Cloned human embryos successfully reprogrammed using human eggs

16 February 2009

Advanced Cell Technology, Inc. and its collaborators have reported that human oocytes (or ‘eggs’) have the capacity to extensively reprogram adult human cells. The research, which appears online ahead of print in the journal Cloning and Stem Cells demonstrates that although human-to-human clones (human clones) and human-to-animal clones (hybrids) 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.

Since the cloning of Dolly the Sheep over a decade ago, somatic cell nuclear transfer (SCNT) has been considered a promising way to generate personalized stem cells to repair the body without fear of tissue rejection. Due to the serious shortage of human donor eggs, cows, rabbits, and other animals have long been considered an attractive surrogate source of eggs.

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 — and to what extent — the donor DNA was reprogrammed.

This new study looked at the reprogramming of human cells using eggs obtained from human and animal sources, and shows for the first time that the donor DNA in the cloned human embryos is extensively reprogrammed through extensive up-regulation (‘turning on’ of genes) with similar expression patterns to normal human embryos. Nearly all of the key differentially-expressed genes were activated in the human clones. In distinct contrast, the majority of these genes were down-regulated or silenced in the human-animal hybrids.

“We examined the factors recently used to reprogram skin cells (to induce pluripotent stem cells),” said Robert Lanza, MD, Chief Scientific Officer at ACT, and senior author of the study. “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 — 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.”

Previous studies have 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.

Our group and others have successfully used eggs to clone closely-related species (for instance, we cloned two endangered species — the gaur and banteng — using cow eggs). Rabbit eggs have also been used to generate embryos using cells from cats and panda, among others.

However, it remains unknown whether the DNA in the later combinations was fully reprogrammed. Importantly, except for a study carried out in China (which to-date has proven irreproducible despite attempts by numerous groups in the last half-decade), 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. These new results suggest that while bovine and rabbit eggs are capable of supporting limited cell division, specific reprogramming towards the normal human embryonic state does not occur.

Wide scale application of stem cell technology will require a solution to the problem of rejection. This report suggests that adult cells can be successfully reprogrammed using human eggs, and that scientists may soon have two ways (SCNT and induced pluripotent stem cell technology) to reprogram adult cells into stem cells. However, until this is achieved, clinical trials are likely to be limited to immune-privileged sites in the body, such as the use of cells in the central nervous system, or the transplantation of ACT’s retinal cells into the eye to prevent blindness.

“Producing millions of patient-specific stem cell lines is commercially unviable,” stated William M. Caldwell, CEO and Chairman of ACTC. “However, we are optimistic that we will soon have at least two different methods to create stem cells banks to match patients. We estimate that a bank of 100 different lines could furnish a complete tissue (HLA haplotype) match for half of the US population. This will allow us to expand the range of possible clinical therapies to include diseases such as diabetes and heart disease.”

The paper is available free online at 

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