Juxtaposition of the HPFH2 enhancer is not sufficient to reactivate the γ-globin gene in adult erythropoiesis

Previous studies have suggested that juxtaposition of a downstream enhancer to the fetal g-globin gene results in reactivation of the g-gene in adult erythrocytes of individuals with hereditary persistence of fetal hemoglobin (HPFH). To test the hypothesis in a much stricter basis, we produced b locus YAC transgenic mice carrying an exact b locus replicate of a deletional HPFH mutation, HPFH 2. Although the g-globin gene was expressed in the HPFH 2/b locus YAC (HPFH2/YAC) transgenic mice in the early stage of development, it was completely silenced in the adult mice. The failure of g-gene reactivation by the juxtaposed HPFH2 enhancer contradicts the results of previous studies. We speculate that the discrepant results reflect differences in the distance between the locus of region (LCR) and the g-globin gene characteristic of the plasmid, cosmid or YAC constructs used for production of transgenic mice. The difference in the phenotype of the HPFH2/YAC transgenic mice and the humans with HPFH2 mutation suggests that in addition to juxtaposition of HPFH enhancers, the upstream region that is absent in the b-YAC construct might be involved in g-gene reactivation in HPFH individuals. The DNase I hypersensitive sites of the LCR were well formed and the chromatin histones were acetylated. A moderate level of pol II binding was detected in the LCR, despite the fact that no transcription occurred in the globin-genes of the adult HPFH2/YAC transgenic mice. The results suggest that formation of the LCR chromatin structure in erythroid cells is independent of globin-gene transcription in the locus.