Retroviral vector sequences inhibit human beta-globin gene expression in transgenic mice

The DNase I hypersensitive site 5' HS2 of the human beta-globin locus control region confers position-independent, high-level expression on the human beta-globin gene in transgenic mice. When a 5' HS2 beta-globin construct is flanked by retroviral vector sequences derived from Moloney Murine Leukemia Virus (MoMLV), expression of the beta-globin gene is severely inhibited. Apparently, one or more elements within the MoMLV genome is capable of repressing transcription of the human beta-globin gene in transgenic mice. A construct lacking the retroviral enhancer also fails to express the beta-globin gene, indicating that this region of the virus is not essential for repression. Further analysis may permit the identification of specific viral sequences that inhibit gene expression; these sequences could then be deleted or mutated to produce improved viral vectors.