Role of Gene Order in Developmental Control of Human γ- and β-Globin Gene Expression

Abstract

To determine the effect of gene order on globin gene developmental regulation, we produced transgenic mice containing two tandemly arranged γ- or β-globin or γ β- and β γ-globin genes linked to a 2.5-kb cassette containing sequences of the locus control region (LCR). Analysis of constructs containing two identical γ or β genes assessed the effect of gene order on globin gene expression, while analysis of constructs containing tandemly arranged γ and β genes assessed any additional effects of the trans-acting environment. When two γ genes were tandemly linked to the LCR, expression from the proximal γ gene was three- to fourfold higher than expression from the distal γ gene, and the ratio of proximal to distal gene expression remained unchanged throughout development. Similarly, when two β genes were tandemly linked to the LCR, the proximal β gene was predominantly expressed throughout development. These results indicate that proximity to LCR increases gene expression, perhaps by influencing the frequency of interaction between the LCR and globin gene promoters. An arrangement where the γ gene was proximal and the β gene distal to the LCR resulted in predominant γ-gene expression in the embryo. When the order was reversed and the γ gene was placed distally to the LCR, γ-gene expression in the embryo was still up to threefold higher than expression of the LCR-proximal β gene. These findings suggest that the embryonic trans-acting environment interacts preferentially with the γ genes irrespective of their order or proximity to the LCR. We conclude that promoter competition rather than gene order plays the major role in globin gene switching.