Control of organ-specific demethylation by an element of the T-cell receptor-alpha locus control region

DNA methylation is important for mammalian development and the control of g ene expression, Recent data suggest that DNA methylation causes chromatin c losure and gene silencing. During development, tissue specifically expresse d gene loci become selectively demethylated in the appropriate cell types b y poorly understood processes. Locus control regions (LCRs), which are cisa cting elements providing stable, tissue-specific expression to linked trans genes in chromatin, may play a role in tissue specific DNA demethylation, W e studied the methylation status of the LCR for the mouse T-cell receptor a lpha/delta locus using a novel assay for scanning large distances of DNA fo r methylation sites. Tissue-specific functions of this LCR depend largely o n two DNase I-hypersensitive site clusters (HS), HS1 (T-cell receptor cy en hancer) and HS1', We report that these HS induce lymphoid organ-specific DN A demethylation in a region located 3.8 kilobases away with little effect o n intervening, methylated DNA. This demethylation is impaired in mice with a germline deletion of the HS1/HS1' clusters. Using 5'-deletion mutants of a transgenic LCR reporter gene construct, we show that HS1' can act in the absence of HS1 to direct this tissue-specific DNA demethylation event. Thus , elements of an LCR can control tissue-specific DNA methylation patterns b oth in transgenes and inside its native locus.