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Preimplantation Genetic Testing

Genetic Diseases

Preimplantation Genetic Testing (PGT) is a pre-implantation genetic diagnosis procedure that allows the evaluation of embryonic health in all couples undergoing in-vitro fertilisation, even if they do not have a genetic disease.

It also makes it possible to identify the presence of monogenic diseases or chromosomal alterations in embryos generated in-vitro by couples at high reproductive risk, prior to their implantation in the uterus.

Evolution

In the past, couples with a child or family member with a hereditary disease have had the following alternatives available to reduce their risk:

  1. decide against having children and opt for adoption;
  2. accept the risk;
  3. opt for the use of medically assisted reproduction (MAR) techniques using oocytes from a donor;
  4. undergo prenatal diagnosis.

Prenatal diagnosis involves the use of techniques such as chorionic villus sampling or amniocentesis, and allows the identification of genetic abnormalities within the first 10-16 weeks of pregnancy in those couples who risk transmitting a genetic disease to their children.

The two procedures involve the sampling of foetal cells from which DNA is extracted to perform a mutation analysis of specific genes and/or to determine the foetal karyotype.

Having an alternative to prenatal diagnosis is of great benefit to couples at genetic risk, as it prevents them from having to resort to abortion.

Prenatal diagnosis is not accepted certain sections of the population due to ethical, moral or religious objections to pregnancy termination.

The evolution of in-vitro fertilisation (IVF) techniques, and the possibility of obtaining gametic and embryonic cells that can be used for the diagnosis of genetic diseases, has made it possible to shift the time of diagnosis from the 'post-implantation' phase to the 'pre-implantation'phase.

What does the PGT involve?

This pre-implantation genetic diagnosis procedure requires an in-vitro fertilisation cycle to be performed.

Subsequently, the embryos obtained will undergo biopsy and subsequent genetic analysis.

At the CMR Clinic, as in most specialised clinics, the material on which the genetic examination is performed consists of several cells taken at the fifth/sixth day of in-vitro culture (trophectoderm) at the blastocyst stage.

This stage of embryonic development ensures greater accuracy of the genetic analysis, but also greater resistance of the embryo to the stress suffered during the procedure. The material collected will then be sent to the GENOMA molecular genetics laboratory in Rome, which will perform the analysis. Embryos which can be deemed normal upon genetic analysis, previously cryopreserved at the end of the biopsy, can then be transferred into the uterus.

The CMR centre has been performing PGT since 2013.

Chromosomal and monogenicalterations

During fertilisation, genetic material recombination processes take place, which in some cases can generate chromosome alterations.

Chromosomal alterations can be numerical (when there are more or fewer chromosomes than normal), or structural (when a portion of a specific chromosome is missing, if it is located on another, or if it has been inverted).

Sometimes the alteration may occur in a specific gene (monogenic), resulting in a particular disease that can be transmitted to offspring in different ways.

Applications and indications of the PGT

There are different forms of application of Preimplantation GeneticTesting:

PGT-A, Preimplantation Genetic Testing of Chromosomal Aneuploidies
In this case, the choice of embryos to be transferred into the uterus is based not only on their morphological appearance, but also on their (numerical) chromosomal framework, making it possible to transfer embryos that are found to be chromosomally normal upon genetic analysis and with a high potential of achieving birth.

Examples of disorders determined by chromosomal aneuploidies that can be identified with PGT- A are Down syndrome, which occurs when three chromosomes 21 (trisomy 21) are present instead of two, trisomy 18, trisomy 13, 45X (Turner syndrome), 47XXY (Klinefelter syndrome), 47XYY and 47XXX.

cariotipo normale test genetico preimpianto
normal karyotype
cariotipo normale test genetico preimpianto
normal karyotype

The PGT-A is accessible to all couples who wish to know the state of health of the embryos that will be transferred, and is also suitable for:

  • - Women in so-called advanced maternal age, i.e. aged between 36 and 43.
  • - Women who have had two or more miscarriages.
  • - Women who have had two or more previous failed in-vitro fertilisation cycles.
  • - Male factor: highly altered parameters in semen can be indicative of the presence of an altered chromosome number in the sperm.
  • - Couples with a previous pregnancy with an altered chromosome number.
  • - Couples carrying numerical abnormalities of sex chromosomes (X or Y).

PGT-M, Preimplantation Genetic Testing of Monogenic Abnormalities

It allows the detection, in embryos generated in-vitro from couples at high reproductive risk, of the alteration or mutation of a specific gene that would cause a monogenic or Mendelian disease in the couple's children.

To date, diagnostic protocols exist for more than 200 monogenic, autosomal dominant, recessive or X-chromosome-related diseases.

Very common genetic diseases in Italy, in which the PGT-M has a valid application are: Beta- Thalassaemia, Sickle Cell Anaemia, Haemophilia A and B, Duchenne-Becker Muscular Dystrophy, Myotonic Dystrophy, Marfan Syndrome, Cystic Fibrosis, Spinal Muscular Atrophy (SMA), Huntington's Chorea and X-Fragile Syndrome.

PGT-M is suitable in the following cases:

  • - Diagnosis of a genetic disease in one of the couple.
  • - Couples in which both members are healthy carriers of the same genetic condition.
  • - Family history of a genetic disease.
Albero trasmissione malattia Test Genetico Preimpianto
Disease transmission tree

PGT-SR, Preimplantation Genetic Testing for Structural Chromosome Abnormalities

It allows the detection of structural chromosome abnormalities caused by the incorrect breaking or joining of chromosome segments, ultimately leading disease.

There are many types of structural alterations: translocations, deletions, duplications, insertions, ring chromosomes or inversions.
These alterations may be balanced or unbalanced.

PGT-SR is suitable:
- For couples in which one member carries a structural chromosome alteration.

Test Genetico Preimpianto per anomalie strutturali