Ethically controversial human-animal embryos could provide cures for a number of intractable diseases.
The UK is poised to forge ahead in the field of hybrid human-animal embryo research following the recent House of Commons vote against a proposal to ban such work. The vote, part of the debate over the Human Embryology and Fertilisation Bill, preserved the UK's position as one of the world's most liberal regimes for fundamental embryonic and stem cell-related research aimed at developing cures for a number of intractable diseases that have a high genetic component, such as Alzheimer's and Parkinson's.
The vote was highly significant because it represents a defeat for the last stand in the UK of pro-lifers and the Catholic Church in the debate over use of embryos in general, although the battles are still to be fought elsewhere, notably in the US and in the rest of the EU. Pro-lifers seeking to ban all research involving embryos had seized on the hybrid issue as the one most likely to attract public support to their campaign, hoping to play on concerns that this would violate human dignity.
However, they probably overplayed their hand, emboldened by the position of the Catholic Church which holds that life begins at the point when egg meets sperm. This position looks increasingly absurd in the face of accumulating knowledge of embryology and also of the potential ability that all cells have to be reprogrammed into 'pluripotent' stem cells capable of generating embryos in the right conditions, as was noted by Professor John Burn, head of the Institute of Human Genetics in Newcastle.
"If stable embryos can be generated using the molecular reprogramming route, the opposition stance remains untenable since this makes every skin cell/liver cell/white cell etc a potential human - all you need is four activated genes and a friendly mother who doesn't mind risking a few hundred malformed offspring and a prison term!" says Burn.
Burn's Institute was one of two centres granted a provisional licence (the Commons vote could have overturned them) to conduct work on hybrids generated by taking an animal cell, removing the nucleus containing its genetic material, and replacing that with human DNA.
Such considerations encouraged MPs to support hybrid embryo research, following a well-informed and perceptive debate, according to Stephen Minger, director of King's College Stem Cell Biology Laboratory in London, the other place granted a provisional licence for research on this type of animal/human hybrid embryo. "It gives us some certainty our licences are valid," says Minger.
The immediate effect will be to accelerate the two hybrid programmes already underway, although as Minger pointed out, the whole field of human embryonic stem cell research, and, in particular, the hybrid area, suffers from a shortage of skilled staff that will take a while to remedy. Both groups will have to tackle the most fundamental technical or engineering challenge, which lies in learning how to turn on cell reprogramming almost like a tap and produce specific types of stem cell to order.
Most scientists strongly believe that this will be possible, and that, at some point, fine control over the differentiation and programming processes will be attained.
"It's highly probable we will get there or else there wouldn't be so much excitement in the stem cell biology field," says Alastair Kent, director of the Genetic Interest Group (GIG), the UK alliance of charities and support groups for people affected by genetic disorders.
Minger and colleagues are pushing ahead with work on these genetic malfunctions, focusing on neurological diseases such as Parkinsons and Alzheimers that are genetic in the sense that they are caused by an aberrant form of a protein arising in people with a particular variant (allele) of a specific gene.
The point is, as Minger notes, that there is then a target protein to work with. Stem cells comprising this variant can be created to study interactions involving the protein in order to understand how and why it causes disease.
The hope then is that the disease process could be targeted and disrupted by a specific drug with little or no side effect.
Alternatively, stem cells with the normal version of that particular gene could be targeted to that site in the hope of regenerating healthy tissue. There is accumulating evidence from stroke victims that stem cells do accumulate in areas of damage within the brain, and it is also known that such people sometimes partially recover over time from the resulting disabilities.
However, it is unclear whether the recovery is through actual regeneration of tissue, or rewiring of the brain, or a combination of both.
It is known that some higher animals, such as song birds, are much more capable of reprogramming their brains than others, and it is obvious that stem cell processes are involved. There is great hope of exploiting knowledge of these processes in treatment of neurological diseases for which there is currently no effective treatment, as well as others such as heart disease and Type 1 diabetes.
The other great hope for stem cell research lies not in direct therapy, but in screening drugs.
Currently, pharmacological drugs typically take 15 years from conception to full availability after completion of clinical trials. This protracted process usually involves animal testing, sometimes several rounds starting with mice and culminating in primates, before clinical trials on humans.
But the human immune system differs significantly from all other mammals, and animal testing does not always eliminate drugs with serious side effects, as was shown in the catastrophic trial in March 2006 in the UK of the drug known as TGN1412 designed for some immune system disorders. It left six men fighting for their lives, although all survived. In that case, the drug had successfully passed the primate stage, but then elicited a serious immune system response in humans.
Stem cells derived from humans could be used to provide either an additional round of screening for such drugs, or even to replace some of the animal testing, which itself would be a step forward.
"One way is to differentiate the cells into particular cell types from a drug discovery viewpoint, for example a neuron, or heart cell, and use them in high throughput screening in big compound libraries (this is automated testing of large numbers of cells against many different potential drug variants)," says Minger.
It is not possible yet, but the great appeal lies in the potential to regenerate the same target cell every time the test or screening is to be performed. This means that when a desirable compound for a drug has been identified, the screening can be repeated against the same candidate cells in order to be sure that it achieves the desired goal without lethal side effects.
For the pharmacological industry, both these lines of research are hugely promising. No cure has yet emerged from human embryonic stem cell work, but the field is only five years old.
Meanwhile, admix embryos promise a secure and readily available source of stem cells, with the possibility of yielding insights over the reprogramming process that cells derived from pure human embryos might not.
Theory and practice
The first scientific goal is to establish that hybrid embryos really are capable of producing human stem cells that are genetically identical to those derived from pure human embryos and capable of being used to study disease or test drugs.
The original Chinese study suggested that this is the case. It was backed up by some evidence from animal-animal hybrids, such as a mouse-cow embryo created again by SCNT.
Indeed, it is fairly certain these hybrids can in principle yield all the human stem cells that researchers want. As the embryo develops, the products expressed by genes - proteins and intermediate RNA molecules - become more purely human.
The only animal components remaining after embryos have proceeded to the 14-day limit are around 13 proteins encoded by the animal mitochondria, the energy producing organelles in the cell's cytoplasm. This is where a significant problem arises, for cells of the developing embryo need mitochondria to develop and replicate.
The key point is that proteins encoded by genes in the nucleus, coming from humans, and the mitochondria, coming from animals, must interact successfully in order for the embryo to develop. The danger is that the evolutionary distance between humans and cows may be such that the latter's mitochondria are insufficiently compatible with the human-derived nucleus.
But all is not quite what it seems in these hybrid embryos, for they do contain some human mitochondria that must have been associated with the nucleus and carried with it into the enucleated animal cell, even though that was not intended - after all, the mitochondria are mostly in the cytoplasm, rather than attached to the nucleus.
The incorporation of some human mitochondria in the hybrids may be a fortunate artefact of the procedure, for it is possible that subsequently they are in some way favoured over the more preponderant animal mitochondria and preferentially encouraged to replicate by the human nucleus.
Whether this is the case, it is obvious that there is plenty more work to be done studying the animal hybrids to learn what exactly is going on at the molecular level and then to make the process more efficient.