Preimplantation Genetic Diagnosis (PGD)

Max Huxham
father of Helena and clinical scientist
© April 2003

What is PGD?

Preimplanation Genetic Diagnosis (PGD), first developed in the 1980's, is the application of genetic testing on a live embryo to determine the presence, absence or change in a specific gene or chromosome prior to the placement of the embryo in the womb.

The first step to PGD is the creation of embryos outside the body by in vitro fertilisation (IVF). These embryos are cultured until they are between 6-10 cells in size so that one or two cells can be removed (cell biopsy) for genetic analysis. The removal of these cells does not appear to be detrimental to the development of the embryo. The 'sampled' embryo can then be grown a little longer in culture to let it recover from the procedures and to allow time for the genetic testing of its removed cells to take place. The advances in genetic technology may mean that the amount of material required for diagnosis in the future may be reduced.

One of two techniques are then applied to the biopsy cells - flourescent in situ hybridisation (FISH) for chromosome disorders or - polymerase chain reaction (PCR) for single gene defects, such as SMA. PCR is used for the detection of genetic errors which occur at the level of the gene, so that the DNA from the single biopsy cell is incubated with molecules which bind specifically to the region of the DNA which has the gene sequences associated with the gene of interest (primers).

Using a soup of other complex molecules and enzymes, thousands of copies are made of the relevant section of DNA in the test tube so that sufficient quantity can be made for accurate analysis, in order to determine whether or not the single cell was at risk of the genetic disease. Embryos which do not carry the genetic disorder can then be transferred to the uterus in the hope that a normal pregnancy will develop. Further analysis later in the pregnancy can then be offered to the couple by sampling the placental material (chorionic villus sampling) of the developing conceptus to confirm the PGD analysis.

Current Use of PGD

Four centres in the UK are licensed to carry out PGD and one centre for the biopsy part of the procedure only. This includes Guy's & St Thomas' NHS Trust in London who have already applied the technique successfully to SMA families. More than 4000 PGD cycles have been performed worldwide, so the technique is no longer considered an experimental research technique, forming an important aspect of clinical assisted reproduction technology.

Some 40 centres worldwide have carried out PGD, although the four centres in Chicago, St Barnabas, Bologna and Brussels account for most PGD cycles. Other centres in Eastern Mediterranean, SE Asia and Australia are emerging as credible centres, and it is likely that as the demand increases, the technique will expand to others centres along with allied science, efficacy, safety and (in the UK) further legislation.

Interestingly, in the UK it is legal to apply for a license to carry out PGD for identification of recessive gene disease such as SMA, but the Human Fertilisation and Embryology Authority (HFEA) who regulate all embryological procedures in the UK cannot grant licenses for PGD use for susceptibility testing i.e. for individuals whose Mendelian characteristics make them more likely that average to get a serious disease later in life. This therefore might include late onset dominant forms of SMA.

Why use PGD?

Because it works! Individual families known to be carriers of genetic disorders are always faced with difficult decisions when contemplating the start or the continuation of family life. They can decide not to have a (another) child, they can opt to have a pregnancy without genetic testing (for SMA this is 1 in 4), or they can proceed with a pregnancy and opt for testing of the fetus using prenatal diagnosis (PND) with a view to requesting termination (within the provisions of current abortion legislation), or they can consider use of IVF in conjunction with PGD.

The financial costs of IVF & PGD (currently about £5-8K per cycle in the UK) to individual families is clearly large if no financial provision for such fertility services is made in their local Health Authority Region. The emotional costs to the family are also a fine balancing act - weighing the chances of a successful pregnancy against the probability of disappointment at each stage of the procedure for each route.

For PGD at Guy's & St Thomas' the overall pregnancy rate for PGD IVF has recently been quoted as 24% per cycle (60 couples, 20 different conditions). This equates to about the same pregnancy rate for IVF without PGD in the UK generally, and is roughly equal to the chances of the average couple getting pregnant if no attempt is made to reduce that chance!

Limitations of PGD

The nature of science means that there is a possibility of misdiagnosis, either because of the failure of the technique involved or because the biopsied material was not representative of the the embryo, or had become contaminated with non-embryonic material. Embryos can also be subject to mosaicism (each cell containing different genetic complements) so that the biopsied cell is apparently normal but the remaining cells of the embryo are affected with the gene defect.

The possibility of this sort of misdiagnosis is significantly reduced if more than one cell of each embryo is used for analysis. Some embryos may be damaged following the cell sampling procedure. Experience at two centres in the UK have shown that in approximately one third of PGD cases, only one embryo is diagnosed as suitable for transfer.

Current practice for IVF in the UK recommends that 2 embryos be replaced per cycle. This number maximises the chance of pregnancy and at the same time minimises the chance of multiple pregnancy. The chances of success via IVF are dependent on many factors including maternal age, previous fertility and the number of embryos available. If more than two unaffected embryos of sufficient quality are available in any one cycle, then such surplus embryos can be frozen for future attempts at a pregnancy, but the freezing process itself will reduce the number of subsequent embryos for replacement attempts.

It is important to realise that IVF is a physically and mentally demanding procedure for couples without any guarantee of success and should not be adopted lightly and without professional support.

Legal, ethical and research considerations

It has been suggested that using PGD and PND devalues those affected by the condition. Others maintain that affected and unaffected individuals have the same value and rights. Saying that it would be better for a child to be free of the disease does not necessarily reflect on attitudes towards those with the disease. The fear is that an increase in genetic testing and its availability via PGD might change people's attitudes towards disability by creating a climate where genetic disability is perceived as largely preventable. If you choose to utilise PGD to prevent SMA you might be perceived to support this notion.

For anyone who knows what it is like to care for a severely disabled child, the difference between having a child with and without a condition is not one of love and care of the child but about the impact that the extreme disability has on the family and affected child. What is more important is to allow them to make an informed choice of their own.

PND is currently available for couples at risk of a having a child with a serious genetic disorder. A legal framework for termination of the pregnancy is available if the couple so choose. PGD provides an opportunity to begin a pregnancy knowing that the probability of having an affected child is drastically reduced before it has a chance to implant in the uterus. This can be done within the legal framework of the HFEA and Welfare of the Child Act formulated for IVF procedures. Some couples may find the PGD route more acceptable than the PND route.

PGD techniques can also identify carriers of recessive conditions such as SMA. At present, the decision to replace a carrier embryo rests with the patient in consultation with the clinical team. Couples may therefore choose not to replace carrier embryos as part of their treatment. It may be found that their choice is limited only to replacement of carrier embryos. In addition, during PGD analysis, it is possible that of the unaffected embryos available all are affected by another unrelated disease. This makes the couples' choice very difficult indeed.

In all cases, the question arises whether it is right deliberately to cause a child to be born with a disability. If already pregnant, a woman found to be carrying a foetus with a known disability is not duty bound to ask for termination. In the case of PGD however, a pregnancy has not yet been established so the nature of the choice to be made is very different. The situation is complicated by the Welfare of the Child Act to which the clinicians and embryologists involved in treatment are bound. There is no legal obligation on the part of a woman to have an embryo replaced. Thus, can the welfare of the child principle ever be compatible with a decision to attempt pregnancy knowing that a child will be born with a genetic disorder.

What are the implications for carrier status? Although currently very difficult and impractical it may become possible to test for multiple disorders. Individual couples presently retain control over the use of their gametes (eggs and sperm) for IVF and PGD and give informed consent as to their fate and the tests that should be carried out. They also retain confidentiality on the use of all genetic information obtained.

As research into SMA continues, further issues arise relevant to PGD. If we are able to treat known affected embryos in vitro at an early age with for example agents which can permanently up-regulate SMN2 production in the embryo to alleviate the disease in the child, then accurate determination of carrier status become a priority. If we can treat the pregnant woman in the same way without unduly affecting either affected or unaffected embryos then PGD becomes irrelevant!

Currently, the choice is yours.