Cell types could explain some of clones' sickly symptoms, say Japanese researchers. But getting to the root of cloned animals' problems remains a distant prospect, fear scientists.

Dolly the sheep limped back into the limelight last week having developed arthritis. Her condition is another example of clones' general ill-health; numerous others have sickened or died early.

Many believe that clones have trouble because their imprinted genes - ones in which only the mother's or father's copy are normally switched on ­ behave abnormally.

Atsuo Ogura of the National Institute of Infectious Diseases in Tokyo contests this idea. His team cloned mice from fresh adult and fetal cells. Less than 3% survived until birth. But the imprinted genes of those that did were correctly expressed and the animals seemed healthy.

This contrasts with a study last year that found irregularities in the imprinted genes of mice cloned from embryonic stem (ES) cells grown in the lab2. Mutations may have accumulated in these ES cells, Ogura's group propose.

This idea is "right on the money", says Randall Prather of the University of Missouri in Columbia. Very little is known about how to keep cells healthy before cloning. Prather's team is one of two that last week reported the birth of cloned pigs with organs genetically tweaked for possible human transplant3. "The cells are dying on us and yet we're trying to make pigs from them," he says.

Pinning down some of the variation between clones to the type of cell used to create them makes these "very exciting results", says Ian Wilmut of Edinburgh's Roslin Institute, which is famed for cloning Dolly. Unlike stem cells, which can adapt into many cell types, other cells probably have more stable gene activity.

By cloning, researchers can convert cells genetically manipulated to carry desirable characteristics into whole animals. Human embryos made by therapeutic cloning may yield stem cells for the growth of tissue-matched grafts. Before these ambitions can be realized, current technical and biological problems must be ironed out.

Stunning stuff

Calling the Japanese mice healthy is "stunning", argues Rudolf Jaenisch of the Massachusetts Institute of Technology. He says that the patterns of gene activity show "the clones are utterly abnormal".

The animals' placentas were overgrown and the activity of three out of six placental imprinted genes there was low. But because four other, non-imprinted genes were also affected, Ogura's team feels a more general mechanism regulating gene activity could be the cause.

"The cloning process still has some problems producing a true copy of donor animals," admits Ogura. And the animals might develop health problems in the future, he concedes.

A new programme

Difficulties in 'developmental reprogramming' are thought to underlie clones' survival and health problems. When nuclei are transferred from a cell into an egg stripped of its nucleus, they must erase previous patterns of gene activity and start up new ones that drive embryo growth.

Understanding this reprogramming will be an extensive project. Systematic studies will help, says Wilmut, and suggest ways in which difficulties can be overcome. "It should keep a lot of us in business for a long time."

References
Inoue, K. et al. Faithful expression of imprinted genes in cloned mice. Science, 295, 11, (2002).
Humpherys, D. et al. Epigenetic instability in ES cells of cloned mice. Science, 293, 95 - 97 (2001).
Lai, L. Production of a-1,3-galactosyltransferase knockout pigs by nuclear transfer cloning. Sciencexpress, 10.1126/science, (2002).