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Mol. Cell. Biol., Aug 1996, 4555-4565, Vol 16, No. 8
PM Vertino, RW Yen, J Gao and SB Baylin
Recent studies showing a correlation between the levels of DNA
(cytosine-5-)-methyltransferase (DNA MTase) enzyme activity and
tumorigenicity have implicated this enzyme in the carcinogenic process.
Moreover, hypermethylation of CpG island-containing promoters is associated
with the inactivation of genes important to tumor initiation and
progression. One proposed role for DNA MTase in tumorigenesis is therefore
a direct role in the de novo methylation of these otherwise unmethylated
CpG islands. In this study, we sought to determine whether increased levels
of DNA MTase could directly affect CpG island methylation. A full-length
cDNA for human DNA MTase driven by the cytomegalovirus promoter was
constitutively expressed in human fibroblasts. Individual clones derived
from cells transfected with DNA MTase (HMT) expressed 1- to 50-fold the
level of DNA MTase protein and enzyme activity of the parental cell line or
clones transfected with the control vector alone (Neo). To determine the
effects of DNA MTase overexpression on CpG island methylation, we examined
12 endogenous CpG island loci in the HMT clones. HMT clones expressing >
or = 9-fold the parental levels of DNA MTase activity were significantly
hypermethylated relative to at least 11 Neo clones at five CpG island loci.
In the HMT clones, methylation reached nearly 100% at susceptible CpG
island loci with time in culture. In contrast, there was little change in
the methylation status in the Neo clones over the same time frame. Taken
together, the data indicate that overexpression of DNA MTase can drive the
de novo methylation of susceptible CpG island loci, thus providing support
for the idea that DNA MTase can contribute to tumor progression through CpG
island methylation-mediated gene inactivation.
Copyright © 1996, American Society for Microbiology
De novo methylation of CpG island sequences in human fibroblasts overexpressing DNA (cytosine-5-)-methyltransferase
Oncology Center, Johns Hopkins University School of Medicine, Baltimore, Maryland 21231, USA.
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