Engineering pigs for xenotransplantation

Abstract

Xenotransplantation using tissues and organs from genetically modified pigs has the potential to fulfil the serious shortage of material available for human transplantation. However, considerable immunological barriers must be overcome before such discordant grafts can be used. The generation of α(1,3)‐galactosyltransferase knock out pig lines has surmounted the initial obstacle of hyperacute rejection. Further improvements are now necessary to provide adequate protection against acute humoral xenograft rejection. The expression of human complement regulator transgenes in existing transgenic animals is inadequate, and a means of achieving abundant uniform expression in the organ is required. Recent findings have also highlighted the need for additional transgenes to alleviate incompatibilities between the porcine and human anticoagulation and anti‐inflammatory systems.Generating multi‐transgenic animals by breeding individual transgenic animals is inefficient and incurs several problems, including: independent Mendelian segregation of multiple transgenes; failure of different transgenes to co‐express at the right time or in the correct tissue; and an increased risk of insertional mutagenesis due to multiple integration events. A novel approach is therefore required to produce what are likely to be increasingly complex multi‐transgenic animals. Ideally, all transgenes should reside at a single Mendelian locus. They should not be subject to the position effect. There should be no limitation on transgene size to allow inclusion of substantial regulatory regions, or genes as multiple copies to obtain abundant expression. Finally, insertional mutagenesis should be excluded if possible. One possible way to achieve these is to use artificial chromosome vectors. De novo formed human artificial chromosomes are being developed as vectors for human gene therapy. The feasibility of this and alternative approaches to deliver sets of xenoprotective transgenes into pigs is being explored.This work is being carried out in close collaboration with the Xeno‐Forschergruppe (FOR 535) and is supported through funding provided by the DFG.