title: Inactivation of Dnmt3b in mouse embryonic fibroblasts results in DNA hypomethylation, chromosomal instability, and spontaneous immortalization.
abstract: DNA hypomethylation is a hallmark of many types of solid tumors. However, it remains elusive how DNA hypomethylation may contribute to tumorigenesis. In this study, we have investigated how targeted disruption of the DNA methyltransferases Dnmt3a and Dnmt3b affects the growth of mouse embryonic fibroblasts (MEFs). Our studies led to the following observations. 1) Constitutive or conditional deletion of Dnmt3b, but not Dnmt3a, resulted in partial loss of DNA methylation throughout the genome, suggesting that Dnmt3b, in addition to the major maintenance methyltransferase Dnmt1, is required for maintaining DNA methylation in MEF cells. 2) Dnmt3b-deficient MEF cells showed aneuploidy and polyploidy, chromosomal breaks, and fusions. 3) Inactivation of Dnmt3b resulted in either premature senescence or spontaneous immortalization of MEF cells. 4) The G(1) to S-phase checkpoint was intact in primary and spontaneously immortalized Dnmt3b-deficient MEFs because the p53 protein was inducible by DNA damage. Interestingly, protein levels of the cyclindependent kinase inhibitor p21 were increased in immortalized Dnmt3b-deficient MEFs even in the absence of p53 induction. These results suggest that DNA hypomethylation may induce genomic instability, which in turn leads to spontaneous immortalization or premature senescence of Dnmt3b-deficient MEFs via a p53-independent mechanism.
authors.affiliation: Cardiovascular Research Center and Cancer Research Center, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129, USA.
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Dodge
authors.forename: Masaki
authors.initials: M
authors.lastname: Okano
authors.forename: Fred
authors.initials: F
authors.lastname: Dick
authors.forename: Naomi
authors.initials: N
authors.lastname: Tsujimoto
authors.forename: Taiping
authors.initials: T
authors.lastname: Chen
authors.forename: Shumei
authors.initials: S
authors.lastname: Wang
authors.forename: Yoshihide
authors.initials: Y
authors.lastname: Ueda
authors.forename: Nick
authors.initials: N
authors.lastname: Dyson
authors.forename: En
authors.initials: E
authors.lastname: Li
mesh_terms: D000818:Animals; D002461:Cell Line, Transformed; D002478:Cells, Cultured; D043171:Chromosomal Instability / Q000502:physiology*; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000037:antagonists & inhibitors* / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D019175:DNA Methylation; D000090205:DNA Methyltransferase 3A; D005347:Fibroblasts / Q000201:enzymology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout
chemical_list: C000715535:Dnmt3a protein, mouse; D004247:DNA; D004248:DNA (Cytosine-5-)-Methyltransferases; D000090205:DNA Methyltransferase 3A