The NIH already withholds funding for research involving the creation of human–primate chimeras or breeding of animals where hPSCs could enter the germline. However, it announced in September 2015 it would start a process to review the science, ethical issues and animal-welfare concerns around the use of hPSCs to produce chimeras and abruptly stopped considering funding applications for such research.
The researchers, primarily from Stanford University, California, argue in a letter to Science that chimeric research has considerable benefits. These include the potential for developing human-specific models for better understanding drug actions, living models for understanding inherited diseases, and the possibility of developing replacement human organs using pig or sheep models.
'We believe that this notice poses a threat to progress in stem cell biology, developmental biology, and regenerative medicine,' they write.
'A continued dialogue between scientists and bioethicists regarding human/non-human chimera studies is critical for advancing human health through basic science.'
Associate Director for Science Policy at the NIH, Dr Carrie Wolinetz, told NPR the decision to suspend funding was prompted by an increasing number of applications in the field. The NIH wanted to 'make sure that we are fully prepared from a policy and guidance point of view' before making decisions about such research, she said.
One ethical concern is that animal brains could contain human cells, raising the possibility that 'this will somehow give the animal a human consciousness', said Professor Hank Greely, a bioethicist at Stanford.
But the authors of the letter argue that numerous studies introducing human neurons to mouse brains have found no evidence for this. 'There's been nothing like a mouse with human thoughts,' co-author Dr Sean Wu, told STAT.
A further issue is the possibility of stem cells creating human eggs and sperm in the animals. Professor Greely told NPR, 'If you had a male mouse that had human sperm in it, that's going to be a concern to some people, especially if it's anywhere near a female mouse that has human eggs in it.'
The NIH is not the only source of funding for regenerative medicine of this nature. However, as a federal funding body it is very influential. The authors of the Science letter argue that withdrawal of NIH funding 'casts a shadow of negativity' over chimerism research and threatens scientific progress.
On 6 November the NIH held a workshop in Bethesda, Maryland, involving researchers, bioethicists and lawyers to begin their process of policy evaluation.