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Human-animal cimeras: the new future in studying human disease?

Written by Science

A team of Spanish researchers based in China have stirred up an ethical debate once again as they were able to create chimeras of monkey embryos with human cells.  

A chimera is an organism which is formed by the combination of two or more “individuals”. Scientifically speaking, chimeras contain at least two different sets of DNAs with each of them capable of forming a different organism.  

The team of scientists, based in China but from the Salk Institute in the United States, have produced monkey-human chimeras according to Spanish publication El Pais. The research was done in China due to their concerns over legal issues that might arise in other countries.  

Chimeras are not a new or a recent phenomenon. In fact, some chimeras may occur naturally. In humans, that can happen when a foetus absorbs its twin. This can occur with fraternal twins, if one embryo dies very early in pregnancy, and some of its cells are “absorbed” by the other twin. The remaining foetus will have two sets of cells: its own original set, plus the one from its twin. There was even an episode of the medical drama House MD based on exactly this phenomenon. Often, these people do not know they are chimeras and can be perfectly normal until genetic testing is done for some medical condition. A famous case was that of Karen Keegan in 2002 where during genetic testing for kidney transplantation, it was found that her DNA did not match her sons. Eventually it was figured out that she was a chimera.  

Other chimeras which can happen include those that are formed after bone marrow transplants – where the red blood cells formed post-transplant resemble the donor and not the recipient – and after pregnancy when some foetal cells can find their way into the maternal bloodstream.  

Chimeras are seen as a possible solution to address the lack of organs that are available for transplantation. The idea is that particular organs matched to the genetic nature of specific individuals could one day be grown inside the animals. The method involves the reprogramming of adult human cells into stem cells which can then give rise to any organ cells in the body under the right conditions. Prof Juan Carlos Izpisúa Belmonte, the leader of this particular project, has previously also been involved in generating chimeras with pig and sheep embryos.  

The ethical implications of such projects, however, is not as concerning because they have not been allowed to get to the stage of organ formation. If they do get there, however, the ethical questions loom large over such projects. Prof Robin Lovell-Badge, a developmental biologist from London’s Francis Crick Institute, said to the Guardian. “I don’t think it is particularly concerning in terms of the ethics, because you are not taking them far enough to have a nervous system or develop in any way – it’s just really a ball of cells.”  

News of human-monkey chimeras closely follows on the heels of reports that the Japanese government gave its support to researchers like Professor Hiromitsu Nakauchi to create mouse-human chimeras. Combined with the lifting of the ban on allowing these embryos to develop beyond 14 days, this could potentially mean that these chimeras can be brought to life although the Japanese professor has no such plans of doing so.  

Aside from helping with organ transplants, chimeras could also potentially be good subjects to study diseases that afflict humans. This is especially true of neurosciences where studying the complex effects of the central nervous system and the diseases affecting it would be made much easier if it were studied with a developed nervous system.  

However, the possibility of this coming to fruition is low given the evolutionary gap between the different species involved. The gap between humans and mice in terms of evolution is about 10 million years which is three to four times less than the gap between humans and monkeys – and given that the efficiency of human-mice chimera are low, it seems quite unlikely that human-monkey chimeras would be realistic in the near future. 

 

Last modified: 22nd September 2019

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