Genetic modification of pigs for xenotransplantation 30/7/03. By Richard Twyman Pigs can be genetically modified to make organs tailored for transplantation to humans.

The best treatment for organ failure or trauma is organ replacement by transplantation. However, due to a chronic lack of suitable human donors, it may be necessary in the future to use animals as a source of replacement organs – the transfer of tissues and organs between species being known as xenotransplantation.

Background: Xenotransplantation

While primates, as our closest relatives, would seem the most logical organ donors, they cannot be bred in captivity in large numbers. There is also some concern that the use of primate organs could promote the spread of human immunodeficiency virus (HIV). It is thought that HIV evolved from the related simian immunodeficiency virus (SIV) that infects non-human primates.

Pigs are regarded as the next-best source of replacement organs because porcine organs are similar in size to their human counterparts. However, there are several technical problems hindering the use of pig organs all of which reflect the fact that the human immune system recognises pig organs as foreign.

Recently, these problems have been addressed by the genetic modification of pigs to alter the antigens displayed on the cell surface. The aim is to fool the human immune system into believing the pig organs are made of genuine human cells.

The genetic modification of pigs is very similar in principle to the genetic modification of mice (see Transgenic mice ) although a great deal more difficult in practice. The first transgenic pigs modified for organ compatibility were generated in 1995. More recently, gene knockout techniques have been used in pigs to remove genes encoding antigens that provoke a strong immune response. Again, this technique was first perfected in mice (see Knockout mice ) but can be used in other mammals, albeit with a much lower efficiency.

The first immune response to xenotransplantation is hyperacute rejection, in which antibodies recognise porcine antigens on the donor organ surface and recruit complement, a group of proteins that break open the membranes of foreign cells and destroy them. Various different genetic modification strategies have been used to make pig organs compatible with humans, and most of these have focussed on the prevention of hyperacute rejection. An early strategy was the expression of human decay accelerating factor (DAF), a protein that attaches to the cell surface and protects cells from complement.

Interestingly, one of the major barriers to pig-human organ transplants is a particular type of sugar molecule, Gal-alpha(1,3)-Gal, which is added to proteins on the surface of pig cells but not human ones. Attempts have been made to reduce the amount of this sugar molecule by expressing antibodies against it, inhibiting the enzyme that makes it (an enzyme called alpha-1,3-galactosyltransferase that is only present in pigs), or using additional enzymes to modify it.

Most recently, two research groups have succeeded in completely knocking out the alpha-1,3-galactosyltransferase gene, producing pigs that cannot make this sugar at all. While these steps bring pig-human xenotransplantation much closer, there are still further technical hurdles as well as legal and ethical implications to consider.

Image credit: Caroline Gunn

Further reading

Dai Y, et al. Targeted disruption of the alpha1,3-galactosyltransferase gene in cloned pigs. Nature Biotechnology 2002, 20: 251–255. Abstract

Lai L, et al. Production of alpha-1,3-galactosyltransferase knockout pigs by nuclear transfer cloning. Science 2002, 295: 1089–1092. Abstract

Phelps CJ, et al. Production of alpha 1,3-galactosyltransferase-deficient pigs. Science 2003, 299: 411–414. Abstract