METHYLATION-ASSOCIATED TRANSCRIPTIONAL SILENCING OF THE MAJOR HISTOCOMPATIBILITY COMPLEX-LINKED HSP70 GENES IN MOUSE-CELL LINES

The MHC-linked hsp70 locus consists of duplicated genes, hsp70.1 and h sp70.3, which in primary mouse embryo cells are highly heat shock-indu cible. Several mouse cell lines in which hsp70 expression is not activ ated by heat shock have been described previously, but the basis for t he deficiency has not been identified. In this study, genomic footprin ting analysis has identified a common basis for the deficient response of the hsp70.1 gene to heat shock in four such cell lines, viz., the promoter is inaccessible to transcription factors, including heat shoc k transcription factor. Southern blot analyses reveal extensive CpG me thylation of a 1.2-kilobase re region spanning the hsp70.1 transcripti on start site and hypermethylation of the adjacent hsp70.3 gene, which is presumably also inaccessible to regulatory factors. Of four additi onal, randomly chosen mouse cell lines, three show no or minimal hsp70 .3 heat shock responsiveness and CpG methylation of both hsp70 genes, and two of the three lines exhibit a suboptimal hsp70.1 response to he at shock as well. In all three lines, the accessibility of the hsp70.1 promoter to transcription factors is detectable but clearly diminishe d (relative to that in primary mouse cells). Our results suggest that the tandem hsp70 genes are concomitantly methylated and transcriptiona lly repressed with high frequency in cultured mouse cells.