Mosaic methylation in clonal tissue.

Current models suggest that de novo methylases add methyl groups to mammalian DNA early in development, establishing cell-specific patterns of methylation, and that these patterns are maintained by maintenance methylases that copy them onto newly replicated DNA strands. To test the prediction that clonal populations of histologically homogeneous cells should, therefore, have homogeneous methylation patterns, we studied methylation in leiomyomas. Despite the clonality and histological homogeneity of these solid tumors, we found that cells were heterogeneously methylated at a number of genomic sites. The heterogeneity was not caused by random methylation events within the leiomyomas because methylation patterns were similar in the core and periphery of a given tumor, and similar also among samples of independent leiomyomas and surrounding myometrial tissues extracted from a single uterus. Our results also showed that methylation of a site in the YNZ22 locus--in leiomyomas and in smooth muscle--was determined independently from the methylation of a neighboring site. Similar results were obtained for the IGH locus in colon and in several tumor tissues. These data indicate that methylation patterns are not identical in progeny cells, as current models suggest. Instead, it seems likely that methylation of a specific site reflects an equilibrium frequency defined by a continual loss and gain of methyl groups. Hence, the specificity found for the methylation of mammalian tissues is not achieved by strictly determining the methylation fate of individual cells, but by determining the overall methylation frequencies for individual sites.