title: 5-Azacytidine induced methyltransferase-DNA adducts block DNA replication in vivo.
abstract: 5-Azacytidine (aza-C) and its derivatives are cytidine analogues used for leukemia chemotherapy. The primary effect of aza-C is the prohibition of cytosine methylation, which results in covalent methyltransferase-DNA (MTase-DNA) adducts at cytosine methylation sites. These adducts have been suggested to cause chromosomal rearrangements and contribute to cytotoxicity, but the detailed mechanisms have not been elucidated. We used two-dimensional agarose gel electrophoresis and electron microscopy to analyze plasmid pBR322 replication dynamics in Escherichia coli cells grown in the presence of aza-C. Two-dimensional gel analysis revealed the accumulation of specific bubble and Y molecules, dependent on overproduction of the cytosine MTase EcoRII (M.EcoRII) and treatment with aza-C. Furthermore, a point mutation that eliminates a particular EcoRII methylation site resulted in disappearance of the corresponding bubble and Y molecules. These results imply that aza-C-induced MTase-DNA adducts block DNA replication in vivo. RecA-dependent X structures were also observed after aza-C treatment. These molecules may be generated from blocked forks by recombinational repair and/or replication fork regression. In addition, electron microscopy analysis revealed both bubbles and rolling circles (RC) after aza-C treatment. These results suggest that replication can switch from theta to RC mode after a replication fork is stalled by an MTase-DNA adduct. The simplest model for the conversion of theta to RC mode is that the blocked replication fork is cleaved by a branch-specific endonuclease. Such replication-dependent DNA breaks may represent an important pathway that contributes to genome rearrangement and/or cytotoxicity.
authors.affiliation: Department of Biochemistry, Duke University, Durham, NC 27710, USA.
authors.forename: H Kenny
authors.initials: HK
authors.lastname: Kuo
authors.forename: Jack D
authors.initials: JD
authors.lastname: Griffith
authors.forename: Kenneth N
authors.initials: KN
authors.lastname: Kreuzer
mesh_terms: D001374:Azacitidine / Q000494:pharmacology*; D018736:DNA Adducts / Q000378:metabolism*; D004261:DNA Replication / Q000187:drug effects*; D015257:DNA-Cytosine Methylases / Q000378:metabolism*; D004926:Escherichia coli; D008954:Models, Biological; D010957:Plasmids / Q000737:chemistry / Q000378:metabolism; D011938:Rec A Recombinases / Q000502:physiology