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JUST OVER three years ago, in November 1986, about 30 scientists and
clinicians met at the CIBA Foundation in London to discuss whether it was
technically possible to diagnose genetic disease in the ‘preimplantation’
embryo. If this could be done in the laboratory at this very early stage
– when the embryo consists of a few cells, and before the time of intimate
contact with the uterus – doctors could then initiate a pregnancy in the
mother by transfer of only those embryos known to be free of that disease.
Couples at risk of transmitting a severe genetic disease to their offspring
might prefer such a procedure to the later methods of prenatal diagnosis
now available, which rely on abortion to prevent a baby being born with
the disease.

Only one year earlier, everyone thought that such ‘preimplantation diagnosis’
was not a practicable proposition. The outcome of the CIBA meeting would
prove otherwise. The meeting provided an opportunity for the bringing together
of new ideas from the three different areasof research which would make
preimplantation diagnosis feasible.

The first crucial requirement was that the procedures for in vitro fertilisation
(IVF) and embryo transfer, as a treatment for infertility, were well established.
These involve the recovery of several eggs from the woman, fertilisation
of the eggs in the laboratory by the partner’s sperm, and transfer of usually
three of the resulting four- to eight-cell embryos to the the woman’s uterus.
More than 10,000 ‘IVF babies’ have been born in this way to infertile couples.
These same procedures provide access to the very early human embryo for
the very early diagnosis of genetic disease.

Developments in DNA technology, the second area of rapid progress, have
enabled us to understand the molecular basis of many inherited diseases,
and the list of diseases that can be accurately diagnosed increases daily.
This knowledge also enables potential parents, who have relatives suffering
from an inherited disease, to be tested to see if one or both of them carries
that genetic defect. Such tests can often establish a couple’s risk of having
a diseased baby, and allow them to choose one of a number of options to
cope with their predicament.

The third area where we have made the technical advances required for
preimplantation diagnosis is in the handling and micromanipulation of tiny
preimplantation embryos, and in the development of highly sensitive microassays
to detect metabolic products and specific proteins in their cells. Around
the time of the CIBA conference, another major technological advance influenced
the picture; researchers at the Cetus Corporation in the US developed the
immensely powerful procedure called the polymerase chain reaction. This
allows scientists to make millions of copies of a specific gene sequence,
representing less than a millionth of the total DNA, so that it can be analysed
easily. This procedure is sensitive enough to detect a chosen sequence in
a single cell.

Our research at the Medical Research Council’s Mammalian Development
Unit in London is involved in this third area. With others, we have developed
highly sensitive techniques over the past decade to enable molecular studies
of development in mammals. Such studies were previously considered impossible
because of the tiny amounts of material in the minute embryos. It was unthinkable
to try to do biochemistry or molecular biology on preimplantation mouse
embryos because the assays available at the time would have required embryonic
tissue from large numbers of mice. The logical step was to modify the assay
techniques, to make them sufficiently sensitive to analyse single preimplantation
embryos, consisting of between 1 and 100 cells.

So, for the past 10 years, our work has focused on the development of
techniques to detect enzymes and specific DNA sequences in minute amounts
of embryological material. The long-term aims of our research were academic,
directed towards understanding how cells regulate the expression of their
genes during the specialisation and growth of particular tissues. It is
the nature of academic research that it sometimes leads to a breakthrough
that was never expected. In this case, the microassays that we developed
for the mouse were the first to be used to show that preimplantation diagnosis
of genetic disease in the human could work.

Thus the time was right, at the end of 1986, to discuss the possibility
of preimplantation diagnosis of genetic diseases. But just because a particular
medical procedure becomes possible to perform does not necessarily make
it ethically, socially or morally right. The appropriateness of a new technology
must always be scrutinised in the context of current developments, the needs
of society and future implications.

About 1 per cent of babies born are afflicted with a serious genetic
disease. The risk of transmitting a genetic disease to a child is high if
both parents carry the same recessive single gene defect (on average, one
in four of their children will be affected), or if one parent carries a
dominant gene defect (one in two children will be affected). In many cases,
parents will know whether there is a risk, either because they havealready
had an affected child, or have a family history of thedisease, and both
partners have been found to be carriers ofthe same defect. Some genetic
diseases are not so severe thatthere will be cause for too much concern;
mild forms ofhaemophilia, for example, which can be easily managed(although
sometimes with unexpected side effects). Otherdiseases are more severe and
result in much suffering, bothfor the child and for the extended family,
and often in theearly death of the affected child.

Often, a couple will be well aware of how much suffering is involved
because of their first-hand experience with an affected child or relative.
In my own family, a niece and her husband suffered through the death of
their baby when he was seven months old. He had contracted pneumonia because
of inheriting severe combined immunodeficiency disease. The tragedy is not
only the loss of a much loved child; there is also the suffering that has
to be endured by the child and the parents, as doctors attempt to prolong
life – an often futile task. There is as yet no cure for most severe genetic
diseases. Often the treatments can only prolong life by a few years and
the treatments themselves may be painful and traumatic. Following such an
experience, it is not hard to imagine the anxiety the parents then endure
as they contemplate another pregnancy, hoping once again to have a normal
healthy baby.

What can be done? Currently, if a couple is at risk of having a diseased
baby, doctors can test the fetus during pregnancy by amniocentesis at around
16 weeks of pregnancy, or earlier, at 8 weeks, by chorionic villus sampling.
Both procedures also carry risks, albeit low, for the normal fetus. If the
fetus proves to be affected, the couple has the option of terminating the
pregnancy by abortion. However, abortion of a wanted pregnancy is traumatic
for all concerned – the woman, the family and the medical staff. Moreover,
because their risk of passing on a genetic disease is high, some couples
may go through this trauma several times in succession in their attempts
to have a normal baby. Very rarely does a couple choose to separate and
find other partners, to remain childless, or to have the affected child.
Most couples choose to have the fetus tested before birth and to terminate
the pregnancy if the fetus has a severe genetic disease. Many people, even
if they are not directly concerned, can understand why this is so.

Preimplantation diagnosis is a form of prenatal diagnosis carried out
before the embryo implants in the uterus. It would enable doctors to place
in the mother’s uterus embryos that do not have the defect under test. Only
those embryos without the disease would then initiate implantation and pregnancy.
In this way, abortion would no longer be necessary to avoid genetic disease
in the children. On the other hand, in order to provide enough embryos (say
four to eight) to test for normal or defective genetic status for a particular
disease, the couple would have to be prepared to produce their embryos by
the current procedures used for infertile couples: namely, byin vitro fertilisation
followed by embryo transfer, withthe inherent low rate of successful pregnancy
(about 15 percent, although success is perhaps more likely for thesecouples
whose problem is not infertility but genetic riskin the offspring).

We have developed procedures for preimplantation diagnosis in mice.
We found that we could remove a cell from a preimplantation mouse embryo
(say, from the embryo at the eight-cell stage) without destroying its ability
to continue to develop normally.

The next step was to carry out a genetic diagnosis on the sampled cell
or cells. Two approaches have been developed in my laboratory. First, we
can aim to test for the absent or altered gene product (a protein) in the
single sampled cell. In this case, the product must be manufactured by the
embryonic genome at this early stage of development. Secondly, we can try
to test for the actual gene defect in the DNA, the mutation itself, in the
single sampled cell. We have shown both these approaches to be possible
and, crucially, reliable: they work in nearly all the single cells tested.
Stringent controls are included to ensure that there is no risk of false
positive or false negative results.

As a model for preimplantation diagnosis, we used a strain of mice with
a genetic defect equivalent to that causing Lesch-Nyhan disease in people.
This is a devastating disease affecting half the boy children of a mother
carrying this genetic defect. Affected boys suffer from spastic cerebral
palsy and obsessive self-mutilation, and they die around the age of puberty.
Using mice, we were able to reliably detect the diseased embryos that did
not produce the gene product – in this case an enzyme, hypoxanthine phophoribosyl
transferase. We could discover which embryos lacked the enzyme by testing
a single cell taken from an eight-cell embryo or a few cells taken from
a blastocyst, made up of 100 to 150 cells. We confirmed the diagnosis by
replacing the embryos in mouse foster mothers and checking the resulting
offspring for the enzyme.

Finding the needle in the haystack

We also developed sensitive biochemical microassays for several other
enzymes associated with genetic diseases in humans, most notably, adenosine
deaminase (deficiency of which causes severe combined immunodeficiency disease).
We again showed these assays to be sensitive enough to detect the missing
enzyme in a single cell of the mouse preimplantation embryo. These early
experiments showed that accurate diagnoses could be done, given a sensitive
assay for the enzyme.

It was time to see if we could apply these procedures to the human.
We analysed a series of unfertilised human eggs and very early embryos from
the infertility clinic of Peter Braude, Martin Johnson and their colleagues,
in Cambridge, to see if we could detect enzymes in a single human cell.
The aim was to determine whether it was feasible to carry out a preimplantation
diagnosis for Lesch-Nyhan disease in people in this way. We found that,
although we could readily detect the enzyme in a single cell of the human
preimplantation embryo, the test could not be used for diagnosis because
the enzyme that we had measured was not actually produced by the embryo’s
own genes. Rather, enzyme produced in the egg before fertilisation was present
in such large amounts that it obscured any new enzyme made as a result of
the activity of the embryo’s genes.

This initial work highlights two important facts: first, the need for
research on human embryos if preimplantation diagnosis is to be a medical
option in the future, and, secondly, the danger of extrapolating from animals
to people without first showing that it is valid to do so. Further research
could show whether direct biochemical microassay would be the best route
for other diseases. For example, we know that we can test a mouse preimplantation
embryo for an enzyme deficiency causing severe combined immunodeficiency
disease. We do not yet know whether this is possible in the human.

The other approach we, and others, have taken, is to detect the actual
genetic mutation in the single cell. Researchers at the Hammersmith Hospital
and St Mary’s Hospital, both in London, have shown that it is possible to
detect sequences identifying Duchenne muscular dystrophy and cystic fibrosis
in a human egg. We have again used a mouse model, this time for beta thalassaemia,
to show that we can reliably diagnose mutant and normal embryos, by the
detection of the presence or absence of the actual mutation in a single
cell taken from the eight-cell mouse embryo. We used a modification of the
polymerase chain reaction to amplify a specific gene sequence, and devised
stringent procedures to ensure there is no contamination – a great danger
at this supreme level of sensitivity. We are developing the same approach
for the possible preimplantation diagnosis of defects in the human beta
haemoglobin gene, which cause beta thalassaemia, using, initially, human
blood cell DNA, or the few cells derived from individual human hair roots.
This research shows that it will be possible to develop procedures for preimplantation
diagnosis of any human genetic disease for which a single defectivegene
is the cause, and for which the molecular basis of thedefect is known.

Preimplantation diagnosis offers the possibility of avoiding pregnancy
with a seriously diseased fetus. To establish the procedures, it was necessary
to do preliminary research on mice; in fact, initially, it was the desire
to limit research on mice that led to the development of the microassays
for enzymes. Research on human one-cell to 100-cell embryos from the infertility
clinics is also necessary, to confirm that we can apply these microassays
to the early human embryo. At present, all research on human embryos in
Britain is regulated by the Interim Licensing Authority. The ILA prohibits
obviously undesirable research aims, such as interfering with an embryo’s
genetic makeup, attempts at cloning, and mixing human and animal cells in
developing embryos. Research is regulated by the guidelines of the Interim
Licensing Committee, agreed by all the scientists and clinicians concerned.
With the forthcoming legislation, MPs will vote whether to ban preimplantation
embryo research or to allow limited embryo research of the type I have described.
The research would be carried out under licence from a Statutory Licensing
Authority that will take over from the present interim body.

There is no turning aside from the joy of new life which has been made
possible through the research on human embryos that has led to current treatments
of infertility, and from the further implications for safer and healthier
pregnancies in the future. But a central question remains. Is research on
the preimplantation embryo (from the one-cell to the 100-cell stage) ethically
acceptable? Some might consider such research absolutely wrong. Others might
consider research to be permissible provided that strict regulations are
observed. Others try to decide when a human being begins: to define a ‘line’
which could be used to make research on embryos acceptable before this time,
or stage, of development.

Personally, I do not know when life begins. Different aspects of being
human happen at different times. In the treatment of infertility, the fertilisation
of the egg by the sperm is separated from the act of physical union of the
man and woman, and pregnancy is initiated at the stage of implantation rather
than at fertilisation. But in any case, the question of when ‘life’ begins
may well be irrelevant to the couple concerned. They are choosing among
three options: starting a pregnancy with an embryo which is not affected
by the disease genes they carry; risking the birth of an affected child;
or prenatal diagnosis and the option of terminating the pregnancy.

We should remember that preimplantation diagnosis is not an experiment
in itself. It is a diagnostic technique applied to embryos destined to be
replaced in the uterus. The development of the procedures for this, however,
requires limited research on embryos. Preimplantation diagnosis is not a
cure; it is not a treatment; it is the prevention of pregnancy with a diseased
embryo, and an alternative to abortion of a diseased fetus following an
unfortunate result from prenatal diagnosis.

As human beings, we consider each individual child to be very precious.
Many families in Britain could not afford to consider having more than two
children. There are other factors as well: the long gestation period, the
birth of a single baby and the prolonged period of dependence of the human
offspring, well beyond puberty. Knowing how precious thehuman baby is tends
to lead us to take a similar attitude to the human preimplantation embryo.

But facts of our biology and social life can give us another perspective.
Because we have so much invested in the production and rearing of only a
few offspring, it is even more important to invest that energy in offspring
who will be free from serious disease. This is not to say that the life
of those individuals afflicted with severe genetic or other disease is invalid.
Nothing could be further from the truth. The appropriate question to ask
an afflicted person is, if they were to have a child, and with the current
technology available, would they want to be able to choose to have a baby
free from the disease that they themselves have suffered? Legislation is
necessary to regulate research on embryos, to protect both the integrity
of reproductive medicine and, in these days of legal action, to protect
the scientist and clinician. Legislation should also allay the fears of
the public. When we cannot understand the technology that is meant to serve
us, we begin to fear it and its possible side effects. Although some people
may be prepared to agree to embryo research because they see benefits in
specific areas, such as infertility and genetic disease, they are uneasy
because they fear that reproductive technology may take more sinister directions,
such as genetic interference in the embryo. However, there would be no purpose
in attempting to genetically cure embryos diagnosed as diseased, because
the diagnostic test itself would simultaneously identify the normal embryos
without the defect. Characteristics influenced by many genes, such as intelligence,
appearance or ability, cannot be identified or genetically engineered in
the human embryo. In addition, the licensing authority and ethical committees
would ban genetic interference as well as selection of embryos on trivial
grounds, either by preimplantation, or by the current procedures of postimplantation,
diagnosis.

An absolute ban on research may allay the fears of the public on the
issue of the ‘slippery slope’, assuring that we do not get onto the slope
in the first place. But why should we assume that the new technology would
be used for ill, when it could be used to so much good? To use knowledge
wisely is our task; to apply our conscious reasoning, rather than habitual
response and unevaluated intuitive feelings, to the issues in hand. To do
this we are obliged to keep ourselves informed of the advances in technology
and medical expertise that ultimately affect all of us.

The argument for an absolute ban on research is also a valid position
for those whose religious or moral conviction is that a human being begins
at conception and that the conceptus must be protected at all costs. In
the case of severe genetic disease, the quality of the life of the human
being who eventually emerges is the issue – whether the degree of suffering
entailed must be endured. If preimplantation diagnosis becomes a reality
in the future, it will be only as one of several available options for the
couples concerned, and the decision will be theirs alone. With further advances
in the application of this new technology, a couple at risk may prefer to
start a pregnancy with an embryo free from the disease, rather than anxiously
wait for the results of a later prenatal diagnosis during pregnancy, and
face abortion if the test shows that the fetus is affected.

Dr Marilyn Monk is a senior research scientist at the Medical Research
Council’s Mammalian Development Unit in London.