Interclonal variation in methylation patterns for expressed and non-expressed genes.

A mass culture of human diploid fibroblasts, and eight clones isolated from that mass culture, were examined for methylation patterns in several regions of DNA. Plasmid-inserted cDNA sequences were used as probes for alpha-hCG, beta-globin, A gamma- and G gamma-globin, and beta- and gamma-actin gene regions. Each probe revealed a different clone-specific pattern of DNA methylation, indicating a striking degree of inter-clonal heterogeneity, for those gene regions which are not normally expressed in diploid fibroblasts (alpha-hCG, gamma-globin and beta-globin). Intra-clonal variation was also evident in many instances, implying that heterogeneity could arise de novo in pure cell clones during serial passage. Thus methylation patterns, in particular for repressed genes, appear to be unstably inherited in these cells, and this instability may lead to random derepression in some cell lineages during mitotic growth.