ABSTRACT

192Evidence is reviewed and new data presented indicating progressive clonal drift of gene-region DNA methylation patterns in human diploid fibroblasts. Losses of methylation generally exceed gains, but these events appear to be random and, at times, to produce methylation increases locally or even globally. A similar trend toward DNA hypomethylation evidently prevails in rodent tissues in vivo. Although senescence-associated gene derepressions have been reported both in vitro and in vivo, the mechanisms are unknown. The loss of methylcytosines from cultured cells cannot be due to insufficient methylase, since its activities increased, relative to the extent of cell cycling, at late passage in a fibroblast strain shown to undergo progressive DNA hypomethylation during its limited replicative life span in vitro. Marked elevation was seen in “maintenance” CpG-methylase activity (conversion of hemimethylated sites to full methylation, as in newly replicated DNA) and also in methylase activities which are inappropriate to the maintenance function: de novo methylation, defined on unmethylated DNA, and activity in nondividing cells for both hemimethylated and unmethylated DNA substrates. Quantitative changes in methylase enzyme activity also could not account for the independent occurrence of drift in DNA methylation patterns, observed at different gene loci in several clonal lineages. Such observations implicate factors acting locally — e.g., accessibility and/or activity of methylase within specific nuclear chromatin domains.