C. Colussi, S. Fiumicino, A. Giuliani, C. Macrí, M. Crescenzi, M. Bignami, Laboratory of Comparative Toxicology and Ecotoxicology, Istituto Superiore di Sanitá, Rome, Italy; S. Rosini, P. Musiani, Department of Oncology and Neurosciences, G. D'Annunzio University, Chieti, Italy; C. S. Potten, Paterson Institute for Cancer Research, Christie Hospital Trust, Manchester, U.K.
Correspondence to: Margherita Bignami, Ph.D., Laboratory of Comparative Toxicology and Ecotoxicology, Istituto Superiore di Sanitá, Viale Regina Elena 299, 00161 Rome, Italy (e-mail: bignami{at}iss.it).
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Loss of MMR also confers resistance to the toxic effects of methylating agents (8,9). These compounds are carcinogenic, mutagenic, and cytotoxic because they methylate guanine bases in DNA to produce O6-methylguanine (O6-MeG). O6-MeG can be removed from DNA by a specific repair enzyme, the O6-methylguanine DNA methyltransferase (MGMT) (10), although MGMT levels are low in some human tissues (11). O6-MeG in DNA may be recognized by MMR (12,13), and this interaction activates apoptotic cell death (1416). MMR-defective human or mouse cells are, consequently, resistant (tolerant) to the cytotoxic effects of methylating agents, and O6-MeG can accumulate in the DNA of MSH2-, MSH6-, MLH1-, or PMS2-defective cells without lethal effects [for a review, see (17)]. Persistent DNA O6-MeG produces G-to-A transitions (18). Thus, MMR normally plays an important role in the elimination of potentially mutated cells (19).
It is not presently known whether the loss of O6-MeG-related apoptosis, which accompanies tolerance to methylation damage, plays a role in the increased cancer risk associated with defective MMR. Many methylating agents are powerful carcinogens that generate O6-MeG in DNA. O6-MeG has been found in DNA of normal human colon (20), suggesting that colon cells might be at risk of persistent DNA methylation. An association between low MGMT levels and colon cancer risk has even been postulated (2022).
We have investigated a possible contribution of DNA methylation damage to the development of colorectal cancer in MMR-defective mice. As an experimental model, we used mice that were null (msh-/-), heterozygous (msh+/-), or wild-type (msh+/+) for the MMR gene msh2. Tumor induction by repeated exposures to the methylating carcinogen 1,2-dimethylhydrazine (DMH) was compared in the three genotypes. DMH is a procarcinogen that, after metabolic activation, induces formation of O6-MeG in the DNA of several tissues, including colon, ileum, and liver (23), and induces tumors in the colorectum (24,25).
The tumors that arise spontaneously in msh2-/- knockout mice are overwhelmingly lymphomas (5,26,27). The few surviving animals develop adenocarcinomas of the duodenum and jejunum and skin neoplasms (27,28). We hypothesized that the increased resistance to methylating agent-induced apoptosis is a factor in tumor susceptibility of MMR-defective mice. We examined the effect of DMH, an acknowledged colon carcinogen, in msh2-deficient animals. The incidence of both colorectal tumors and lymphoma and the time to death were analyzed, and the apoptotic response in the proliferating cells of the colonic epithelium was examined.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
MMR-deficient mice (provided by Professor H. te Riele, The Netherlands Cancer Institute, Amsterdam) carrying a targeted disruption in exon 12 of the msh2 gene were described previously (5). 129OLA/FVB msh2-/- mice were crossed five times with FVB mice to obtain an almost 95% FVB genetic background. All of the animals were fed a standard laboratory diet and water ad libitum and were maintained on a 12-hour light12-hour dark schedule. The animal experimental protocol has been reviewed and approved by the Italian Ministry of Health.
Genotyping of mice by polymerase chain reaction (PCR).
Pieces of tail were digested overnight at 53 °C in 700 µL of lysis buffer (i.e., 50 mM Tris [pH 8], 100 mM EDTA, 100 mM NaCl, 1% sodium dodecyl sulfate, and 1 mg/mL proteinase K). DNA was extracted by standard procedures and amplified in a three-primer PCR reaction (50 µL) containing 125 pmol of each primer, 200 µM deoxynucleoside triphosphates, 2 mM MgCl2, and 1.25 U of Taq polymerase. Primers were as described previously (28). The reaction conditions were 94 °C for 5 minutes, 60 °C for 2 minutes, and 72 °C for 2 minutes (for one cycle); 94 °C for 1 minute, 60 °C for 2 minutes, and 72 °C for 2 minutes (for 40 cycles); and 94 °C for 1 minute, 60 °C for 2 minutes, and 72 °C for 10 minutes (for one cycle). The wild-type and targeted msh2 alleles generate a 164- and a 194-base-pair PCR product, respectively.
Induction of tumors.
The carcinogenic treatment was performed as described previously (24). Briefly, DMH (Sigma Chemical Co., St. Louis, MO) was dissolved (2 mg/mL) immediately before use in 1 mM EDTA (pH 7). Groups of mice, 45 weeks of age (12 msh2+/+, 12 msh2-/-, and 21 msh2+/- animals), received weekly subcutaneous injections of DMH (20 mg/kg body weight) until tumors appeared. Control animals (12 msh2+/+, 12 msh2-/-, and 15 msh2+/-) received mock injections of 1 mM EDTA.
Pathologic analysis.
Complete autopsy was performed on all mice that were found dead or that were killed by cervical dislocation when they were moribund. Before being embedded in paraffin blocks, tumors and tissues were removed and fixed in 10% formaldehyde in phosphate-buffered saline. The sections were stained with hematoxylineosin and examined.
Statistical analysis.
A two-sided MannWhitney U test on the rank sums was used to compare intervals from the beginning of treatment to the time of death between treated and untreated mice (29). This test was adopted to avoid the imposition of any distributional hypothesis on the data. The probability of survival was also estimated by the standard KaplanMeier parametric approach, and the difference in median time to death between groups was analyzed with two-sided Wilcoxon and log-rank tests. Computation was performed by the NPAR1WAY procedure of the SAS system 8.00 version for personal computers (Statistical Analysis Software, Inc., Cary, NC). All statistical tests were considered to be statistically significant at P<.05.
Immunohistochemistry.
Paraffin-embedded intestinal sections (5 µm) were baked at 65 °C for 1 hour, deparaffinized, and rehydrated through a series of graded alcohols. Sections were incubated overnight at 4 °C with the msh2 antiserum (a gift from Professor J. Jiricny, University of Zurich, Zurich, Switzerland) in 0.1 M sodium phosphate (pH 8) with 0.5% Nonidet P-40 and 5% nonfat dry milk. The sections were then incubated with fluorescein-conjugated, goat anti-rabbit immunoglobulin antiserum and counterstained with Hoechst 33258.
Apoptosis scoring.
Animals (1012 weeks of age) were killed 8 or 16 hours after injection with DMH (80 mg/kg). Small and large intestines were removed, flushed with water, fixed in Carnoy's fixative for 30 minutes, and stored in 70% ethanol. Apoptosis was scored in histologic sections as described previously (30). Briefly, 25 good longitudinal sections were selected per animal and divided down the long axis into two half crypts, which were scored separately; 50 such half-crypt sections per mouse were scored. Starting at the base of the crypt column, the cells were numbered up each side, and data were recorded on a cell-position-by-cell-position basis. The location of each apoptotic event was then recorded in term of its position. The results are presented as plots of frequency of apoptotic events at each position in the crypt. Each frequency plot consists of the pooled data from three mice.
Microsatellite analysis.
The tumor was microdissected from formalin-fixed, paraffin-embedded specimens, and DNA was extracted by incubating individual 5-µm histologic sections in 0.5 mL of buffer containing 10 mM Tris (pH 8.3), 50 mM KCl, 1.5 mM MgCl2, bovine serum albumin (10 mg/mL), 1% Tween 20, and proteinase K (10 µg/mL). The sections were incubated overnight at 55 °C, boiled for 5 minutes, and then cooled on ice. Normal tissue DNA was extracted from the tails. DNA aliquots were amplified with end-labeled primers and analyzed by electrophoresis on denaturating 6% polyacrylamide gels.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Four- to five-week old, wild-type, msh2+/- or homozygous msh2-/- knockout mice were treated weekly with DMH. The times at which tumor-bearing animals died or were killed are summarized in Fig. 1, A. DMH-treated msh2-/- animals developed tumors, with a median time to death of 83.5 days (95% confidence interval [CI] = 62 to 112 days) from the start of the treatment. This latent period was statistically significantly reduced (P<.0002, MannWhitney U test) compared with untreated knockout animals, all of which developed lymphoblastic lymphoma after a median time of 151.5 days (95% CI = 131 to 181 days) (P<.001, log-rank and Wilcoxon tests). Pathologic analysis indicated a clear difference in the tumor load of DMH-treated and untreated knockout animals. Although all knockout animals developed lymphomas, colorectal adenocarcinomas were present in six of 12 DMH-treated msh2-/-mice. The adenocarcinomas, which were located predominantly in the lower part of the colon or in the rectum, were not observed in untreated knockout animals (none of 12). No difference in tumor distribution was seen between the sexes (carcinomas were in three males and three females). The lower part of the colon and the rectum showed multiple papillary or sessile adenomas with areas of dysplasia and carcinoma in situ close to overt, usually exophytic, adenocarcinomas. Multiple areas of atypia were found, suggesting that several initiating events were induced by the carcinogen. In a few cases, multiple separate tumors were observed. Histologically, the colorectal carcinomas were moderately to well differentiated, and their growth was frequently limited to the mucosa (Fig. 2
), although submucosal and lymphatic invasion was evident in some mice.
|
|
In contrast to the msh2-/- mice, neither heterozygous nor wild-type mice were prone to lymphoma development. DMH-treated msh2+/- mice died at a median time to death of 204 days (Fig. 1, B). All (21 of 21) heterozygous mice had colon adenocarcinomas at death; a lymphoma was present in a single case. There were two examples of lung metastasis and two of angiosarcomas of the kidney capsule, which, although uncommon, have been reported previously in mice exposed to DMH (31) (Table 1
). Two mice also developed trichofolliculomas resembling the sebaceous tumors that form a subset of spontaneous tumors in msh2-/- animals (26). These trichofolliculomas are reminiscent of the skin malignancies associated with the MuirTorre variant of HNPCC (2).
|
The absence of msh2, therefore, increases the tumorigenicity of DMH in mice. This absence affects the colon, and DMH-induced colorectal adenocarcinomas developed in msh2-/- animals in about one third of the time required in wild-type animals. A statistically significant difference in the times to death (P<.0001, log-rank, Wilcoxon, and MannWhitney U tests) was observed between DMH-treated msh2-/- and msh2+/+ mice with a median time to death of 83.5 days (95% CI = 62 to 112 days) and 209 days (95% CI = 196 to 217 days), respectively. The lymphomas to which msh2-/- mice are already predisposed also exhibited an earlier onset following DMH treatment. The accelerated induction of colorectal tumors and lymphomas by DMH is dependent on the inactivation of both msh2 alleles and was not observed in msh2+/- mice. The somewhat broader spectrum of tumors in the DMH-treated heterozygous animals is consistent with a modest general increase in carcinogen susceptibility as a consequence of the retention of a single active msh2 allele.
The similarity in response of the msh2+/- and wild-type mice suggests that heterozygosity for msh2 is not an important factor in DMH-induced carcinogenesis. In agreement with this suggestion, we found no evidence for microsatellite instability in DMH-induced tumors from heterozygous animals. Four colorectal adenocarcinomas from DMH-treated msh2+/- mice were analyzed at the D17mit123 locus, which is closely linked to msh2. The D18mit14 locus was also examined in two of the four tumors. Neither locus was mutated in any of the tumors. Four spontaneous lymphomas from msh2-/- mice displayed the expected microsatellite instability. D17mit123 and at least one other microsatellite locus were mutated in each case (Fig. 3).
|
Msh2 Expression and Apoptosis in the Gastrointestinal Tract of Wild-Type and msh2 Knockout Animals
Msh2 protein was detected in the lower two thirds of immunohistochemically stained colonic crypts, from position 1 to approximately position 30, from both msh2+/+ and msh2+/- animals (Fig. 4, A and B). msh2 was detected uniformly in the nucleus; the intensity or distribution of msh2 protein in heterozygous and wild-type mice was similar (Fig. 4
, A and B). As expected, no staining was detected in crypts from homozygous knockout animals (Fig. 4
, C).
|
|
Thus, with the exception of a wave in colonic crypt cells at early times after treatment, DMH-induced apoptosis in the small or large intestine was dependent on functional msh2 protein.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The organ specificity of DMH is not, however, related in a simple way to apoptosis. Although an early wave of apoptosis in colonic crypt cells was largely independent of msh2, most DMH-induced cell death in both colon and small intestine was msh2 dependent. This finding is consistent with the report of Toft et al. (28), which indicated that methylating agent-induced apoptosis in mouse small intestinal crypt cells is dependent on functional msh2. Despite this association, we did not observe DMH-induced tumors of the small intestine. It appears likely that msh2, by controlling apoptosis, provides a general protection mechanism against DNA methylation damage in all mouse tissues. Additional factors, such as MGMT level and turnover rate or the renewal rate of the cell population affected by DMH, may underlie the colon specificity of this carcinogen.
Differences in apoptosis clearly do not explain the organotropism of DMH carcinogenesis. The overwhelming tendency of DMH to induce colorectal tumors is apparently independent of the msh2 background. We did observe, however, accelerated colorectal carcinogenesis in msh2 knockout animals. This finding may reflect the decreased susceptibility to O6-MeG-induced apoptosis that accompanies loss of msh2. Thus, increased resistance to methylation-induced apoptosis (1416,35) and increased spontaneous and DNA damage-related mutation rates (19) might contribute to the development of colorectal tumors. It is important also to stress that inactivation of msh2 dramatically enlarges the number of colonic crypts cells that are target for neoplastic transformation by DMH.
The pattern of tumors in the homozygous msh2-/- mice reflected the expected colorectal specificity of DMH and the spontaneous susceptibility of msh2-/- animals to lymphoma. In the msh2+/- animals, however, the pattern was more complex. Although the majority of DMH-treated heterozygous mice died of colorectal carcinoma, a substantial minority developed tumors at additional sites. Angiosarcoma of the kidney capsule in two of 21 msh2+/- mice suggests that heterozygosity for msh2 might increase the susceptibility to DMH carcinogenesis in tissues other than the colon. Angiosarcoma of the kidney capsule is associated, although rarely, with DMH exposure (31,36). Furthermore, the skin neoplasms and the lymphomas are reminiscent of spontaneous tumors associated with complete loss of Msh2 function (26). A similar heightened susceptibility to induction of nonhematologic malignancies by N-ethyl-N-nitrosourea, a primarily lymphomagenic carcinogen, might underlie the more heterogeneous pattern of tumors in msh2+/- animals (27). Thus, although a single active msh2 allele is compatible with a normal life span and is sufficient to provide protection against endogenous carcinogens, it may be insufficient to protect against high doses of a potent carcinogen. MGMT levels, which differ widely among organs, might be the crucial factor in determining tumor susceptibility (32).
In agreement with previous studies (27,37), we found no evidence of early inactivation of the active MMR allele in the tumors that developed in heterozygous mice. This finding contrasts dramatically with results seen with HNPCC, in which inactivation of the functional allele is considered to be a probable early event in tumor development. Retention of an active msh2 allele suggests that selection for resistance to methylation damagethe starting point for this studymight not underlie the emergence of tumors in heterozygous individuals with HNPCC. There are two obvious caveats, however. First, the mice were exposed repeatedly to high doses of a colon-specific carcinogen. This dosing contrasts with the chronic, low-level exposure to DNA-methylating agents that human colon cells might encounter and that could provide a selective pressure for MMR loss. In addition, mouse cells are generally less sensitive to killing by methylating agents than are their human counterparts, and loss of MMR confers only a modest (twofold) increased resistance (38). This small increase compares with the greater than 50-fold higher resistance of MMR-defective human cells. The selective advantage provided by inactivation of the second MMR allele is, therefore, minor in mouse cells, and other targets of mutational inactivation may contribute more substantially to tumor development.
DMH also markedly decreases the latent period for lymphoma development in msh2-/- mice. This finding suggests that MMR provides substantial protection against lymphomagenesis by alkylating carcinogens (27,32). We note that the constitutional absence of MMR predisposes humans to hematologic malignancy (non-Hodgkin's lymphomas and acute myeloid leukemia) and type I neurofibromatosis (39,40). Thus, although the parallels between heterozygous msh2 mice and HNPCC are not strong, homozygous knockout mice are good models for their human counterparts.
DNA methylation damage has long been suspected to be a contributor to human colon cancer. We conclude that MMR constitutes a powerful defense against colorectal cancer induced by an exogenous methylating carcinogen. Our data support the notion that failure by MMR to elicit the apoptotic pathway associated with persistence of this methylated base is associated with an increased risk of colon cancer.
![]() |
NOTES |
---|
Supported by a grant from Associazione Nazionale Ricerca sul Cancro and Ministero della Sanitá. C. Colussi was partially supported by a fellowship from Federazione Italiana Ricerca sul Cancro.
We are grateful to P. Karran (Imperial Cancer Research Fund, U.K.) for helpful discussions and Professor Mariani Costantini (University of Rome "La Sapienza," Italy) for critical review of the manuscript. M. Bignami thanks her mentors, Umberto Saffiotti and Ruggero Montesano, for guidance and support early in her career.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1 Modrich P, Lahue R. Mismatch repair in replication fidelity, genetic recombination, and cancer biology. Annu Rev Biochem 1996;65:10133.[Medline]
2 Lynch HT, Kimberling W, Albano WA, Lynch JF, Biscone K, Schuelke GS, et al. Hereditary nonpolyposis colorectal cancer (Lynch syndromes I and II). I. Clinical description of resource. Cancer 1985;56:9348.[Medline]
3 Buermeyer AB, Deschenes SM, Baker SM, Liskay RM. Mammalian DNA mismatch repair. Annu Rev Genet 1999;33:53364.[Medline]
4 Reitmair AH, Schmits R, Ewel A, Bapat B, Redston M, Mitri A, et al. MSH2 deficient mice are viable and susceptible to lymphoid tumours. Nat Genet 1995;11:6470.[Medline]
5 de Wind N, Dekker M, Berns A, Radman M, te Riele H. Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer. Cell 1995;82:32130.[Medline]
6 Edelmann W, Yang K, Umar A, Heyer J, Lau K, Fan K, et al. Mutation in the mismatch repair gene Msh6 causes cancer susceptibility. Cell 1997;91:46777.[Medline]
7 Prolla TA, Baker SM, Harris AC, Tsao JL, Yao X, Bronner CE, et al. Tumour susceptibility and spontaneous mutation in mice deficient in Mlh1, Pms1 and Pms2 DNA mismatch repair. Nat Genet 1998;18:2769.[Medline]
8 Branch P, Aquilina G, Bignami M, Karran P. Defective mismatch binding and a mutator phenotype in cells tolerant to DNA damage. Nature 1993;362:6524.[Medline]
9 Kat A, Thilly WG, Fang WH, Longley MJ, Li GM, Modrich P. An alkylation-tolerant, mutator human cell line is deficient in strand-specific mismatch repair. Proc Natl Acad Sci U S A 1993;90:64248.[Abstract]
10 Pegg AE. Mammalian O6-alkylguanine-DNA alkyltransferase: regulation and importance in response to alkylating carcinogenic and therapeutic agents. Cancer Res 1990;50:611929.[Medline]
11 Grafstrom RC, Pegg AE, Trump BF, Harris CC. O6-alkylguanine-DNA alkyltransferase activity in normal human tissues and cells. Cancer Res 1984;44:28557.[Abstract]
12 Griffin S, Branch P, Xu YZ, Karran P. DNA mismatch binding and incision at modified guanine bases by extracts of mammalian cells: implications for tolerance to DNA methylation damage. Biochemistry 1994;33:478793.[Medline]
13
Duckett DR, Drummond JT, Murchie AI, Reardon YT, Sancar A, Lilley DM, et al. Human MutS recognizes damaged DNA base pairs containing O6-methylguanine, O4-methylthymine, or the cisplatin d(GpG) adduct. Proc Natl Acad Sci U S A 1996;93:64437.
14
D'Atri S, Tentori L, Lacal PM, Graziani G, Pagani E, Benincasa E, et al. Involvement of the mismatch repair system in temozolomide-induced apoptosis. Mol Pharmacol 1998;54:33441.
15 Meikrantz W, Bergom MA, Memisoglu A, Samson L. O6-alkylguanine DNA lesions trigger apoptosis. Carcinogenesis 1998;19:36972.[Abstract]
16
Hickman MJ, Samson LD. Role of DNA mismatch repair and p53 in signaling induction of apoptosis by alkylating agents. Proc Natl Acad Sci U S A 1999;96:107649.
17 Bignami M, O'Driscoll M, Aquilina G, Karran P. Unmasking a killer: DNA O6-methylguanine and the cytotoxicity of methylating agents. Mutat Res 2000;462:7182.[Medline]
18 Ellison KS, Dogliotti E, Connors TD, Basu AK, Essigmann JM. Site-specific mutagenesis by O6-alkylguanines located in the chromosomes of mammalian cells: influence of the mammalian O6-alkylguanine-DNA alkyltransferase. Proc Natl Acad Sci U S A 1989;86:86204.[Abstract]
19
Andrew SE, McKinnon M, Cheng BS, Francis A, Penney J, Reitmair AH, et al. Tissues of MSH2-deficient mice demonstrate hypermutability on exposure to a DNA methylating agent. Proc Natl Acad Sci U S A 1998;95:112630.
20 Hall CN, Badawi AF, O'Connor PJ, Saffhill R. The detection of alkylation damage in the DNA of human gastrointestinal tissues. Br J Cancer 1991;64:5963.[Medline]
21 Jackson PE, Hall CN, Badawi AF, O'Connor PJ, Cooper DP, Povey AC. Frequency of Ki-ras mutations and DNA alkylation in colorectal tissue from individuals living in Manchester. Mol Carcinog 1996;16:129.[Medline]
22
Jackson PE, Hall CN, O'Connor PJ, Cooper DP, Margison GP, Povey AC. Low O6-alkylguanine DNA-alkyltransferase activity in normal colorectal tissue is associated with colorectal tumours containing a GCAT transition in the K-ras oncogene. Carcinogenesis 1997;18:1299302.[Abstract]
23 Swenberg JA, Cooper HK, Bucheler J, Kleihues P. 1,2-Dimethylhydrazine-induced methylation of DNA bases in various rat organs and the effect of pretreatment with disulfiram. Cancer Res 1979;39:4657.[Abstract]
24 Thurnherr N, Deschner EE, Stonehill EH, Lipkin M. Induction of adenocarcinomas of the colon in mice by weekly injections of 1,2-dimethylhydrazine. Cancer Res 1973;33:9405.[Medline]
25 Diwan BA, Dempster AM, Blackman KE. Effects of methylazoxymethanol acetate on inbred mice: influence of genetic factors on tumor induction. Proc Soc Exp Biol Med 1979;161:3479.
26 Reitmair AH, Redston M, Cai JC, Chuang TC, Bjerknes M, Cheng H, et al. Spontaneous intestinal carcinomas and skin neoplasms in Msh2-deficient mice. Cancer Res 1996;56:38429.[Abstract]
27 de Wind N, Dekker M, van Rossum A, van der Valk M, te Riele H. Mouse models for hereditary nonpolyposis colorectal cancer. Cancer Res 1998;58:24855.[Abstract]
28
Toft NJ, Winton DJ, Kelly J, Howard LA, Dekker M, te Riele H, et al. Msh2 status modulates both apoptosis and mutation frequency in the murine small intestine. Proc Natl Acad Sci U S A 1999;96:39115.
29 Lehmann EL. Nonparametrics: statistical methods based on ranks. San Francisco (CA): Holden-Day; 1975.
30 Li YQ, Fan CY, O' Connor PJ, Winton DJ, Potten CS. Target cells for the cytotoxic effects of carcinogens in the murine small bowel. Carcinogenesis 1992;13:3618.[Abstract]
31 Turusov VS. Role of host factors in carcinogenesis induced in mice by 1,2-dimethylhydrazine. IARC Sci Publ 1983;51:3948.[Medline]
32
Kawate H, Sakumi K, Tsuzuki T, Nakatsuru Y, Ishikawa T, Takahashi S, et al. Separation of killing and tumorigenic effects of an alkylating agent in mice defective in two of the DNA repair genes. Proc Natl Acad Sci U S A 1998;95:511620.
33 Qin X, Liu L, Gerson SL. Mice defective in the DNA mismatch gene PMS2 are hypersensitive to MNU induced thymic lymphoma and are partially protected by transgenic expression of human MGMT. Oncogene 1999;18:4394400.[Medline]
34 Potten CS, Li YQ, O'Connor PJ, Winton DJ. A possible explanation for the differential cancer incidence in the intestine, based on distribution of the cytotoxic effects of carcinogens in the murine large bowel. Carcinogenesis 1992;13:230512.[Abstract]
35
Kawate H, Itoh R, Sakumi K, Nakabeppu Y, Tsuzuki T, Ide F, et al. A defect in a single allele of the Mlh1 gene causes dissociation of the killing and tumorigenic actions of an alkylating carcinogen in methyltransferase-deficient mice. Carcinogenesis 2000;21:3015.
36 Toth B, Malick L, Shimizu H. Production of intestinal and other tumors by 1,2-dimethylhydrazine dihydrochloride in mice. I. A light and transmission electron microscopic study of colonic neoplasms. Am J Pathol 1976;84:6986.[Abstract]
37
Qin X, Shibata D, Gerson SL. Heterozygous DNA mismatch repair gene PMS2-knockout mice are susceptible to intestinal tumor induction with N-methyl-N-nitrosourea. Carcinogenesis 2000;21:8338.
38
Humbert O, Fiumicino S, Aquilina G, Branch P, Oda S, Zijno A, et al. Mismatch repair and differential sensitivity of mouse and human cells to methylating agents. Carcinogenesis 1999;20:20514.
39
Ricciardone M, Ozcelik T, Cevher B, Ozdag H, Tuncer M, Gurgey A, et al. Human MLH1 deficiency predisposes to hematological malignancy and neurofibromatosis type I. Cancer Res 1999;59:2903.
40
Wang Q, Lasset C, Desseigne F, Frappaz D, Bergeron C, Navarro C, et al. Neurofibromatosis and early onset of cancers in hMLH1-deficient children. Cancer Res 1999;59:2947.
Manuscript received January 29, 2001; revised July 2, 2001; accepted August 16, 2001.
This article has been cited by other articles in HighWire Press-hosted journals:
![]() |
||||
|
Oxford University Press Privacy Policy and Legal Statement |