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Methylation of the O 6 ‐methylguanine‐DNA methyltransferase promoter suppresses expression in mouse skin tumors and varies with the tumor induction protocol
Author(s) -
AbdelFattah Rana,
Glick Adam,
Rehman Ishtiaq,
Maiberger Patrick,
Hennings Henry
Publication year - 2005
Publication title -
international journal of cancer
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.475
H-Index - 234
eISSN - 1097-0215
pISSN - 0020-7136
DOI - 10.1002/ijc.21316
Subject(s) - methylation , dna methylation , methyltransferase , o 6 methylguanine dna methyltransferase , cancer research , microbiology and biotechnology , biology , carcinogenesis , dmba , dna methyltransferase , cpg site , cancer , gene expression , dna , gene , genetics
Hypermethylation of CpG sites within the promoter region of the O 6 ‐methylguanine‐DNA methyltransferase ( MGMT ) gene occurs frequently in human cancer, preventing both MGMT expression and repair of alkylation damage. To assess the role of MGMT in the development of mouse skin tumors induced by initiation‐promotion protocols, methylation of the MGMT promoter was examined in tumor DNA using methylation‐specific PCR. To determine whether MGMT promoter methylation was affected by the tumor induction protocol, tumors were initiated by N ‐methyl‐ N ′‐nitro‐ N ‐nitrosoguanidine (MNNG) or 7,12‐dimethylbenz[a]anthracene (DMBA) and promoted by 12‐ O ‐tetradecanoylphorbol‐13‐acetate (TPA) or mezerein. Although the MGMT promoter was not methylated in normal skin, promoter methylation was found in 56 of 136 papillomas (41.2%) and in 19 of 37 squamous cell carcinomas (51.4%). When methylation of the MGMT promoter was compared in the 4 treatment groups, hypermethylation was found more frequently in tumors initiated by DMBA and promoted by mezerein, a protocol associated with a high frequency of malignant conversion. Methylation was found in some tumors as early as 5 weeks after initiation, but the methylation frequency increased with time. MGMT promoter methylation reduced MGMT expression as determined by immunohistochemistry. Although MGMT promoter methylation was not generally correlated with ras mutations, the frequency of MGMT methylation was higher in MNNG‐initiated, mezerein‐promoted papillomas with mutations in Ha‐ ras compared to papillomas with Ki‐ ras . Methylation of the MGMT promoter, associated with reduced MGMT expression, is found in nearly half of mouse skin tumors, but varies with both the tumor initiator and tumor promoter, and may be a key step in the progression from papillomas to carcinomas. © 2005 Wiley‐Liss, Inc.