Human gamma- to beta-globin gene switching using a mini construct in transgenic mice.

The developmental regulation of the human globin genes involves a key switch from fetal (gamma-) to adult (beta-) globin gene expression. It is possible to study the mechanism of this switch by expressing the human globin genes in transgenic mice. Previous work has shown that high-level expression of the human globin genes in transgenic mice requires the presence of the locus control region (LCR) upstream of the genes in the beta-globin locus. High-level, correct developmental regulation of beta-globin gene expression in transgenic mice has previously been accomplished only in 30- to 40-kb genomic constructs containing the LCR and multiple genes from the locus. This suggests that either competition for LCR sequences by other globin genes or the presence of intergenic sequences from the beta-globin locus is required to silence the beta-globin gene in embryonic life. The results presented here clearly show that the presence of the gamma-globin gene (3.3 kb) alone is sufficient to down-regulate the beta-globin gene in embryonic transgenic mice made with an LCR-gamma-beta-globin mini construct. The results also show that the gamma-globin gene is down-regulated in adult mice from most transgenic lines made with LCR-gamma-globin constructs not including the beta-globin gene, i.e., that the gamma-globin gene can be autonomously regulated. Evidence presented here suggests that a region 3' of the gamma-globin gene may be important for down-regulation in the adult. The 5'HS2 gamma en beta construct described is a suitable model for further study of the mechanism of human gamma- to beta-globin gene switching in transgenic mice.