There are more than 6,000 serious diseases that can be passed on from parents to children. When a family member is a carrier of one of these hereditary genetic alterations, there is an increased risk of another family member having a child with the same disease, even if both parents are completely healthy. However, there is a technique that eliminates this risk: the Preimplantation Genetic Diagnosis (PGD).
What is Preimplantation Genetic Diagnosis?
PGD is a laboratory technique whereby the DNA of embryos is analysed to ensure that they are free of genetic alterations, whether hereditary diseases or chromosomal abnormalities.
To achieve this, embryos are generated from the couple (sometimes directly from the eggs) by means of a conventional In Vitro Fertilisation (IVF) cycle. Once the embryos have been obtained, their genetic load is analysed using PGD to determine which of them would have the disease and which would not. Subsequently, only those that do not present the disease are transferred to the woman. In this way, the children are born without the risk of contracting the diseases that their parents or some of their relatives have.
What is the procedure for Preimplantation Genetic Diagnosis or PGD?
To do PGD on embryos, the following steps are carried out:
- Step 1. Genetic study on the parents. This involves an analysis of the progenitors to detect the diseases that must be prevented from reproducing in the embryos.
- Step 2. Genetic study of the embryo. After in vitro fertilisation, and before transfer to the uterus, the embryo’s genetic material is studied to detect genetic alterations. Each embryo is biopsied and those with a specific congenital disease are ruled out.
- Step 3. Transfer. Between one and two healthy embryos are transferred. Healthy embryos can be frozen.
What are the benefits of using this technique?
Current Spanish regulations allow the use of PGD when certain requirements are met. Thus, it is provided for in the case of serious, early onset and non-curable diseases (such as cystic fibrosis or polycystic kidney disease). The use of this procedure is also permitted when there is a factor that reduces the viability of the embryo, for example, the possibility of the embryo reaching full term once it has been transferred to the mother’s uterus, such as alterations in the number of chromosomes.
As well as helping people with hereditary diseases, PGD has also been a great advance for healthy people who have certain infertility problems. This is the case of couples in which repeated miscarriages occur, because, even though they have no problems conceiving, one of the parents carries a genetic alteration that results in a higher rate of miscarriage.
What types of tests are there and what diseases does it detect?
PGD can be carried out in two different ways, using either embryos or eggs.
On the one hand, PGD on embryos is performed when the embryos are preferably at the blastocyst stage (which occurs between days five and six after fertilisation) or at the 6-8 cell stage, depending on embryo quality and quantity. To study their genetic material, each embryo is biopsied, and those with genetic disease are ruled out and only healthy embryos are selected for transfer to the uterus.
On the other hand, PGD using eggs makes it possible to detect genetic or chromosomal diseases in the egg, before the embryo is formed. Only a part of the egg is analysed, the so-called polar body, so it can only detect hereditary pathologies on the mother’s side. A biopsy is performed, and once the polar body is removed, the oocytes are inseminated by the intracytoplasmic microinjection of a spermatozoon (ICSI). After two days, the genetic result is obtained and embryos from healthy oocytes are selected for transfer.
Through this test, genetic anomalies can be detected, but also diseases of chromosomal origin. Among the former are achondroplasia; Huntington’s disease; Neurofibromatosis type 1 and 2; Retinitis Pigmentosa; Lynch syndrome; Spinal Muscular Atrophy; Alpha-thalassaemia; Beta-thalassaemia; Gaucher disease; Cystic Fibrosis; Rett syndrome; Haemophilia; Alport syndrome.
Preimplantation Genetic Diagnosis or PGD is a test that analyses the DNA of the embryos (sometimes only the eggs) in order to ensure that they have the correct genetic load.
For this purpose, the In Vitro Fertilisation technique is used so that, once the embryos have been obtained, they are analysed to see if any of them are carriers of a genetic or chromosomal disease and only those that do not present the disease are transferred to the woman.
This test can be used to detect genetic anomalies, namely those that appear in embryos after fertilisation (achondroplasia; Huntington’s disease; Alpha-thalassaemia; Beta-thalassaemia; Cystic Fibrosis; Rett Syndrome, among others); as well as chromosomal diseases, that is, those that are passed on from parents to children (Down Syndrome or Turner’s Syndrome).
In addition, PGD has meant a major breakthrough for healthy people, but who have certain infertility problems, for example, for couples suffering from recurrent miscarriages.