Select Committee on Science and Technology Written Evidence


Memorandum 20

Submission from Dr Lyle Armstrong, University of Newcastle upon Tyne

  In response to the points made in paragraph 2.85, in which the government states its intention to propose that the creation of hybrid and chimera embryos in vitro, should not be allowed, I would like to put forward my case for creating such entities for the purpose of embryonic stem cell research. There are several advantages to the use of animal oocytes that I would like the Science and Technology select committee to take into account.

(A)  BACKGROUND INFORMATION

1.   Embryonic stem cells

  Embryonic stem cells (ESC) are valuable because they can differentiate into any cell type found in the adult body and they can also keep doing this indefinitely. There seems to be no limit to their ability to grow in culture while still retaining the ability to differentiate into any cell type. This property is known as pluripotency.

  ESC are derived from very early pre-implantation stage embryos which are obtained from IVF clinics subject to licensing by the HFEA. A typical blastocyst stage embryo is shown in figure 1 and ESC are derived from the area highlighted by an arrow which is called the inner cell mass. This small group of cells would normally produce the entire organism (the other cells are destined to become parts of the placenta) but at this point in time they are all equally capable of becoming nerve, muscle, blood, etc and so we think of them as being pluripotent. ESC effectively represent a "snapshot" of this stage of development which we can maintain indefinitely by choosing culture media that prevent the cells from differentiating as they would in the developing embryo. This allows us to replicate embryonic development in the laboratory which is enormously useful since it should enable us not only to produce cells from later stages of development but also to answer many scientific questions about how human development works.

Figure 1

BLASTOCYST STAGE EMBRYO AND DERIVATION OF EMBRYONIC STEM CELLS


1.1  Embryonic stem cells may be useful in treating disease

  Producing large numbers of differentiated cells may form the basis of cell replacement therapies which could be used to treat serious diseases such as Parkinson's disease, heart disease, etc. Details of how we might reasonably expect to develop ESC based therapies have been dealt with in more detail by other researchers so I do not intend to dwell on this area however, there is a major problem with current ESC derived cells in that a patients immune system will probably perceive them as foreign to the host tissues and try to destroy them. This effect can be treated by immunosuppression but this has considerable medical complications in its own right and should be avoided if at all possible. It may be possible to lessen the effects of immune rejection by "matching" the immune characteristics of the patient to specific ESC lines but this will require more cell lines than are currently available to represent a broader range of "immunophenotypes". Whilst it is certainly possible to generate more ESC lines, it would be unlikely to eliminate the immune rejection problem completely. Tissue matching in an organ donation scenario can slow down rejection but it does not stop it completely and immunosuppression is still required.

2.   Getting around the immune rejection problem

2.1  Can we make the immune system accept cells derived from ESC?

  Several solutions have been proposed to circumvent immune rejection of ESC derived cells. It may be possible to genetically engineer existing ESC lines to alter their expression of the major histocompatibility complexes I and II to restrict the ability of the immune system to detect them. This possibility is unproven and since genetic engineering is quite difficult (though not impossible and still easier than engineering adult stem cells) with human ESC lines, the chances of success are not high. In addition, do we really want cells in the patient that the immune system cannot detect?

  The immnune system may be educated to tolerate the foreign cells by using a technique known as haematopoietic chimerism. This would require pre-conditioning the immune system by transplanting blood stem cells from a donor into the patient some time before the planned transplant of ESC derived cells. In theory, this method may be able to deplete T-lymphocytes that would normally attack the non-host cells but it may also induce a condition known as graft versus host disease (this results from the transplant of the donors T-lymphocytes along with the blood stem cells and these attack their new host which they perceive as foreign) which can be fatal. We may be able to avoid GVHD by deriving the necessary blood stem cells from ESC but this adds an additional complication to the disease treatment process and there is no guarantee that it will work.

2.2  Using genetically identical ESC lines will eliminate the immune response

  The best way to prevent rejection altogether is to transplant cells that are genetically identical to the recipient. This is analogous to transferring organs between identical twins which does not normally induce an immune response.

  This requires a genetically identical ESC line and to date the only way we have to make such lines is the process of somatic cell nuclear transfer more commonly known a cloning. The use of this technique to produce ESC lines has been referred to as "therapeutic cloning". Figure 2 shows how this might be applied.

  Essentially, this process involves removing the genetic material from an oocyte and replacing it with the genetic material of the patient then instructing this new entity to start development as though it were a normal embryo. If development proceeds to the blastocyst stage, we can derive ESC in the same way as from an IVF embryo and importantly, these cells all contain the patients genome. Any differentiated cells derived from these lines will be genetically indistinguishable from the patients own cells.

Figure 2

THE SOMATIC CELL NUCLEAR TRANSFER PROCESS


2.3  Nuclear transfer reprograms the genome of the donor cell

  The nuclear transfer process can work because every cell of an adult's body contains all of the information needed to build a completely new organism. They are of course never called upon to do this because higher animals such as humans are organised into specific compartments that do designated jobs necessary for the functioning of the animal as a whole and the reproductive function is the speciality of the gamete cells. Gametes have their DNA organised in a very precise manner that allows them to express all the genes needed for embryonic development in a highly orchestrated manner. All of these genes are still present in, for example, a skin cell but they are not organised according to the same gene "architecture" as the gametes and so they are not expressed. This is quite a good thing actually since such genes would interfere with a skin cell's ability to perform skin related functions.

  The problem for the nuclear transfer embryo is getting those genes to work in the same way they would in an embryo resulting from a normal fertilisation. In order to do this it has to remove whatever controls the skin cell genome and tells it to express only skin cell specific genes and then replace it with a program which is specific to an early pre-implantation embryo. Fortunately, we are discovering quite a lot of information about how such programs are applied in the differentiated cells of the body and this has given rise to an area of science known as epigenetics. As its name suggests, this is concerned with factors controlling gene expression and cell functions which are "outside" genetics ie they are not directly connected to the sequences of base pairs written into the DNA. Most of this control comes from the attachment of a wide range of different proteins to specific locations along the DNA and /or chemical modifications of those proteins. The pattern of modifications effectively tells the cell what it can do with the information encoded in the DNA. This allows the cell to impose a specific pattern of gene expression and to repress genes or areas of DNA that are not needed for its function as say a skin cell however, the fact that this control does not require any changes to the basic information encoded in the DNA suggests that it might be reversible under certain circumstances.

  There is evidence to suggest that normally fertilised embryos also need to reprogram the genes that they get from the male sperm and female oocyte. These reproductive cells need to use information encoded in their genes to maintain their own stuctures and biochemical processes which allows them to exist as either oocytes or sperm. The genes in these cells are organised so that they can be used as rapidly as possible during embryo development but there are some genes that need to be suppressed in the oocyte. It is even worse in the sperm because the DNA needs to be tightly packaged into a very small space so it needs very specific chemical modifications to allow this to happen. This means that even gametes have a specific epigenetic program that needs to be removed before embryo development can take place. It has become apparent that there are factors in the oocyte that are capable of altering the epigenetic structure of the genome to reset this to a state where genes needed for development are expressed. It is also apparent that these factors are needed to reprogram the genes of the gamete cells after fertilisation and that "cloning" works because these factors attempt to do the same job of reprogramming the genes of the donor cell whatever its original program was. A skin cell for example, would have a radically different expression program to a cell from some other organ but cloning attempts to reprogram them both. It doesn't always do this perfectly but it surprising that it can do it at all given the fact that the donor cell type is very different from that which the oocyte is "expecting" to work with.

  Animal cloning experiments in which nuclear transfer embryos have been implanted into surrogate females and develop to term have highlighted a number of problems. Cloned animals suffer from a variety of malformations attributed to inappropriate expression of a small percentage of their genes. This has been further attributed to incomplete reprogramming of the genes of the donor cell used to derive the clone in the first place. However, the same may not be true for producing embryonic stem cells by method since the derivation process may select for those cells in the embryo's inner cell mass in which reprogramming has taken place to completion. This suggests that clone derived ESC would be equivalent to those derived from IVF embryos.

(B)  THE CASE FOR USING ANIMAL OOCYTES TO MAKE "INTERSPECIES" OR ANIMAL-HUMAN HYBRID EMBRYOS

1.   Why do we need animal-human hybrids?

1.1  Human oocytes are in short supply

  My laboratory has also been involved in the derivation of ESC from spare IVF embryos and we have found that on average, 15% of embryos will give rise to a ESC line. Cloning technology will at best produce embryos of the correct stage for ESC derivation in about 20-30% of cases so a worst case scenario is that 3% of cloned embryos would be able to produce ESC. This means that we would need in excess of 30 oocytes to have a reasonable chance of producing an ESC line for each patient so if large numbers of individuals were to require cell replacement therapies in the future, the demands for human oocytes would be large indeed.

1.2  Animal oocytes are readily available

  It has been suggested that replacing human oocytes by those of animal species such as cows and rabbits will greatly facilitate production of human ESC. This will rely upon similarity between the reprogramming factors in human and animal oocytes but there is evidence in the scientific literature to suggest that ESC can be derived from animal-human hybrid embryos and that these may have similar differentiation abilities to IVF derived ESC. It is clear of course that this requires a great deal more investigation before animal-human hybrid embryos can be used for this purpose but they have the great advantage that much larger numbers of experiments can be performed using readily available animal oocytes. To set this in context, hundreds of cow oocytes can be obtained from a single slaughterhouse every day. An IVF clinic would struggle to collect more than 10-20 donated human oocytes in one week.

1.3  The use of animal oocytes will conserve supplies of human oocytes

  Moreover, the use of animal oocytes for cloning means that precious human oocytes are not diverted from other projects where they are absolutely needed. It probably represents a much better use of resources if IVF patients are allowed to freeze oocytes for later use in IVF treatment than simply to donate them to a research programme that would not stand a high chance of success with the small numbers of oocyte that patient would normally have to donate. There are other important research applications such as investigations of how to solve problems associated with mitochondrial mutations in fertility and how ageing affects a woman's ability to conceive that cannot be performed in animal species and so have an absolute requirement for human oocytes. Production of ESC using cloning would possibly be much easier if there was an unlimited supply of human material but since we can carry out the necessary experiments using animal oocytes, it seems more logical to adopt this strategy which will save human oocytes for applications where they are really needed.

2.   Using animal-human hybrids to understand the reprogramming phenomenon might end reliance on cloning

  A crucial part of our investigations into somatic cell nuclear transfer will be to understand how the oocyte (animal or human) goes about reprogramming the genes of the incoming donor cell. We need to know the molecules that remove the epigenetic patterns from the genes and the chemical mechanisms they use to do this. At the same time, we need to learn how the gene expression program required for embryonic development is imposed.

  If we can understand how this process occurs in the cloned embryos, we might be able to reproduce it, or something like it, in the laboratory and the possible implications of such understanding are great indeed. The ability to produce ESC from differentiated cells without having to use an oocyte to produce an embryo that is subsequently destroyed for ESC harvesting would circumvent all of the ethical objections to ESC technology. This would leave us in a similar ethical position to the proposed use of a stem cells harvested from adult tissue which after expansion in culture could be injected back into the same patient with the aim of treating disease. This scenario will of course be many years in the future but if we can understand how reprogramming is effected in cloned embryos we will have a fighting chance of being able to make it a reality. The large numbers of experiments we need will only be possible using readily available animal oocytes.

2.1  Why not use animal-animal hybrids?

  Many questions about the reprogramming phenomenon can be answered by examining the response of animal cells after injection into animal eggs but ultimately we need to understand the response of human genes in order to produce human ESC lines. Furthermore, other research groups plan to use ESC from animal-human hybrid embryos to model the development of serious diseases by taking donor cells from such patients and creating disease specifc ESC lines. Availability of hES cell lines from patients with Alzheimer's disease, type I diabetes, or many other complex diseases would provide a source of cells that could be differentiated into appropriate cell types; and the progression of the disease could then be modeled and potentially modified in culture. Given the complex interplay between genotype and environment that typifies complex chronic diseases, the availability of cell-line models would provide major new tools for diagnosis and therapy. In this context, hES cells are research tools for the study of disease, not therapeutic agents themselves however, it would be impossible to model human diseases using purely animal ESC lines.

2.2  Ethical status of animal-human hybrid embryos

  Somatic cell nuclear transfer will replace the genetic material of an animal oocytes with a nuclear genome that is completely human in origin thus, apart from the presence of animal mitochondria derived from the oocytes cytoplasm, the resulting embryo will eventually derive all of the gene products needed for its development from the human genome. In this respect, creation of an animal-human hybrid embryo using somatic cell nuclear transfer is no different to creating a totally human embryo by transferring a human donor cell into a human oocyte. The resulting identities would be identical except for the presence of animal mitochondria.

  The government is prepared to accept that somatic cell nuclear transfer of human cells into human oocytes is allowable subject to the award of a licence by the Human fertility and embryology authority and that such work is necessary to increase our knowledge of embryonic development and serious disease. In view of this, I would like to suggest that creation of animal-human hybrid embryos should be allowed according to the same licensing protocol. This is in accord with the recommendation of the House of Commons Science and Technology committee detailed in paragraph 2.84 of the white paper which states, "that revised legislation should permit the creation of hybrid and chimera embryos for research provided they are destroyed in line with the 14 day rule applicable to human embryos".

January 2007





 
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