The health and viability of cloned mammals was again in the spotlight last week, after the sudden death of three cloned pigs was reported. A joint venture between teams based in Connecticut, US and Taiwan produced four live-born cloned transgenic piglets from three surrogate pigs. Of the four piglets, one died within seven days and the other three have all since died suddenly of heart failure, before the age of six months. Team leader Dr Yang said 'it was totally shocking' and termed the fatalities 'adult clone sudden death syndrome'.
The cloning of pigs has proved to be a technical challenge, as pigs require more than one healthy fetus to sustain a pregnancy. Scientists eventually hope to harvest organs from cloned pigs containing human transgenes - which would label the organs as 'human' to the human immune system - paving the way for pig to human organ transplants. The Edinburgh-based firm PPL therapeutics (creators of Dolly the sheep, the first cloned mammal) managed to create five cloned piglets in March 2000. They used the SCNT(somatic cell nuclear transfer ) method, whilst the Connecticut-Taiwan team used a variation of the technique, in which they fused a whole adult cell, rather than just a donor nucleus, with an enucleated egg. This simpler approach is believed to be the reason for the increased success rate of cloned embryo formation (37 per cent), as it requires less manipulation of the donor cells. Yang is still optimistic that despite the recent setbacks, his team's new method will prove to be a valuable technique for researchers in reproductive cloning.
A number of different mammals have been cloned, including cattle, pigs, mice, rabbits and domestic cats. However, researchers have been frustrated by a high rate of early fatality and illness, with animals suffering from afflictions such as enlarged organs, immune system problems (Dolly suffered from arthritis, an autoimmune disease) and obesity. Although it is hard to link specific illnesses and congenital defects to the cloning procedure, scientists believe that the adult DNA from donor cells is not properly reprogrammed to its embryonic state during or after the transfer.