Preimplantation Genetic Testing (PGT) is a technique that has been around for almost 30 years – and for almost all this time it has been a topic for a hot debate. In this webinar, Dr Luca Gianaroli, Scientific Director of S.I.S.Me.R. (Italian Society for the Study of Reproductive Medicine), is explaining PGT in details, together with its possible results and the indications for such a test.
In medicine, there are two important concepts to differentiate between: a diagnosis, which means looking for a specific disease or a specific disorder, and a screening, meaning testing individuals in order to find out if there is something that is not working properly. Dr Luca Gianaroli thinks that such a differentiation is crucial in order to understand the idea of PGT properly.
Until a few years ago, there were two terms in common use in the assisted reproduction field: pre-implantation genetic diagnosis (PGD) – this was referred to when fertility specialist were looking for some specific genetic disorders; and PGS – when they were looking for chromosomal abnormalities that could impair the patient’s chance not only to get pregnant but also to carry the pregnancy to full term. Dr Gianaroli says that nowadays both of these definitions have been abandoned and there is only one term that is being used – namely, preimplantation genetic testing (PGT). PGT can be done for different reasons – and that’s why there are different types of it.
Preimplantation Genetic Testing for Aneuploidy Screening (PGT-A)
Aneuploidy (the opposite of euploidy) is the presence of an abnormal number of chromosomes in a cell. It can be due to oocyte and/or sperm abnormalities or by segregation errors during the early stages of the embryo development – even if both spermatozoa and oocyte were perfectly normal. Dr Gianaroli differentiates between three types of aneuploidies: monosomy (when only one copy of a chromosome is present), trisomy (when there are three copies of the same chromosome) and nullisomy ( when both chromosomes of the same number are not present at all). PGT-A is a procedure aimed at detecting aneuploidy (any potential extra or missing chromosomes in embryos) before the transfer. It is a screening procedure and – what has to be highlighted – it is not aimed at detecting a specific pathology.
PGT-A is generally indicated in all those cases when it is more difficult to get a patient pregnant via IVF or ICSI. However, from the clinical point of view, there are five categories in which performing PGT-A is especially beneficial.
- advanced maternal age,
- a severe male factor of infertility,
- high incidence of sperm chromosomal abnormalities (detected by the FISH test),
- repeated implantation failures and recurrent miscarriages.
Patients in all those categories are usually able to produce embryos but the majority of these embryos are chromosomally abnormal.
What dr Gianaroli stresses here is the fact that PGT-A is not going to increase the quality of the genetic material – it is only going to shorten the time to pregnancy thanks to selecting the best embryos to transfer. At the same time, it helps to exclude the embryos that are chromosomally abnormal and may either not implant or cause a miscarriage.
The way PGT-A is performed
The analysis aimed at detecting potential aneuploidy is done via biopsy. It can be done at the level of oocytes by removing the polar bodies or when the embryo is in day 3 of culture – by removing one or two blastomeres. It can also be performed at the stage of a blastocyst – then a few cells are being removed from the trophectoderm (the outer cell layer that surrounds the cell which will give rise to the embryo). Dr Luca Gianaroli says that recently a biopsy has also been done in a less invasive way – by removing the blastocoelic fluid instead of cells. The blastocoelic fluid contains several substances including DNA and thanks to the fact that it is located in the inner compartment of the blastocyst, in direct contact with trophectoderm and inner cell mass, it should be informative of the embryonic condition. The procedure of removing the blastocoelic fluid is much simpler and it does not involve any specific training for embryologists.
Out of the sample of the genetic material, DNA is extracted and amplified. It is repeatedly replicated and once there is sufficient amount of DNA, the aneuploidy screening is done. It is performed using array and next-generation sequencing (NGS) solutions which help to classify the embryo as either transferrable (with a normal number of chromosomes) or non-transferrable (with a chromosome loss or a chromosome gain). According to dr Gianaroli, no matter which sophisticated technique is used to perform PGT-A, the main aim remains the same: it is about excluding all the samples that have a very limited chance of implanting or – if they implant – they can cause a miscarriage or give rise to an affected individual.
Taking all the above into account, it is clear that Preimplantation Genetic Testing for Aneuploidy Screening (PGT-A) has one undoubted advantage: it increases the chances of pregnancy thanks to selecting the embryos which do not have chromosomal abnormalities. Furthermore, by avoiding the transfer of abnormal embryos, it is possible to reduce implantation failures and miscarriages, treatment cost and the time to pregnancy. Dr Gianaroli stresses that, independently of the performance of the IVF centre, the technique used and the risk of multiple pregnancies, if you have a control group in which PGT-A is not done, there are many more cycles needed to get the same results in terms of live births and deliveries.
Preimplantation Genetic Testing for Monogenic Diseases (PGT-M) or Structural Rearrangements (PGT-SR)
Dr Luca Gianaroli says that in case of PGT-M or PGT-SR we’re not talking about infertility anymore. It is a procedure aimed at detecting a specific disorder or a pathology – in this case single gene disorders or structural rearrangements. The former refers to hereditary diseases caused by the mutation of a single gene (e.g. cystic fibrosis, haemophilia, spastic paraparesis, Fragile-X, thalassemia, etc.) while the latter are chromosomal abnormalities involving a change in the structure of the native chromosome, that may generate gametes missing genetic material (e.g translocations). These abnormalities can cause infertility, miscarriages or hereditary diseases.
PGT-M works differently than PGT-A. Before performing the treatment, it is necessary to carry out a pre-clinical study of maternal and paternal DNA (either in the form of a blood sample or a buccal swab) aimed at increasing the accuracy of the subsequent analysis. Dr Gianaroli stresses that is possible to generate a specific probe with markers able to detect the genetic disease even in a very small amount of DNA collected during the embryo biopsy. Once DNA is amplified, the search for a specific mutation begins. When it is detected, the embryo that is affected by it is not being transferred.
Monogenic disorders can be divided into dominant and recessive. If they’re dominant, a mutation in a gene in a single chromosome is sufficient to cause the full-blown disease. If they’re recessive, the disease is full-blown only in case of a mutation of a gene in both copies of a chromosome. If the mutation is present only in one chromosome, the individual will be a carrier. During the DNA analysis, the embryo – just like in case of PGT-A – is classified either as transferable (when its karyotype is normal or it inherited a balanced form of the translocation) or non-transferrable (when there is a loss or gain of one or more chromosomes).
From this point of view, selecting embryos that do not have the mutation associated with a specific disease makes it possible to prevent its transmission to the offspring. Dr Gianaroli says that if we go back to the concept of aneuploidy and combine it with the search for a specific monogenic disorder, we will avoid transferring abnormal embryos and manage to prevent almost totally the birth of affected children and miscarriages as well as avert termination of pregnancies in case of transmission of the pathology.
According to dr Luca Gianaroli, if one considers PGT-A as a selection technique, they have to have a high number of embryos available for the analysis. Only in this way they can increase the likelihood of obtaining transferrable embryos. That’s why, sometimes more than one cell collection is required and – in order to achieve the goal – embryo banking strategies could be recommended.
In order to be able to undergo PGT treatment, a referral from a specialist is necessary. In case of the PGT-M/PGT-SR program, it is important to have a frank discussion with a geneticist who will provide a patient with all pros and cons of such a decision. Because, as dr Gianaroli stresses, none of the testing and diagnostic techniques gives us 100% of true results. However, thanks to sophisticated technologies available nowadays, PGT-A offers even up to 98-99% of accuracy. With such help and information, it can simply be easier for a couple to make the right reproductive choice and decide how they want to navigate further in their fertility journey.