Pigs, sheep, rabbits with a human gene.

Pigs, sheep, rabbits with a human gene

While successes mount for genetic engineering in mice, the practical applications for farm animals lag far behind. But now a collaboration of experts in gene transfer and animal husbandry report the first demonstration that a gene of a foreign species, in this case a human growth hormone gene, can be injected into embryos of farm animals so that the gene joins a chromosome. In some cases the resultant animals make the protein that the gene encodes. But in no case so far reported is the animal's growth measurably enhanced.

One reason researchers have been slow to shift from mice to cows or sheep is that success rates are low; many eggs must be injected to produce an animal with a foreign gene incorporated into its chromosomes. Collecting eggs and providing "foster mothers' require far fewer animals with mice than with sheep.

Nevertheless, in the current work, about 2,000 rabbit embryos, 2,000 pig embryos and 1,000 sheep embryos were each injected with a few hundred copies of the human growth hormone gene fused to a mouse regulatory region, report Robert E. Hammer and Ralph L. Brinster of the University of Pennsylvania in Philadelphia, Vernon G. Pursel of the USDA Agricultural Research Service in Beltsville, Md., Richard D. Palmiter of University of Washington in Seattle and their colleagues in the June 20 NATURE. About 10 percent of the embryos injected survived to a full-term birth.

The gene-integration results in sheep were disappointing--only 1 percent. But in rabbits and pigs, the frequency of gene integration was considered good-- about 12 percent. Among the animals with copies of the human growth hormone gene in their chromosomes, four rabbits and 11 pigs showed human gene activity.

The scientists attribute their success to the techniques they used to see into cells--methods that allowed them to inject the genes directly into nuclei or pronuclei (the nuclei of sperm and egg before they fuse in the fertilized egg). The eggs of many farm animals have an opaque cytoplasm, so a type of microscopy called interference contrast was used. In addition, the pig eggs were centrifuged to remove the darkest material from the part of the egg containing the nuclei.

The scientists conclude, "These experiments demonstrate that foreign genes can be introduced into several large animal species by microinjection of ova.'

Photo: Key to success: Inject genes directly into nucleus. This interference-contrast micrograph shows a rabbit fertilized egg.