Here we describe a procedure for cloning pigs by the use of in vitro culture systems. Four healthy male piglets from two litters were born following nuclear transfer of cultured somatic cells and subsequent embryo transfer. The initiation of five additional pregnancies demonstrates the reproducibility of this procedure. Its important features include extended in vitro culture of fetal cells preceding nuclear transfer, as well as in vitro maturation and activation of oocytes and in vitro embryo culture. The cell culture and nuclear transfer techniques described here should allow the use of genetic modification procedures to produce tissues and organs from cloned pigs with reduced immunogenicity for use in xenotransplantation.

The ability to modify animal genomes rapidly at a specific locus would be valuable both for research purposes and in the development of animals suitable for xenotransplantation. In a proof-of-concept study, we used a unique, homology-dependent strand transferase protein called drosophila recombination-associated protein (DRAP) and DNA oligonucleotides to modify the porcine gene encoding alpha 1,3 galactosyl transferase (GGTA1). This gene is responsible for generating xenotransplantation antigens resulting in hyperacute rejection. Pronuclear injection of DRAP and mutant oligonucleotides yielded piglets with heritable, modified alleles of GGTA1 in a direct, rapid and efficient manner. Cells derived from these piglets had markedly reduced alpha 1,3 galactosyl sugar epitopes. The simplicity of this method should permit rapid sequential or simultaneous modification of the various genes encoding or producing antigens that impose limits on xenotransplantation as they are discovered.

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