Meta- and pooled analysis of GSTT1 and lung cancer: a HuGE-GSEC review.

Lung cancer is the most common malignancy in the Western world, and the main risk factor is tobacco smoking. Polymorphisms in metabolic genes may modulate the risk associated with environmental factors. The glutathione S-transferase theta 1 gene (GSTT1) is a particularly attractive candidate for lung cancer susceptibility because of its involvement in the metabolism of polycyclic aromatic hydrocarbons found in tobacco smoke and of other chemicals, pesticides, and industrial solvents. The frequency of the GSTT1 null genotype is lower among Caucasians (10-20%) than among Asians (50-60%). The authors present a meta- and a pooled analysis of case-control, genotype-based studies that examined the association between GSTT1 and lung cancer (34 studies, 7,629 cases and 10,087 controls for the meta-analysis; 34 studies, 7,044 cases and 10,000 controls for the pooled analysis). No association was observed between GSTT1 deletion and lung cancer for Caucasians (odds ratio (OR) = 0.99, 95% confidence interval (CI): 0.87, 1.12); for Asians, a positive association was found (OR = 1.28, 95% CI: 1.10, 1.49). In the pooled analysis, the odds ratios were not significant for either Asians (OR = 0.97, 95% CI: 0.83, 1.13) or Caucasians (OR = 1.09, 95% CI: 0.99, 1.21). No significant interaction was observed between GSTT1 and smoking on lung cancer, whereas GSTT1 appeared to modulate occupational-related lung cancer.

Polymorphisms in CYP1A1, GSTM1, GSTT1 and lung cancer below the age of 45 years.

BACKGROUND: A genetic component of early-onset lung cancer has been suggested. The role of metabolic gene polymorphisms has never been studied in young lung cancer cases. Phase 1 and Phase 2 gene polymorphisms are involved in tobacco carcinogens' metabolism and therefore in lung cancer risk. METHODS: The effect of metabolic gene polymorphisms on lung cancer at young ages was studied by pooling data from the Genetic Susceptibility to Environmental Carcinogens (GSEC) database. All primary lung cancer cases of both sexes who were Caucasian and

CYP2D6 gene polymorphism in caucasian smokers: lung cancer susceptibility and phenotype-genotype relationships.

Individual susceptibility to smoking-related cancers is proposed to partly depend on a genetically determined ability to metabolise tobacco carcinogens. We previously reported on the association between the activity of the xenobiotic-metabolising enzyme CYP2D6 and lung cancer risk in a hospital-based case-control study among French Caucasian smokers. Here we extended the study to address the effect of four gene-inactivating mutations (CYP2D6(*)3, (*)4, (*)5 and (*)16) and the gene duplication of the CYP2D6 gene (CYP2D6(*)2x2 or CYP2D6(*)1x2) on lung cancer risk in the same population (150 patients with primary lung carcinoma of squamous cell or small cell histology and 172 controls). The risk of lung cancer associated with the CYP2D6 poor metaboliser genotype (odds ratio 1.5, 95% confidence interval 0.5-4.3) did not differ from that in the reference category of extensive metaboliser and ultra-rapid metaboliser genotypes combined. Lung cancer risks for the CYP2D6 PM genotype amongst light smokers (tobacco consumption 20 g/day) were not significantly different. The present findings agree with the discrepancy between the phenotype-based and genotype-based studies indicated by the recent meta-analyses.

p53 mutations and exposure to environmental tobacco smoke in a multicenter study on lung cancer.

Biomarker data may provide a way to strengthen the link between environmental tobacco smoke (ETS) exposure and lung cancer shown in epidemiological studies. We conducted a multicenter case-control study to investigate the association between ETS exposure and lung cancer in never-smokers using p53 mutations as a biomarker of tobacco-related carcinogenesis. Paraffin-embedded tissue or fresh tissue samples from 91 never-smokers and 66 smokers with histologically confirmed lung cancer and interview data about smoking habits and ETS exposure were analyzed for mutations in the p53 gene. Statistical analysis was performed using multivariate logistic regression. Among the lifelong nonsmokers, the overall mutation prevalence was 10% (nine cases). Among 48 never-smokers ever exposed to spousal ETS, 13% (six cases) showed mutations. Smokers exhibited 17 (26%) mutations. A 3-fold [odds ratio, 2.9; 95% confidence interval (CI), 1.2-7.2] increased risk of p53 mutation was observed for smokers as compared with all never-smokers combined (i.e., irrespective of ETS exposure). The increase was 4.4-fold (95% CI, 1.2-16.2) when compared with never-smokers without ETS exposure. Among never-smokers, the risk of mutation was doubled (odds ratio, 2.0; 95% CI, 0.5-8.7) for exposure to spousal ETS only, based on 6 exposed cases with mutation and 42 exposed cases without mutation. The risk was 1.5 (95% CI, 0.2-8.8) for those ever exposed to spousal or workplace ETS as compared with those never exposed to spousal or workplace ETS. For smokers, the most common mutation type was G:C to T:A transversion (31%), whereas G:C to A:T transitions were predominant among never smokers (57%). In conclusion, our study indicates a significant 3-4-fold increased risk of p53 mutation in smoking lung cancer cases, and it suggests that mechanisms of lung carcinogenesis in ETS-exposed never-smokers include mutations in the p53 gene, similar to that seen in smokers. However, the mutation patterns observed also suggest a difference between smokers and never-smokers. Clearly, additional investigations of the role of p53 mutation as a biomarker for tobacco-related carcinogenesis, including that related to ETS, are indicated.

Survival in operable non-small-cell lung cancer: role of p53 mutations, tobacco smoking and asbestos exposure.

Validated markers are needed to identify operable lung cancer patients with poor prognosis. About one-half of non-small-cell lung cancers (NSCLCs) carry a mutation in the p53 tumor-suppressor gene. We examined 101 NSCLC patients for surgical stage, completeness of resection, tobacco smoking, asbestos exposure, age, gender and p53 gene mutations as prognostic factors after a follow-up period of 4 years. Cox's multivariate regression model was applied to quantify the associations with overall and cancer-related survival. Patients with a wild-type p53 gene had an overall 4-year survival of 43% and those with a mutated p53 gene, 35%. In squamous-cell carcinoma, stage and heavy smoking, defined as the median of pack-years smoked, had prognostic significance for overall survival. Only stage was associated with poor cancer-related survival. Asbestos exposure was not associated with overall survival or cancer-related survival in squamous-cell carcinoma or adenocarcinoma. In adenocarcinoma, p53 mutation, in addition to stage, emerged as a significant predictor of poor cancer-related survival.

CYP2D6 ultrarapid metabolizer genotype as a potential modifier of smoking behaviour.

Some 3-10% of Caucasians are deficient in CYP2D6 metabolism (poor metabolizers), due to inheritance of two defective alleles, whereas amplification of the CYP2D6 gene results in ultrarapid metabolism in 1-2% of Caucasian populations. To examine the possible association between CYP2D6 polymorphism and individual smoking behaviour, we analysed the prevalence of CYP2D6 genotypes among 292 long-term heavy smokers, 382 individuals with more variable smoking histories, and 302 never-smokers. The prevalence of ultrarapid metabolizers in heavy smokers (7.9%) was twofold compared to individuals with variable smoking habits (3.7%; odds ratio 2.3, 95% confidence interval 1.2-4.4), and fourfold compared with never-smokers (2.0%) (odds ratio 4.2, 95% confidence interval 1.8-9.8). The frequency of poor metabolizer genotype was approximately 2%, in each smoker group. However, when men and women were studied separately, the prevalence of poor metabolizer genotype was higher in male never-smokers (3.6%) than in variable smokers (2.7%) and heavy smokers (2.2%). Moreover, a trend test, adjusted by age, gender and cancer status, revealed a significant trend for the increased tobacco usage with increased metabolic capacity. Our results are in agreement with the assumption that increased CYP2D6 activity may contribute to the probability of being addicted to smoking.

A broad amplification pattern at 3q in squamous cell lung cancer--a fluorescence in situ hybridization study.

Frequent DNA copy number gain at 3q, with minimal overlapping area at 3q24-qter, has previously been reported in squamous cell carcinoma of the lung (SQCC), implicating the importance of genes at 3q in the tumorigenesis of SQCC. To further characterize the gain of DNA sequences at 3q, we performed interphase fluorescence in situ hybridization (FISH) analysis on 16 paraffin-embedded SQCC tumor samples that had previously been studied by comparative genomic hybridization (CGH). Eleven yeast artificial chromosome (YAC) probes located at 3q25-q27 and a chromosome 3-specific centromeric probe were used in the analysis. All SQCC tumors showed increase in DNA sequence copy number with 9-11 probes. In 5 tumors (31%) the number of centromeric signals varied from 3 to 5 and the YAC/centromeric signal ratio was 1.0, suggesting that the increase in DNA sequence copy number at 3q in these cases resulted from polysomy of chromosome 3. In 11 tumors (69%), the YAC/centromeric signal ratio varied between 1.5 and 4.7, indicating that the increase in DNA sequence copy number was due to intrachromosomal gain of DNA sequences at 3q. In each case, several YACs showed increased number of signals, demonstrating that the gained area was relatively large. Our findings therefore suggest that multiple genes located at 3q25-q27 are involved in the tumorigenesis of SQCC.

Lung cancer and past occupational exposure to asbestos. Role of p53 and K-ras mutations.

Studies on somatic mutations in lung cancers associated with cigarette smoking and asbestos exposure are few. We investigated prevalence of mutations in the p53 and K-ras genes in lung tumors from smokers with and without asbestos exposure at work. For K-ras mutations, the study was an extension of an earlier analysis. Nearly all of the 105 consecutive patients examined were smokers and had non-small-cell carcinoma of the lung with squamous-cell carcinoma or adenocarcinoma histology. Exposure to asbestos was estimated by pulmonary fiber counts and occupational histories. A pulmonary burden of >/= 1 x 10(6) asbestos fibers per gram of lung tissue, indicating work-related exposure, was found in 32% of the patients for whom fiber-analysis data were available (33 of 102 patients, all men). The statistical analysis showed pulmonary fiber count as the only significant predictor of adenocarcinoma histology, in contrast to squamous-cell carcinoma (smoking-adjusted odds ratio [OR] 3.0, 95% confidence interval [CI] 1.1 to 8.5). The frequency of p53 mutations was 39% (13 of 33) among the asbestos-exposed cases, as compared with 54% (29 of 54) among the nonexposed cases; the difference was not significant, however. In male ever-smokers, a long duration of smoking was associated with p53 mutation (OR 3.2, 95% CI 1.2 to 8.8). In adenocarcinoma, p53 mutations were less prevalent (10 of 30, 33%) as compared with squamous-cell carcinoma (28 of 46, 61%; P = 0.02), whereas a strong and significant association was found between adenocarcinoma and K-ras mutation (OR 37, 95% CI 5.8 to 232, adjusted for smoking and asbestos exposure). Asbestos exposure alone was not significantly associated with increased occurrence of K-ras mutations. In conclusion, the results may primarily reflect the observed excess of adenocarcinoma in the asbestos- exposed patients, and hence the decrease in p53 mutations and increase in K-ras mutations.

LKB1 somatic mutations in sporadic tumors.

Germline mutations of LKB1/Peutz-Jeghers syndrome gene predispose carriers to hamartomatous polyposis of the gastrointestinal tract as well as to cancer of different organ systems. Although Peutz-Jeghers syndrome patients frequently present with neoplasms of the colon, stomach, small intestine, pancreas, breast, ovaries, and cervix, somatic mutations appear to be rare in the sporadic tumor types thus far studied (colorectal, gastric, testicular, and breast cancers). To evaluate whether somatic mutations of LKB1 contribute to the tumorigenesis of yet unstudied tumor types, we screened 14 cell lines and 129 tumor specimens from different cancers for a genetic defect in LKB1. Six melanoma and eight myeloma cell lines were scrutinized for LKB1 somatic mutations by genomic sequencing. No changes were found in the coding LKB1 sequence and exon/intron boundaries. Next, we analyzed 12 pancreatic, 8 gastric, 12 ovarian granulosa cell, 26 cervical, 28 lung, 24 soft tissue, and 19 renal tumors by single-strand conformational polymorphism analysis. Three changes in LKB1 coding nucleotide sequence were identified. One base pair deletion at A957 and G958 substitution by T occurred in a cervical adenocarcinoma sample, resulting in a frameshift and premature stop codon at position 335. Substitution of A581 by T occurred in a lung adenocarcinoma sample, resulting in the change of aspartic acid at position 194 to valine. A loss of another allele was detected in this sample. One silent change, C1257T, was found in a pancreatic carcinoma sample. The changes were not present in the matched normal tissue DNA samples. Our results suggest that mutational inactivation of LKB1 is a rare event in most sporadic tumor types.

Induction of UDP-glycosyltransferase family 1 genes in rat liver: different patterns of mRNA expression with two inducers, 3-methylcholanthrene and beta-naphthoflavone.

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), presently called UDP-glycosyltransferases, catalyse the detoxification of many toxic and carcinogenic compounds. Glucuronidation is also a major metabolic pathway for numerous drugs. The UGT1A6 gene (formerly known as UGT1*06 and UGT1A1) has been suggested to belong to the aryl hydrocarbon (Ah) gene battery, which consists of several genes encoding for drug-metabolising enzymes regulated by dioxin and other ligands of the Ah receptor. In this study, we analysed the localisation of UGT1A6 expression in rat liver by in situ hybridisation to mRNA. Two different RNA probes were used, one which was specific to UGT1A6 and the other against the C terminal sequence shared by all UGT1 genes. In this study, no UGT1A6 mRNA was detected in the control animals. However, other gene(s) of the UGT1 family were expressed in the perivenous region surrounding the central veins as detected by hybridisation with the probe against the common region of the UGT1 genes. Treatment with the lower dose (5 mg/kg) of 3-methylcholanthrene (3MC) induced expression of UGT1A6 perivenously. Treatment with the higher dose (25 mg/kg) of 3-Methylcholanthrene resulted in a more panacinar expression pattern. In contrast to the perivenous induction observed with 3-methylcholanthrene, treatment with 15 mg/kg of beta-naphthoflavone (BNF) resulted in strong induction in the periportal region. The results reveal an inducer-specific pattern of UGT1A6 expression similar to that demonstrated earlier for other Ah battery genes, namely CYP1A1, CYP1A2, GSTYalpha and ALDH3. The finding further supports the notion that common factors regulate the regional hepatic expression of Ah battery genes.

N-acetyltransferase NAT1 and NAT2 genotypes and lung cancer risk.

Acetyltransferases, encoded by the NAT1 and NAT2 genes, are involved in the activation/inactivation reactions of numerous xenobiotics, including tobacco-derived aromatic amine carcinogens. Several allelic variants of NAT1 and NAT2, which cause variations in acetylation capacity, have been detected. The NAT2 slow acetylator phenotype/genotype has been inconsistently associated with lung cancer and, to date, the role of NAT1 polymorphism in lung cancer has not been reported. The effect of NAT1 and NAT2 genetic polymorphisms on individual lung cancer risk was evaluated among 150 lung cancer patients and 172 control individuals, all French Caucasian smokers. The NAT1 alleles (*3, *4, *10, *11, *14, and *15) and the NAT2 alleles (*4, *5, *6, *7) were differentiated by polymerase chain reaction-based restriction fragment length polymorphism methods using DNA extracted from peripheral white blood cells. Genotypes were classified according to current knowledge of the functional activity of the variant alleles. The NAT1*10 and NAT1*11 alleles were considered as rapid alleles, the NAT1*4 and the NAT1*3 as normal alleles and NAT1*14 and NAT1*15 as slow-acetylation alleles. Logistic regression analyses were performed taking into account the age, sex, smoking and occupational exposures. A significant association was observed between lung cancer and NAT1 genotypes (P(homogeneity) < 0.02) with a gene dose effect (P(trend) < 0.01); compared with homozygous rapid acetylators, the lung cancer risk was 4.0 (95% confidence interval 0.8-19.6) for heterozygous rapid acetylators, 6.4 (95% confidence interval 1.4-30.5) for homozygous normal acetylators and 11.7 (95% confidence interval 1.3-106.5) for heterozygous slow acetylators. None of the individuals were homozygous slow acetylators. Similar results were obtained whatever the adjustment considered. No significant association was found between NAT2 genotype and lung cancer. The NAT1 polymorphism may thus be an important modifier of individual susceptibility to smoking-induced lung cancer.

Combined effect of polymorphic GST genes on individual susceptibility to lung cancer.

Glutathione S-transferases (GSTs) are known to take part in detoxification of many potentially carcinogenic compounds. Therefore, polymorphisms of the GST genes have been considered as potentially important modifiers of individual risk of environmentally induced cancers. The association between lack of glutathione S-transferase M1 gene (GSTM1 null genotype) and susceptibility to smoking-related lung cancer has been actively studied, with contradictory results. In contrast, little is known about the more recently found polymorphisms in GSTM3, GSTP1 and GSTT1 genes with respect to individual responses to environmental exposures. In this study, we determined the genotype distribution of all these genes, and their combinations, among 208 Finnish lung cancer patients and 294 population controls. None of the genotypes studied had a statistically significant effect on lung cancer risk, when studied separately. However, a significant association was observed for concurrent lack of the GSTM1 and GSTT1 genes and susceptibility to squamous cell carcinoma. For that cell type, the risk was more than 2-fold when compared with that of individuals having other genotype combinations (OR = 2.3; 95% CI = 1.0-5.3; p = 0.05). Moreover, the risk was mostly attributable to patients with smoking history of 40 pack-years or less (OR = 2.9; 95% CI = 1.1-7.7; p = 0.03). In contrast, this genotype combination did not affect the risk for other histological types of lung cancer, and the other genotype combinations had no effects on individual susceptibility to this malignancy. The overall role of GST polymorphisms in modifying the lung cancer risk may therefore be more limited than has been so far anticipated.

Localization of CYP1A1 mRNA in human lung by in situ hybridization: comparison with immunohistochemical findings.

Cytochrome P4501A1 (CYP1A1) is involved in the bioactivation of polycyclic aromatic hydrocarbons into their reactive epoxide metabolites. CYP1A1 is considered to be important with regard to individual susceptibility to lung cancer since phenotypic and genotypic polymorphisms of CYP1A1 have been associated with an increased risk of lung cancer in a number of studies. We examined here the expression and localization of CYP1A1 mRNA in human lung tissue using in situ hybridization with a CYP1A1-specific RNA probe. A centrilobular expression of CYP1A1 mRNA was observed in the peripheral lung. The expression was intense in bronchiolar epithelium of peripheral lung, especially in terminal cuboidal epithelium. Type II alveolar epithelial cells were also intensely labelled. Type I alveolar epithelial cells and vascular epithelium exhibited binding but the hybridization signals were less intense. Our results are in good agreement with our previous work on immunohistochemical localization of CYP1A protein, in which we used the 1-7-1 MAb that recognizes both CYP1A1 and CYP1A2. In serial sections analyzed with in situ hybridization and immunohistochemistry, a similar distribution of CYP1A1 mRNA and CYP1A protein was observed. CYP1A1 mRNA is thus expressed in human lungs and the expression is particularly intense in the cell types involved in the development of peripheral lung cancers.

DNA gains in 3q occur frequently in squamous cell carcinoma of the lung, but not in adenocarcinoma.

We performed a comparative genomic hybridization study on 25 samples of adenocarcinoma and 19 samples of squamous cell carcinoma of the lung to detect recurrent changes in the genetic material. DNA copy number changes were found in 16 squamous cell carcinoma samples and 17 adenocarcinoma samples. The most common changes were gains of DNA sequences in 3q (43%), 1q (34%), 8q (32%), 5p, (30%), 7p (25%), and 12p (25%). Of the squamous cell carcinoma samples with DNA copy number changes, 94% (15/16) had a gain in 3q (minimal common region of overlap q24-qter), whereas only 24% (4/17) of the adenocarcinoma samples with DNA copy number changes showed a gain in 3q (q22-qter) (P < 0.001). Six high-level amplifications in 3q (q26.2-q26.3) were detected in the squamous cell carcinoma samples but none were observed in the adenocarcinoma samples. Our results suggest that amplification of genes in 3q may be important in the tumorigenesis of squamous cell carcinoma but not necessarily of adenocarcinoma.

Glutathione S transferase M1 andT1 genotypes and susceptibility to smoking related larynx cancer.

Susceptibility to smoking related larynx cancer has been suggested to be associated with genetically determined differences in the ability to detoxify carcinogens present in tobacco smoke. The genetic polymorphisms of glutathione S-transferases, involved in the metabolic inactivation of, for example, tobacco derived carcinogens, have been recognized as potential risk modifiers in various environmentally induced malignancies, including larynx cancer. We employed PCR-based methods to determine the distribution of the GSTM 1 and G STT1 null genotypes in 171 larynx cancer patients and 180 controls to examine further their potential role in individual susceptibility to this neoplasm. The GSTM 1 null genotype was found in 49 1 % of the cases and 57 7 % of the controls and the GSTT1 null genotype in 17 5 % of the cases and 21 7 % of the controls, respectively. Larynx cancer risk associated with the lack of GST M 1 (OR = 0 7; 95 % CI: 0 5-1 1) or GSTT1 (OR = 0 8; 95 % CI: 0 5-1 3) was not significantly affected by age, smoking status, or cancer progression. Although this study thus suggests no role for the G STM 1 and GSTT1 gene polymorphisms in individual susceptibility to smoking-related larynx cancer, due to its relatively small sample size more data are required before any definite conclusions can be drawn.

Molecular analysis of occupational cancer: infrequent p53 and ras mutations in renal-cell cancer in workers exposed to gasoline.

Occupational exposure to gasoline has been identified in several studies as a risk factor for renal-cell cancer. Cases of renal-cell cancer with and without work-related exposure to gasoline or gasoline and diesel fuel were studied for the presence of mutations in the tumour-suppressor gene p53 (n = 23 exposed and 30 non-exposed cases studied) and ras oncogene (n = 30 exposed and 36 non-exposed cases studied). An average cumulative exposure was estimated at 10 ppm-years benzene among the exposed. Three p53 mutations were detected by denaturing-gradient gel electrophoresis (DGGE) among the 23 exposed cases (3/23, 13%). Of the non-exposed referent cases, 4 had a mutation (4/30, 13%). All but one of the cases with a p53 mutation had smoked. A ras gene (K-ras or N-ras) mutation was found in 3 (3/66, 4.5%) cases, all of whom were smoker referents. We conclude that p53 and ras mutations are infrequent in renal-cell cancer associated with occupational exposure to gasoline. However, the majority of the mutations (6/7 for p53, and 3/3 for ras genes) were seen in smokers.

Interaction between dose and susceptibility to environmental cancer: a short review.

Increased risk of environmentally induced cancer is associated with various types of exposures and host factors, including differences in carcinogen metabolism. Since many carcinogenic compounds require metabolic activation to enable them to react with cellular macromolecules, individual features of carcinogen metabolism may play an essential role in the development of environmental cancer. In this context, cigarette smoking has often been the main type of carcinogenic exposure examined in human studies. Increasing attention has recently been paid to the dose level at which individual susceptibility may be observed. Present studies on increased risk of smoking-related lung cancer associated with phenotypic or genotypic variation of the genes encoding for CYP1A1 or CYP2D6 enzymes are summarized. Similarly, higher risks of lung or bladder cancer seen at various levels of smoking in association with polymorphism of the glutathione S-transferase gene GSTM1 or NAT1 and NAT2 genes involved in N-acetylation are reviewed. Finally, the influence of CYP2E1, GSTM1, or the combined at-risk genotype on the risk of hepatocellular carcinoma in smokers is briefly discussed.

No evidence of microsatellite instability but frequent loss of heterozygosity in primary resected lung cancer.

We examined microsatellite instability and loss of heterozygosity (LOH) in primary lung tumors from 93 cancer patients, using 16 microsatellite markers. The cases studied included 87 non-small-cell lung cancers (NSCLC) and six small-cell lung cancers (SCLC). All the patients except two were current or former smokers. The microsatellite markers were all dinucleotide repeat sequences from chromosomal locations 1p, 3p, 5q, 8p, 9p, 10p, 11p, 13q, and 17q. None of the tumors showed microsatellite instability (0/93). In NSCLC, 28% (24/87) of the cases showed LOH in at least one locus, whereas, in SCLC, 67% (4/6) had allelic losses. The frequency of LOH differed between the various cell types of NSCLC. The highest frequency was seen in large cell carcinoma (3/6, 50%) followed by squamous cell carcinoma (16/43, 37%) and adenocarcinoma (5/35, 14%). The most common site of LOH was 3p, where markers D3S1284, D3S659, D3S1289, D3S966, D3S647, and D3S1038 were studied. LOH, studied with 9p markers (D9S126, D9S171, D9S162), was less common. The present results, together with earlier reports, suggest that smoking-related primary lung cancers seldom show microsatellite instability but are characterized by frequent LOH.