Paper in Cloning and Stem Cells shows that animal eggs do not support specific reprogramming toward the normal human embryonic state.
Investigators at Advanced Cell Technology (ACTC) have found that human oocytes have the capacity to extensively reprogram adult human cells. The research also demonstrates that although human-to-human clones and human-to-animal clones appear similar, the pattern of reprogramming of the donor human cell is dramatically different. In contrast to the human-animal hybrids, the gene-expression pattern of the human clones was highly similar to normal human embryos.
Although previous reports have documented the formation of cloned embryos using both human and animal eggs, to date there has been no data indicating whether the donor DNA was actually reprogrammed, according to the company.
“We examined the factors recently used to reprogram skin cells (to induce pluripotent stem cells),” says Robert Lanza, M.D., CSO at ACTC. “At the center of cellular reprogramming lies the activation of the transcription factors Oct4, Sox2, and nanog. These core factors were activated in both the normal and cloned human embryos.
“In striking contrast, the human-animal hybrids showed no difference or a down-regulation of these critical pluripotency genes—effectively silencing them and thus making the generation of stem cells impossible. Without appropriate reprogramming, these data call into question the potential use of animal egg sources to generate patient-specific stem cells. It also renders the moral controversy surrounding the use of human-animal hybrids moot.”
Prior studies confirmed the ability of animal eggs to support interspecies cell division to the embryo stage and in a few closely related bovid species, successful development to term. However, there are clear differences in compatibility. Distantly related animal combinations generally arrest at the cleavage stage, although there have been reports of blastocyst formation.
The research group and others successfully have in the past used eggs to clone closely related species: the gaur and banteng using cow eggs and rabbit eggs used to generate embryos using cells from cats and panda. However, it remains unknown whether the DNA in the later combinations was fully reprogrammed.
Importantly, the researchers note, except for a study carried out in China, which has proven irreproducible, there is no evidence that patient-specific stem cells can be generated using animal eggs. This is consistent with studies that indicate that eggs support nuclear remodeling but not reprogramming of discordant animal combinations.
Studies using cow and rabbit eggs clearly suggest that DNA methylation/demethylation of the donor DNA occurs in a species-specific way and that the eggs might lack the ability to demethylate repetitive sequences from other species. While cleavage division relies on maternal factors in the egg, further development requires activation of the embryonic genome to ensure correct progression of the cell cycle.
This research appears online ahead of print in Cloning and Stem Cells.