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 gene’s sporadic expression in cloned mouse embryos casts fresh doubt on prospects for reproductive human cloning.
The findings, by a team led by Hans R. Schöler of Penn’s 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 cell’s 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.
Schöler’s 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 Schöler, professor of animal biology and director of Penn’s 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, Schöler’s 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 human cloning, Schöler’s work offers new support for the feasibility of therapeutic cloning using embryonic stem cells.
"The small number of clones that did adequately express Oct4 were capable of forming embryonic stem cell lines," Schöler said, "supporting existing evidence of the scientific feasibility of therapeutic cloning even as it shows the infeasibility of reproductive human cloning."
Schöler’s team chose Oct4 as a marker because its expression is tightly regulated throughout the mammalian life cycle. Oct4, which encodes a protein critical to normal embryonic development, is expressed only in the portion of the embryo that eventually gives rise to fetal tissues; in the adult, Oct4 is expressed only in germ cells.
Although improper expression of Oct4 can single-handedly obstruct embryonic development, Schöler said Oct4 is likely not the only gene expressed incorrectly in cloned embryos.
"When we started the study, we thought Oct4 misexpression might account for a fraction of clone failures," Schöler said. "The big surprise was that Oct4 alone can explain most of the failures, although Oct4 is probably just one gene of many whose misexpression can cause cloned embryos to fail."
Schöler was joined in the Genes & Development paper by Michele Boiani, Sigrid Eckhardt and K. John McLaughlin, all of the Center for Animal Transgenesis and Germ Cell Research at Penn’s School of Veterinary Medicine. Their work is supported by the Marion Dilley and David George Jones Funds and the Commonwealth and General Assembly of Pennsylvania.
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