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Mol. Cell. Biol., Sep 1997, 5612-5619, Vol 17, No. 9
GS Watts, RO Pieper, JF Costello, YM Peng, WS Dalton and BW Futscher
O6-Methylguanine DNA methyltransferase (MGMT) repairs the mutagenic and
cytotoxic O6-alkylguanine lesions produced by environmental carcinogens and
the chemotherapeutic nitrosoureas. As such, MGMT-mediated repair of
O6-alkylguanine lesions constitutes a major form of resistance to
nitrosourea chemotherapy and makes control of MGMT expression of clinical
interest. The variability of expression in cell lines and tissues, along
with the ease with which the MGMT phenotype reverts under various
conditions, suggests that MGMT is under epigenetic control. One such
epigenetic mechanism, 5-methylation of cytosines, has been linked to MGMT
expression. We have used an isogenic human multiple myeloma tumor cell line
model composed of an MGMT-positive parent cell line, RPMI 8226/S, and its
MGMT-negative variant, termed 8226/V, to study the control of MGMT
expression. The loss of MGMT activity in 8226/V was found to be due to the
loss of detectable MGMT gene expression. Bisulfite sequencing of the MGMT
CpG island promoter revealed large increases in the levels of CpG
methylation within discrete regions of the 8226/V MGMT CpG island compared
to those in 8226/S. These changes in CpG methylation are associated with
local heterochromatinization of the 8226/V MGMT transcription start site
and provide a likely mechanism for the loss of MGMT transcription in
8226/V.
Copyright © 1997, American Society for Microbiology
Methylation of discrete regions of the O6-methylguanine DNA methyltransferase (MGMT) CpG island is associated with heterochromatinization of the MGMT transcription start site and silencing of the gene
Bone Marrow Transplant Program, Arizona Cancer Center, and Department of Pharmacology, University of Arizona, Tucson 85724, USA.
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