Failure of gene activation may be cause of cloning problems

The failure of gene activation might be responsible for problems that have been associated with reproductive cloning in animals. Many cloned animals have suffered from genetic abnormalities or premature death. Many cloned embryos are not viable, a lot of clone pregnancies miscarry and those animals that are born often have medical disorders.

However, a study published in the journal Nature Genetics suggests that there may be a genetic reason for this. Researchers led by Xiangzhong Yang of the regenerative biology centre at the University of Connecticut, US, compared 10 genes on the X chromosome in nine cloned calves - four of which were alive and five of which had died - with the same genes in non-cloned cows.

In the normal cows and the healthy clones, all of the genes were correctly expressed, or 'turned on'. But in the dead cloned cows, nine out of the ten genes had 'failed to perform normally'. The scientists believe that this caused the cows' deaths. In normal reproduction, genes on the two X chromosomes carried by female embryos are active during embryonic growth. One of the chromosomes is later 'deactivated'. But female clones have one X chromosome that is already deactivated, and only one that is active. The researchers believe that for successful development, the deactivated chromosome must be reactivated during the cloning process so the embryo develops normally. Later, one of the chromosomes can be deactivated as normal.

Cindy Tian, a member of the research team, said that the study ought to be a further warning to anyone who wanted to attempt human reproductive cloning, saying the procedure would be 'very dangerous'. Rudolph Jaenisch, from the Massachusetts Institute of Technology said that the chromosomal problem would not account for all abnormalities, adding that the gene inactivation would only cause problems for reproductive cloning and should not affect the use of therapeutic cloning.