PGD gives couples known to be at high risk of having children affected by an inherited disease the option of using IVF, with embryo biopsy and single cell genetic analysis to selectively transfer unaffected embryos and avoid the possibility of terminating an affected pregnancy only diagnosed at a later stage. It was in April 1990 that Professor Lord Robert Winston and I reported the first pregnancies following PGD to identify the gender of embryos from several couples at risk of having children with an X-linked inherited disease (which typically only affect males) and, 20 years ago this month, the first twin girls following this procedure were born at Hammersmith Hospital, London.
Since then, thousands of children have been born following PGD treatment at hundreds of clinics worldwide and it was a pleasure to be reunited at the meeting with so many ex-students, friends and colleagues who have contributed to the development of PGD over the years and to celebrate what has been achieved. The pleasure was tinged with sadness, however, by the absence of Yury Verlinsky, one of the pioneers and most passionate advocates of PGD, who died last year, and whose achievements notably included the first successful attempts to use PGD to HLA (human leukocyte antigen) match (also known as tissue typing) embryos as potential umbilical cord blood stem cell donors for existing affected children in the family.
The history of PGD can in fact be traced back to the 1930s and has its origins in attempts to predetermine the gender of offspring in domestic species for agricultural purposes. Today this can be achieved by fluorescence activated cell sorting following labelling of sperm DNA, with both cow and human sperm, which enables the small percentage difference in DNA content between X and Y chromosome-bearing sperm cells to be distinguished. However, it was only in the late 1960s that Richard Gardner and Robert Edwards (who would later pioneer IVF) reported that cells could be biopsied from the outer cell layer of rabbit blastocysts and the gender identified by detecting the presence of an inactive X in the nucleus of cells from female embryos. Male and female embryos were then transferred to the womb and the gender was confirmed at birth. With remarkable prescience for the time, the authors were, I believe, the first to speculate that a similar approach with human embryos might be possible to avoid X-linked disease - though this would take another 20 years to achieve!
PGD could only become a clinical reality after the establishment of IVF as a treatment for infertility in the late 1970s and early 1980s and was particularly stimulated by the discovery of the polymerase chain reaction (PCR) for the amplification of short fragments of DNA. For the first time, it seemed that it might be possible to detect the mutations known to cause genetic disease in small numbers of cells removed from the early human embryo before implantation. Nevertheless, it would take almost five years of preliminary work, mainly in animal models, to establish the proof of principle. In the early days, amplification from single cells was plagued by errors caused by amplification failure from one or both parents' DNA or conversely amplification of contaminating DNA. But there have been huge technical advances and we are now able to amplify reliably many DNA fragments in a single reaction allowing diagnosis not only by mutation detection but also using multiple informative markers surrounding the relevant gene.
More recently, advances in amplifying the whole genome from single cells have opened up new opportunities to use genome wide analytical tools including microarrays of different kinds which promise to provide universal testing for diagnosis of virtually any genetic defect known to be carried by the parents as well as chromosomal abnormalities which affect the viability of the embryo. Ultimately it may even be possible to combine whole genome amplification with so called 'third generation' sequencing technologies to rapidly sequence the entire genome of single cells. If this ever becomes technically feasible, this will raise challenging medical, social and ethical issues.
Clinically, PGD is now well established in many centres worldwide with pregnancy rates broadly similar to those of infertile couples except where the proportion of suitable embryos for transfer is particularly low as, for example, in HLA matching cases in which the aim is also to exclude a single gene defect such as beta thalassaemia. Starting about ten years ago, efforts have been made through the ESHRE PGD Consortium to follow up pregnancy outcomes and the children born. Although there is a significant increase in prematurity, mainly associated with multiple births, the incidence of congenital abnormalities is not increased in comparison with children born following assisted conception generally. Paediatric assessment of the development of several hundred children has also been reassuring. The only significant difference in outcome reported so far is an increased incidence of still births in multiple pregnancies following PGD.
For me, the biggest disappointment after 20 years is that PGD still only contributes a small fraction to prenatal diagnosis generally. There are many reasons for this, of course, and there has been progress with, for example, increasing NHS funding in the UK. But given that pregnancy rates following IVF have almost doubled over the period, with rates in women under 35 reaching almost 50 per cent, and that diagnostic accuracy has been transformed, I really believe it is time that PGD is accepted into mainstream clinical genetics and proactively promoted as an effective strategy, particularly for younger couples.
Furthermore, with microarray-based tests now available to screen for hundreds of mutations causing a range of relatively common single gene defects, there is a real prospect of making an impact on the incidence of these diseases which could deliver substantial savings for health systems around the world compared with the cost of treating and caring for affected individuals in some cases over decades at a time of global recession.
Finally, low cost strategies for prevalent diseases, such as the one described by Mark Hughes at the meeting where testing for sickle cell disease could be provided for $75, could bring substantial benefits not just to affluent couples in Western countries but to developing countries where these conditions are a heavy burden on their health systems. So there are plenty of challenges for the next generation to tackle in the next 20 years!