Genetic engineering of pigs for xenogeneic heart transplantation

Given the worldwide shortage of organs for human allotransplantation, the pig is considered a potential candidate as a source of organs for donation because of similarities in size, anatomy and organ physiology between pigs and humans. However, hyperacute rejection as a result of activation of human complement has prevented the use of pigs as organ donors. The action of endogenous complement in humans is usually blocked by cell surface proteins which are regulators of complement activation (RCA). The aim of this research programme was to produce pigs transgenic for human decay accelerating factor (DAF), one of these human RCAs. A 6.8kb DNA construct was prepared; it included the 4kb genomic DNA fragment incorporating the 5' untranslated and signal peptide sequence of the human DAF gene. The first exon and intron of the gene are linked to the main cDNA for hDAF by use of this hDAF minigene. This construct was then microinjected into the pronucleus of fertilised ova collected from mature Large White gilts. 49 transgenic pigs incorporating between 1 and 30 copies of the DAF minigene in the genome were produced. It was found that hDAF could be expressed in all the different tissues and organs of the pigs without causing any harmful effect to the animal in terms of well-being, growth, sexual maturity and reproduction. In some pigs, hDAF levels expressed were higher than those found in the equivalent human tissue. Subsequently, hearts from hDAF transgenic pigs were transplanted both with and without immunosuppression into non-human primates. These studies show the following: (i) hearts from transgenic pigs are not hyperacutely rejected by cynomolgus monkeys; and (ii) survival of such hearts over 2 months can be achieved by immunosuppressive treatment of the recipient.