The position of integration affects expression of the γ-globin-encoding gene linked to HS3 in transgenic mice

Proper expression of the human β-globin (βGlb) locus is dependent on the presence of a major regulatory element located upstream from the βGlb gene cluster, the locus control region (LCR). The LCR, as well as the individual DNase-I-hypersensitive sites from which it is composed, have been shown to provide position-of-integration-independent expression in transgenic mice. Here, we report that a transgenic founder carrying multiple integrations of a hypersensitive site 3::^Aγ globin gene (HS3::^Aγ) construct produced three types of progeny, one with zero ^Aγ expression in the adult stage, one with minimal ^Aγ expression (1% of ^Aγ-expressing cells) and one with abundant ^Aγ expression (100% ^Aγ-expressing cells). The possibility that these phenotypes were due to parental imprinting or to DNA rearrangements of the transgene or to point mutations of the HS3 core or the ^Aγ promoter were excluded. The pattern of inheritance of the three HS3::^Aγ transgene phenotypes indicate that the transgene has integrated into three different chromosomes. These results provide direct evidence that the HS3 of the LCR is not sufficient to protect the ^Aγ gene from position effects excerted by the surrounding chromatin.