Dolly the sheep celebrated her fifth birthday yesterday. Most cloned animals aren't so lucky: they rarely reach adulthood, or even birth. US researchers now have an idea why the success rate of cloning is so low.
Cloning's unpredictability seems to lie in certain unstable genes. The activation of these genes depends on which parent they hark from. Which of these 'imprinted' genes are switched on and off varies greatly between cloned and normal embryos.
Rudolf Jaenisch of the Massachusetts Institute of Technology, Cambridge,
and colleagues cloned mice from the nuclei of embryonic stem cells, which have
a better record of success than adult cells
But the use of stem cells makes it unclear how general an explanation this is for the inefficiency of cloning, says Alan Colman, research director at PPL Therapeutics in Roslin, Scotland, where Dolly was cloned.
Larger animals such as sheep and pigs have to be cloned from adult cells, not embryonic stem cells. These should have a full and correct set of imprinted genes, Colman says, and "are more hardy, and less prone to lose their imprinting in culture". Colman believes that this work needs to be reproduced in other animals, using adult cells, to get the full picture.
The researchers looked at six imprinted genes. To their surprise, they found no cloned mouse in which all six were working normally. There are between 100 and 200 imprinted genes in the genome, meaning, says Jaenisch, that "one can't expect to have normal clones - even if they appear healthy, they may have abnormal gene expression."
Abnormal gene-expression patterns persisted into the mice's adulthood. This might not matter in a mouse or a domestic animal such as Dolly, but it is another reason why cloning humans might not be a good idea. On the positive side, says Jaenisch, the results also show how tolerant development is to errors in gene regulation.
There was no particular pattern to the misfiring of imprinted genes. Genetic instability in stem cells seems to be a random process; developmental abnormalities are the cumulative effect of many malfunctions.
Could stem cells be repaired before cloning? Jaenisch is pessimistic: "I'd never say never, but genetic programming is such a complicated process, I can't see how you'd fix it."
Embryonic stem cells used for therapeutic transplantation into mice don't have this problem; being surrounded by normal cells seems to put the cells back on track. Difficulties arise when the cell is sent back to square one and asked to make a whole organism. Again, says Colman, it would be useful to know whether stem cells taken from adults have similar difficulty in reprogramming.