Fidelity of the methylation pattern and its variation in the genome

Abstract

The methylated or unmethylated status of a CpG site is copied faithfully from parental DNA to daughter DNA, and functions as a cellular memory. However, no information is available for the fidelity of methylation pattern in unmethylated CpG islands (CGIs) or its variation in the genome. Here, we determined the methylation status of each CpG site on each DNA molecule obtained from clonal populations of normal human mammary epithelial cells. Methylation pattern error rates (MPERs) were calculated based upon the deviation from the methylation patterns that should be obtained if the cells had 100% fidelity in replicating the methylation pattern. Unmethylated CGIs in the promoter regions of five genes showed MPERs of 0.018-0.032 errors/site/21.6 generations, and the fidelity of methylation pattern was calculated as 99.85%-99.92%/site/generation. In contrast, unmethylated CGIs outside the promoter regions showed MPERs more than twice as high (P < 0.01). Methylated regions, including a CGI in the MAGE-A3 promoter and DMR of the H19 gene, showed much lower MPERs than unmethylated CGIs. These showed that errors in methylation pattern were mainly due to de novo methylations in unmethylated regions. The differential MPERs even among unmethylated CGIs indicated that a promoter-specific protection mechanism(s) from de novo methylation was present.

Figure 1.

Strategy of cell culture. A single HMEC was inoculated in a well by limiting dilution, and the cell was expanded up to approximately 10⁶ cells. Based on the plating efficiencies during the two transfers and the actual final cell count, the number of cells that should have been produced at the time of harvest and the number of generations observed were calculated. DNA was extracted from the final cells, and used for bisulfite sequencing. Six independent cultures were performed.

Figure 2.

Structures and expressions of the genes analyzed. (A) Schematic representation of the genomic regions analyzed. Regions analyzed by bisulfite sequencing are shown by closed boxes, and designations A–L correspond to panels in Fig. 3. CGI-P: a CGI in the promoter regions; CGI-outside: a CGI outside the promoter regions; Non-CGI: CpG sites outside CGIs; and DMR: differentially methylated region. All panels are drawn to the same scale. (B) Expression levels of the seven genes in HMECs.

Figure 3.

Distribution of unmethylated and methylated CpG sites shown by bisulfite sequencing. Unmethylated and methylated CpG sites are shown by open and closed circles, respectively. (A)–(C) A CGI in the promoter region, a CGI outside the promoter region and CpG sites in non-CGIs of the E-cadherin gene. (D)-(F) A CGI in the promoter region, a CGI outside the promoter region and CpG sites in non-CGIs of the p41-Arc gene. (G), (H) A CGI in the promoter region and a CGI outside the promoter region of the SIM2 gene. (I) A CGI in the promoter region of the 3-OST-2 gene. (J) A CGI in the promoter region of the Cyclophilin A gene. (K) A CGI in the promoter region of the MAGE-A3 gene, which is normally methylated. (L) A CGI in the differentially methylated region of the H19 gene.