TRANSIENT DNA DEMETHYLATION IN DIFFERENTIATING MOUSE MYOBLASTS CORRELATES WITH HIGHER ACTIVITY OF 5-METHYLDEOXYCYTIDINE EXCISION-REPAIR

It has been recently shown that in developing chicken embryonic nuclea r extracts there is a 5-methyldeoxycytidine excision repair activity ( Jost, J. P. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 4684-4688). We show that in differentiating mouse myoblasts, a similar enzymatic reac tion may be responsible for the genome-wide DNA demethylation (up to 5 0% of all CmCGG) occurring between the 3rd and 5th days of differentia tion. Furthermore, in differentiating myoblasts, there is first a 50% transient decrease in DNA methyltransferase activity and a 90% drop in the rate of DNA synthesis, followed by an increase in 5-methyl-CpG en donuclease and 5-methyldeoxycytidine excision repair activities. As te sted in vitro, the maximal activity of the 5-methyldeoxycytidine excis ion repair coincides with the maximal in vivo genome-wide DNA demethyl ation. We also find that 3-aminobenzamide, a potent inhibitor of ADP-r ibosyltransferase, blocks the differentiation of myoblasts, the 5-meth yldeoxycytidine excision repair activity, and the genome-wide demethyl ation.