Just how reliable, asks a leading neuroscientist, are claims made for the research benefits of human-animal hybrid embryos - one of the most controversial elements in the Human Fertilisation and Embryology Bill now being debated in the House of Commons?

How many times have you read, during recent discussions about the Human Fertilisation and Embryology Bill, that research using human-animal hybrids is vital to produce cures for Alzheimer's and Parkinson's diseases?

Take Lord Patel: "This bill has the potential to either help the development of new treatments, or [if blocked] set us back decades", with "a real risk that life-saving treatments could be lost". Or Gordon Brown, persuading MPs, citing treatments that "can save and improve the lives of thousands and over time millions of people".

Dazzled by the promises, the public stands by in awe of the science. The Human Fertilisation and Embryology Authority allows everything: it has thus far not ultimately rejected a single embryo-research-related application. Pro-embryo-research scientists have a ready mouthpiece in politicians and journalists beguiled by the claims. How could anyone oppose these miraculous cures? What we have seen in the determined efforts of some of the bill's more politically motivated protagonists is a confusion of the issues and a classic sleight of hand - in two separate ways. Both need exposing if people of conscience are to form honestly informed views.

The first is tacitly to allow the exciting advances in adult stem-cell treatments to illustrate the far more speculative therapeutic potential of embryonic stem cells; to use the former to justify the latter. Thus Gordon Brown: "With adult stem cells already being used as treatments for conditions including leukaemia, severe combined immunodeficiency, and heart disease, scientists are already close to the breakthroughs that will allow embryonic stem cells to be used to treat a much wider range of conditions. Medical researchers now believe that stem-cell therapy has the potential to change dramatically the treatment of many other human afflictions: including not only Parkinson's disease and Alzheimer's but perhaps also cancer, spinal-cord injuries and muscle damage."

Another example was in last Saturday's edition of The Times, a 12-page supplement (sponsored by the Wellcome Trust and - hardly by coincidence - appearing 48 hours before the debate in Parliament), relentlessly trumpeting stem-cell therapies and research with heart-warming stories of stem-cell cures and exciting reports of scientific progress.

Yet quietly submerged was the fact that every one of the stories concerning patients was about adult stem cells; and every report concerning embryonic stem cells was an experimental or animal study, or one speculating on their possible future potential. Not a single patient has been treated, even in trials, with embryonic stem cells: it would be too dangerous.

Medically, these heavily camouflaged truths are hardly surprising. The facts speak for themselves. Embryonic stem cells have an innate and invariable propensity to form tumours. They have genetic and chromosomal instabilities and abnormalities. Donor-derived cells carry the serious double hazards of cross-infection (prion diseases, for example) and tissue incompatibility (rejection). It is impossible to envisage their use in patients in the foreseeable future. Three months ago the highly reputable New England Journal of Medicine - previously a stout defender of cloning and embryonic stem-cell research - lamented: "Perhaps, not surprisingly, the technical difficulties and ethical complexities of this approach [cloned human embryonic stem cells] were always likely to render it impractical."

Conversely, this week has seen yet another positive clinical trial of adult stem cells (controlling damaging immune reactions after tissue transplants). The relative accessibility of adult (say, bone marrow) cells, their known safety, and the ability to use patients' own cells (avoiding rejection and cross-infection) all help explain why successful clinical trials in diseases as diverse as myocardial infarction, diabetes, limb ischaemia, stress incontinence and blindness from corneal disease have already been completed.

But there is a deeper biological aspect to adult stem cells' advantage as therapies. Our scientific approach to regenerative medicine has changed markedly in the last few years. The basic properties of embryonic stem cells - to generate limitless numbers of cells, and to turn into any kind of specialised cell - were considered clearly advantageous only when we thought of cell therapy as simply the replacement of lost cells. In fact, this simplistic notion applies in very few clinical circumstances. Tissue repair is infinitely more complex than this. Expecting implanted stem-cell-derived neurons, for example, to cure Alzheimer's disease would be a little like packing a few cogs and wheels and springs into the back of a broken clock and waiting for it to start working again.

Adult stem cells, present in most if not all specialised organs, have evolved as cells for repair: that is their purpose, and they successfully achieve this in many ways. But all this is barely relevant to the new bill. For here lies the second sleight of hand. The debate has, falsely, been turned into a referendum on all embryonic stem-cell research. What is proposed is actually "only" the licensing of various forms of mixed animal-human embryos as possible new sources of stem cells. But all the justifications for experiments using cybrids (embryos that are largely human but contain a minute quantity of animal material) are based on the falsehood that they are vital for developing embryonic stem-cell-based cures for dreadful diseases as argued by Lord Patel and Gordon Brown.

A broader perspective quickly reveals this as pure spin. The Daily Telegraph's Roger Highfield (generally supportive of embryo research) has forcefully pointed out that cell biologists who understand the complexity of proposed cybrid embryos are profoundly sceptical that they could ever prove remotely informative about human disease.

As James Sherley, from the Program in Regenerative Biology and Cancer, Boston, has said: "Huge volumes of ... basic cellular and molecular biology must be ignored to justify [cybrid] research. Not a single new experiment is necessary to know with certainty that human-animal cloning will not provide faithful models for human-human cloning."

I strongly suspect that it is this false equation - defeat of the bill represents a defeat for all embryo research - that has dog-whistled the British scientific establishment in support of the bill. In truth, few serious embryonic stem-cell scientists will speak in support of cybrid embryos specifically on the basis of their intrinsic potential for therapeutic research; most (obviously) will speak in favour of embryo research in general. (Although even among these, a proportion defends the bill more on the principle that scientists should not have limits set on their work than on the specifics of embryonic stem-cell science.)

And the suggestion that there is "no alternative" to cybrids is not even close to the truth. Rather, clinical scientists around the world have been extraordinarily excited by the emergence in the last year of a new technique for producing so-called "inducible pluripotent stem cells" (IPSCs). Certain genes are artificially activated to make adult cells "de-differentiate", or turn their clock back. IPSCs are virtually identical to embryonic stem cells; and this is a far, far easier technique than human cloning (let alone cybrid cloning). And involves no embryos. IPSCs have already successfully treated mouse disease models, and there are reports that IPSCs have been made from patients with various diseases.

In other words, this approach has quickly and clearly overtaken the cybrid idea. And IPSCs are 100 per cent human. Scientists all over the world are turning to this approach; even British stem-cell scientists say it spells the end of human-embryo research. Nowhere else is the rather bizarre alternative of making cybrids - let alone hybrids - generating any serious interest.

Yet, when three Catholic Cabinet members stood up for their right to vote according to their conscience, there was a near-stampede of panicking scientists, journalists and politicians, railing against the Church. Why the flap? Surely there is little chance of the bill being defeated. Or is there?

The HFE bill is not a referendum on embryonic stem-cell research. Cybrid embryos are a small part of the bill - but a minuscule branch of stem-cell research; most likely a cul-de-sac of slight interest to the scientifically curious, but clinically irrelevant. The alternatives are considered by the overwhelming consensus to be superior. Defeating this part of the bill will have zero impact on the development of stem-cell therapies - and represent a triumph for common sense and for moral responsibility.

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