|[Frontiers in Bioscience 1, e34-41, July 1, 1996]|
PIG alpha1,3GALACTOSYLTRANSFERASE: A MAJOR TARGET FOR GENETIC MANIPULATION IN XENOTRANSPLANTATION
Karen Strahan, Andrew Preece and Kenth Gustafsson
Division of Cell & Molecular Biology, Institute of Child Health, University of London, 30 Guilford St., London WC1N 1EH, UK
Received 06/21/96; Accepted 07/02/96; On-line 07/08/96
The galactosyl (a1,3)galactosyl(b1,4)acetyl-glucosaminyl carbohydrate epitope, hereafter referred to as Gala1,3Gal, is expressed in many mammals, but not in humans, Old World monkeys and the great apes (14-16). Instead, the latter species ubiquitously express high levels of pre-formed 'natural' antibodies (NAb) against this epitope (17, 18). In pigs, as well as in mice, the carbohydrate is present on glycoproteins and glycolipids on a variety of cell types, including endothelial cells (EC)(19). It was therefore a reasonable assumption that the Gala1,3Gal structure would constitute one of the important targets for human NAb in pig-to-primate xenotransplantation. The Gala1,3Gal structure is reminiscent of the ABO-type of carbohydrates in humans, a terminal carbohydrate structure which is polymorphic among individuals of one species (Fig. 1). It is well known that A- or B-type grafts, due to the presence of pre-formed NAb in non-matched recipients, can lead to HAR in human allotransplantation (20).
Fig. 1 Structure of ABO and Gala1,3Gal terminal carbohydrates. The individual carbohydrate components are: Fuc, fucosyl, lined horizontally; GalNAc, acetylgalactosaminyl, lined vertically; Gal, galactosyl, black filled; GlcNAc, acetylglucosaminyl, white filled.
Complex carbohydrates are synthesized in the Golgi via the action of a number of glycosyltransferases (21). The number of glycosyl-transferase genes with different specificity has been estimated to be as many as one hundred or more. The human ABO carbohydrate terminus is synthesized by two alternative enzymes encoded for by an allelic gene locus. The two A- and B- enzymes differ by only a few amino acids, resulting in the addition of a terminal GalNAc and Gal residue, respectively, whereas the O-type allele does not result in the expression of an active enzyme (22). The Gala1,3Gal structure is identical to the (ABO) B-type with the important exception that this structure lacks the fucosylation in an a1,2-linkage (Fig. 1).
We, as well as others, have cloned the cDNA coding for the pig a1-3galactosyl-transferase (a1-3GT) responsible for synthesizing the Gala1,3Gal epitope in pigs (23, 24). We have also mapped (23) and cloned the corresponding gene (GGTA1)(manuscript in preparation). a1,3GT cDNAs and the genes encoding them have previously been cloned both in mouse and cattle (25, 26). An alignment of a1,3GT amino acid sequences with ABO-transferases reveals considerable similarity consistent with the findings of a close functional relationship (27, 28). Using a cDNA clone coding for mouse a1,3GT, Sandrin and colleagues, by transfection of Gala1,3Gal-negative COS cells, have shown that these cells are able to absorb most of the human anti-pig NAb, confirming the previous suspicion that this is the major target for such antibodies (29).