Hereditary persistence of fetal hemoglobin (HPFH) is characterized by high-level expression of the g globin gene in adult patients. HPFH2 contains a genetic deletion of approximately 83.5 kb from the middle of intron 2 of the yb gene to the region 66 kb 3′ to the b gene. The deletion juxtaposes an enhancer located downstream to the 3′ breakpoint to the vicinity of the g gene. To understand the reactivation mechanism of this fetal stage specific globin, we introduced the deletion into a 213 kb b-YAC. Four transgenic lines bearing 1–3 intact copies of the YAC construct were established. Globin gene expression at different developmental stages was measured by RNase protection assays. In the HPFH2 transgenic mice the e and g genes were expressed at high levels similar to the wild type YAC mice in embryonic and fetal stages. Unexpectedly, the g gene was completely silenced in the adult mice. The failure of g gene reactivation by juxtaposition of the HPFH2 enhancer contradicts the results reported by the Forget lab (

Mol. Cell. Biol.
) and the Strouboulis lab (
). The discrepancy could be due to the differences between the constructs used in the different labs. The construct used in the Forget lab was a cosmid containing the mLCR linked a 13 kb Gg and Ag fragment. The cosmid used by the Strouboulis lab contained the 22 kb LCR and a 5 kb Ag fragment. The bYAC construct we used in this study contains the whole b globin locus and spanned from about 40 kb 5′ to the e gene to 100 kb 3′ to the b gene. The failure of g gene reactivation in the HPFH2 YAC mice suggests that additional elements, which are missing in the 213 kb bYAC construct, are involved in reactivation of the g gene in the HPFH patients. Based on data from literature (
Bender et al.,
Mol. Cell
Forrester et al.,
Genes & Dev.
) and our own studies (Ping et al., this meeting) we speculate that the 200 kb upstream region is the candidate for harboring this activity.

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