Interviews for Science Watch

with Professor Sir Alec Jeffreys and Prof. Sir Roy Calne

1. Alec Jeffreys

Since the discovery of the structure of DNA in 1953, knowledge of the composition and organization of the genetic material has accumulated at an astonishing pace. By the early 1980s it had become clear that most human DNA shows very little variation from one person to another. The small percentage that does vary presents enormous potential for fruitful study.

Sir Alec Jeffreys’s involvement with mammalian molecular genetics began in 1975, when, as a postdoc, he moved from Oxford University to the University of Amsterdam to work with Dick Flavell. There, the two and their colleagues tried to clone a mammalian single-copy gene. They failed, but in the process managed to develop the Southern blot hybridization technique to the point where they could directly detect single-copy genes–and, in so doing, discovered one of the first examples of introns.

When Jeffreys moved to the University of Leicester in 1977, he chose to change direction completely and study DNA variation and the evolution of gene families. As a result of this work, his laboratory produced one of the first descriptions of RFLPs–restriction fragment length polymorphisms–a common form of variation in human DNA. The aim of the work was to develop a new breed of markers using DNA to track the position of genes. To develop good markers, the researchers needed to find highly variable regions of DNA.

In 1980, another team made one of the major breakthroughs in the study of DNA polymorphism, with their fortuitous discovery of the first “hypervariable” region of human DNA. These regions were found to consist of short tandem sequences repeated over and over again.

In 1983, Jeffreys found that these repeat sequences, dubbed “minisatellites,” contain certain “core” sequences. This opened the way for the development of probes, containing the core sequences, for detecting many other such regions of variable DNA. One Monday morning in September 1984, Jeffreys and colleague Vicky Wilson successfully tested the effectiveness of such a probe. “The implications for individual identification and kinship analysis were obvious…. It was clear that these hypervariable DNA patterns offered the promise of a truly individual-specific identification system,” Jeffreys wrote later (see A.J. Jeffreys, Am. J. Hum. Genet., 53[1]:1-5, 1993). They had stumbled on DNA fingerprinting, and Jeffreys’s life was changed.

Jeffreys, 45, gained a first-class degree in biochemistry from Oxford University in 1972, and his Ph.D., also from Oxford, in 1975. After working in Amsterdam with Flavell between 1975 and 1977, Jeffreys moved to the University of Leicester as a lecturer in genetics and became a full professor in 1987. He was elected a Fellow of the Royal Society (FRS) in 1986.

Science Watch’s European correspondent Amir Amirani spoke with Jeffreys at his laboratory in Leicester.

SW:Your most-cited paper, “Hypervariable minisatellite regions in human DNA,” appeared in Nature in 1985. Is the paper highly cited because it’s subsequently been used in fingerprinting, or because of the light that the paper shed on the structure of variable DNA?

Jeffreys: The citations, I think, reflect the fact that at the time this was a novel, very powerful generalized technology that could be applied to a wide range of problems in human and nonhuman genetics. The paper described for the first time a general method for getting at large numbers of highly variable regions of human DNA. Also, almost as an accidental by-product, it suggested approaches for not only developing genetic markers for medical genetic research, but for opening up the whole field of forensic DNA typing. And, from the work in that first paper, we could see immediately the potential applications in individual identification and in establishing family relationships–for example in paternity and immigration disputes.

Although it wasn’t mentioned in the paper for patenting reasons, we also saw the potential for exactly the same technology being applied to nonhuman species as well. This opened up all sorts of interesting possibilities in animal breeding, conservation biology, ecological genetics, and the like.

SW:Has the potential for the animal work been fulfilled?

Jeffreys: Very much so. That original DNA fingerprinting system, for example, has been used in a fair number of zoos to try and establish family relationships within captive colonies of endangered species of animals and birds, in particular to identify cases of close relationship–those individuals that you do not want to interbreed. The aim, in other words, is to minimize inbreeding and maintain genetic diversity.

Sir Alec Jeffreys’s Most-Cited Papers

Published Since 1985

(Citations updated through 1996)

Rank Paper Citations through 6/94*           Citations through 12/96

Avg. cites per year through 1996

1          A.J. Jeffreys, V. Wilson, S.L. Thein, “Hypervariable minisatellite regions in human DNA,” Nature, 314(6006):67-73, 1985. 1,407 1,778 148

2          A.J. Jeffreys, V. Wilson, S.L. Thein, “Individual-specific fingerprints of human DNA,” Nature, 316(6023):76-9, 1985.             669     857     71

3          E. Solomon, R. Voss, V. Hall, W.F. Bodmer, J.R. Jass, A.J. Jeffreys, F.C. Lucibello, I. Patel, S.H. Rider, “Chromosome 5 allele loss in human colorectal carcinomas,” Nature, 328(6131):616-9, 1987.    444     485     49

4          A.J. Jeffreys, N.J. Royle, V. Wilson, Z. Wong, “Spontaneous mutation rates to new length alleles at tandem repetitive hypervariable loci in human DNA,” Nature, 332(6161):278-81, 1988. 309     434     48

5          Z. Wong, V. Wilson, I. Patel, S. Povey, A.J. Jeffreys, “Characterization of a panel of highly variable minisatellites cloned from human DNA,” Ann. Hum. Gen., 51:269-88, 1987.             308     375     38

SOURCE: ISI’s Personal Citation Report, 1981-96

*citations reported with original interview

SW:Is there a biological function for mini- and microsatellites?

Jeffreys: That is a very, very tough question. If we look at minisatellites, by and large, there seems to be no obvious biological function. There are a few cases in the human genome, and a fair number of cases outside the human genome, of minisatellites that actually form part of genes. So there are tandem repeated DNA sequences that code for tandem repeated protein sequences. But those are the exception, not the rule. The majority of the minisatellite loci we look at have no obvious function. However, one area that we are very actively examining at the moment is the whole question of how variation arises at these tandem repeat DNA sequences. And that means exploring the mutation processes that go on in sperm and eggs, creating new versions.

What’s come out of that is actually a very surprising result in which the mutation process, rather than just reflecting the instability of tandem repeat DNA, seems to be actively controlled by elements external to the tandem repeats. So it looks as though the tandem repeats themselves are not so unstable, but rather the instability is being directed from a locally acting regulator. We also know that the mutation process is astonishingly complex and operates by a process that is wholly unexpected for minisatellites. We call this process “gene conversion,” and it involves chunks of DNA being shifted from one allele to another during the mutation process.

We also suspect that, in males, the majority of sperm mutations are specific to the male germline and may be meiotic in origin. This suggests a type of recombinational process, controlled by some elements near the minisatellite, and it looks as if it’s meiotic as well. And that really does start raising questions–such as, maybe this mutation process isn’t just some sort of accidental artifact of having tandem repeat DNA, but rather reflects some basic biological process going on in the DNA. One of our main jobs now is to explore this in a lot more detail.

SW:And is that of purely theoretical interest, or are there going to be practical implications as well?

Jeffreys: This is basic biology. As to whether there will be practical implications, I don’t know. However, in the course of our investigations, we’ve developed various new strategies for detecting new mutations in human DNA, and this does, in principle, offer practical applications. By mutations, I don’t mean, for example, a cystic fibrosis mutation, which is actually not a mutation at all but a variant that’s been around for thousands of years. I’m talking about new mutations–actually catching DNA at the point where it has altered its structure. If we can develop methods for measuring mutation rate in an individual undergoing this process–and this is one of my main interests–then we can start asking basic questions about environmental agents, such as ionizing radiation, which might impact upon the mutation rate.

SW:Fingerprinting has been subject to a lot of controversy, something you have alluded to in some of your papers. Do you personally have any reservations about its reliability?

Jeffreys: Before I answer that, we must clear up a point on semantics, and this is not trivial. The original DNA fingerprinting system we developed, which for technical reasons is not that useful in forensic identification, produces patterns that are idiotypes–they are, for all intents and purposes, completely unique to an individual, except for identical twins. There’s no serious dispute about that in my view. Unfortunately, the second generation of DNA typing systems–which is DNA profiling using single-locus probes–do not produce individual-specific patterns test by test. Even with a typical battery of five different tests, they produce patterns where unrelated people are most unlikely, in fact extremely unlikely, to share the same pattern. However, when you come to close relatives, brothers and sisters, there is a real chance, in fact about 1 in 4 to the power of 5 chance, of a brother and sister match, which is 1 in 1,000.

So, for every 1,000 sibling pairs, over five probes, you find a complete match. So they are not DNA fingerprints, not unique to an individual. However, their variability among unrelated people is pretty spectacular over five tests. Unfortunately–and particularly in the United States–the term “DNA fingerprinting,” which we specifically apply to the original multi-locus system in which we look at scores of markers, has been corrupted to be used in almost any DNA typing system. That has created a problem in court, because DNA profiling does not produce DNA fingerprints, but if you call them DNA fingerprints, then the defense lawyer can stand up in court and say, “This is misleading,” and that’s quite right.

So this is a semantic problem, but a serious one. Basically, the terminology that we developed for DNA typing using multi-locus probes has been hijacked, and in a misleading way. Now, if we get rid of that semantic part, we can ask how valid is the huge amount of debate that’s gone on about the reliability of DNA profiling? In the early days, in particular, there was real cause for concern. Some of the laboratories doing this work were carrying out real forensic analysis with technology that had been very poorly validated and hadn’t been standardized.

I think that this issue has been largely addressed now, through quality controls, the adoption of standard operating conditions, blind proficiency trials, and so on. For DNA profiling, the real source of debate now relates to how one estimates the rarity of a set of DNA profiles out in the population, and how one presents that evidence in court. If you say that a DNA profile of a forensic sample matches a given suspect and is very rare in the population, then that, depending on the context, can be pretty damning evidence.

SW:Let’s turn to your current research interests.

Jeffreys: My current interests are in exploring the basics of mutation of human minisatellites. We now know they are mutating by processes that are totally unexpected. These processes are probably of biological significance and may shed light on another fascinating area of human genetics: the whole area of triplet repeat instability diseases. These are microsatellites that go horribly unstable and cause neurological disease, such as Huntington’s chorea, myotonic dystrophy, fragile x syndrome, and so on. These are basically microsatellites, which suddenly become highly unstable, increase their repeat number, become very long, and wreck nearby genes.

And again, for technical reasons, it’s not easy to explore the details of the mutation process going on there, but we can explore in great detail the mutation process going on in minisatellites. We can use a whole battery of techniques that we’ve developed, which explore these bizarre mutation processes. It’s not impossible, though far from guaranteed, that what we discover in minisatellites may actually be applicable to these inherited diseases.

One sort of science-fiction scenario would be this: let’s suppose that what happens in minisatellites also applies to these unstable microsatellites. In other words, instability is conferred upon the array by flanking DNA, which, we suspect, is activating an allele for mutation. It’s basically switching an allele on, perhaps by introducing some kind of DNA damage, such as a double strand break into the DNA.

Now, if that is true for these neurological diseases, and these diseases manifest because of this instability, one could conceivably think of some therapy aimed at blocking that mutation initiation. That’s wild fantasy, but who knows? After all, gene therapy was fairly wild fantasy 20 years ago.

Another area in which we’re very much involved is developing new approaches to DNA typing. We’ve been heavily involved over the last couple of years in an approach called digital DNA typing, where you get a digital readout from the DNA rather than the usual sort of band length measurements in DNA profiling. And that has first of all revealed minisatellites as by far the most variable loci in the human genome. The typical minisatellite has, for example, 100 million different alleles worldwide, and that is astonishingly variable. And that in turn may give us some rather interesting markers for studying recent events in human evolution–by looking at these allele structure and how they’ve changed over time, how they differ between recently split populations, and so on.

SW:The field of DNA fingerprinting is relatively new. How do you expect this technique to develop, and how do you expect DNA structure studies overall to progress?

Jeffreys: The field of DNA fingerprinting has diversified to the point of incoherence. It’s no longer a single unified field. For example, back in 1987-88, when we had our first congress on DNA fingerprinting, the thing that welded it together was that everybody was playing around with minisatellites, DNA fingerprinting, and DNA profiling.

What’s happened since then, of course, is the advent of DNA amplification by polymerase chain reaction, or PCR. This means, first of all, that there is little doubt that in forensic DNA typing within the next few years all the classic systems of DNA fingerprinting and DNA profiling will be totally replaced by PCR-driven systems. Such systems have their powers and their weaknesses as well–contamination and the like. But the advantage of PCR is that it offers great sensitivity, potential for automation, lower costs, and information that is much less ambiguous in terms of a DNA profiling result.

Now, what the ultimate DNA forensic typing system will be, I don’t know. But to suppose that we’ve actually arrived there now is naive in the extreme, bearing in mind that information about PCR, or user-friendly PCR, was published only seven years ago. To pretend that we’ve gone from that to the ultimate DNA typing system is nonsense. There’ll be other ones coming along, and that actually creates a major problem for the forensic scientist who is interested in databasing, because once you go in for very large-scale databasing of many thousands of people–you are trapped in that technology. You cannot change that technology because you’ve got to retype everybody in the database if you do. So the drive towards databasing, I think, is in fundamental conflict with the still rapidly evolving field of forensic DNA typing–the technology itself.

So I see all kinds of developments on the forensic front. People may actually come up with what everyone is talking about: DNA chips, oligonucleotide chips that will be used to interrogate PCR reactions. At the moment, these are not chips at all in the electronic sense. If one could, however, create a chip in which an oligonucleotide could detect and transduce the detection of a product (such as a PCR product) into an electronic signal, that would open up not just forensic typing, but DNA typing, medical diagnostics, and just about everything else one can think of.block-close.gif (38 bytes)

2. Sir Roy Calne

As recently as 40 years ago, organ transplantation was still a distant dream. Since then it has been transformed into a major branch of surgery and a valuable form of treatment.

One of the figures most responsible for this transformation is Sir Roy Calne, Professor of Surgery at the University of Cambridge. Calne began his research on organ transplantation in 1959 at the Royal Free Hospital, and described the first effective immunosuppression for experimental kidney transplantation. He developed this approach further while working as a Harkness Fellow at the Peter Bent Brigham Hospital in Boston, Massachusetts, where it was applied to the treatment of patients in 1962. In 1977, Calne developed the immunosuppressive agent cyclosporine A and introduced it into clinical practice in 1978. This breakthrough is reflected in his highly cited papers–most notably, a 1984 report discussing cyclosporine in renal transplantation. (See R.M. Merion et al., “Cyclosporine: Five years experience in cadaveric renal transplantation,” New Engl. J. Med., 310[3]:148-54, 1984. This paper has been cited nearly 300 times since its publication.)

In 1968 Calne performed the first liver transplant in Europe; in 1987, the world’s first liver, heart, and lung transplant; in 1992, the first intestinal transplant in the U.K.; and in 1994, the first successful combined stomach, intestine, pancreas, liver, and kidney cluster transplant.

In addition to his duties at the University of Cambridge, Calne is also an Honorary Consultant Surgeon at Addenbrooke’s Hospital, Cambridge, and is Immediate Past President of the Transplantation Society. Aside from his research, he has other, wide-ranging interests. For example, he is an artist whose paintings, depicting images of his clinical work, have been exhibited in many countries to help promote awareness of transplantation. In 1994, adding to his roster of textbooks on surgery and transplantation, he wrote a book entitled Too Many People, which warns of the dangers of the continuing, rapid growth in the world’s population.

Calne, 64, was educated at Lancing College and received his medical training at Guy’s Hospital, London. He was elected a Fellow of the Royal Society in 1973, and was knighted in 1986.

Science Watch’s European correspondent, Amir Amirani, met with Calne at his office in Cambridge.

SW Many of your most-cited papers have been concerned with cyclosporine. What is the significance of this research?

Calne: The use of cyclosporine was a watershed in transplantation. I have always felt that my own most interesting work was the introduction of 6-mercaptopurine and Imuran as immunosuppressants in 1959 and 1960. But it was only when Imuran was used together with steroids that it was possible to develop clinical transplantation as a useful therapeutic option. We had to wait 20 years for cyclosporine. The experimental work on cyclosporine with organ grafts in animals was done in Cambridge in my department. We applied our findings in the clinic for the treatment of patients with organ grafts. When cyclosporine was perceived as an important advance in immunosuppression, the whole attitude of the medical profession towards transplantation changed. Before that, it was regarded as an enterprise for mad surgeons ignorant of immunology who really didn’t know what they were doing and who obtained unpredictable results. Subsequently, however, the image changed to that of an extremely valuable form of treatment for a majority of patients–but not all. In the early 1980s there was an important consensus meeting on liver transplantation held by the American National Institutes of Health. It was decided that the procedure was no longer experimental and was the preferred treatment for most forms of liver disease.

SW What is the current status of cyclosporine among the other immunosuppressants?

Calne: Cyclosporine is probably still the pivotal drug of immunosuppression for all organ grafts, except perhaps intestinal grafts, where FK506 seems to have better results. Everything that comes along has to be assessed in relation to cyclosporine, and if a new drug is developed that is clearly much better, cyclosporine will be displaced. But it has not been superseded so far and does not seem likely to be in the immediate future.

SW Is there any discernible trend in the development of drugs in this area?

Calne: Cell biologists have been analyzing the mode of action of cyclosporine. This has led to an unraveling of important intracellular mechanisms of signal transduction, from the antigen appearing on the surface of a lymphocyte to the signal for the lymphocyte to synthesize the powerful cytokine interleukin-2, which causes the clonal proliferation of lymphocytes.

There has been intense activity searching for other molecules, and analogues of cyclosporine. FK506, a macrolide with action similar to cyclosporine, has now been registered, and rapamycin, which is chemically very similar to FK506 but has a different action, is undergoing trials in the laboratory. There are a number of agents in phase II trials. FK506 has recently been licensed for liver and kidney transplantation in the U.K. Mycophenalate, the Syntex drug, which is a purine antagonist, seems to be superior to azathioprine, and I believe the definitive phase III trial on this is soon to be published.

So there is going to be a plethora of agents, which will make it extremely difficult for the clinician to know what to do and how best to treat the patient. I think we will need to train pharmacologists in immunosuppression–a special breed of physician/clinical pharmacologists who will be expert in the use of these drugs in all transplant patients.

Classic Papers by Sir Roy Calne

(Citations updated through June 1997)

Rank Paper             Citations through 12/94*        Citations through 6/97

1          R.Y. Calne, B.D. Pentlow, D.J.G. White, D.B. Evans, P. McMaster, K. Rolles, D.C. Dunn, S. Thiru, G.N. Craddock, “Cyclosporin A in patients receiving renal allografts from cadaver donors,” The Lancet, 2:1323, 1978.      623     682

2          R.Y. Calne, “The rejection of renal homografts: Inhibition in dogs by 6-mercaptoprine,” The Lancet, 1:417, 1960.          174             182

3          R.Y. Calne, R. Williams, “Liver transplantation in man. I. Observations on technique and organization in five cases,” Brit. Med. J., 4:535, 1968          88       99

SOURCE: ISI’s Science Citation Index, 1960-June 1997

* citations reported with original interview

SW You’ve said that the surgical problems of transplantation have essentially been solved, and only immunological problems remain. Could you say what the next major development in the field may be?

Calne: There is a reasonable consensus among surgeons as to the techniques for transplanting a heart, lungs, kidney, and liver. It’s true: the immunology is still the main stumbling block. If I were to crystal-ball gaze, I would predict that tolerance, or “almost tolerance,” would be the next major advance.

There are a number of different models to produce tolerance. They have in common the use of extra donor material, particularly donor bone-marrow-derived cells, and immunosuppression–hopefully for a short period of time–with the aim of manipulating the immune system to be able to accept the graft without any continuous immunosuppression, or with a minimal dose.

The classical Medawar-type tolerance is applicable only to the neonate or to the unborn fetus, but nevertheless it demonstrated that it is possible to manipulate the immune system–at least when it is not fully developed. The question arises, can we render the immune system to a similar pliable state as in the neonate or the embryo? There have been many different approaches to this pioneered by Dr. Anthony Monaco. The idea is to give extra bone-marrow-derived cells and immunosuppression therapy for a short period of time. Dr. David Sacks in Boston has been one of the most active and successful and has produced what’s called “mixed chimerism,” in which the bone marrow becomes populated with cells of both recipient and donor origin.

Dr. Thomas Starzl and his group in Pittsburgh have published a number of papers on “microchimerism,” in which donor-derived bone marrow cells are scattered throughout the body, even in the skin of patients, many years after transplantation. Microchimerism is prominent in the case of livers but also occurs with other organs. Dr. Starzl believes that this microchimerism is the cause of graft acceptance, but most of these patients are still on immunosuppression, so they are not really tolerant. The Pittsburgh group is in the process of a very extensive trial using large amounts of bone marrow from the donor along with a very high dosage of immunosuppression at the time of organ transplantation. Tolerance has not yet been reported in these patients.

SW What is the main focus of your research at the moment?

Calne: For about 25 years, we have been intrigued by the fact that the liver can sometimes produce tolerance in animals without any immunosuppression at all. The tolerance produced by the liver is interesting in that the liver undergoes a rejection crisis and recovers spontaneously, and then the animal will accept another organ from the same donor. From a whole variety of experiments, it would seem that the liver induces tolerance by two mechanisms: 1) the bone-marrow-derived cells in the liver, which include a special population of Kupffer cells, probably establish themselves in the recipient and may be involved in some kind of immunological engagement or conflict that leads to tolerance; and, 2) the liver produces something that maintains tolerance–probably Class-I antigen. This is true in humans as well. In a patient who has had a liver graft, about half of the Class-I antigen in the blood is from the donor.

In man, if a liver is transplanted together with another organ, such as a kidney, the kidney graft is protected from rejection. The longest survivor in the world with a liver transplant (a patient of Dr. Starzl’s), after 24 years, has had no immunosuppression for 14 years.

The liver effect has been reproduced in many different laboratories. We have been struggling with the obvious question: could we mimic the liver effect without having to actually transplant the liver? Can we use, say, ground-up liver cells?

Our hypothesis, and we’re not alone in this, is that there should be a chance for a dynamic immunological engagement to occur between the host and graft–host against graft and graft against host–without destruction of the graft. If we could only inhibit aggressive T-cell activity for a period of time, we might establish tolerance. And the period of time could be very critical, because if we miss that opportunity, we might then be stuck with continuous high-dose immunosuppression as the only way of keeping an organ in place.

We have done a number of experiments in which the strategy has been to leave a window of opportunity–I call it “WOOFIE,” for “Window of Opportunity For Immunological Engagement.” The experiments are simple, and the results are straightforward. I do not know if the interpretations are correct, but the principle is that we give one large dose of immunosuppression–we’ve been using a very large dose of intravenous cyclosporine in the pig–and either donor spleen cells or donor blood. We then leave a space of two or three days without any more immunosuppression, followed by six more daily doses of immunosuppression. The model is a kidney graft between grossly mismatched strains of pigs.

Control subjects reject renal allografts after seven to ten days if we don’t give any immunosuppression. If we give live spleen cells or fresh blood, three out of four animals go on to long-term tolerance beyond a year without any rejection and no chronic rejection. If the spleen cells were irradiated, there were no long-term survivors, so living cells in the spleen would seem to be important. If we eliminated the window and gave continuous immunosuppression, three out of four grafts were rejected. So this proposed window of opportunity does seem to be important. Without any spleen cells there was one long-term survivor out of four. So it’s not an all-or-none thing, but the phenomenon is fairly clear. We now have a protocol that could be used in the clinic, although such a dose of cyclosporine would be too high to give to man. A modified protocol would be needed with a gradual reduction to either minimal dosage or nil. That’s our hypothesis.

SW What’s actually going on in this window of opportunity?

Calne: Imagine an analogy with a football match: you want the match to take place and, at the end of it, for the two teams to shake hands and be friends. Unfortunately, there’ll be football hooligans who’ll cause a great deal of trouble and maybe destroy the match so it cannot take place. The initial dose of immunosuppression would either kill or imprison the football hooligans, allow the match to occur, and permit the shaking of hands–which is analogous to tolerance. The maintenance of tolerance will also be precarious, because there will be recruitment of new hooligans, or they’ll be let out of jail, and that’s the reason for giving another six doses although in man you’d probably need to give it longer, I would think, because human beings are much more complicated. They have alternate strategies of immunity. As to the exact molecular mechanism involved in this window of opportunity: the short answer is that we don’t know. Some kind of contact between donor marrow-derived cells and those of the recipient seems to be necessary in every kind of tolerance. We have experiments that work but we do not know why they work. So that’s my main research interest at the moment: to try and adapt the experiments to a clinical protocol in patients.

SW Aside from the immunological problems, what about the issue of shortage of organs?

Calne: No matter how much propaganda we have and how ever much political pressure is put on people, there will not be enough organs because the indications for transplantation are widening rather then contracting. So the dream of a xenograft becomes more and more attractive.

One would have thought that a xenograft from a close relative such as a nonhuman primate would be much better than a discordant species such as a pig, but the pig is more acceptable on ethical grounds. Also, one can get pig organs of any size. However, the history of xenografting in humans has been depressing.

The theoretical hurdle to be overcome first in a discordant graft is complement activation, which causes an almost immediate graft destruction. David White in my department is producing transgenic pigs with the anticomplementary human gene. I hope that this approach will be successful in stopping complement activation. We will then be in a position to see what the next barrier is.

There could be a whole variety of immunological obstacles and physiological considerations. We do not know if xenograft rejection without complement can be controlled with currently available drugs.

I think it would be facile to assume that there won’t be important physiological barriers, because every mammalian cell produces 1,000 or more proteins, and every protein in the pig is different from the equivalent protein in man; some are only slightly different, others are very dissimilar. These differences are likely to be important at least in the long term, even if they are compensated for in the short term. I don’t think this has been appreciated fully. I would regard this as very interesting science, but still a long way from practical application. Optimists say it’s going to be this year, but I believe that there is much difficult and painstaking work to be done before we get near to using pig organs as a “piggy bank” for humans.