REGULATED EXPRESSION OF THE HUMAN BETA-GLOBIN GENE IN TRANSGENIC MICEREQUIRES AN UPSTREAM GLOBIN OR NONGLOBIN PROMOTER

Transgenic mice have been used extensively to study elements governing the erythroid-specific developmental switch from human fetal gamma to human adult beta globin. Previous work demonstrated that a small cons truct composed of hypersensitive site 2 (HS2) of the locus control reg ion (LCR) linked to the gamma and beta globin genes (HS2-gamma-beta) i s sufficient for correct tissue and temporal expression of these genes , whereas HS2-beta alone is inappropriately expressed in the embryo. T wo models, which are not mutually exclusive, have been proposed to exp lain these results and those of other constructs in transgenic mice. O ne model emphasizes the conserved polarity in the globin locus and sug gests a distance effect whereby the beta globin gene must be removed f rom the LCR/HS2 to prevent an early and incorrect activation of this g ene in the embryonic compartment. A second hypothesis proposes a compe tition between the gamma and beta globin gene promoters for interactio n with the LCR/HS2. The active gamma globin gene promoter positioned b etween the LCR/HS2 and the beta globin gene thereby interacts with the HS2 elements early in erythroid development and is expressed until a change in putative stage-specific nuclear factors makes an interaction with the adult beta globin gene more favorable. In an effort to test the competition model, a construct has been prepared in which a small deletion was produced in the promoter region of the gamma globin gene while in the context of the HS2-gamma-beta plasmid. Analysis of this c onstruct in. transgenic mice reveals a constitutive unregulated expres sion of the human beta globin gene during erythroid development. To de termine if this competition effect is specific for globin genes, a het erologous reporter gene has been substituted for the gamma globin gene in the construct HS2-gamma-beta. In this case, the beta globin gene e xhibits correct developmental expression. This data is consistent with a model in which transcription from a promoter upstream of the beta g lobin gene in some manner protects this adult gene from activation by the LCR/HS2 during early development.