Preimplantation Genetic Diagnosis (PGD) is a technique for the detection of genetic abnormalities in the embryo, prior to its transfer into the woman’s uterus.
In some couples, this technique is recommended to avoid the risk of transmitting a disease or genetic abnormality.
PGD is a procedure that is always implemented as a complement to In Vitro Fertilisation (IVF), whose purpose is to select embryos that are genetically free of the genetic abnormality being studied for their subsequent transfer into the woman’s uterus.
PGD in Spain is allowed but only within a well-defined legal framework, which means that not all cases can be directly accepted without a prior assessment. Example: in Spain the selection of the sex of the baby is not allowed unless there are strong medical grounds for doing so.
What kinds of PGD are there?
There are two types of PGD depending on the abnormality we want to study:
In the human species there are a very large number of genetic diseases that are caused by defects (mutations) in a single gene, which is what we call monogenic diseases. The genes are on the chromosomes, which are grouped in pairs (one comes from the father and one from the mother). This means that we normally have two copies of the same gene (one from the father and one from the mother).
Monogenic diseases have different patterns of inheritance:
- Recessive: TWO copies of the gene must be mutated for the disease to manifest itself. Both the father and the mother must be carriers of the mutated gene.
- Dominant: only ONE copy of the mutated gene is needed for the disease to manifest itself. If one of the two members of the couple is a carrier of the mutation, there is a risk of transmission for the offspring.
PGD for monogenic diseases allows us to identify disorders affecting certain genes.
Humans have a given number of chromosomes amounting to 46 in all. These are grouped in pairs and designated by a number. We have 2 copies of chromosome 1, 2 of chromosome 2…. and so on, up to 22 pairs, which are called autosomes, and 2 copies of sex chromosome XX for females and XY for males.
Any alteration in the number of chromosomes, whether due to too many or because there are too few, can give rise to a clinical disorder, whose severity depends mainly on the chromosome or chromosomes involved. PGD allows us to detect alterations in the number of chromosomes.
There are people who have a change in the structure or arrangement thereof, which is what we call TRANSLOCATION. Translocations can involve a whole chromosome (Robertsonian Translocation) or a fragment of it ( Reciprocal Translocation).
When, in spite of this change, the correct number of chromosomes is maintained (46 in total), we are talking about a balanced Translocation, since, as there is no gain or loss of genetic material, there are no clinical problems.
The problem for carriers of a balanced translocation is that, when their genetic material mixes with that of their partners, it can give rise to unbalanced embryos, with a greater or lesser number of chromosomes.
Couples who are candidates for PGD must undergo an IVF-ICSI cycle, which includes all the stages of ovarian stimulation, oocyte retrieval and embryo development in vitro.
The embryo biopsy is performed 120 hours after the oocytes (gametes) have been retrieved, when the embryo is at the blastocyst stage. In some cases, PGD for monogenic diseases can be performed 72 hours after the embryo develops, with 5-10 cells. It consists of a biopsy of the embryo but without compromising its normal development. Once the biopsy has been carried out, the embryo is returned to the incubator where it will be kept under in vitro culture until the time of transfer.
The cell material obtained in the biopsy is processed for genetic analysis using the technique required in each case (FISH, CGH, array-based CGH, among other techniques). For diagnosis with CGH or array-based CGH, the cell is washed in sterile medium and deposited in a tube for subsequent genomic amplification.
The result of the genetic analysis is conveyed to the IVF laboratory by means of a detailed report a few days after the biopsy and a decision is made, in consultation with the couple in question, as to which embryos are to be transferred on the basis of the genetic constitution and the morphological characteristics for embryo viability.
The overall effectiveness of PGD depends on the effectiveness of the diagnostic method, the number of embryos available and their potential for implantation, the latter factor being closely linked to the age of the woman undergoing the procedure.
Pre-implantation Genetic Diagnosis is a well-established technique that makes it possible to eliminate the transmission of various genetic diseases from parents to children. It is carried out as part of an in vitro fertilisation cycle and through it, we can detect a damaged embryo before it is transferred to the woman’s uterus.
The process consists of a genetic study in which, after in vitro fertilisation and before transfer to the uterus, the embryo’s genetic material is studied to detect possible genetic alterations. Each embryo is then biopsied and those with a specific congenital disease are discarded. Finally, between one and two healthy embryos are transferred.
We at Eugin are experienced in working with this procedure, so we have mastered it and use it in those cases where it is needed or required by patients.
If you still have any questions about Pre-implantation Genetic Diagnosis, you can also have a look at our website to find out more: Ask the expert.