Clones Express Themselves Like Other Embryos

Scientists have cloned a menagerie of different animal species. But cloning is hampered by a very high failure rate, with 99 out of every 100 embryos dying before birth—and the ones that are born often develop health problems. Clones are created when the nucleus of an adult tissue cell is transplanted into an egg cell that has had its own nucleus sucked out, a process called nuclear transfer (NT). According to a leading theory, this is where the trouble starts. Because the genes inside the transferred nucleus come with the programming of an adult tissue cell, they would typically produce, or "express," proteins at completely different levels than the same genes do in an egg. The hope is that the egg cell molecules will reprogram the genes for proper expression, but the 99% failure rate of NT embryos is chalked up to faulty reprogramming. To see if improper gene expression really is at fault, a team led by Xiangzhong Yang, a reproductive biologist at the University of Connecticut, Storrs, used NT to make 15 cow embryos. The researchers compared these to the cells used as nucleus donors for the NT embryos, 15 embryos fertilized by in vitro fertilization (IVF) in a petri dish, and 14 embryos fertilized naturally through artificial insemination (AI). They let the NT and IVF embryos grow for 7 days in the lab, and the AI embryo grow 7 days in the womb, and then harvested RNA from the cells. For a gene to be expressed as a protein, its DNA must first be transcribed into RNA, so the amount of RNA produced by different genes adds up to a kind of gene-expression fingerprint. Compared to their activity in the adult donor cells, 84% of the genes in the NT embryos were expressed at different levels. After 7 days of growth, the genes in the cloned embryos had switched to an expression pattern 99% similar to that of the AI embryos, the team reports this week in the Proceedings of the National Academy of Sciences. In short, the gene activity in the cloned embryos was more like that of other embryos and less like that of adult cells. Surprisingly, the cloned embryos' gene expression came out as an even better match to that of AI embryos than IVF embryos. The study shows that "reprogramming really works," says Thomas Wagner, a cell biologist at Clemson University in Greenville, South Carolina, and the problems with cloning "probably happen later when the embryo interacts with the placenta," perhaps because the NT procedure robs an egg of something it needs for this interaction. Research will now focus on what those missing embryo-to-placenta signals might be, he says. Source:"here":[ http://sciencenow.sciencemag.org/cgi/content/full/2005/1128/1].

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