Cloning: Humans May Have It Easier

By Christine Gorman

Most of the scientists who gathered in Washington earlier this month to talk about human cloning agreed that cloning an entire human being--besides being morally questionable--was fraught with technical obstacles. After all, research into animal cloning has already shown that for every apparent success like Dolly the sheep, there are hundreds of failures, including many badly deformed creatures that were usually miscarried.

Now comes word that it might be easier to clone humans than was previously believed. According to research at Duke University, people have a genetic quirk that might prevent some of the developmental deformities associated with animal cloning. "That doesn't mean there aren't other things that could go wrong," says Randy Jirtle, a professor of radiation oncology at Duke and one of the study's authors (who hastens to add that he has no intention to try such cloning). "But humans may be less susceptible to these kinds of [mishaps]."

To understand why, you need to know about a curious feature of some genes. Except for the genes that occur on the sex-determining X and Y chromosomes, it generally doesn't matter whether you inherit a particular stretch of DNA from your mother or your father. In the past 15 years, though, researchers have learned that at least 50 pairs of these so-called autosomal genes act a little differently. In a process called imprinting, one of each pair is permanently turned on or off, depending on whether it derives from the sperm or the egg.

As a rule, genes imprinted by the father would, if they worked in isolation, favor larger fetuses--presumably to give them a better shot at survival. The mother imprints the genes in such as way as to favor smaller fetuses--presumably so she'll have enough strength to bear more than one child. Usually a stalemate ensues between the competing imprinted genes because one gene is turned on and one turned off, and the babies are born just the right size.

During cloning, however, most of these imprints are wiped out and have to be reset--that is, turned on or off. Chances are that some won't be reset properly, which could lead to severe birth defects.

What the Duke researchers showed is that one gene, called IGF2R, which helps brake growth, is normally imprinted in sheep, cows and mice but not in humans. Human clones would always inherit nonimprinted IGF2R genes, so there would be no chance of a mix-up and, at least in this respect, their growth would be normal. But what of the other 49 or so imprinted genes? No one knows what trouble they might cause. So the fact that humans have one less imprinted gene than mice, sheep or cows means that human cloning might be marginally easier, but not necessarily safer.

--By Christine Gorman