For genetic diseases

People who are carriers of a monogenic disease have a high risk of transmitting this condition to their offspring. The Preimplantational Genetic Test (PGT), which is the technique previously known as Preimplantational Genetic Diagnosis (PGD), makes it possible to have a child without transmitting the genetic disease.

At Institut Marquès, PGT is carried out using the Next Generation Sequencing (NGS) technique, analysing all the chromosomes of the embryo. This method, currently the most innovative applied in the genetic diagnosis of embryos, allows us to know if they have inherited the disease that has been studied, as well as analysing the chromosomal content simultaneously.

In order to evaluate each case in a personalised way and to give a better orientation to the couple about their prognosis, the best option is always to request a visit to our Biology Service.

What are monogenic diseases?

Monogenic diseases are those caused by the alteration or mutation of a specific gene of the affected person. Examples of monogenic diseases are cystic fibrosis, sickle cell anaemia, Tay Sachs disease, myotonic dystrophy or Duchenne muscular dystrophy, to name a few.

There are different types of monogenic diseases:

• Autosomal: The affected gene is located on non-sex chromosomes.
  • Autosomal recessive: 25% of the offspring are affected by the disease (e.g. cystic fibrosis, sickle cell anaemia, thalassaemias, phenylketonuria, metabolic aminoacidopathies, etc.).
  • Autosomal dominant: 50% of the offspring are affected by the disease (Huntington, myotonic dystrophy, Marfan, predisposition to cancer (neurofibromatosis type I, Von-Hippel Lindau, Li-Fraumeni, APC, etc.).
• Sex-linked: The affected gene is located on one of the sex chromosomes.
  • X-linked dominant: X-linked dominant traits are rare, but do exist. They occur in females who have only one copy of the gene and in males who have the mutated gene on the only X chromosome they have. Both sons and daughters of an affected mother have a 50% chance of being affected. Affected males, on the other hand, only pass the disease on to their daughters; their sons will be healthy.
  • X-linked recessive: In these cases, even if the female carries an abnormal gene, she will not have the disease, because the normal X chromosome will compensate for the abnormality. On the other hand, any male who receives the abnormal X chromosome will have the disease. Each male child born to a female carrier of an X-linked recessive disease has a 50% chance of inheriting the defective gene and thus developing the disease. Each of the daughters will have a 50% chance of inheriting the defective gene and being a carrier of the disease. Carriers usually have no symptoms of the disease, but may have an affected child. A man affected by an X-linked disease cannot pass the disease on to his sons, because he carries the Y chromosome, but he will pass it on to all his daughters. Example: haemophilia.

Who is interested in PGT?

All patients who are carriers of a genetic disease. It can be said that in practically all genetic diseases it is feasible to perform a PGT as long as the mutation, the associated gene and the chromosome where the gene is located are known. Examples of the most common are:

• Cystic fibrosis
• Beta thalassaemia
• Duchenne Muscular Dystrophy
• Fragile-X syndrome
• Haemophilia A and B
• Huntington’s disease
• Tay-Sachs
• Spinal muscular atrophy
• Sickle cell anaemia
• Myotonic dystrophy
• Gaucher disease
• Marfan syndrome
• Neurofibromatosis Type I
• Retinitis pigmentosa
• Etc.

How is Preimplantation Genetic Testing performed?

1. In Vitro Fertilisation to obtain the embryos.
2. Embryo biopsy. The biopsy can be performed during the third day of embryo development, or between the fifth and sixth day when the embryo has reached the blastocyst stage.
3. Diagnostic genetic analysis. The cell obtained by biopsy is processed for analysis and subjected to genetic study.
4. Embryo transfer. The result of the genetic analysis is transmitted to the assisted reproduction team by means of a detailed report and a decision is made as to which embryos will be transferred according to the PGT result and the morphological characteristics of embryo viability. If the biopsy is performed on the third day of development, the embryo transfer to the maternal uterus can be performed immediately after the analysis, normally 5 days after the recovery of the oocytes. In cases where the biopsy is performed between the fifth and sixth day of development, it will be necessary to freeze the embryos and schedule the transfer at a later date.

Can PGT damage the embryo?

It is a very sophisticated technique but performed by expert biologists does not adversely affect the development of the embryo.