Human G_γ and A_γ globin gene constructs containing the 3′ A_γ enhancer show persistent fetal expression in transgenic mice

Abstract

Transgenic mice were produced from two 13 kb constructs containing the fetal (γ) globin genes. Each construct consisted of a G_γ globin gene linked to one of two alternative A_γ genes. The first construct contained a normal G_γ gene plus and A_γ gene with the −198T→C hereditary persistence of fetal haemoglobin (HPFH) mutation. In the second, a normal G_γ gene was linked to a A_γ gene with a−222 to −225 four base pair deletion in the promoter. This latter mutation has been associated with low A_γ expression in humans. Both A_γ genes in these constructs also contained the 3′ flanking enhancer. The two different constructs showed expression throughout gestation from day 11 yolk sac, through the fetal period and for a variable time during the first three postnatal weeks. Thereafter, no expression of any of the γ-globin genes was seen in adult erythroid tissues. Transcription from the normal G_γ and HPFH A_γ genes in the first construct paralleled each other in developmental timing, with a proportionate excess of A_γ more evident in later gestation. G_γ and A_γ mRNA transcripts from the normal G_γ+4 bp deletional A_γ construct were unable to be distinguished because of a 3′ G_γ-like conversion in the deletional A_γ gene. Combined γ-globin expression from the two genes in this second construct was detectable until just after birth, as seen with the individual genes in the first construct. Previous work has shown that when individual G_γ and A_γ transgenes are introduced into mice unlinked, without the locus control region (LCR), embryonic expression only is seen. In the present study, γ-globin expression was evident from both constructs throughout the fetal period of erythropoiesis. This suggests that the additional DNA sequences flanking the γ-globin genes included in these larger constructs are capable of supporting fetal recruitment of the γ-globin genes, independently of the LCR.