Effects of human locus control region elements HS2 and HS3 on human β-globin gene expression in transgenic mouse

The locus control region (LCR) is the most important cis-element in the regulation of β-globin gene expression. DNaseI-hypersensitive site (HS) 2 and HS3 are two significant components of β-LCR. To examine the effect of HS2, HS3, and HS2–HS3 (combination of HS2 and HS3) on the spatial and temporal expression of the human β-globin gene, we have produced transgenic mice with constructs, in which the gene encoding enhanced green fluorescent protein (EGFP) is driven by β-globin promoter and under the control of HS2, HS3, and HS2–HS3, respectively. The results showed that HS2 and HS3 each had the same enhancement activity in regulation of β-globin gene expression in transgenic mice. When HS2 and HS3 were in combination (HS2–HS3), the two cis-elements showed a marked synergy in regulating β-globin gene spatial and temporal expression as well as its expression level in transgenic mice although the EGFP expression varied largely among different transgenic mouse litters. The results also showed that HS2 was able to confer β-globin gene expression in embryonic yolk sac, fetal liver, and adult bone marrow, which was not developmentally stage-specific, while HS3 could confer the same β-globin gene expression in the adult. Thus, HS3 was different from HS2, the former being more important for specific expression of β-globin gene in the developmental stages and the switch of γ→β-globin genes. Our results indicate that the mechanism of γ→β switch could be best explained by the “divided model.”