Potentiation of lymphomagenesis by methylnitrosourea in mice transgenic for LMO1 is blocked by O6-alkylguanine DNA-alkyltransferase

Abstract

We evaluated induction of lymphomas by the methylating carcinogen, N-methylnitrosourea [MNU], in transgenic mice expressing both LMO1 and the DNA repair gene, MGMT, in the thymus. The goal was to determine whether environmental mutagens shorten the latency or increase the incidence of LMO1+lymphomas and whether mice transgenic for both LMO1 and MGMT, and thereby able to repair O 6 -methylguanine DNA adducts induced by MNU, would be protected. Mice heterozygous for LMO1 or MGMT were crossed and o spring treated with MNU at 6 weeks of age. MNU induced lymphoma incidence was highest in the LMO1 mice, 91% and lowest in the hMGMT+mice, 15%. MNU induced K-ras mutations in codon 12 in non-MGMT transgenics resulted in a shorter latency of tumors and accounting for half of the early lymphomas in LMO1 mice. The e ect of MNU was abrogated in the LMO1/hMGMT transgenic mice, indicating the ability of MGMT expression to block the carcinogenic e ect of MNU even in cancer prone mice. Thus, methylating agents potentiate lymphomagenesis of LMO1, in part through activation of K-ras and the MAPK pathway, a process which appear to synergize with LMO1 mediated transcription activation. O 6 -alkylguanine DNA-alkyltransferase mediated DNA repair e ectively blocks chemical carcinogenesis in mice carrying the LMO1 oncogene.

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

“…In our study, although K-ras mutations were found, no correlation was present between these mutations and tumor latency in either PMS2 +/+ or PMS2 7/7 mice. This is in contrast to our recent report showing an association between K-ras mutation and shorter latency in LMO1 overexpressing mice (Allay et al, 1997). Thus, while K-ras mutations are observed in some MNU-induced lymphomas, other oncogenes and/or inactivation of unidenti®ed tumor suppressor gene(s) may also be involved, and may complement the presence of K-ras mutations.…”

“…at 5 weeks of age. High expression of hMGMT + reduced the incidence from 100% in the PMS2 7/7 mice to 80% in the PMS2 7/7 /hMGMT + mice (the thymic lymphoma-free survival rate was 0 vs 20% in PMS2 7/7 mice and PMS2 7/7 / hMGMT + mice, respectively, P50.001) and signi®cantly increased the mean latency 81 to 91 days, P50.05) lymphomas in MSH2-de®cient mice (Lowsky et al, 1997) and MNU-induced thymic lymphomas (Allay et al, 1997). Mice groups were analysed for incidence and latency of lymphoma induction.…”

Live and let die: In vivo selection of gene-modified hematopoietic stem cells via MGMT-mediated chemoprotection

“…In practice, there is little evidence that such a chemoprotective strategy would promote leukemia in cells which overexpress MGMT. In fact, gene modified hematopoietic cells demonstrate a significant reduction in mutation frequency and chromosomal aberrations upon challenge with O 6 alkylating agents [ 31,[82][83][84] and mice transgenic for human MGMT show a decrease in cellular transformation frequency upon drug treatment [85][86][87][88][89]. Perhaps most compelling in this regard is the observation that there was no evidence of hematopoietic malignancy following an extended, dose escalating chemotherapeutic regimen in a canine large animal model which enabled selection of gene modified cells [ 62 ].…”

Therapeutic Modulation of DNA Damage and Repair Mechanisms in Blood Cells

“…While this is still an area of active investigation, there is no evidence to date that overexpression of mutant forms of MGMT and exposure to alkylating agents can lead to leukemia. To this end, several groups have demonstrated that hematopoietic cells transduced with retroviral vectors that express MGMTmutant proteins do not show an increased in the frequency of mutations or chromosomal aberrations upon challenge with O 6 alkylating agents (Allay et al, 1997;Chinnasamy et al, 1998a;Chinnasamy et al, 1998b;Dumenco et al, 1993;Fairbairn et al, 2000;Liu et al, 1994;Liu et al, 1999;Reese et al, 2001). Additionally, there have been no signs of hematopoietic clonal expansion in a canine large animal model in which animals transplanted animals MGMT P140K -transduced cells received alkylator therapy over a prolonged time period that also included some escalation in alkylator dose.…”