HLA-E IS THE ONLY CLASS-I GENE THAT ESCAPES CPG METHYLATION AND IS TRANSCRIPTIONALLY ACTIVE IN THE TROPHOBLAST-DERIVED HUMAN CELL-LINE JAR

Polymorphic as well as HLA-F and -G genes are repressed in the human c ell line JAR, derived from a tumor of trophoblast origin. By contrast, the HLA-E gene as well as the non-HLA novel coding-sequence, R1, loca ted 5' to HLA-E, both remain transcriptionally active. We first demons trated the role of DNA methylation in the repression of class I genes (except HLA-E) in JAR by the use of the 5-Azacytidine demethylating ag ent. Following treatment, JAR clones reexpressed polymorphic class I t ranscripts and cell surface alpha chains. Using methylation-sensitive rare cutter enzymes on JAR genomic DNA, followed by classical or pulse field gel electrophesis and hybridization with HLA locus-specific pro bes, we found methylated CpG islands in the 5' region of all class I g enes, except for HLA-E. These results, establishing an inverse relatio nship between states of methylation and transcriptional activity withi n the MHC class I chromosomal region in JAR, and the observations that the HLA-E and R1 genes were ubiquitously expressed, suggest that the HLA-E chromosomal domain might have functional importance including th e presence of housekeeping genes.