Mutagenicity of 5-hydroxycytosine in human cells

Base damage in DNA constitutes a major source of mutations, and consequently leads to cancers. In human cells, 8-oxo-7,8-dihydroguanine (8-hydroxyguanine) induces targeted G→T transversions, and untargeted base substitution mutations at positions distant from the damaged site (action-at-a-distance mutations). OGG1 is a base excision repair enzyme and suppresses the former mutations, but is involved in the latter mutations’ process. In this study, 5-hydroxycytosine (CO), another oxidized base removed by base excision repair, was incorporated into the inside and outside regions of the supF gene, and the CO-plasmid DNAs were transfected into human U2OS cells. The damaged cytosine base caused base substitution mutations at the lesion site, and seemed to induce the action-at-a-distance mutations at a lower frequency than the oxidized guanine base. These results indicated that CO is mutagenic in human cells. In addition, the (6–4) photoproduct of 5'-TpT-3', the lesion repaired by another type of DNA repair pathway, nucleotide excision repair, did not cause the action-at-a-distance mutations.