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How PGD works

Sioban SenGupta, UCL
I’ve been working in the field of pre- implantation genetic diagnosis or PGD. What is PGD? PGD is a means of testing embryos from couples who know that they have a genetic disease, that runs in their family. This allows them to select embryos without the disease and these embryos are transferred back into the female partner so that she can have a pregnancy is free from the disorder that is known to run in that family. This has become something that is used clinically throughout the world. Are we allowed to test for any disorder? No we are not. In the United Kingdom, we are governed by the Human Fertilization and Embryology Authority or the HFEA.
When a couple comes to us with a particular disorder we write a summary about what the disorder involves, how it affects the family how bad the condition can be, and we submit an application to the HFEA. The HFEA then pass on our application to clinical geneticists as well as lay members of the public and groups that work with individuals who suffer from the disorder and they discuss the application and decide whether at this moment in time, this is a reasonable disorder to test by PGD. In the UK a disorder has to be classified as something that is ‘serious.’ And defining what is serious is different from one person to another. Different people have different perspectives.
But also what I would like to point out, most importantly, that over time what is acceptable ten years ago may not be acceptable ten years in the future because we may have developed treatments. So we like to think of PGD as a transient type of treatment for specific disorders. And it should move on to treatment when treatment means become available. In our lab we have specialized in doing PGD for cancer predisposition. Genes such as BRCA1 and 2 that when mutated result in an increased risk of breast and ovarian cancer for example. But this risk is very high.
It is about 80%, because of that risk without PGD for example, people who have these mutations and these genes are offered prophylactic surgery to avoid developing cancer in those organs. So, until treatments are available that possibly are less aggressive, it is a life time worry to these families. And therefore this is a way of completely avoiding surgery and removing the mutation from the family. So those children also do not suffer the problems of carrying the mutations in these genes. With new technologies it is possible to screen oneself and find out if one is a carrier of various recessive disorders.
Obviously this will only manifest as a disease if the partner also is a carrier of that, same disorder and that both parents pass on the mutative copy to the embryo. But because the sequencing technology has become so readily available parents are contemplating screening themselves for a whole variety of disorders and then it becomes a worry that people will just want to remove any kind of carrier status from embryos that are being tested. Already as a scientist doing this work when we test embryos for recessive disorders, some embryos will be completely normal, i.e. free of the mutated copy of the gene. Other embryos will be carriers of a recessive disorder; only have one copy of the mutative gene.
So when there are two embryos like this, do you prioritise the completely normal embryo and do you put the carrier embryo in second place? What we do generally is to look at the quality of the embryo, how it looks and whichever embryo is most likely to implant and lead to a pregnancy will be selected. But, these problems arise the more you can test, the more you can see and the more you might, be biased against a carrier status. And what does that to us in evolutionary terms is difficult to assess.

Dr. Sioban SenGupta, Senior Lecturer at UCL, provides an overview of PGD.

PGD is a technology which allows genetic testing of an embryo prior to implantation and before pregnancy occurs. It is used in conjunction with IVF and allows only those embryos diagnosed as being free of a specific genetic disorder to be transferred into a woman for pregnancy.

Who might benefit from PGD?

Some couples are at risk of transmitting an inherited disease to their children – such as Cystic Fibrosis, Thalassaemia and Fanconi Anaemia.

Until PGD, if couples wished to avoid having a child with a life threatening or severely debilitating disorder, their options were not to become pregnant, or to become pregnant and undergo pre-natal diagnosis via either amniocentesis or chorionic villus sampling (CVS) at between 11 – 16 weeks of pregnancy.

If the fetus was affected then the couple had the difficult decision of whether or not to continue with the pregnancy. PGD is performed before implantation making it the earliest form of prenatal diagnosis before the couple embark on a pregnancy.

More controversially, there is now also carrier screening available, meaning that a woman carrying a risk gene (such as the cancer-causing BRCA mutation) could undergo PGD and have a successful pregnancy.

For your discussion: Take no more than 5 minutes and have a look online to see what kinds of offers fertility clinics have with regards to PGD. How many different abnormalities is currently screened for?

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