A DELETION OF THE HUMAN BETA-GLOBIN LOCUS ACTIVATION REGION CAUSES A MAJOR ALTERATION IN CHROMATIN STRUCTURE AND REPLICATION ACROSS THE ENTIRE BETA-GLOBIN LOCUS

Naturally occurring deletions that remove sequences located ∼60 kb upstream of the human adult β-globin gene result in the failure to transcriptionally activate the cis-linked globin genes in erythroid cells. In addition, transfection, transgenic, and somatic cell hybrid studies have revealed that sequences within this region are essential for the developmentally regulated high-level expression of cis-linked globin genes. This regulatory region located at the 5′ end of the β-globin locus has been termed the locus activation region (LAR). Using somatic cell hybrids, we have studied the chromatin structure and timing of DNA replication of the normal human β-globin locus and a locus containing a de novo 25-kb deletion that removes elements of the LAR. As a result of this deletion, the entire β-globin locus and sequences ∼100 kb 5′ and 3′ of the adult β-globin gene are DNase I-resistant and do not form characteristic distant hypersensitive sites. These sequences also replicate late in S phase in an erythroid cell background. In contrast, the sequences of the normal locus are DNase I sensitive and early replicating. These results suggest that the LAR is required for both the erythroid-specific chromatin structure and timing of DNA replication over a large physical distance.