Preimplantational Genetic Diagnosis

Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis (PGD) is the study of DNA from human embryos, with the intention of selecting those that meet certain characteristics and/or eliminating those that carry some type of birth defect. Thus, couples at risk of transmitting any abnormality or genetic disease to their offspring are able to continue the pregnancy knowing that the embryo should not be affected by this genetic abnormality.

What is the legal framework?

The legal framework of PGD falls within the Law 14/2006 (March 26), on human assisted reproduction techniques, specifically article 12 which indicates when a PGD cycle is allowed.

What does PGD consist of?

To carry out PGD it is necessary to undergo IBF treatment to obtain a specific number of oocytes, which are fertilized using an intracytoplasmic spermatozoid injection technique (ICSI). The embryos obtained are cultured in the laboratory and on the third day post-fertilization, when the embryo consists of six to eight cells, the embryonic biopsy is performed. The biopsy consists of the extraction of one cell from each embryo to analyze its genetic content. Two techniques may be applied based on the genetic alteration to be determined: specifically FISH (fluorescent in situ hybridization) may be carried out to search for chromosome alterations or PCR (polymerase chain reaction) may be done if possible mutations of the genes which cause monogenic diseases are to be analyzed.

Once the results of PGD are obtained the embryos which are considered viable according to the preimplantational study and are therefore not affected by the alterations or genetic diseases studied will be transferred.

What are the current conditions for performing PGD?

The initial conditions for carrying out PGD have been extended over the years and a cycle of IVF-PGD is currently prescribed in the following cases:

1. Progenitors carrying chromosomal alterations or severe hereditary diseases.

a) Chromosomal alterations. These anomalies affect the structure of the chromosome in the linear arrangement of the genes. It aims to reduce the risk of generating chromosomally unbalanced embryos.

b) Monogenic diseases. Hereditary diseases are caused by mutation or alteration of the DNA sequence of a single gene. Some of the most important are Huntingdon’s disease, cystic fibrosis, fragile X syndrome and Steinert’s disease

2. In very selected cases PGDF may be used for the study of possible alterations in the number of chromosomes (aneuploidies) of embryos of patients with repeated miscarriages or in patients with repeated implantation failure, that is, in patients who become pregnant but in whom the pregnancy is repeatedly interrupted in a very early phase. In these cases all of the chromosomes that may be implicated in the most frequent chromosomal diseases will be studied (X,Y,13,15,16,27,18,21 and 22).

How reliable is the technique?

Over 90% reliability rates are reported. In any cases prenatal diagnosis studies are recommended during gestation with the aim of minimizing the possibility of a false negative result.