Mono-allelic expression of variegating transgene locus in the mouse.

We have generated transgenic mice which express an ovine beta-lactoglobulin transgene during lactation. In two transgenic lines, BLG/7 and BLG/45, beta-lactoglobulin protein levels vary between siblings, reflected at the cellular level by a mosaic transgene expression pattern in the mammary tissue that is reminiscent of position effect variegation. To investigate whether this variegating expression profile can be affected by the introduction of an identical variegating locus on the homologous chromosome, we compared the beta-lactoglobulin expression profiles in mice hemizygous or homozygous for the transgene locus. In BLG/45 mice, milk protein analysis revealed that transgene expression was effectively doubled in homozygous compared to hemizygous mice. In contrast, beta-lactoglobulin protein in hemizygous and homozygous BLG/7 mice displayed a similar range; although minimum expression levels were doubled in the homozygous population, the maximum level of expression was indistinguishable between the two populations. Fluorescent in situ hybridisation (FISH) for transgene mRNA indicated that for a given protein level, the extent of cellular expression is similar in both BLG/7 populations. In homozygous mice genomic DNA and nuclear RNA FISH demonstrated that only one of the two BLG/7 loci is active in expressing cells, while two transcription foci were present in BLG/45 homozygous mice. This mono-allelic transgene expression pattern is not inherited through the germline, as hemizygous mice bred from homozygous parents expressed at the expected hemizygous population level. We discuss these observations in the context of known epigenetic events such as imprinting and trans-inactivation.

Multiple effects of genetic background on variegated transgene expression in mice.

BLG/7 transgenic mice express an ovine beta-lactoglobulin transgene during lactation. Unusually, transgene expression levels in milk differ between siblings. This variable expression is due to variegated transgene expression in the mammary gland and is reminiscent of position-effect variegation. The BLG/7 line was created and maintained on a mixed CBA x C57BL/6 background. We have investigated the effect on transgene expression of backcrossing for 13 generations into these backgrounds. Variable transgene expression was observed in all populations examined, confirming that it is an inherent property of the transgene array at its site of integration. There were also strain-specific effects on transgene expression that appear to be independent of the inherent variegation. The transgene, compared to endogenous milk protein genes, is specifically susceptible to inbreeding depression. Outcrossing restored transgene expression levels to that of the parental population; thus suppression was not inherited. Finally, no generation-dependent decrease in mean expression levels was observed in the parental population. Thus, although the BLG/7 transgene is expressed in a variegated manner, there was no generation-associated accumulated silencing of transgene expression.