Application of transgenesis in livestock for agriculture and biomedicine.

Microinjection of foreign DNA into pronuclei of a fertilized oocyte has predominantly been used for the generation of transgenic livestock. This technology works reliably, but is inefficient and results in random integration and variable expression patterns in the transgenic offspring. Nevertheless, remarkable achievements have been made with this technology. By targeting expression to the mammary gland, numerous heterologous recombinant human proteins have been produced in large amounts which could be purified from milk of transgenic goats, sheep, cattle and rabbit. Products such as human anti-thrombin III, alpha-anti-trypsin and tissue plasminogen activator are currently in advanced clinical trials and are expected to be on the market within the next few years. Transgenic pigs that express human complement regulating proteins have been tested in their ability to serve as donors in human organ transplantation (i.e. xenotransplantation). In vitro and in vivo data convincingly show that the hyperacute rejection response can be overcome in a clinically acceptable manner by successful employing this strategy. It is anticipated that transgenic pigs will be available as donors for functional xenografts within a few years. Similarly, pigs may serve as donors for a variety of xenogenic cells and tissues. The recent developments in nuclear transfer and its merger with the growing genomic data allow a targeted and regulatable transgenic production. Systems for efficient homologous recombination in somatic cells are being developed and the adaptation of sophisticated molecular tools, already explored in mice, for transgenic livestock production is underway. The availability of these technologies are essential to maintain "genetic security" and to ensure absence of unwanted side effects.