Long-Distance Control of Origin Choice and Replication Timing in the Human β-Globin Locus Are Independent of the Locus Control Region

Abstract

DNA replication in the human β-globin locus is subject to long-distance regulation. In murine and human erythroid cells, the human locus replicates in early S phase from a bidirectional origin located near the β-globin gene. This Hispanic thalassemia deletion removes regulatory sequences located over 52 kb from the origin, resulting in replication of the locus from a different origin, a shift in replication timing to late S phase, adoption of a closed chromatin conformation, and silencing of globin gene expression in murine erythroid cells. The sequences deleted include nuclease-hypersensitive sites 2 to 5 (5′HS2-5) of the locus control region (LCR) plus an additional 27-kb upstream region. We tested a targeted deletion of 5′HS2-5 in the normal chromosomal context of the human β-globin locus to determine the role of these elements in replication origin choice and replication timing. We demonstrate that the 5′HS2-5-deleted locus initiates replication at the appropriate origin and with normal timing in murine erythroid cells, and therefore we conclude that 5′HS2-5 in the classically defined LCR do not control replication in the human β-globin locus. Recent studies also show that targeted deletion of 5′HS2-5 results in a locus that lacks globin gene expression yet retains an open chromatin conformation. Thus, the replication timing of the locus is closely correlated with nuclease sensitivity but not globin gene expression.