Methylation of deoxycytidine in replicating cells treated with ultraviolet radiation and chemical carcinogens

5-Methyl-2'-deoxycytidine (m⁵dC) levels were measured in DNA from three types of cultured cells following treatment with u.v. radiation and two chemical carcinogens, N-methyl-N-nitrosourea (MNU) and N-acetoxy-2-acetylaminofluorene (NA-AAF). Control values for m⁵dC in Raji cells (a human lymphoblastoid cell line), S49 cells (a mouse thymic lymphoma cell line) and human diploid fibroblasts are 3.6%, 3.6% and 3.2%, respectively. None of the damaging agents produced a detectable change in methylation levels of newly replicated DNA, even at levels of damage that inhibited replication by 95%. In contrast, treatment with 5-aza-2'-deoxycytidine, a known methyltransferase inhibitor, transiently reduced genomic methylation by 89% and 74% in Raji and S49 cells, respectively. Although other investigators have found a marked reduction in m⁵dC in DNA replicated after carcinogen treatment, our experiments indicate that extensive demethylation is not a necessary consequence of DNA damage.