Hypomethylation of DNA in Raji cells after treatment with N-methyl-N-nitrosourea.

Human Raji lymphoblast-like cells were propagated in the presence of various concentrations of N-methyl-N-nitrosourea (MNU) and the degree of enzymatic methylation of newly synthesized DNA was analysed by two independent methods. The overall extent of enzymatic DNA methylation was measured on the basis of [14C]deoxycytidine derived radioactivity incorporated into DNA 5-methylcytosine and cytosine residues. Enzymatic methylation of internal cytosines at 5'-CCGG-3' sequences of Raji DNA was analysed by use of the bacterial restriction enzyme HpaII and its isoschizomer MspI. The data obtained by both methods indicate that the treatment with MNU causes a lower level of enzymatic methylation of newly synthesized DNA. This lower extent of DNA methylation persists in the absence of the carcinogen in the cell cycles following the treatment.