Transgenic animals in medicine: integration and expression of foreign genes, theoretical and applied aspects.

The production of different pharmaceutically important human proteins in the mammary gland of transgenic animals constitutes an important field of modern biotechnology. In addition, transgenic animals are used to develop suitable models of various human diseases and the possibility of using transgenic technologies to adapt pigs for xenotransplantation of their organs to humans is widely discussed. All these practical applications depend on the availability of reliable techniques to obtain transgenic animals with the necessary spectrum of transgene(s) expression. In the present review we discuss different aspects of producing transgenic animals by microinjection procedure, including both technical aspects and theoretical issues. As far as the technical aspects are concerned, special emphasis is placed on DNA preparation, microinjection timing, vectors permitting delivery of large genomic fragments, and approaches used to increase integration frequency. The molecular basis of the advantage of transgene microinjection into a male pronucleus is discussed. Finally, different strategies used to obtain polytransgenic animals are critically reviewed. In the section devoted to theoretical issues, the mechanisms of integration and molecular systems regulating transgene expression are extensively discussed. In particular, the importance of the domain-level regulatory systems is outlined and experimental approaches permitting the use of the domain-level regulatory systems of the genes encoding milk proteins are described and discussed.