A distinct sequence (ATAAA)(n) separates methylated and unmethylated domains at the 5 '-end of the GSTP1 CpG island

What defines the boundaries between methylated and unmethylated domains in the genome is unclear. In this study we used bisulfite genomic sequencing t o map the boundaries of methylation that flank the 5'- and 3' ends of the C pG island spanning the promoter region of the glutathione S-transferase (GS TP1) gene. We show that GSTP1 is expressed in a nide range of tissues inclu ding brain, lung, skeletal muscle, spleen, pancreas, bone marrow, prostate, heart, and blood and that this expression is associated with the CPG islan d being unmethylated, In these normal tissues a marked boundary was found t o separate the methylated and unmethylated regions of the gene at the 5'-fl ank of the CpG island, and this boundary correlated with an (ATAAA)(19-24) repeated sequence. In contrast, the 3'-end of the CpG island was not marked by a sharp transition in methylation but by a gradual change in methylatio n density over about 500 base pairs. In normal tissue the sequences on eith er side of the 5'-boundary appear to lie in separate domains in which CpG m ethylation is independently controlled. These separate methylation domains are lost in all prostate cancer where GSTP1 expression is silenced and meth ylation extends throughout the island and spans across both the 5'- and 3'- boundary regions.