Expression of green fluorescent protein under the regulation of human locus control region elements HS2 and HS3 in transgenic mice.

Human beta-globin locus control region (LCR) is composed of five DNase I hypersensitive sites (HSs). We previously demonstrated that when HS2 and HS3 were constructed together in one vector and integrated into one position in transgenic mice, the two cis-elements showed a marked synergy in regulating the spatial and temporal expression of beta-globin transgene. This study is to investigate whether these two elements still show a synergy or superposition when they are not located in one construct. HS2/GFP and HS3/GFP transgenic mice, with the transgene encoding green fluorescent protein (GFP) driven by beta-globin promoter and under the control of HS2 (HS2/GFP mice) and HS3 (HS3/GFP mice), were mated to generate double-heterozygotes (HS2/GFP + HS3/GFP mice). The expression and integration status of GFP genes in transgenic mice were analyzed. Our data showed that there was no difference in the percentages of GFP positive cells in peripheral blood between double-heterozygotes and their corresponding founders (HS2/GFP or HS3/GFP). Interestingly, it was observed that exogenous GFP genes were highly expressed in another transgenic mouse pedigree with HS2 element integrated twice into the same chromosome at two different loci. However, when one site was lost in some offspring because of recombination, the percentages of GFP positive cells in peripheral blood as well as in some erythroid tissues were decreased obviously. These results demonstrated that HS2 and HS3 elements would not show any synergy or superposition if they were located on different chromosomes in transgenic mice, and that HS2 element containing the core and flanking sequences might not be able to overcome the position effects and control position dependent expression of linked genes in transgenic mice.