Li E, Bestor TH, Jaenisch R:
Targeted mutation of the DNA methyltransferase gene results in embryonic lethality.
Cell 1992 Jun 12;69(6):915-26
ABSTRACT
Gene targeting in embryonic stem (ES) cells has been used to mutate
the murine DNA methyltransferase gene. ES cell lines homozygous for
the mutation were generated by consecutive targeting of both
wild-type alleles; the mutant cells were viable and showed no
obvious abnormalities with respect to growth rate or morphology, and
had only trace levels of DNA methyltransferase activity. A
quantitative end-labeling assay showed that the level of m5C in the
DNA of homozygous mutant cells was about one-third that of wild-type
cells, and Southern blot analysis after cleavage of the DNA with a
methylation-sensitive restriction endonuclease revealed substantial
demethylation of endogenous retroviral DNA. The mutation was
introduced into the germline of mice and found to cause a recessive
lethal phenotype. Homozygous embryos were stunted, delayed in
development, and did not survive past mid-gestation. The DNA of
homozygous embryos showed a reduction of the level of m5C similar to
that of homozygous ES cells. These results indicate that while a
3-fold reduction in levels of genomic m5C has no detectable effect
on the viability or proliferation of ES cells in culture, a similar
reduction of DNA methylation in embryos causes abnormal development
and embryonic lethality.
