Jj. Wu et al., EXPRESSION OF PROKARYOTIC HHAI DNA METHYLTRANSFERASE IS TRANSFORMING AND LETHAL TO NIH 3T3 CELLS, Cancer research, 56(3), 1996, pp. 616-622
In neoplastic cells, levels of DNA methyltransferase activity are ofte
n increased, and evidence is accruing to suggest an important role for
this event in tumorigenesis, To evaluate this possibility further, an
d to investigate the contribution of increasing de novo, as opposed to
maintenance, DNA methylation in mammalian cells, we expressed the bac
terial HhaI methyltransferase in cultured murine fibroblasts. This enz
yme is a pure de novo DNA methyltransferase that methylates the intern
al C in the sequence GCGC. We find that both constitutive and induced
expression of the wild-type HhaI results, primarily, in lethality to t
he cells, However, surviving cell clones that express low levels of M,
HhaI demonstrate increased tumorigenicity as assessed by soft agar cl
oning efficiency (8.6% for sense HhaI-transduced PA 317 cells versus 0
.4% for antisense controls; 1.7% for sense HhaI-transfected NIH 3T3 ce
lls versus 0% for a mutant HhaI control) and tumorigenicity in nude mo
use heterotransplants (75% for sense HhaI-transduced PA 317 cells vers
us 18.5% for antisense controls), DNA isolated from the clonogenic sen
se HhaI clones, versus clones expressing the mutant HhaI gene, has no
increase in overall CpG methylation but an average of 27% (range, 16.7
-38.9) increase in methylcytosine content at GCGC sites, These finding
s suggest that eukaryotic cells tolerate a narrow window of increased
ne novo DNA methylating capacity, above which cell death occurs and wi
thin which cell transformation results, Our results further emphasize
the potential role of increased DNA methyltransferase activity in the
evolution of cancer.