Animal eggs no good for cloning?
Gene expression differences suggest cloned human embryos are pluripotent but human-animal hybrids are not
Cloned human embryos express the genes required for pluripotency, but animal-human hybrids do not, according to a study published today (Feb. 2nd) in the journal__ linkurl:Cloning and Stem Cells.;http://www.liebertonline.com/clo __The findings pave the way for isolating human embryonic stem cells from therapeutic cloning -- a landmark that has never been achieved after linkurl:Woo-suk Hwang's discredited cloning experiments;http://www.the-scientist.com/news/display/22933/ -- but call into question the utility of interspecies embryos.
"These eggs simply do not reprogram," lead author linkurl:Robert Lanza,;http://www.robertlanza.com/ chief scientific officer of Advanced Cell Technology in Worcester, Mass., said of the human-animal hybrid embryos. "That puts the nail on the coffin for that whole line of work."
In the face of a shortfall of human egg donors, many researchers hope that by injecting human nuclei into animal eggs they will be able to obtain patient-specific human embryonic stem cells. Although this type of work is not currently funded by the US government, it is becoming an increasingly popular line of research around the world, including in the UK where linkurl:three biologists have been granted licenses;http://www.the-scientist.com/blog/display/54814/ to study human-animal hybrid embryos.
"You have to start saying, 'Well, maybe this isn't a viable approach for generating stem cells,'" linkurl:Keith Latham,;http://www.temple.edu/medicine/faculty/l/lathamk.asp?pms=(latham%20KE[au]%20Temple%20University[affiliation]) a developmental biologist at Temple University School of Medicine in Philadelphia, who was not involved in the research, told __The Scientist__.
Not all researchers agree. "These authors argue that reprogramming isn't taking place, but other manipulations need to be carried out in order to promote the chances of getting good development," said linkurl:Justin St. John,;http://www2.warwick.ac.uk/fac/med/staff/stjohn one of the three British license holders from Warwick University. "Everything depends on the compatibility between the transferred nucleus [of] one species and the cytoplasm [of] another," linkurl:Josef Fulka Jr.,;http://www.vuzv.cz/old/molgen/fulka.htm a reproductive biologist at the Institute of Animal Science in Prague, Czech Republic, wrote in an email. "It may well be that another combination will give us much better results."
Lanza and his colleagues analyzed genome-wide gene expression in early stage embryos derived from either human in vitro fertilization or from human-human, human-cow, and human-rabbit clones generated using somatic cell nuclear transfer -- the technique used to clone Dolly the sheep. Although the interspecies embryos looked similar to human ones, they did not show the same expression patterns.
More than 2,300 genes were differentially expressed in the human-animal hybrids, the majority of which were down-regulated. Notably, only the human embryos up-regulated the critical pluripotency genes Oct-4, Sox-2, and nanog -- three key genes involved in cell reprogramming. "Instead of turning on the right genes, the animal is actually turning them off or silencing them," said Lanza.
A 2008 linkurl:study;http://www.liebertonline.com/doi/abs/10.1089/clo.2007.0084 in the same journal from Hui Sheng of the Shanghai Jiao Tong University in China, however, disputes these findings. Sheng's team found that five pluripotency-associated genes, including the three tested by Lanza's group, were activated in later-stage human-cow hybrid embryos, but not earlier on.
King's College London's linkurl:Stephen Minger,;http://www.kcl.ac.uk/schools/biohealth/research/wolfson/sminger.html another UK hybrid license holder who was also not involved in the work, said that Lanza's team is "making a leap" in concluding that animal eggs don't support reprogramming just because the hybrid embryos don't express certain genes at the early stages. "Maybe in hybrid embryos these genes come on much later in development than human-human embryos -- they just haven't looked for long enough," he said.
Lanza dismisses Sheng's findings as an "artifact." He doubts that Sheng's team actually produced human-cow later-stage embryos, noting that in 2003 Sheng also linkurl:reported;http://www.nature.com/cr/journal/v13/n4/full/7290170a.html generating human embryonic stem cells from rabbit eggs, although the work has never been replicated despite multiple attempts. "There is no 'later stage in hybrid embryos,'" he said. "No matter how long you wait, these hybrids die."
In addition to refuting the utility of hybrid embryos, the study also provides further evidence that human therapeutic cloning will enable the production of tailor-made embryonic stem cells. "For the first time, not only are we able to show that the core genes necessary for cellular reprogramming are up-regulated, but the donor DNA was extensively reprogrammed and the reprogramming was along the same pathway as normal embryos," said Lanza. "Now that we know we can get reprogramming hopefully soon we'll be able to get stem cells from this technology."
The gene expression profiles now "lay the foundation" for other detailed molecular analyses of human-human clones with an eye toward isolating embryonic stem cells, said Latham. St. John agreed that the paper's most important finding was the detailed characterization of human-human embryos, not the limited human-animal hybrid data. "That in and of itself is a success," he said. "I'm not sure why they weren't selling that point more... They seem to spinning a negative result instead of spinning a positive result."
**__Related stories:__***linkurl:Cracking Cloning;http://www.the-scientist.com/article/display/53224/
[June 2007]*linkurl:Cloned, fertilized stem cells look the same;http://www.the-scientist.com/news/display/22971/
[17th January 2006]*linkurl:Cloning Capsized?;http://www.the-scientist.com/article/display/12542/
[20th August 2001]