Position Independence and Proper Developmental Control of γ-Globin Gene Expression Require both a 5′ Locus Control Region and a Downstream Sequence Element

Abstract

We have analyzed the expression of human γ-globin genes during development in F₂ progeny of transgenic mice carrying two types of constructs. In the first type, γ-globin genes were linked individually to large (~4-kb) sequence fragments spanning locus control region (LCR) hypersensitive site 2 (HS2) or HS3. These LCR fragments contained not only the core HS elements but also extensive evolutionarily conserved flanking sequences. The second type of construct contained tandem γ- and β-globin genes linked to identical HS2 or HS3 fragments. We show that γ-globin expression in transgenic mice carrying ΗS2γ or ΗS3γ constructs is highly sensitive to position effects and that such effects override the cis regulatory elements present in these constructs to produce markedly different developmental patterns of γ-globin expression in lines carrying the same transgene. In contrast, γ-globin expression in both ΗS2γβ and ΗS3γβ mice is sheltered from position effects and the developmental patterns of γ-globin expression in lines carrying the same transgene are identical and display stage-specific regulation. The results suggest that cis regulatory sequences required for proper developmental control of fetal globin expression in the presence of an LCR element reside downstream from the γ genes.