The incidence of abnormal gene expression in cloned mammals is more extensive than widely believed, Rudolph Jaenisch and colleagues reported this week in the Proceedings of the National Academy of Sciences. Furthermore, many abnormalities are a consequence of the cloning procedure, whereas others derive from the donor nucleus, said the authors from the Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology.

Cloning mammals involves taking a mature donor nucleus and reprogramming it to a state that supports embryonic development, while suppressing previously transcribed genes involved in cell differentiation in the donor. The majority of cloned mammals die before or soon after birth. The few that do survive, however, are affected by developmental abnormalities to varying degrees.

The extent of these abnormalities was revealed by Jaenisch and colleagues using oligonucleotide microarray analysis, which enabled them to evaluate the expression of more than 10,000 genes in the livers and placentas of cloned mice. Donor nuclei were obtained from embryonic stem cells cultured in the lab and cumulus cells (cells in the ovary that nourish the developing egg and are easiest to clone) obtained directly from the mice. Until now, gene expression analysis in cloned animals has only looked at a small number of genes from preimplantation embryos.

What they found was worse than anyone anticipated. Hundreds of genes of the 10,000 studied were dysregulated, comprising roughly 4% of the mouse genome. Significantly, abnormal expression of many of the same genes was found whether the nuclei were obtained from cultured cells or taken directly from the mice, fingering the nuclear transfer technique and not cell culture as a significant cause of many abnormalities. "It means that clones that develop despite enormous dysregulation are not normal," said Jaenisch.

This news won't bode well for biotech companies intent on making hay from cloning mammals, or rogue groups claiming to be able to clone human beings. "What is true with other mammals will be true with humans," Jaenisch explained. "It's totally irresponsible to think you can use the nuclear transfer procedure to produce any normal individuals. Those clones will suffer the same fate and be as abnormal as [cloned] mice."

Nor does he think that tide will turn anytime in the foreseeable future. "Reprogramming the donor nucleus during egg maturation is the principle barrier to cloning," said Jaenisch. "There is no good [alternative] way to accomplish this and no progress to improve the procedure of cloning."