Mouse mammary tumor virus promoter directs high‐level expression of bovine αS1 casein in the milk of transgenic heterozygous and homozygous mice

Expression of a bovine kappa-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 kappa-casein (kappa-CN) cDNA under the control of the goat beta-CN 5' promoter elements and 3' flanking regions into pronuclear-stage embryos. The gene construct targeted the expression of bovine kappa-CN RNA to the mammary gland and secretion of bovine kappa-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 kappa-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 kappa-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 kappa-CN expressed in mouse milk had the same molecular mass and immunoactivity with polyclonal antibodies as did kappa-CN from bovine milk. A high degree of variation in the production of bovine kappa-CN within each of the transgenic lines was observed.

Variation in expression of a bovine alpha-lactalbumin transgene in milk of transgenic mice

Transgenic mice were produced to study the production of bovine alpha-LA in their milk. A 7.6-kb fragment containing a bovine alpha-LA gene was purified and microinjected into pronuclear stage mouse embryos. This fragment contained 2.0 kb of 5' flanking region, the 1.7-kb coding region, and 2.7 kb of 3' flanking region. Out of 121 potential transgenic founder mice, 3 were identified as being transgenic by the polymerase chain reaction. Multiple mice from the second, third, and fourth generation from each line were milked, and the milk was analyzed using an ELISA assay and Western blots to determine the presence of bovine alpha-LA. Bovine alpha-LA was present at concentrations up to 1.5 mg of protein/ml of mouse milk. The high degree of expression variation between mice within each of the transgenic lines was a characteristic that has not been reported in other studies of transgene expression in milk. Production of bovine alpha-LA in the milk of these transgenic mice showed a high degree of variation both within a lactation and between mice within a line. The bovine alpha-LA concentration in a single line of transgenic mice exhibited as much as a 10-fold variation between mice. Variations as high as 3-fold were detected within a single lactation in the same mouse. These differences in expression appeared to be correlated with mouse milk production; bovine alpha-LA was higher on d 10 and 15 of lactation than on d 5. Transgenic mice that show variation in expression of a bovine gene might offer a unique system for studying quantitative traits in a laboratory model.

Production of pharmaceutical proteins from transgenic animals

Different systems are being studied and used to prepare recombinant proteins for pharmaceutical use. The blood, and still more the milk, from transgenic animals appear a very attractive source of pharmaceuticals. The cells from these animals are expected to produce well-matured proteins in potentially huge amounts. Several problems remain before this process becomes used in a large scale. Gene transfer remains a difficult and costly task for farm animals. The vectors carrying the genes coding for the proteins of interest are of unpredictable efficiency. Improvement of these vectors includes the choice of efficient promoters, introns and transcription terminators, the addition of matrix attached regions (MAR) and specialized chromatin sequences (SCS) to enhance the expression of the transgenes and to insulate them from the chromatin environment. Mice are routinely used to evaluate the gene constructs to be transferred into larger animals. Mice can also be utilized to prepare amounts as high as a few hundred mg of recombinant proteins from their milk. Rabbit appears adequate for amounts not higher than 1 kg per year. For larger quantities, goat, sheep, pig and cow are required. No recombinant proteins extracted from the blood or milk of transgenic animals are yet on the market. The relatively slow but real progress to improving the efficiency of this process inclines to be reasonably optimistic. Predictive reports suggest that 10% of the recombinant proteins, corresponding to a 100 million dollars annual market, will be prepared from the milk of transgenic animals by the end of the century.