Action-at-a-Distance Mutations Induced by the True Abasic Site, Uracil, and Nicks: Unexpectedly Lower Mutagenicity of the True Abasic Site.

A major oxidized base, 8-oxo-7,8-dihydroguanine (G, 8-hydroxyguanine), is involved in cancer initiation. G induces untargeted base substitution (action-at-a-distance) mutations as well as targeted G→T transversions in human cells. Uracil also induces similar untargeted mutations. An abasic site is the common product of their specific DNA glycosylases (OGG1 and UNG2, respectively) and is an expected intermediate of the untargeted mutation pathway. Subsequently, the DNA strand is nicked by AP endonuclease. In this study, a shuttle plasmid containing a true (natural) abasic site was introduced into human U2OS cells. The frequency of action-at-a-distance mutations was much lower for the abasic site than that for uracil, although the abasic site is considered the mutational intermediate of the latter. Moreover, nicked DNA containing a 5'-phosphate induced untargeted mutations less frequently than that without a 5'-phosphate, and their mutation frequencies were much higher than that of the abasic site. The unexpectedly low mutagenic potential of the true abasic site suggests that the mechanism of action-at-a-distance mutations is complex.