Overexpression of the major DNA methyltransferase Dnmt1 is cytotoxic and has 
been hypothesized to result in aberrant hypermethylation of genes required for 
cell survival. Indeed, overexpression of mouse or human Dnmt1 in murine and 
human cell lines decreased clonogenicity. By frame-shift and deletion 
constructs, this effect of mouse Dnmt1 was localized at the N-terminal 124 amino 
acid domain, which mediates interaction with proliferating cell nuclear antigen 
(PCNA). Mutation of the PCNA-binding site restored normal cloning efficiencies. 
Overexpression of Dnmt3A or Dnmt3B, which do not interact with PCNA, yielded 
weaker effects on clonogenicity. Following introduction of the toxic domain, no 
significant effects on apoptosis, replication, or overall DNA methylation were 
observed for up to 3 d. Suppression of clonogenicity by Dnmt1 was also observed 
in cell lines lacking wild-type p53, p21(CIP1), or p16(INK4A). Suppression of 
clonogenicity by Dnmt1 overexpression may act as a fail-safe mechanism against 
carcinogenicity of sustained Dnmt1 overexpression.