Back in the mid 1970s, when genetic engineering was still new andscary, a journalist put this question to Nobel-winning geneticistJoshua Lederberg: "What's to stop a vicious dictator from cloning anarmy of mindless, ruthless killers?"
To which the scientist replied: "Hitler did that without benefit of genecloning."
Now, embryologists in Scotland have, for the first time, createdliving, breathing mammals _ identical lambs _ from scratch, that is,by manipulating early embryonic sheep cells.
Today's Nature records this feat in an article titled: "Sheep cloned bynuclear transfer from a cultured cell line." Its senior author, Scottishembryologist Ian Wilmut, told BioWorld Today: "This is the firsttime that offspring have been born after nuclear transfer from anestablished cell line."
Wilmut heads the nuclear transfer laboratory at Edinburgh's RoslinInstitute, a research affiliate of Pharmaceutical Proteins Ltd. (PPL).That company pioneered a decade ago in producing high-valueproteins in the milk of transgenic ewes.
In the proof-of-principle experiment reported in Nature, Wilmut andhis team detected a total of eight fetuses in seven pregnant ewes(including one pair of twins). Of this number, five lambs, all female,were born, but only two lived longer than a few neonatal days. Nownearly one year old, these two survivors "are apparently normal andhealthy."
To explain the purpose of their experiment, Wilmut said: "There aretwo reasons why you might think of doing this: The obvious one isthat you could make a number of genetically identical animals with ahigh level of performance.
"From the point of view of biotechnology," he added, "the moreimportant application is in the fact that you can introduce a precisegenetic change into those [starter] cells before you use them asnuclear donors." This will involve gene-targeting techniques,"probably quite similar to those being used with mouse embryonicstem cells," he suggested, adding, "With gene targeting, one canactually change existing genes, so it opens up an entirely newopportunity as far as modification of livestock is concerned."
However, Wilmut surmised that "The first application is likely to bein biotechnology. Because of the fact that one can make very precisemodifications, those companies will probably use the nuclear transfertechnology first of all, because they can justify the expense of a newexpensive technique like this."
Alluding to neighboring PPL's focus on making pharmaceuticalproteins in sheeps' milk, he observed, "Because of the fact thatyou're making a genetic change in a different way, you have anopportunity to improve the quality of the proteins produced in milk,because you have a more precise control over the system."
He also suggested that "companies thinking of transplant organdonations _ xenografts _ might find it useful."
As for advantages to agriculture, "In ten or more years, by the timethe genome-mapping project has identified the genes that regulateanimal production, people may come to play that for modifying[livestock] performance."
Lambs Now, Calves Next?
On this score, developmental biologist Davor Solter, who wrote aneditorial on "Lambing by nuclear transfer" to accompany the Roslinreport in Nature, observed to BioWorld Today: "It has the potentialto produce a lot of identical, superior sheep. But cows are notnegligible either."
Solter, who is on sabbatical at the National Institutes of Health, fromthe Max-Planck Institute of Immunology, in Freiburg, Germany, drewthe line at mass-producing champion race horses. "Horses," he said,"can't be genetically manipulated this way. They don't even supportartifical fertilization."
He pointed out that a useful veterinary application would be "tointroduce genes for disease resistance." But attempting to improve,say, the quality and quantity of wool on a sheep must wait, "becausethese traits are governed by multiple genes."
Wilmut observed that "There are methods at present of makinggenetic changes in livestock. They involve injecting DNA into thecell nucleus, but this is a very haphazard process because the genesstick in at random and sometimes damage endogenous genes _ inother words, cause mutations. And the ultimate limitation is that thistechnique is able to add only one gene."
Just as it takes two to tango, or beget, Wilmut's nuclear transferprocedure requires two cell types, donor and recipient.
The latter, he related, "is an unfertilized egg recovered from Scottishblack face ewes. We remove their genetic material by aspirating thechromosomes." In its place, the Roslin team fuses the nucleus fromthe donor cell.
"In these experiments, for the very first time," he continued, "we'veused donor cells that we've induced to become quiescent, stopdividing. We do this by reducing their culture serum concentration."
The embryos came from pregnant sheep, nine days after mating. "Thecells from their embryonic disk," Wilmut went on, "were maintainedin culture by passaging approximately every week." This allowedthem "to produce thousands of genetically identical copies, which inprinciple have the ability to be used as nuclear donor cells."
Solter observed: "If the cells are behaving identically, regardless ofpassage number, this implies that they are very stable, which I thinkis the main point of this paper."
The embryonic disk consists of several thousand still-totipotent cells,at a stage of embryogenesis when the very first differentiation takesplace. "They are not like mouse embryonic stem cells," Wilmutexplained. "They will become the first membranes from which thewhole conceptus will develop."
He and his team release them by freeing up their clumping withenzymes, then insert them one at a time in the uterus of surrogateewes.
The Roslin embryologist estimates that perhaps "10 or a dozen"laboratories in the world are attempting mammalian cloning. "We'rethe only people," he observed, "who have been able to use cells thathave been in culture for a very prolonged period. That's thedistinction between us and everybody else."
Right now, Wilmut's laboratory is quiescent. "It's the middle of thesheep-breeding season," he said, "so my colleagues are repeatingexperiments of the same cell type, and beginning to use other celltypes derived from embryos." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.