January 26, 2017

expert reaction to study attempting to create human/pig chimera embryos using stem cells

Scientists publishing in Cell attempted to make interspecies chimeras – both rat/mouse chimera embryos, and human/pig chimera embryos, and reported both the successes and difficulties they faced.  These techniques are of interest because they may advance understanding of the developing embryo, and may in future offer the possibility of growing human tissues in animals for transplants, although this is not yet possible.

Prof. Darren Griffin, Professor of Genetics, University of Kent, said:

“The work has the ambition of understanding better the mechanisms by which stem cells differentiate in different tissues.  The work will also help us better understand evolution, development and disease.  The long term aim is further to develop the scientific basis that may, in the future, support xenotransplantation, e.g. growing human organs in large mammals such as pigs as a means of addressing the existing shortage in donor organs.

“Both xenotransplantation itself and the creation of pig/human chimeras raises complex ethical issues and are rightly subject to robust regulation.  In this study, the authors followed existing legal and ethical guidelines, allowing the embryos to develop to the maximum time allowed.  It is important that any further research is conducted with full transparency so as to allow public scrutiny and debate.”

Prof. Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said:

“An ability to make interspecies chimeras would be valuable in terms of providing basic understanding of species differences in embryo development and organ function, and if human cells are incorporated then this offers the possibility of using such chimeras to study not just normal development, but the etiology of congenital defects; to test the effects of exogenous agents on human development, from chemicals to viruses (such as Zika); and to test potential therapies.  It would also offer the possibility of growing human tissues or organs in animals for transplants – although this is still a long way off.  The goals of this study are therefore highly laudable.

“The paper is a bit of a mixed bag, which left me a little frustrated by what had not been done.  However, although successful in places, it highlights several areas where it is clear that much more research is needed in order to realise the goals outlined above.

“The experiments involving rat-mouse chimeras are similar to those reported by Kobayashi et al in 2010, and to Yamaguchi et al, published in Nature this week.  This includes the use of Pdx1 mutants to eliminate the pancreas in the host mouse embryos, and therefore allow the rat pluripotent stem cell (PSC) derivatives to give rise to this developing organ without having to compete with mouse cells: ‘complementation’.  The use of CRISPR/Cas9 to generate such mutants (rather than via breeding heterozygous mice together) is novel and worth doing as a proof of principle approach that could be used in other types of animal.  But then I was expecting this to have been used in the studies with pig host embryos, with either rodent or human PSC, but it wasn’t.  The similar findings using Pax6 mutants reinforce the complementation strategy, but again I wanted to know more details and not just the contribution to the eye, but to other tissues which depend on Pax6, such as parts of the brain.  The data on the Nkx2.5 mutant host embryos suggest that the same approach may also be useful to obtain hearts – clearly of relevance to humans if this could be made to work in suitable chimeras.  However, they failed to get live born rat-mouse chimeras suggesting that the hearts, which should be mostly rat cells, fail to function properly in the later embryo.  Perhaps they don’t beat as fast as mouse hearts.

“The data on human PSC in pig and cow blastocysts revealed the best type of human PSC to use to allow a chance of later chimeras containing sufficient human cells.  However, the data on post-implantation development is a little disappointing.  The rates of development after transfer of unmanipulated pig blastocysts was much higher than those injected with human PSC, which suggests that the problems of retarded development and loss of embryos is due to the human PSCs.  The number of controls was low, but if the authors’ conclusions are correct, then this says that more work is needed to find the best type of human PSC to use and perhaps a different large animal to provide the host embryos.  However, this is where the complementation strategy would have been useful.  Perhaps it is too much to expect the human PSC to give sufficient numbers of cells in tissues where they have to compete with the pig cells, but if the pig cells were absent from a developing organ, such as the pancreas, then the human cells may have been able to give rise to the entire organ.

“There is currently much interest in these kinds of approaches, particularly with respect to animals containing human cells or tissues and how far these should go.  Experiments involving chimeras, whether they are animal to animal or animals containing human material, are subject to regulation in the UK via the Home Office.  The authors of this study, who are based in the USA, have been careful to follow guidelines issued by the ISSCR (International Society for Stem Cell Research), which match well with the UK regulations.”

Prof. Bruce Whitelaw, Interim Director of The Roslin Institute, and Professor of Animal Biotechnology, University of Edinburgh, said:

“This is an exciting publication. It clearly demonstrates that human stem cells introduced into the early pig embryo can form a human-pig chimera.  This is the first scientific publication to achieve this result.  It follows earlier successes with rat stems cells introduced into mice.  In these earlier studies, live rat-mice were produced.  In the current pig project, experiments were terminated at the embryo stage before the potential chimeric animal was born.  The 10 years between these two studies is a testament of how difficult it has been to achieve the human-pig result.  The scientific trick appears to be the use of intermediate-stage stem cells – but this still remains inefficient at various levels.  Nevertheless, this is a first in the development of chimeric animal production and paves the way for significant advances in our understanding of cell lineage development in the embryo and hints towards future novel biotech applications.”

* ‘Interspecies chimerism with mammalian pluripotent stem cells’ by Jun Wu et al. published in Cell on Thursday 26 January 2017.

Declared interests

Prof. Darren Griffin: “No conflicts.”

Prof Robin Lovell-Badge: “Robin Lovell-Badge is a Senior Group Leader at the Francis Crick Institute in London. Some of his research touches on the methods used in this paper, but not on the specific topic, and he has no conflicts of interest or financial interest in the study. He was a member of the Academy of Medical Sciences Working Group on: “Animals containing human material” (2009 – 2011), whose recommendations were adopted into UK law in 2015.

Prof. Bruce Whitelaw: “I receive funding from the BBSRC and InnovateUK for research into genome editing applications in livestock, and I am a member of the Scientific Advisory Boards of Immunogenes AG and Recombinetics Inc.”