Expression of active human erythropoietin in the mammary gland of lactating transgenic mice and rabbits.

Transgenic mice and rabbits were generated using a chimeric gene comprising the human erythropoietin (hEPO) cDNA under the 5' and 3' regulatory sequences of the rabbit whey acidic protein gene. Transgenic mice expressed hEPO at levels of 0.01 mg/l in the milk of lactating females showing that the genetic construct was functional. Reverse transcriptase polymerase chain reaction with RNA from various tissues showed that this transgene was expressed mainly in the ovary and mammary gland. In rabbits, we demonstrated the germ line transmission of the transgene. The hEPO was obtained in the milk of lactating females at levels of up to 0.0003 mg/l. Although the expression levels were low, biologically active hEPO was obtained in the milk of transgenic rabbits without any apparent detrimental effect for the animals. In vitro, the specific activity of the rabbit-derived hEPO was higher than that reported for the natural hEPO, thus suggesting differences in the glycosylation pattern in at least part of the molecules secreted by the mammary gland of transgenic rabbits.