The DNA of cloned embryos has been found to contain abnormalities. This could explain why so many cloned embryos die, says the South Korean team who did the research.
Only a few per cent of cloned embryos survive to term, and some of those that are born are unhealthy. Researchers have suspected that misplaced methyl (CH3) tags on DNA, which help regulate gene expression, may be partly to blame.
Even culturing embryos in the lab can disrupt methylation (快猫短视频, 3 February, p 7). And the nuclear transfer technique used in cloning involves not only culturing cells, but also adding DNA from a donor cell to an egg stripped of its own genetic material.
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Yong-Mahn Han of the Korea Research Institute of Bioscience and Biotechnology compared the methylation of sections of DNA from cow embryos created by nuclear transfer, with DNA from embryos produced by normal IVF.
As in naturally fertilised embryos, Han found that most methylation marks are erased by the time the IVF embryos become a ball of cells called a blastocyst. But in the cloned embryos, Han鈥檚 team saw that some DNA sites still had methyl tags at the blastocyst stage.
Even more striking were methylated sites that weren鈥檛 cleaned up at all as the embryo developed, but appeared to be actively maintained. 鈥淭his was very exciting for us to see,鈥� says Han.
鈥淭hey鈥檝e done a good job at showing there are clear differences in nuclear-transfer embryos. It鈥檚 intriguing,鈥� says William Rideout of the Whitehead Institute in Cambridge, Massachusetts.
The DNA sequences Han has looked at so far aren鈥檛 thought to contain or control genes. And it鈥檚 not known if abnormal methylation causes problems later on. But he believes the results illustrate a basic problem in cloned embryos. 鈥淚t鈥檚 an important clue to why a lot of nuclear-transfer embryos fail to develop,鈥� he says.
More at: Nature Genetics (vol 28, p 173)