Gene thats key to cloning success also hints at serious hurdles to reproductive cloning

PHILADELPHIA -- Scientists at the University of Pennsylvania have found that the activity of a single gene is a powerful predictor of whether newly cloned mammalian embryos will survive and thrive, but the genes sporadic expression in cloned mouse embryos casts fresh doubt on prospects for reproductive human cloning.

The findings, by a team led by K. John McLaughlin and Hans R. Schler of Penns School of Veterinary Medicine, are described in the May 15 issue of the journal Genes & Development.

Despite the successful cloning of sheep, pigs and cats, mammalian cloning -- in which an ordinary cells nucleus is transferred to an egg whose nucleus has been removed -- remains remarkably inefficient. Fewer than three in 100 cloned mouse embryos survive to birth.

McLaughlin and Schlers group showed that the activity of the gene Oct4 correlates strongly with the viability of cloned embryos but also found that in only one-tenth of cloned mouse embryos is the gene expressed at the right level in the right place at the right time. Without Oct4, embryos cannot survive; even if Oct4 expression is a tad high or low, an embryo will die.

"Cloning requires the precise reprogramming of the nucleus inserted into an enucleated egg," said Schler, professor of animal biology and director of Penns Center for Animal Transgenesis and Germ Cell Research. "This nucleus must abandon its former genetic program and adopt the genetic profile of an embryonic nucleus; failure to do so dooms the embryo."

To evaluate the accuracy of this genetic reprogramming, The Penn group analyzed Oct4 expression in cloned mouse embryos derived from cells that surround ovulated eggs in adult mice, cells that would not normally express Oct4. The result: Only 34 percent of embryonic cells correctly reprogrammed to express Oct4, and just 10 percent showed levels of Oct 4 expression conducive to further development.

Even as it suggests new hurdles for reproductive

Contact: Steve Bradt
University of Pennsylvania

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