Expression of a Bovine κ-CN cDNA in the Mammary Gland of Transgenic Mice Utilizing a Genomic Milk Protein Gene as an Expression Cassette

Transgenic mice were produced by microinjection of a DNA construct composed of the bovine κ-casein (κ-CN) cDNA under the control of the goat β-CN 5′ promoter elements and 3′ flanking regions into pronuclear-stage embryos. The gene construct targeted the expression of bovine κ-CN RNA to the mammary gland and secretion of bovine κ-CN in the milk. In the three lines studied (BC-7, BC-31 and BC-67) the transgene was stably integrated and propagated as a Mendelian locus. Expression of the bovine protein in lactating mice from the three transgenic lines was demonstrated by northern and western blots. In ten different tissues analysed by northern blotting, expression was confined to the mammary gland of lactating transgenic mice from line BC-7, with low-level expression also observed in the salivary gland of lines BC-31 and BC-67. Transgene expression in the mammary gland paralleled normal casein gene expression during lactation and was not observed in virgin females. The level of bovine κ-CN mRNA expression on day 10 of lactation in hemizygous transgenic females in relation to endogenous mRNA of whey acid protein (WAP) gene expression was 14%, 69% and 127% in lines BC-7, BC-31 and BC-67, respectively. No association between transgene copy number and expression was observed. The bovine κ-CN concentration in milk on day 10 of lactation ranged from 0.94 to 3.85 mg of protein per ml of milk. The bovine κ-CN expressed in mouse milk had the same molecular mass and immunoactivity with polyclonal antibodies as did κ-CN from bovine milk. A high degree of variation in the production of bovine κ-CN within each of the transgenic lines was observed.