Variegated transgene expression in mice

• Main Author: Opsahl, Margaret L.
• Published: University of Edinburgh 2001
• Subjects: 591.35
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.660214

This thesis investigates expression instability, focusing on mice carrying a transgene encoding sheep β-lactoglobulin (BLG), the major whey protein BLG transgenes inserted as multicopy arrays within the mouse genome are robustly expressed in mammary epithelium and the product is exported into milk (Simons <i>et al</i> 1987). In two of the three lines studied, milk BLG levels differed markedly between individuals. This was attributed to mosaic expression due to stochastic inactivation of the transgene (Dobie <i>et al</i> 1996), and suggestive of position effect variegation as seen in <i>Drosophila. </i>The highly variegating line, BLG/7, was chosen for further studies. The original line was created on a mixed genetic background. First, to address a possible link between genetic background and variegation, BLG/7 animals were backcrossed to inbred mouse strains CBA and C57BL/6. Transgene expression continued to variegate after 13 backcross generations. However, inbreeding reduced the absolute expression levels: the mean and variance differed significantly from the parental population. Levels were restored by intercrossing inbred strains. Second, homozygous BLG/7 mice were studied. Variegated expression was maintained, but maximum expression levels were indistinguishable from heterozygote levels. Fluorescent <i>in situ</i> hybridisation for nuclear BLG transcription revealed that only one transgene locus is active per cell. This is suggestive of a <i>trans-</i>homology effect for the BLG/7 transgene. Previously, similar events have only been reported in plants and insects. Finally, mosaic expression could reflect clonal expansion of committed progenitor cells. To address this issue, we sought to exploit X-inactivation. A lacZ reporter gene under BLG promoter control was targeted to the X-linked hypoxanthine-xanthine phosophoribosyl transferase (HPRT) gene of ES cells. To date, chimeric animals have been obtained.