ELEMENTS WITHIN THE BETA-LACTOGLOBULIN GENE INHIBIT EXPRESSION OF HUMAN SERUM-ALBUMIN CDNA AND MINIGENES IN TRANSFECTED CELLS BUT RESCUE THEIR EXPRESSION IN THE MAMMARY-GLAND OF TRANSGENIC MICE

Two new beta-lactoglobulin (BLG)/human serum albumin (HSA) hybrid gene vectors were constructed and tested for expression in COS-7 cells and in transgenic mice. The HSA sequences were inserted between the secon d and sixth BLG exons. Transient transfection experiments with these v ectors as well as a series of additional vectors with either the BLG 5 '- or 3'-intragenic sequences revealed that sequences within BLG exon 1/intron 1/exon 2 abrogated BLG-directed HSA expression in vitro, rega rdless of the presence of HSA introns or the origin of the 3' polyaden ylation signal. In contrast, the same BLG expression cassette enabled the efficient expression of HSA cDNA or minigene in the mammary gland of transgenic mice with subsequent secretion of the corresponding prot ein into the milk of 56 and 82%, respectively of the mouse strains at levels up to 0.3 mg/ml. Previous attempts to express HSA cDNA inserted into exon 1 of the BLG gene had failed [Shani,M., Barash,I., Nathan,M ., Ricca,G., Searfoss,G.H., Dekel,I., Faerman,A, Givol,D. and Hurwitz, D.R. (1992) Transgenic Res. 1, 195-208]. The new BLG expression casset te conferred more stringent tissue specific expression than previously described BLG/HSA constructs [Barash,I., Faerman,A., Ratovitsky,T., P uzis,R., Nathan,M. Hurwitz,D.R. and Shani,M. (1994) Transgenic Res. 3, 141-151]. However, it was not able to insulate the transgenes from th e surrounding host DNA sequences and did not result in copy number dep endent expression in transgenics. Together, the in vitro and in vivo r esults suggest both positive and negative regulatory elements within t he BLG intragenic sequences evaluated. The new BLG construct represent s an extremely valuable vector for the efficient expression of cDNAs i n the mammary gland of transgenic animals.