The image shows the development of a human-bovine cloned embryo (top) and a human-rabbit cloned embryo (bottom). Credit: CLONING AND STEM CELLS, 2009 Mary Ann Liebert Inc.

A shortage of human eggs has been the major impediment to human cloning, so scientists have been trying to use animal eggs instead, a controversial approach that has raised fears of human-animal hybrids. Now new research suggests that using animal eggs as surrogates won't be successful.

In therapeutic cloning (or somatic cell nuclear transfer), scientists transplant DNA from an adult skin cell into an egg that has had its DNA removed. Unknown factors in the egg reprogram the adult DNA to resemble embryonic DNA, and, in theory, the cell begins to develop like a normal embryo. Scientists would like to create stem cells from cloned human embryos, both for research and potentially for therapy: the cells would be genetically matched to their human donors and thus could be transplanted without fear of rejection. But no one has yet accomplished this with human cells and eggs.

The creation of human-animal hybrids has been a subject of great debate in the United Kingdom, where scientists won permission to use rabbit and cow eggs in human cloning experiments in 2007. (This Q&A with Ian Wilmut, the biologist who spearheaded the cloning of the now renowned sheep Dolly, explores the controversy.) Similar research involving rabbit eggs has taken place in the United States, but with little government regulation here, there has been much less public debate.

A paper published today in Cloning and Stem Cells could make the debate moot. A comparison of gene expression in human cells transplanted into both human eggs and animal eggs suggests that animal eggs simply don't have the power to reprogram human DNA. Here's an extract from a press release issued by Advanced Cell Technology (ACT), which sponsored the research.

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. This study . . . 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.

Wilmut, who edits the journal, said in a statement, "This very important paper suggests that livestock oocytes are extremely unlikely to be suitable as recipients for use in human nuclear transfer. This is very disappointing because it would mean that production of patient-specific stem cells by this means would be impracticable."

In the last year, scientists have been experimenting with a new method of reprogramming, which skips the egg altogether and instead uses several genetic factors to directly modify DNA. The ACT researchers also examined expression of these key genes and found that they were activated in both normal and cloned human embryos but not in the human-animal hybrids. "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," said Robert Lanza, chief scientific officer at ACT, in an e-mail.

Some say that the characterization of reprogramming in human clones is the most interesting aspect of the research. According to an article in the Scientist,

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 [Keith Latham, a developmental biologist at Temple University School of Medicine, in Philadelphia, who was not involved in the research]. [Justin St. John of Warwick University] 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 [be] spinning a negative result instead of spinning [a] positive result."