HIGH-LEVEL PRODUCTION OF HUMAN GROWTH-HORMONE IN THE MILK OF TRANSGENIC MICE - THE UPSTREAM REGION OF THE RABBIT WHEY ACIDIC PROTEIN (WAP) GENE TARGETS TRANSGENE EXPRESSION TO THE MAMMARY-GLAND

The 5' flanking region (6.3 kb) of the rabbit WAP (rWAP) gene possesse s important regulatory elements. This region was linked to the human g rowth hormone (hGH) structural gene in order to target transgene expre ssion to the mammary gland. Thirteen lines of transgenic mice were pro duced. Milk could be collected from six lines of transgenic mice. In f ive of them, hGH was present in the milk at high concentrations rangin g from 4 to 22 mg ml(-1).hGH produced by the mammary gland comigrated with hGH of human origin. It was biologically active, and through its prolactin-like activity induced lactogenesis when introduced into mamm ary culture media. Two of these mouse lines were studied further. hGH mRNA was only detected in the mammary gland during lactation. In the s even other transgenic lines, hGH was present in the blood of cyclic fe males. The prolactin-like effect of hGH in these mice probably induced female sterility, and milk could therefore not be obtained. In two li nes studied in more detail, the mammary gland was the main organ produ cing hGH, even in cyclic mice. Low ectopic expression was detected in other organs which varied from one line to the other. This was probabl y due to the influence on the transgene of the site of integration int o the mouse genome. In the 13 lines studied, high mammary-specific hGH expression was not correlated to the transgene copy number. The rWAP- hGH construct thus did not behave as an independent unit of transcript ion. However, it can be concluded that the 6.3 kb flanking region of t he rWAP gene contains regulatory elements responsible for the strong m ammary-specific expression of hGH transgene, and that it is a good can didate to control high levels of foreign protein gene expression in th e mammary gland of lactating transgenic animals.