Genetic polymorphisms in carcinogen metabolism and their association to hereditary nonpolyposis colon cancer

A review on role of ATM gene in hereditary transfer of colorectal cancer

Colorectal cancer found to be the most commonly occurring cancer worldwide which can be prevented by screening and its curable if diagnosed early. Lynch syndrome/HNPCC being an autosomal genetic disease and propensity in forming colorectal cancer is inherited wherein genomic instabilities and epigenetic changes are being the characteristic forms in hereditary cancers. It is very important to determine the polymorphism in several DNA repairing genes such as ATM, RAD51, XRCC2, XRCC3 and XRCC9 to study the risk exploring both the prognosis and the developing of colorectal cancer. The role of ATM gene has been studied which involves in the hereditary transfer of colorectal cancer associated with other related cancers such as stomach, lung and breast cancers. ATM found to be the mutation target and also a modifier gene with more risk of developing the disease by its polymorphism in variant of ATM D1853N. It was identified that ATM gene polymorphism did not drastically change HNPCC age of onset. ATM expression levels were studied and it has been concluded that the complete loss of ATM expression resulted in a propensity of worse survival and no better prognosis with increase in mortality rate. This ATM gene might be considered to be a predicted biomarker in colorectal cancer.

Risk of colorectal cancer for people with a mutation in both a MUTYH and a DNA mismatch repair gene

The base excision repair protein, MUTYH, functionally interacts with the DNA mismatch repair (MMR) system. As genetic testing moves from testing one gene at a time, to gene panel and whole exome next generation sequencing approaches, understandin g the risk associated with co-existence of germline mutations in these genes will be important for clinical interpretation and management. From the Colon Cancer Family Registry, we identified 10 carriers who had both a MUTYH mutation (6 with c.1187G>A p.(Gly396Asp), 3 with c.821G>A p.(Arg274Gln), and 1 with c.536A>G p.(Tyr179Cys)) and a MMR gene mutation (3 in MLH1, 6 in MSH2, and 1 in PMS2), 375 carriers of a single (monoallelic) MUTYH mutation alone, and 469 carriers of a MMR gene mutation alone. Of the 10 carriers of both gene mutations, 8 were diagnosed with colorectal cancer. Using a weighted cohort analysis, we estimated that risk of colorectal cancer for carriers of both a MUTYH and a MMR gene mutation was substantially higher than that for carriers of a MUTYH mutation alone [hazard ratio (HR) 21.5, 95% confidence interval (CI) 9.19-50.1; p < 0.001], but not different from that for carriers of a MMR gene mutation alone (HR 1.94, 95% CI 0.63-5.99; p = 0.25). Within the limited power of this study, there was no evidence that a monoallelic MUTYH gene mutation confers additional risk of colorectal cancer for carriers of a MMR gene mutation alone. Our finding suggests MUTYH mutation testing in MMR gene mutation carriers is not clinically informative.

Genetic modifiers of cancer risk in Lynch syndrome: a review

The report by Aldred Scott Warthin in 1913 of a cancer family history and expanded on by Henry T. Lynch demonstrated one of the most enduring traits observed in patients with Lynch syndrome. The recognition of a variety of malignancies occurring at differing ages within a single family suggested the role of genetic variance on disease expression in an autosomal dominantly inherited genetic condition. With the identification of the genetic basis of Lynch syndrome and the subsequent collection of families and their medical records it has become possible to identify subtle genetic effects that influence the age at which disease onset occurs in this cancer predisposition. Knowledge about genetic modifiers influencing disease expression has the potential to be used to personalise prophylactic screening measures to maximise the benefits for family members and their carers.

Colorectal and other cancer risks for carriers and noncarriers from families with a DNA mismatch repair gene mutation: a prospective cohort study

PURPOSE

To determine whether cancer risks for carriers and noncarriers from families with a mismatch repair (MMR) gene mutation are increased above the risks of the general population.

PATIENTS AND METHODS

We prospectively followed a cohort of 446 unaffected carriers of an MMR gene mutation (MLH1, n = 161; MSH2, n = 222; MSH6, n = 47; and PMS2, n = 16) and 1,029 their unaffected relatives who did not carry a mutation every 5 years at recruitment centers of the Colon Cancer Family Registry. For comparison of cancer risk with the general population, we estimated country-, age-, and sex-specific standardized incidence ratios (SIRs) of cancer for carriers and noncarriers.

RESULTS

Over a median follow-up of 5 years, mutation carriers had an increased risk of colorectal cancer (CRC; SIR, 20.48; 95% CI, 11.71 to 33.27; P < .001), endometrial cancer (SIR, 30.62; 95% CI, 11.24 to 66.64; P < .001), ovarian cancer (SIR, 18.81; 95% CI, 3.88 to 54.95; P < .001), renal cancer (SIR, 11.22; 95% CI, 2.31 to 32.79; P < .001), pancreatic cancer (SIR, 10.68; 95% CI, 2.68 to 47.70; P = .001), gastric cancer (SIR, 9.78; 95% CI, 1.18 to 35.30; P = .009), urinary bladder cancer (SIR, 9.51; 95% CI, 1.15 to 34.37; P = .009), and female breast cancer (SIR, 3.95; 95% CI, 1.59 to 8.13; P = .001). We found no evidence of their noncarrier relatives having an increased risk of any cancer, including CRC (SIR, 1.02; 95% CI, 0.33 to 2.39; P = .97).

CONCLUSION

We confirmed that carriers of an MMR gene mutation were at increased risk of a wide variety of cancers, including some cancers not previously recognized as being a result of MMR mutations, and found no evidence of an increased risk of cancer for their noncarrier relatives.

Age-related macular degeneration and genetic polymorphisms of glutathione S-transferases M1 (GSTM1) and T1 (GSTT1)

The aim of this study is to understand the multifactorial causes of age-related macular degeneration (ARMD), and, therefore, it is reasonable to investigate whether genetic polymorphisms of antioxidant enzymes (GSTM1 and GSTT1) contribute to the development of ARMD. This study consisted of 112 subjects (44 females, 68 males) with exudative ARMD, who were recruited from Khalili Hospital ophthalmic clinic in Shiraz (southern Iran), referred by vitreoretinal surgeon. Also 112 sex-matched controls (44 females, 68 males) were randomly selected from unrelated volunteers in the same clinic. We excluded patients and controls with cataract or past history of cataract surgery, asthma, past history of malignancy, cardiovascular disease that on medication and known cases of glaucoma, because these traits were associated with GSTM1 and/or GSTT1 polymorphisms. There was no association between polymorphisms of neither GSTM1 nor GSTT1 and risk of ARMD. The combination genotypes of GSTM1 and GSTT1 were not associated with the risk of ARMD. We considered the time of deterioration of vision as the time of onset of exudative ARMD. The Kaplan-Meier analysis revealed that there was significant difference between genotypes of GSTM1 (log rank statistic = 7.03, df = 1, P = 0.008). The age at onset among GSTM1 null genotype was lower than the active genotype of GSTM1. Our results support the hypothesis that the protein encoded by the GSTM1 gene might have a protective function against oxidative stress in retina. Since the age at onset is influenced by the GSTM1 polymorphism, this implies that GSTM1 is a modifier gene.

Genetic polymorphisms of glutathione S-transferase genes and susceptibility to colorectal cancer: a case-control study in an Indian population

BACKGROUND

Susceptibility to sporadic colorectal cancer is multifactorial and arises from interactive combinations of allelic variants in low-penetrance genes and relevant environmental risk factors. Genetic polymorphisms in metabolic enzymes as gene susceptibility factors may modify colorectal cancer risk. We evaluated the risk of colorectal cancer associated with respective or combined glutathione S-transferase (GST) polymorphisms and assessed the interactions between genes and environmental factors in a case-control study in an Indian population.

METHODS

The study included 59 colon and 243 rectal cancer cases, and 291 cancer-free healthy controls. GST genotypes were detected by multiplex PCR-based and PCR-RFLP methods. The risk of cancer associated with GST polymorphisms was estimated by calculation of odds ratios (ORs) and confidence intervals (95% CIs) using unconditional logistic regression.

RESULTS

The GSTM1 null genotype was found to be associated with a significantly increased rectal cancer risk (OR=1.55; 95% CI, 1.05-2.30), while the GSTT1 null genotype with a greater risk of colon cancer (OR=2.15; 95% CI, 1.04-4.32). A substantial increase of both colon (OR=10.81; 95% CI, 1.11-107.22) and rectal (OR=4.80; 95% CI, 0.94-35.91) cancer risk was shown for the combination of GSTM1 null, GSTT1 null and GSTP1 105Val allele. The combined GSTM1 null and GSTP1 114Val allele also revealed an increased risk for either colon cancer (OR=4.69; 95% CI, 0.84-23.87) or rectal cancer (OR=5.68; 95% CI, 1.79-22.16). Furthermore, the combination of GSTM1 null, GSTT1 null and GSTP1 114Val allele was found in 2 rectal cancer cases.

CONCLUSION

Our results suggest that co-exist of GSTM1 null, GSTT1 null and the variant GSTP1 105Val or 114Val allele may be predisposing risk factors for colorectal cancer in Indian population.

GSTM1, GSTT1, GSTP1, GSTA1 and colorectal cancer risk: a comprehensive meta-analysis

Glutathione S-transferases (GSTs) catalyse reactions between glutathione and lipophilic compounds with electrophilic centres, leading to neutralisation of toxic compounds, xenobiotics and products of oxidative stress. Controversy exists about whether GST polymorphisms (GSTM1 null/present genotype, GSTT1 null/present genotype, GSTP1 Ile105Val and GSTA1 *A/*B) represent risk factors for colorectal cancer. This meta-analysis aims to examine the associations between the above-mentioned polymorphisms and colorectal cancer risk. Forty-four studies were eligible for GSTM1 (11,998 colorectal cancer cases, 17,552 controls), 34 studies for GSTT1 (8596 cases, 13,589 controls), 19 studies for GSTP1 (5421 cases, 7671 controls) and four studies for GSTA1 polymorphism (1648 cases, 2039 controls). Pooled odds ratios (ORs) were appropriately derived from fixed-effects or random-effects models. Separate analyses were conducted on Caucasian and Chinese populations. Where appropriate, sensitivity analysis concerning the deviation of genotype frequencies in controls from the Hardy-Weinberg equilibrium was performed. GSTM1 null allele carriers exhibited increased colorectal cancer risk in Caucasian populations (pooled OR=1.150, 95% confidence interval (CI): 1.060-1.248, random effects); no significant association was detected for Chinese subjects (pooled OR=1.025, 95% CI: 0.903-1.163, fixed effects). Similarly, GSTT1 null allele carriers exhibited increased colorectal cancer risk in Caucasian populations (pooled OR=1.312, 95% CI: 1.119-1.538, random effects); the association in Chinese subjects was not significant (pooled OR=1.068, 95% CI: 0.788-1.449, random effects). Concerning GSTP1 Ile105Val no significant associations were demonstrated in either race. GSTA1 *A/*B polymorphism was not associated with colorectal cancer risk. GSTM1 and GSTT1 null genotypes confer additional risk for colorectal cancer in Caucasian populations.

Genetic epidemiology studies in hereditary non-polyposis colorectal cancer

Genetic epidemiology studies in hereditary non-polyposis colorectal cancer (HNPCC) have the potential to radically improve assessment of disease risk such that more individualised information can be provided to patients susceptible to developing disease. Studies of HNPCC initially focused on disease associations and the definition of the disease and its association with different cancers within the context of an inherited predisposition. With the identification of the genetic basis of HNPCC, new insights into the disease have been forthcoming and many advances in our understanding have been made. There have been many reports examining potential modifier genes in HNPCC, yet the results remain controversial as many findings have not been replicated and therefore no clear consensus as to the role of specific modifier genes has been reached. This review focuses on some of the factors associated with disease risk in HNPCC and where some of the difficulties lie in assessing the value of genetic epidemiology studies in this disorder.

Genetic variation in genes for the xenobiotic-metabolizing enzymes CYP1A1, EPHX1, GSTM1, GSTT1, and GSTP1 and susceptibility to colorectal cancer in Lynch syndrome

Individuals with Lynch syndrome are predisposed to cancer due to an inherited DNA mismatch repair gene mutation. However, there is significant variability observed in disease expression likely due to the influence of other environmental, lifestyle, or genetic factors. Polymorphisms in genes encoding xenobiotic-metabolizing enzymes may modify cancer risk by influencing the metabolism and clearance of potential carcinogens from the body. In this retrospective analysis, we examined key candidate gene polymorphisms in CYP1A1, EPHX1, GSTT1, GSTM1, and GSTP1 as modifiers of age at onset of colorectal cancer among 257 individuals with Lynch syndrome. We found that subjects heterozygous for CYP1A1 I462V (c.1384A>G) developed colorectal cancer 4 years earlier than those with the homozygous wild-type genotype (median ages, 39 and 43 years, respectively; log-rank test P = 0.018). Furthermore, being heterozygous for the CYP1A1 polymorphisms, I462V and Msp1 (g.6235T>C), was associated with an increased risk for developing colorectal cancer [adjusted hazard ratio for AG relative to AA, 1.78; 95% confidence interval, 1.16-2.74; P = 0.008; hazard ratio for TC relative to TT, 1.53; 95% confidence interval, 1.06-2.22; P = 0.02]. Because homozygous variants for both CYP1A1 polymorphisms were rare, risk estimates were imprecise. None of the other gene polymorphisms examined were associated with an earlier onset age for colorectal cancer. Our results suggest that the I462V and Msp1 polymorphisms in CYP1A1 may be an additional susceptibility factor for disease expression in Lynch syndrome because they modify the age of colorectal cancer onset by up to 4 years.

Polymorphisms inGSTT1andp53and urinary transitional cell carcinoma in south-western Taiwan: A preliminary study

Little is known about the relevance of genetic polymorphisms to arsenic-related bladder cancer. A preliminary case-control study was conducted to explore the association between genetic polymorphisms of GSTT1, p53 codon 72 and bladder cancer in southern Taiwan, a former high arsenic exposure area. Fifty-nine urinary transitional cell carcinoma (TCC) patients from a referral centre in south-western Taiwan and 81 community controls matched on residence were recruited from 1996 to 1999. A questionnaire was administered to obtain arsenic exposure and general health information. Genotypes of p53 codon 72 and GSTT1 were analysed by polymerase chain reaction-restriction fragment length polymerase. The combined variant genotypes (heterozygous or homozygous variant) of p53 codon 72 and GSTT1 null were observed in 29% of cases and in 44% of controls, respectively. In this preliminary study, bladder cancer risk was slightly elevated for subjects carrying the variant genotype of p53 codon 72 or in subjects carrying the GSTT1 null genotype. Variants in p53 codon 72 increased the risk of bladder cancer among smokers. However, the results were not statistically significant and larger confirmatory studies are needed to clarify the role of candidate gene polymorphisms and bladder cancer risk in arsenic exposed populations.

Gene-gene, gene-environment & multiple interactions in colorectal cancer

This review comprehensively evaluates the influence of gene-gene, gene-environment and multiple interactions on the risk of colorectal cancer (CRC). Methods of studying these interactions and their limitations have been discussed herein. There is a need to develop biomarkers of exposure and of risk that are sensitive, specific, present in the pathway of the disease, and that have been clinically tested for routine use. The influence of inherited variation (polymorphism) in several genes has been discussed in this review; however, due to study limitations and confounders, it is difficult to conclude which ones are associated with the highest risk (either individually or in combination with environmental factors) to CRC. The majority of the sporadic cancer is believed to be due to modification of mutation risk by other genetic and/or environmental factors. Micronutrient deficiency may explain the association between low consumption of fruit/vegetables and CRC in human studies. Mitochondrial modulation by dietary factors influences the balance between cell renewal and death critical in colon mucosal homeostasis. Both genetic and epigenetic interactions are intricately dependent on each other, and collectively influence the process of colorectal tumorigenesis. The genetic and environmental interactions present a good prospect and a challenge for prevention strategies for CRC because they support the view that this highly prevalent cancer is preventable.

Low colonic glutathione detoxification capacity in patients at risk for colon cancer

BACKGROUND

Colon carcinogenesis is a multifactorial process influenced by hereditary as well as environmental factors. The glutathione/glutathione S-transferase detoxification system in the colon is important for protection against carcinogens. We investigated the levels of glutathione/glutathione S-transferase in normal colon mucosa of patients with colorectal cancer and in patients at high risk for colorectal cancer compared with those in healthy controls.

MATERIALS AND METHODS

Glutathione content was analyzed by high-performance liquid chromatography, and glutathione S-transferase enzyme activity by spectrophotometric determination with 1-chloro 2,4-dinitrobenzene. Normal colon tissue of patients with colon adenoma (n = 64), colorectal cancer (n = 37), familial adenomatous polyposis (FAP; n = 19), hereditary non-polyposis colorectal cancer families with (HNPCC+Ad; n = 34) or without (HNPCC-Ad; n = 33) adenoma was investigated.

RESULTS

Glutathione levels were significantly lower in the normal colon mucosa of patients with cancer, FAP, HNPCC-Ad or HNPCC+Ad compared with adenoma patients or healthy controls. Glutathione S-transferase enzyme activity in the distal colon was significantly lower in patients with cancer or FAP compared with the adenoma patients or healthy controls, whereas values in carcinoma patients were significantly lower compared with both the HNPCC-Ad and HNPCC+Ad groups.

CONCLUSIONS

An association of low colonic glutathione/glutathione S-transferase activity levels and high clinical risk for the development of colorectal cancer was observed. This low glutathione detoxification capacity might contribute to the colon cancer risk.

p53 codon 72 and MDM2 SNP309 polymorphisms and age of colorectal cancer onset in Lynch syndrome

PURPOSE

The Arg/Pro polymorphism in codon 72 of p53 was recently associated with age of onset of colorectal cancer in Lynch syndrome. A novel polymorphism in the promoter region of MDM2 was associated with age of cancer onset in Li-Fraumeni syndrome. We studied the influence of both polymorphisms on age of onset in Lynch syndrome and of the p53 polymorphism also in sporadic colorectal cancer.

EXPERIMENTAL DESIGN

We genotyped p53 codon 72 in 193 individuals with Lynch syndrome mutations, 93 patients with sporadic microsatellite unstable colorectal cancer, and 93 patients with sporadic microsatellite stable colorectal cancer from Finland and 323 Finnish controls. We genotyped 30 colorectal cancer patients with Lynch syndrome mutations from Ohio and 118 U.S. controls. We genotyped SNP309 of MDM2 in the Lynch syndrome groups. We used chi2 test, Kaplan-Meier statistics, and Cox regression model to analyze the data.

RESULTS

Allele frequencies of both polymorphisms were similar in subjects and controls from both populations and showed Hardy-Weinberg equilibrium. Neither polymorphism was associated with age of colorectal cancer onset in any of the subject groups.

CONCLUSIONS

This study failed to show any role of the p53 polymorphism on age of colorectal cancer onset in Lynch syndrome and sporadic colorectal cancer. The polymorphism in the MDM2 promoter had no affect on age of onset in Lynch syndrome. Accurate information about age of onset is important in clinical practice, especially in high-risk conditions. As association studies are vulnerable to biologically insignificant variation, both positive and negative findings need to be reported to enable unbiased assessment of the significance of putative risk variants.

Genetic polymorphisms of flavin monooxygenase 3 in sulindac-induced regression of colorectal adenomas in familial adenomatous polyposis

Sulindac is a nonsteroidal antiinflammatory drug with a chemopreventive effect in patients with familial adenomatous polyposis (FAP). In vivo, the active form of sulindac is sulindac sulfide, which is inactivated by the hepatic microsomal enzyme, flavin monooxygenase 3 (FMO3). In humans, numerous polymorphisms exist in FMO3, which alter enzymatic activity and subsequent substrate metabolism. We recently showed that certain polymorphic forms of FMO3 with reduced activity were associated with a more favorable response to sulindac in preventing the formation of adenomas in patients with FAP without polyps at baseline. Here, we determined whether these FMO3 polymorphisms correlated with the ability of sulindac to regress polyposis in patients with FAP who had polyps prior to treatment. Nineteen patients were treated with 150 mg sulindac twice a day for 6 months. The size and number of polyps in each patient was assessed at baseline (prior to the administration of sulindac), and at 3 and 6 months. Genotyping was done on seven established FMO3 polymorphisms with functional significance-M66I, E158K, P153L, V257M, E305X, E308G, and R492W. Statistical analyses were done with Wilcoxon rank sum test. Of the loci examined, only E158K and E308G showed polymorphic changes. Six patients exhibited polymorphisms in both E158K and E308G loci and were designated as genotype combination 1. The remaining patients were designated as genotype combination 2. Over the course of treatment, patients with genotype combination 1 had a greater reduction in both the size and number of polyps than those with genotype combination 2. These results suggest that combined polymorphic changes in the E158K and E308G alleles may protect against polyposis in patients with FAP treated with sulindac.

N-acetyltransferase (NAT) 2 acetylator status and age of onset in patients with hereditary nonpolyposis colorectal cancer (HNPCC)

N-acetyltransferase (NAT) 2 is an essential polymorphic enzyme involved in the metabolism of various xenobiotics, including potential carcinogens. The individual differences in the NAT2 metabolic capacity are caused by allelic variants of the NAT2 gene which are determined by a pattern of single nucleotide polymorphisms (SNPs) resulting in slow (SA), intermediate (IA) or rapid acetylator (RA) phenotypes. Highly penetrant germline mutations in mismatch repair (MMR) genes are the cause of the disease in hereditary nonpolyposis colorectal cancer (HNPCC). There is no strict correlation between the type of germline mutation in MMR genes and the HNPCC phenotype, but age of tumor onset (AO) in HNPCC has been associated at least in part with different variants in apoptosis-related genes. To clarify the potential modifying role of the NAT2 acetylator status in HNPCC, we performed a multicenter study in 226 individuals with colorectal cancer carrying exclusively pathogenic germline mutations in MSH2 or MLH1. We did not observe any significant difference in the NAT2 acetylator status frequency between HNPCC patients and 107 healthy controls (P=0.156), and between MLH1 and MSH2 mutation carriers (P=0.198). Multivariate Cox regression analysis revealed that male patients had a significantly increased risk to develop CRC compared to females during any interval (P=0.043), while the NAT2 acetylator status (P=0.447) and the mutated gene (MLH1 or MSH2) (P=0.236) were not risk factors for AO. The median AO in HNPCC patients was 39 years in patients with RA as well as with SA status (P=0.347). In MLH1 mutation carriers, the median AO was 38 years in RA and 36 years in SA status patients (P=0.901), whereas in MSH2 mutation carriers, the median AO was 39 years in RA and 42 years in SA status patients (P=0.163). Log-rank test revealed a significantly lower age of CRC onset in male compared to female HNPCC patients (P=0.0442). These data do not support the hypothesis that the NAT2 acetylatorship acts as a modifying factor on AO in HNPCC-associated CRC.

Genotype and phenotype in hereditary nonpolyposis colon cancer: a study of families with different vs. shared predisposing mutations

Hereditary nonpolyposis colorectal cancer (HNPCC) is a multi-organ cancer syndrome associated with heritable mutations in DNA mismatch repair genes, particularly MLH1 (MutL Homologue 1) and MSH2 (MutS Homologue 2). We took advantage of the unique characteristics of the Finnish HNPCC families to assess genotype- phenotype correlations in this disorder. We studied 295 mutation carriers (10 mutations in MLH1 and 3 in MSH2) segregating in 55 families. In addition to the comparison of families with different mutations, the enrichment of two MLH1 mutations, one affecting exon 16 (29 families, 186 individuals) and another one affecting exon 6 (10 families, 45 individuals) allowed the comparison of kindreds with identical predisposing mutations. Extracolonic cancers were more common in MSH2 than MLH1 mutation carriers, with the ratios of 0.48 and 0.64, respectively, of colorectal cancer to all cancers (P = 0.076). Within MLH1, two mutations affecting only the amino terminal portion showed a significant association with late onset of cancer as compared to the remaining mutations. Importantly, families with the MLH1 exon 16 mutation displayed significant variation (P = 0.012) in the age at onset of colon cancer, despite shared predisposition. We conclude that even though characteristics of the inherited mutations may explain part of the observed clinical variation, other factors have a significant impact on HNPCC phenotype determination.

Genotype and phenotype in hereditary nonpolyposis colon cancer: a study of families with different vs. shared predisposing mutations

Hereditary nonpolyposis colorectal cancer (HNPCC) is a multi-organ cancer syndrome associated with heritable mutations in DNA mismatch repair genes, particularly MLH1 (MutL Homologue 1) and MSH2 (MutS Homologue 2). We took advantage of the unique characteristics of the Finnish HNPCC families to assess genotype- phenotype correlations in this disorder. We studied 295 mutation carriers (10 mutations in MLH1 and 3 in MSH2) segregating in 55 families. In addition to the comparison of families with different mutations, the enrichment of two MLH1 mutations, one affecting exon 16 (29 families, 186 individuals) and another one affecting exon 6 (10 families, 45 individuals) allowed the comparison of kindreds with identical predisposing mutations. Extracolonic cancers were more common in MSH2 than MLH1 mutation carriers, with the ratios of 0.48 and 0.64, respectively, of colorectal cancer to all cancers (P = 0.076). Within MLH1, two mutations affecting only the amino terminal portion showed a significant association with late onset of cancer as compared to the remaining mutations. Importantly, families with the MLH1 exon 16 mutation displayed significant variation (P = 0.012) in the age at onset of colon cancer, despite shared predisposition. We conclude that even though characteristics of the inherited mutations may explain part of the observed clinical variation, other factors have a significant impact on HNPCC phenotype determination.

Are there two sides to colorectal cancer?

Colorectal carcinomas (CRC) that arise proximal (right) or distal (left) to the splenic flexure exhibit differences in incidence according to geographic region, age and gender. Together with observations that tumours in the hereditary cancer syndromes HNPCC and FAP occur predominantly in the right and left colon, respectively, the existence of 2 categories of CRC based on site of origin in the large bowel was proposed more than a decade ago. Differences between normal right and left colonic segments that could favour progression through different tumourigenic pathways are summarized in this review. Accumulating evidence suggests that the risk of CRC conferred by various environmental and genetic factors is different for proximal and distal tumours. Right- and left-sided tumours also exhibit different sensitivities to fluorouracil-based chemotherapy. Such differences are probably related to the molecular characteristics of the tumours, with the microsatellite instability and CpG island methylator phenotypes being associated with right-sided tumours and chromosomal instability with left-sided tumours. Future molecular-based classification systems for CRC that rely upon distinctive gene expression patterns may allow a clearer discrimination of subgroups than that provided by tumour site alone. Until then however, the existence of 2 broadly different groups of cancer defined by site of origin in the colon should be considered in the design of future epidemiologic studies as well as in the design of new clinical trials aimed at testing novel adjuvant therapies.

Arylamine N-acetyltransferase type 2 and glutathione S-transferases M1 and T1 polymorphisms in familial adenomatous polyposis

Familial adenomatous polyposis (FAP) exhibits a variable phenotype even in carriers of the same adenomatous polyposis coli germline mutation. Xenobiotic-metabolizing enzymes such as N-acetyltransferases (NATs) and glutathione S-transferases (GSTs) were reported to modify the individual risk for colorectal cancer. We examined whether the polymorphisms of the NAT2, GSTM1, and GSTT1 enzymes affect age at diagnosis of first colorectal adenomas or extracolonic manifestations in 411 FAP patients. Neither age at diagnosis of colorectal adenomas nor occurrence of extra-intestinal tumors differed significantly between genotypes at the NAT2 and GSTM1 loci, whereas GSTT1 polymorphism showed an uncertain association with extra-intestinal manifestations. Combinations of supposed at risk genotypes of the three enzymes showed no significant differences either. Thus, NAT2, GSTM1, or GSTT1 are unlikely to modify the disease phenotype in FAP patients.

Mammalian class theta GST and differential susceptibility to carcinogens: a review

Glutathione S-transferases (GSTs) are an important part of the cellular detoxification system and, perhaps, evolved to protect cells against reactive oxygen metabolites. Theta is considered the most ancient among the GSTs and theta-like GSTs are found in mammals, fish, insects, plants, unicellular algae, and bacteria. It is thought that an ancestral theta-gene underwent an early duplication before the divergence of fungi and animals and further duplications generated the variety of the other classes of GSTs (alpha, mu, phi, etc.). The comparison of the aminoacidic homologies among mammals suggests that a duplication of an ancient GST theta occurred before the speciation of mammals and resulted in the subunits GSTT1 and GSTT2. The ancestral GST theta has a dehalogenase activity towards several halogenated compounds, such as the dichloromethane. In fact, some aerobic and anaerobic methylotrophic bacteria can use these molecules as the sole carbon and energy source. The mammalian GST theta cannot sustain the growth of bacteria but still retains the dehalogenating activity. Therefore, although mammalian GST theta behaves as a scavenger towards electrophiles, such as epoxides, it acts also as metabolic activator for halogenated compounds, producing a variety of intermediates potentially dangerous for DNA and cells. For example, mice exposed to dichloromethane show a dose-dependent incidence of cancer via the GSTT1-1 pathway. Because GSTT1-1 is polymorphic in humans, with about 20% of Caucasians and 80% of Asians lacking the enzyme, the relationship between the phenotype and the incidence of cancer has been investigated extensively in order to detect GSTT1-1-associated differential susceptibility towards endogenous or exogenous carcinogens. The lack of the enzyme is related to a slightly increased risk of cancer of the bladder, gastro-intestinal tract, and for tobacco-related tumors (lung or oral cavity). More pronounced risks were found in males with the GSTT1-null genotype for brain diseases and skin basal cell carcinomas not related to sunlight exposures. Moreover, there was an increased risk of kidney and liver tumors in humans with the GSTT1-1 positive genotype following exposures to halogenated solvents. Interestingly, the liver and kidney are two organs that express the highest level of GST theta in the human body. Thus, the GSTT1-1 genotype is suspected to confer decreased or increased risk of cancer in relation to the source of exposure; in vitro studies, mostly conducted on metabolites of butadiene, confirm the protective action of GSTT1-1, whereas, thus far, experimental studies prove that the increasing risk is limited.

Genetics of hereditary colon cancer - a basis for prevention?

Hereditary colorectal cancer syndromes are among the best in vivo models to study colorectal carcinogenesis and the influence of putative modifiers of the cancer risk. The present knowledge regarding the wide range of colorectal cancer (CRC) susceptibilities and the histological and molecular changes they elicit is leading to a very dynamic and integrated concept of tumorigenesis in the colon and to new views about prevention and early treatment of cancer.

Genetic polymorphism and cancer risk

Inter-individual variability in carcinogen metabolism has been attributed in part to the polymorphic expression of several phase I and II detoxification enzymes. The role of these genetic polymorphisms in cancer susceptibility has been most extensively evaluated for isozymes of cytochrome P450 (CYP1A1, CYP2D6, and CYP2E1), N-acetyltransferase (NAT1 and NAT2), glutathione S-transferase (GSTM1, GSTT1, and GSTP1), microsomal epoxide hydrolase, and NAD(P)H:quinone oxidoreductase. Our understanding of the genetic basis of cancer risk has been enhanced most recently by establishment of genotype-phenotype correlations in humans and identification of numerous diverse factors, both genetic and environmental, that can modify risk.

Glutathione S-transferase: genetics and role in toxicology

The glutathione S-transferases (GST) are a supergene family of dimeric, enzymes that catalyse the conjugation of glutathione (GSH) to a variety of electrophiles including arene oxides, unsaturated carbonyls, organic halides and other substrates. Their importance is suggested by the finding that GST enzymes are expressed in probably all life forms. In humans, polymorphism in GST genes has been associated with susceptibility to various diseases though some recent data indicate that these genotypes modify disease phenotype. Thus, GST genotypes alone and in combination have been linked with clinical outcome.