EVIDENCE FOR GENE SILENCING BY DNA METHYLATION IN NORMAL HUMAN-DIPLOID FIBROBLASTS

Human diploid fibroblasts, strain MRC-5, were permeabilized by electro poration and treated with 5-methyl deoxycytidine triphosphate (5-methy l dCTP) in the S phase of the cell cycle. The frequency of TG(R) HPRT( -) cells was increased up to 20-fold in comparison to control untreate d cultures. Representative TG(R) clones were unable to grow in HAT, an d these were treated with 5-azacytidine (5-aza-CR). In many cases subs equent growth in HAT medium was observed, but in others it is likely t hat the cells had run out of growth potential. The results provide the first evidence of the silencing and reactivation of a gene in normal diploid mammalian cells.