Production of transgenic cattle expressing a recombinant protein in milk

Transgenic technology has been applied with some success in all of the majo r agricultural species. However, its application in cattle has been relativ ely slow due to the greater technical, logistical and financial challenges encountered in working with this species. Sustained efforts to produce tran sgenic cattle have concentrated on producing valuable biomedical proteins i n milk rather than modifying agricultural production traits. The overall ai m of the work described here was to enhance the nutritional value of cow's milk for human consumption by expressing human alpha-lactalbumin (the major whey protein in human milk) in the milk of transgenic cattle. To this end, we sought an efficient system in which to generate healthy, transgenic cal ves from microinjected zygotes. Embryo production and transfer were perform ed continuously over a period of 5 months. Zygotes for microinjection were generated by in vitro maturation and fertilization of abattoir-derived oocy tes. To evaluate the effect of fetal calf serum (FCS) on embryo development , pregnancy rate, calving rate and birthweight, some embryos were cultured with or without FCS from day 5 to day 7 or 8 post-insemination (p.i.). Of 1 1,507 injected zygotes, 1011 (9%) developed to the compact morula or blasto cyst stage. After non-surgical embryo transfer of 478 embryos to recipient heifers (one embryo/recipient), 155 recipients (32%) were pregnant at day 3 0 of gestation and 90 (19%) produced calves (including one set of identical twins). Most (97%; 88/90) calves were delivered by scheduled Caesarean sec tion at about day 274 of gestation. Mean (range) calf birthweight was 41.83 (29.09-60.45) kg. The male:female sex ratio was 1.44:1 (59% male:41% femal e) and did not differ from the expected ratio of 1:1 (P>0.05). Perinatal ca lf survival was high (90/91; 99%). Under the conditions of these experiment s, the presence of FCS in embryo culture medium did not affect (P>0.05) emb ryo development, pregnancy rate, calving rate, birthweight or perinatal sur vival. Nine (10%) of the calves were transgenic. All transgenic calves were born healthy. A female calf, induced to lactate, expressed human a-lactalb umin in her milk at a level of 2.4 mg ml(-1).