Bovine alpha s1-casein gene sequences direct high level expression of human granulocyte-macrophage colony-stimulating factor in the milk of transgenic mice.

The generation is reported of transgenic mice expressing human granulocyte-macrophage colony-stimulating factor (GM-CSF) or human erythropoietin (EPO) under the control of bovine alpha s1-casein regulatory sequences. GM-CSF expression was specific to the mammary gland, and levels of human GM-CSF in transgenic mouse milk were in the range of mg ml-1. The specific activity of the milk GM-CSF was similar to that of the recombinant protein produced in Escherichia coli, and the glycosylation-derived size heterogeneity corresponded to that of the native human protein. In spite of the identical bovine regulatory sequences of the fusion genes, the levels of human EPO in transgenic mouse milk were 10(3)-10(6) times lower than those of GM-CSF, ranging from 0.003 to 3 micrograms ml-1. There appeared to be a positive correlation between the amount of EPO in the milk of lactating females and blood haematocrit values. In view of this, other type of constructs should be used to achieve more efficient EPO expression and to circumvent concomitantly-occurring adverse effects. In contrast, the high-level production of recombinant GM-CSF, its resemblance to the native mammalian protein, and mild adverse consequences of transgene expression imply that the current construct could be used for generation of larger GM-CSF transgenic animals to produce this protein in quantities sufficient for therapeutic purposes.