[Impact of transgenes and cloning on xenografts]

Transgenesis can theoretically add a foreign gene or specifically replace an endogenous gene by another gene. Gene addition in mammals is generally achieved by DNA microinjection into one-cell embryos. Gene replacement implies homologous recombination in cultured cells which must be selected and remain capable of generating a living organism. The use of totipotent cells can, though currently in the mouse only, lead to the generation of chimeric animals transmitting the genetic modification to offsprings. The embryo cloning technique has recently allowed the use of somatic fetal cells, in which gene replacement occurred to generate living sheeps. This technique is being extended to other domestic ruminants, to pigs and rabbits. The mouse, rat and rabbit are being used as models to define the genes which should be added or inactivated to reduce rejections of xenografts. Transgenic pigs harbouring the human CD59 or the DAF genes have been obtained by several groups. Heart, kidney and isolated cells from these animals are more resistant to hyperacute rejection when grafted to experimental primates. Additional genes are to be added in the future to inhibit the other rejection mechanisms as soon as their action has been demonstrated in laboratory animals. Experiments in progress aim at inactivating the gene for Gal: 1-3-galactosyl transferase by homologous recombination in the pig genome. Transgenesis might also be used to prevent expression of endogenous pig retroviral vectors and to prepare recombinant proteins having antirejection activity from the milk of animals.