptibility to head and neck squamous cell carcinoma (HNSCC) have examined gene-environment interaction in their detoxification of tobacco-associated carcinogens. Little work has been done to ask if these variant genes also modify the interaction of tobacco and alcohol in the development of HNSCC. To test this hypothesis, we conducted a case-control study, enrolling 692 incident cases of HNSCC and 753 population controls. Information about lifetime tobacco and alcohol use was ascertained through questionnaires, and genotypes for GSTM1, GSTT1, and GSTP1 were determined from constitutional DNA. Genotype frequencies were compared among cases and controls, and the association between genotypes and tobacco use was evaluated on cancer risk through logistic regression. Deletion of GSTM1 was associated with an increased risk for HNSCC [odds ratio (OR), 1.3; 95% confidence interval (95% CI), 1.0-1.6]. GSTT1 deletion was associated with a slight decreased HNSCC risk (OR, 0.8; 95% CI, 0.6-1.0). Among those with GSTM1 present, the OR of cancer for heavy smoking was 2.6 (95% CI, 1.6-4.3) compared with 4.2 for those with the GSTM1 deleted (95% CI, 2.6-6.7). The combination of consuming 10 to 20 alcohol drinks weekly and smoking >45 pack-years was associated with a 13-fold elevated risk (OR, 12.6; 95% CI, 4.0-40.2) among the GSTM1 deleted subjects compared with an OR of 3.6 (95% CI, 1.5-8.7) among the GSTM1 present individuals. These data (showing that the GSTM1 deletion affects on the tobacco and alcohol synergy) suggest that the interaction of these carcinogens is, at least in part, driven by alcohol, enhancing the carcinogenic action of tobacco smoke.

AMINO ACID RESIDUE ILE211 IS ESSENTIAL FOR THE ENZYMATIC ACTIVITY OF HUMAN UDP-GLUCURONOSYLTRANSFERASE 1A10 (UGT1A10)

Conjugation of exogenous and endogenous compounds by uridine diphosphoglucuronosyltransferases (UGTs) is a pathway catalyzing the transfer of a glucuronic acid molecule from UDP glucuronic acid to lipophilic aglycones, which become more polar and more easily excretable in the bile or urine. UGTs are divided into two major families, UGT1 and UGT2. The isoform UGT1A10, along with UGT1A7 and UGT1A8, is expressed exclusively in extrahepatic tissues, notably in the gastrointestinal tract. Here, we report the isolation of a mutant clone of the human UGT1A10, at position 211 of the protein, where a threonine residue replaces an isoleucine residue (allele Thr211). Because the isoleucine is conserved among many UGT1A isoforms, we proceeded to the analysis of the activity of the wild-type UGT1A10 (T211I) and compared it with that of the variant enzyme (I211T(*)). In vitro assays with microsomal extracts from stably expressing human embryonic kidney 293 (HEK293) cells showed that the mutant enzyme lost all detectable activity toward major substrates, which demonstrate that the residue isoleucine at position 211 is essential for UGT1A10 activity. Mutant UGT1A10 (I211T(*)) also lost all detectable activity toward mycophenolic acid. Genomic DNA from 103 unrelated individuals was sequenced for this mutation, and two heterozygous genotypes were detected for this mutation (frequency: 2 per 100 individuals). Because UGT1A10 appears to be expressed in all gastrointestinal tissues and is active toward a wide range of substrates, lack of activity of this isoform may have an impact on individual glucuronidation efficiency.

Gastric cancer and other organ cancer history in the parents of patients with gastric cancer

A case-control study in a population from the Eastern Black Sea region of Turkey was performed to evaluate the risk of gastric cancer development in patients with gastric cancer reporting gastric cancer or other organ cancer history in their parents. Gastric cancer and/or other organ cancer history in the parents were found in 215 of 1240 patients with gastric cancer versus 73 of 1240 controls (odds ratio (OR) 3.35, P<0.001). The frequency of gastric cancer history among the parents was significantly different between the patients and the controls (148 versus 25, respectively; OR 6.59, P<0.001). The frequency of other organ cancer history was not significantly different between the patients and the controls (63 versus 48, respectively; OR 1.33, P>0.05). The frequency of gastric cancer history was significantly higher than the frequency of other organ cancer history in the parents of the patients (OR 4.51, P<0.001). Gastric cancer history was significantly higher in first- to third-degree relatives of the patients reporting gastric cancer and/or other organ cancer history in their parents than in the controls (OR 14.72, P<0.001). Familial clustering of gastric cancer defined by the presence of at least four cancer cases in family members, including parents, was reported by 12% of the patients. Overall, the result of this study of gastric cancer and other organ cancer history in the parents of patients with gastric cancer may suggest a genetic susceptibility for gastric carcinoma. A high risk of gastric cancer occurred in subjects reporting a gastric cancer history in their parents, and there was a higher predisposition to gastric cancer compared with other organ cancer in relatives and a familial clustering of the disease.

Lung cancer andCYP1A1 orGSTM1 polymorphisms

Most chemical carcinogens are metabolized and activated in vivo by phase I enzymes including the microsomal cytochromes P450 and epoxide hydroxylases. The carcinogens and their metabolites are detoxified by phase II enzymes that in clude various transferases such as glutathion-S-transferases (GST). Increasing numbers of studies have demonstrated the association of the polymorphisms inGSTM1 (a member of GST) andCYP1A1 genes with the susceptibility to lung cancer. Subsequently, the polymorphisms appear to be important biomarkers that provide information for assessment of exposure and total burden of environmental carcinogens. Therefore, the investigation of the polymorphisms in these genes will provide information not only for the prediction of individual cancer risk but also for the prevention of cancer. In this review, we will summarize the polymorphisms in theGSTM1 andCYP1A1 genes and their relation to lung cancer susceptibility.

Relationship of folate to colorectal and cervical cancer: review and recommendations for practitioners

Evidence suggests that folate may play a role in cancer prevention. A plausible mechanism for prevention lies in the integral role that folate plays in deoxyribonucleic acid (DNA) synthesis and methylation. DNA methylation most likely regulates gene expression. Abnormal methylation, specifically hypomethylation, has been associated with tumorigenesis. The availability of methyl groups needed for adequate DNA methylation may be negatively influenced by low folate status, alcohol intake, or genetic polymorphisms that affect folate metabolism. Observational studies evaluating the association between folate and risk for colorectal and cervical cancers or precancerous conditions have produced conflicting results, and clinical trial data are needed to confirm a cause-and-effect relationship. However, several studies show interesting associations between cancer risk and factors that influence methyl group availability. Although data relating folate to cancer risk remain equivocal, when coupled with the other potential health benefits associated with folate, evidence supports recommending that people consume folate-rich foods such as fruits and vegetables. People consuming alcohol on a daily basis may especially benefit from additional folate in their diets.

Genetic regulation of ionizing radiation sensitivity and breast cancer risk

Genetic variability in DNA repair may contribute to hypersensitivity to ionizing radiation (IR) and susceptibility to breast cancer. We used samples collected from a clinic-based breast cancer case-control study to test the working hypothesis that amino acid substitution variants of DNA repair genes may contribute to prolonged cell-cycle delay following IR and breast cancer risk. Fluorescence-activated cell sorter (FACS) analysis was used to measure cell-cycle delay. PCR-restriction fragment length polymorphism (RFLP) assays were used to determine four genotypes of three DNA repair genes: XRCC1, 194 Arg/Trp and 399 Arg/Gln; XRCC3, 241 Thr/Met; and APE1, 148 Asp/Glu. The data showed that breast cancer patients had a significantly higher delay index than that of controls (P < 0.001); the means +/- SD for cases and controls were 36.0 +/- 13.1 (n = 118) and 31.4 +/- 11.5 (n = 225), respectively. There was a significant dose-response relationship between delay index, categorized into quartiles, and an increasing risk of breast cancer (crude odds ratios: 1.00, 1.00, 1.27, and 2.46, respectively; P(trend) = 0.002). In controls, prolonged cell-cycle delay was significantly associated with the number of variant alleles in APE1 Asp148Glu and XRCC1 Arg399Gln genotypes (P(trend) = 0.001). Although larger studies are needed to validate the results, our data suggest that an inherited hypersensitivity to IR may contribute to human breast carcinogenesis.

Diet and cancer prevention

Research from several sources provides strong evidence that vegetables, fruits, and whole grains, dietary fibre, certain micronutrients, some fatty acids and physical activity protect against some cancers. In contrast, other factors, such as obesity, alcohol, some fatty acids and food preparation methods may increase risks. Unravelling the multitude of plausible mechanisms for the effects of dietary factors on cancer risk will likely necessitate that nutrition research moves beyond traditional epidemiological and metabolic studies. Nutritional sciences must build on recent advances in molecular biology and genetics to move the discipline from being largely 'observational' to focusing on 'cause and effect'. Such basic research is fundamental to cancer prevention strategies that incorporate effective dietary interventions for target populations.

Nutrition, genetics, and risks of cancer

Dietary patterns, nutrients, and other constituents of food are major components of the environmental influences that contribute to risk for cancer, and the study of interactions between nutritional and genetic factors is a new and important area or research. This review describes the concepts and principles underlying this area of study and types of relationships between nutritional and genetic factors, and it provides examples of specific diet-gene interactions that are of current interest, with an emphasis on implications for cancer prevention and public health. Polymorphisms exist in the genes for the activating and conjugating metabolizing enzymes, and the induction of metabolizing enzyme activity by nutritional factors may result in either the activation of a carcinogen or the detoxification of a reactive intermediate metabolite. The relationship between the methylenetetrahydrofolate reductase gene and dietary folate is an example of a diet-gene interaction that involves a polymorphism in a vitamin metabolism gene, and the presence of the variant appears to influence both risk for cancer and folate requirements. Diet-gene interactions likely contribute considerably to the observed inter-individual variations in cancer risk in response to exposures to the nutritional factors that have the potential to promote or protect against cancer. Insights into mechanisms by which nutritional factors affect the process of carcinogenesis are provided by knowledge of the targeted gene function and enzyme activity. Increased knowledge in this area will allow a more refined approach to reducing risk for cancer, with diet interventions targeted toward individuals and subgroups that are genetically susceptible and responsive to the effects of nutritional factors.

Human DNA repair genes

DNA repair systems are essential for the maintenance of genome integrity. Consequently, the disregulation of repair genes can be expected to be associated with significant, detrimental health effects, which can include an increased prevalence of birth defects, an enhancement of cancer risk, and an accelerated rate of aging. Although original insights into DNA repair and the genes responsible were largely derived from studies in bacteria and yeast, well over 125 genes directly involved in DNA repair have now been identified in humans, and their cDNA sequence established. These genes function in a diverse set of pathways that involve the recognition and removal of DNA lesions, tolerance to DNA damage, and protection from errors of incorporation made during DNA replication or DNA repair. Additional genes indirectly affect DNA repair, by regulating the cell cycle, ostensibly to provide an opportunity for repair or to direct the cell to apoptosis. For about 70 of the DNA repair genes listed in Table I, both the genomic DNA sequence and the cDNA sequence and chromosomal location have been elucidated. In 45 cases single-nucleotide polymorphisms have been identified and, in some cases, genetic variants have been associated with specific disorders. With the accelerating rate of gene discovery, the number of identified DNA repair genes and sequence variants is quickly rising. This report tabulates the current status of what is known about these genes. The report is limited to genes whose function is directly related to DNA repair.

Stomach cancer history in the siblings of patients with gastric carcinoma

A case-control study in the population of the Eastern Black Sea region of Turkey was conducted to learn the incidence of stomach cancer in the siblings of patients with gastric carcinoma. Among 1240 patients with gastric carcinoma, 168 had sibling(s) with a history of stomach cancer versus 19 cases in the control group matched according to age and gender (OR 10.07, P < 0.0001). The frequency of a history of stomach cancers and cancer of other organs in first- to third-degree relatives was 60.7% and 38.0%, respectively, of 168 sibling cases with gastric carcinoma (P < 0.0001). Fifty-two point three per cent of sibling cases having a history of cancer in other organs in their relatives also reported stomach cancer in the same-degree relatives. The number of stomach cancers in the first- to third-degree relatives of sibling cases was higher than the number of other organ cancers in the same-degree relatives (P < 0.01). Familial clustering of stomach cancer was reported in 12.5% of sibling cases. The study of stomach cancer history in the siblings suggests: the presence of a genetic susceptibility, high risk of the disease occurrence in the siblings of patients, higher predisposition to gastric than to other organ cancers in the relatives, and not infrequent familial clustering.

Human UDP-glucuronosyltransferases: metabolism, expression, and disease

In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.