DNA adducts, genetic polymorphisms, and K-ras mutation in human pancreatic cancer

Polymorphic variation in hOGG1 and risk of cancer: a review of the functional and epidemiologic literature

The gene encoding human 8-oxoguanine glycosylase 1 (hOGG1) is involved in DNA base excision repair. The encoded DNA glycosylase excises 7,8-dihydro-8-oxoguanine (8-OHdG), a highly mutagenic base produced in DNA as a result of exposure to reactive oxygen species (ROS). Polymorphisms in this gene may alter glycosylase function and an individual's ability to repair damaged DNA, possibly resulting in genetic instability that can foster carcinogenesis. In order to elucidate the possible impact of polymorphisms in hOGG1, we performed a literature review of both functional and epidemiologic studies that assessed the effects of these polymorphisms on repair function, levels of oxidative DNA damage, or associations with cancer risk. Fourteen functional studies and 19 epidemiologic studies of breast, colon, esophageal, head and neck, lung, nasopharyngeal, orolaryngeal, prostate, squamous cell carcinoma of the head and neck (SCCHN), and stomach cancers were identified. Although the larger functional studies suggest reduced repair function with variant alleles in hOGG1, the evidence is generally inconclusive. There is some epidemiologic evidence that risk for esophageal, lung, nasopharyngeal, orolaryngeal, and prostate is related to hOGG1 genotype, whereas risk of breast cancer does not appear related. In studies that explored potential interactions with environmental factors, cancer risk for hOGG1 genotypes differed depending on exposure, especially for colon cancer. In summary, there is limited evidence that polymorphisms in hOGG1 affect repair function and carcinogenesis. Larger, well-designed functional and epidemiologic studies are needed to clarify these relationships, especially with respect to interactions with other DNA repair enzymes and interactions with environmental factors that increase carcinogenic load.

Patterns of K-ras codon 12 and 13 mutations found in pancreatic adenocarcinoma of 30 Chinese patients by microdissection, PCR and direct sequencing

BACKGROUND AND AIM

To our knowledge there are few reports on the K-ras mutation pattern of pancreatic adenocarcinoma from Chinese mainland patients. We examined surgically resected formalin-fixed, paraffin-embedded primary pancreatic adenocarcinoma tissue blocks for the presence of activating point mutations at codon 12 and 13 of the K-ras gene.

METHODS

Mutations were detected through the use of microdissection, polymerase chain reaction (PCR) and direct sequencing. The results were confirmed by reverse sequencing.

RESULTS

The combination of microdissection, PCR and direct sequencing techniques resulted in a rapid and sensitive detection of K-ras mutations at codon 12 and 13. Twenty-five (83%) of the 30 pancreatic adenocarcinomas examined harbored K-ras mutation. Among the 25 pancreatic adenocarcinomas, 24 showed K-ras mutation at codon 12 (11 with GGT-GTT, seven with GGT-GAT, four with GGT-CGT, and two with GGT-TGT), and only one showed a GGC-TGC mutation at codon 13. In this study most of K-ras mutations at codon 12 were at the second base (72%, 18/25) with a transition/transversion ratio of 1 : 1.57 (7/11).

CONCLUSIONS

The mutation profiles of K-ras at codon 12 in our pancreatic adenocarcinoma samples were significantly different from those of European and Japanese samples.

Endometrial K-ras mutations in postmenopausal breast cancer patients treated with adjuvant tamoxifen or toremifene

PURPOSE

Long-term use of tamoxifen is associated with a two- to threefold increased risk of endometrial cancer in postmenopausal women. Toremifene is another triphenylethylene antiestrogen, which is as effective as tamoxifen in postmenopausal breast cancer. Thus far, its use has not been associated with an increased risk of endometrial cancer. K-ras codon 12 mutations seem to be important in endometrial carcinogenesis, and these mutations have been found in endometrial samples of patients on tamoxifen. The present study was undertaken to investigate if there is any difference in the frequency of endometrial K-ras mutations among patients treated with tamoxifen or toremifene.

METHODS

Endometrial samples were taken from 23 postmenopausal breast cancer patients (tamoxifen, n = 11; toremifene, n = 12) before and after 36 months of treatment. DNA was isolated from formalin-fixed paraffin-embedded samples using a routine proteinase K digestion protocol. K-ras mutations in codon 12 were screened using real-time PCR and melting curve analysis in LightCycler equipment. Wild-type PNA oligomer was used to increase the sensitivity of the assay.

RESULTS

All baseline samples contained wild-type K-ras, while 10/23 (43%) of the follow-up samples carried a codon 12 mutation. Mutations were identified in 3 of the 11 in the tamoxifen group and in 7 of the 12 in the toremifene group. Seven were transitions (G-->A), and three were transversions (two G-->T, one G-->C). One of the mutations in the toremifene group was associated with a polypoid endometrium. All the other mutations were found in an atrophic (n = 6) or proliferative (n = 3) endometrium.

CONCLUSIONS

Both tamoxifen and toremifene induce endometrial K-ras codon 12 mutations. The significance of this finding to endometrial carcinogenesis remains to be elucidated.

Pancreatic cancer

Pancreatic cancer remains a major unsolved health problem, with conventional cancer treatments having little impact on disease course. Almost all patients who have pancreatic cancer develop metastases and die. The main risk factors are smoking, age, and some genetic disorders, although the primary causes are poorly understood. Advances in molecular biology have, however, greatly improved understanding of the pathogenesis of pancreatic cancer. Many patients have mutations of the K-ras oncogene, and various tumour-suppressor genes are also inactivated. Growth factors also play an important part. However, disease prognosis is extremely poor. Around 15-20% of patients have resectable disease, but only around 20% of these survive to 5 years. For locally advanced, unresectable, and metastatic disease, treatment is palliative, although fluorouracil chemoradiation for locally advanced and gemcitabine chemotherapy for metastatic disease can provide palliative benefits. Despite pancreatic cancer's resistance to currently available treatments, new methods are being investigated. Preoperative chemoradiation is being advocated, with seemingly sound reasoning, and a wider role for gemcitabine is being explored. However, new therapeutic strategies based on the molecular biology of pancreatic cancer seem to hold the greatest promise.

Molecular epidemiology of pancreatic cancer

Pancreatic cancer is the fourth leading cause of cancer-related death in the United States. Currently there is no early diagnostic test and no effective treatment options for this deadly disease. Prevention of pancreatic cancer is difficult because little is known about its etiology. The main modifiable risk factors for pancreatic cancer include cigarette smoking and dietary factors. Information from molecular epidemiological study of pancreatic cancer is very limited. DNA adducts derived from exposure to polycyclic aromatic hydrocarbon, aromatic amines, and heterocyclic amines have been detected in human pancreatic tissues. DNA damages derived from oxidative stress and lipid peroxidation are also present in the pancreas. No study has demonstrated a main effect of carcinogen-metabolizing genes and DNA repair genes on the risk of pancreatic cancer thus far. However, significant effects of these genes have been observed among individuals with known carcinogen exposure, such as smoking. A number of environmental and lifestyle factors, such as smoking, alcohol, coffee consumption, and exposure to organochlorine or hydrocarbon solvent, have been associated with the frequency and spectrum of K-ras mutation in pancreatic tumors. Dietary folate intake and serum levels of folate have been associated with the risk of pancreatic cancer among male smokers. These findings demonstrate the potential of the molecular epidemiology approach in understanding the etiology of pancreatic cancer. Further efforts should be made to understand the interactive relationship between genetic and environmental factors in the etiology of pancreatic cancer, which will in turn be important in identifying the high-risk population for the primary prevention of this deadly disease.

K-ras mutation in tamoxifen-related endometrial polyps

BACKGROUND

K-ras mutation is thought to occur at an early stage of neoplastic progression in the endometrium. The authors investigated mutations in codon 12 of K-ras in tamoxifen (TAM)-related endometrial polyps.

METHODS

DNA was extracted from 11 frozen endometrial polyps from TAM-treated patients with breast carcinoma. Mutations were detected using the mutant allele-specific amplification method. The results subsequently were analyzed for correlations with immunohistochemical data that were obtained using antibodies against estrogen receptors (ERs; alpha and beta forms), progesterone receptors (PRs; A and B forms), and Ki-67.

RESULTS

Mutations in codon 12 of K-ras were observed in 7 of 11 TAM-related endometrial polyps. Expression levels of ER-alpha and PR-B were high in the glandular epithelium and low in the stroma. PR-A expression was high in both the glandular epithelium and the stroma. In the glandular epithelium, expression of ER-beta appeared to be lower than expression of ER-alpha. The Ki-67 index in the glandular epithelium ranged from 2 to 38, whereas the index ranged from 0 to 4 in the stroma (P < 0.01).

CONCLUSIONS

The incidence of mutations in codon 12 of K-ras in TAM-related endometrial polyps (64%) was greater than the incidence of these same mutations in sporadic endometrial hyperplasias (4.5-23%). High expression levels of ER-alpha, PR-A, and PR-B in the glandular epithelium were observed in all polyps, regardless of K-ras codon 12 mutation status and Ki-67 index. The authors' findings may support the hypothesis that the polyp-carcinoma sequence partly indicates the development of endometrial carcinoma in postmenopausal women who have been treated with TAM.

Occupational exposure to dyes, metals, polycyclic aromatic hydrocarbons and other agents and K-ras activation in human exocrine pancreatic cancer

ras genes are known critical DNA targets for chemical carcinogens. Exocrine pancreatic cancer (EPC) is the human tumor with the highest prevalence of K-ras mutations at diagnosis. We analyzed the relationship between past occupational exposure to dyes, metals, polycyclic aromatic hydrocarbons (PAHs) and other agents and mutations in codon 12 of the K-ras gene in 107 incident cases of EPC. Information on occupational and life-style factors was obtained from personal interviews conducted during hospital stay. Occupational exposures were examined using industrial hygienists (IH) assessment and the Finnish job-exposure matrix (Finjem). Specific occupational exposures among K-ras mutated EPC cases (n = 83) were compared to those of K-ras wild-type EPC cases (n = 24) (case-case analysis). Multivariate-adjusted odds ratios (OR) and their corresponding 95% confidence limits were estimated by unconditional logistic regression. Cases with K-ras mutations were significantly more likely than wild-type cases to have been exposed to dyes and organic pigments (OR 4.8; p<0.05). There was some indication of weaker associations between K-ras mutations and occupational exposure to lead, PAHs, benzo[a]pyrene, gasoline, nickel, inhalatory exposure to chromium and sedentary work. The association with chromium compounds was stronger for G to T transversions, a finding compatible with experimental studies on mutation spectra for chromium. Results lend moderate support to the hypothesis of indirect relationships between occupational exposure to dyes and organic pigments and the activation of the K-ras gene in the etiopathogenesis of human exocrine pancreatic cancer.

Exploring environmental causes of altered ras effects: fragmentation plus integration?

Mutations in ras genes are the most common abnormality of oncogenes in human cancer and a major example of activation by point mutation. Experimental and epidemiological studies support the notion that Ki-ras activation and expression may be chemically related. We discuss the potential role of several environmental compounds in the induction or promotion of ras mutations in humans, with a focus on exocrine pancreatic cancer, the human tumor with the highest prevalence at diagnosis of Ki-ras mutations. Organochlorine compounds, organic solvents, and coffee compounds may play an indirect role in causing Ki-ras mutations, rather than as direct inducers of the mutations. Although for some organochlorine compounds the induction of point mutations in ras oncogenes cannot be excluded, it seems more likely that the effects of these compounds are mediated through nongenomic or indirectly genotoxic mechanisms of action. Organic solvents also may act via enzymatic induction of ras mutagens or by providing a proliferation advantage to ras-mutated cell clones. In exocrine pancreatic cancer, caffeine, other coffee compounds, or other factors with which coffee drinking is associated could modulate Ki-ras activation by interfering with DNA repair, cell-cycle checkpoints, and apoptosis. Asbestos, cigarette smoking, and some dietary factors also may be involved in the initiation or the promotion of Ki-ras mutations in lung and colon cancers. Further development of the mechanistic scenarios proposed here could contribute to a meaningful integration of biological, clinical, and environmental knowledge on the causes of altered ras effects.

K-ras gene mutation in the diagnosis of ultrasound guided fine-needle biopsy of pancreatic masses

AIM: To investigate the utility of K-ras mutation analysis of ultrasound guided fine-needle aspirate biopsy of pancreatic masses.

METHODS: Sixty-six ultrasound guided fine-needle biopsies were evaluated by cytology, histology and K-ras mutation. The mutation at codon 12 of the K-ras oncogene was detected by artificial restriction fragment length polymorphisms using BstN I approach.

RESULTS: The presence of malignant cells was reported in 40 of 54 pancreatic carcinomas and K-ras mutations were detected in 45 of the 54 FNABs of pancreatic carcinomas. The sensitivity of cytology and K-ras mutation were 74% and 83%, respectively. The speciality of cytology and K-ras mutation were both 100%. The sensitivity and speciality of K-ras mutation combined with cytology were 83% and 100%, respectively.

CONCLUSION: High diagnostic accuracy with acceptable discomfort of FNAB make it useful in diagnosis of pancreatic carcinoma. Ultrasound guided fine-needle biopsy is a safe and feasible method for diagnosing pancreatic cancer. Pancreatic carcinoma has the highest K-ras mutation rate among all solid tumors. The mutation rate of K-ras is about 80%-100%. The usage of mutation of codon 12 of K-ras oncogene combined with cytology is a good alternative for evaluation of pancreatic masses.

DNA adduct burden and tobacco carcinogenesis

DNA adducts associated with tobacco smoking could provide a marker of biologically effective dose of tobacco carcinogens and improve individual cancer risk prediction. A significant number of clinical and epidemiologic studies have reported associations of increased DNA adduct levels with the occurrence of the prevalent tobacco related cancers including cancer of the lung, head and neck, and bladder. The inducibility of DNA adducts following in vitro treatments using blood lymphocytes also appears to be a risk factor in the development of lung and head and neck cancer. Corroborative evidence pointing to the importance of DNA adducts in tobacco carcinogenesis include numerous studies showing associations of tobacco smoke exposure with the induction of DNA adducts in humans in vivo. Further effort is necessary, however, to more fully characterize the dose-response relationship between smoking and DNA adducts in exposed target and surrogate tissues. The relationship between gene polymorphisms thought to modify tobacco-related cancer risk and DNA adduct levels is complex. Results of some DNA adduct studies (both in vitro and in vivo) appear inconsistent with the epidemiologic findings. This is evident for polymorphisms involving both carcinogen metabolism (e.g. GSTP1) and DNA repair (e.g. XRCC1). Molecular studies of human tumors suggest associations of p53 mutation with DNA adducts and have revealed correlations of DNA adduct levels with somatic alterations (e.g. 3p21 LOH) that are thought to occur at the very earliest stages of tobacco carcinogenesis. More research is needed to assess the relationship between endogenous sources of DNA adducts and tobacco smoke exposure and the relative oncogenic effects of chemically stable versus unstable DNA adducts. Many potentially fruitful new avenues of cancer research are emerging that integrate DNA adduct analyses with assessments of smoking, genetics, diet and ambient air quality. These investigations aim to understand the multifactorial nature of interindividual variability in response to tobacco carcinogens. As these trends continue a variety of innovative study designs and approaches will become important in human populations.

Analyses of bulky DNA adduct levels in human breast tissue and genetic polymorphisms of cytochromes P450 (CYPs), myeloperoxidase (MPO), quinone oxidoreductase (NQO1), and glutathione S-transferases (GSTs)

Environmental carcinogens are converted into DNA-reactive metabolites by phase I and phase II enzymes that are involved in the activation and detoxification of xenobiotics. Several of these enzymes display genetic polymorphisms that alter their activity leading to individual variation in DNA damage levels and thus cancer susceptibility. We investigated the relationship between DNA adduct levels and genetic polymorphisms in key enzymes of chemical carcinogenesis: CYP1A1, CYP1A2, GSTT1, GSTM1, GSTP1, NQO1 and MPO. Levels of DNA adducts were determined in human breast tissue using the 32P-postlabeling method. A significantly higher adduct level was observed for individuals with the A-463 variant in the MPO gene (P=0.008), providing the first observation of an association between a predicted reduced MPO gene transcription and a higher level of DNA adducts. Furthermore, levels of DNA adducts were about 45% higher in individuals with either GSTP1*B or GSTP1*C variants compared to those homozygous for the wild-type allele. When the MPO and GSTP1 were examined together, individuals with these combined variant genotypes had significantly higher adduct levels than all other genotype combinations (P=0.003).