Preimplantation genetic diagnosis

All human cells except the sex cells, which are the ovum and the sperm, have a total of 46 chromosomes grouped in 23 pairs:

• 23 chromosomes from the father
• 23 coming from the mother.

Of these 23 pairs, 22 of them are autosomes, the non-sexual chromosomes, named from 1 to 22, and a pair of chromosomes are the sex chromosomes, XX in the case of a female and XY in the case of a male. When an alteration occurs in a gene, certain diseases can develop. This is where preimplantation genetic diagnosis is at stake.

What is the preimplantation genetic test?

Preimplantation genetic diagnosis (PGD) is the technique that allows study chromosomal and genetic alterations in the embryos before being implanted. The first step consists of a genetic study of the parents of the possible diseases to be diagnosed.

Subsequently, the couple must undergo a in vitro fertilization supplemented with sperm microinjection1 (ICSI). The embryos2 are left in culture in the laboratory and on the third day after fertilization, when the embryo is in a 6-10 cell stage, the embryo biopsy. This biopsy consists of the extraction of a cell from the embryo without affecting its later development.

How it is performed?

The genetic study is then carried out; can be applied two different techniques according to the genetic alteration to be diagnosed:

• FISH (fluorescent in situ hybridization): FISH is used to diagnose numerical or structural abnormalities and consists of the application of DNA probes that allow the number of copies of the chromosomes analyzed to be counted. There are probes for the detection of chromosomes 13, 15, 16, 17, 18, 21, 22 X and Y.
• PCR (polymerase chain reaction): PCR is a technique that, by amplifying DNA sequences, is able to detect an altered gene. This technique has a reliability of approximately 90%. In any case, it is recommended to carry out prenatal diagnostic studies during pregnancy to minimize the risk of an affected fetus as much as possible. The biopsy has a low possibility of damaging the embryo but it is never without risk so the viability of the embryo can be altered.

After the embryo biopsy, the embryo transfer, selecting them according to the results of the biopsy and their viability. Leftover embryos that have not been transferred can be frozen by slow freezing or vitrification for later transfer.

Who is recommended to do it?

This technique has specific indications, such as the couples carrying chromosomal abnormalities that suppose a high risk to generate chromosomally unbalanced embryos. It is also used in the cases of couples who are carriers of a genetic disease as long as the mutation, the associated gene and the chromosome where the gene is found are known. PGD ​​is also used in patients from IVF programs and with advanced maternal age (over 37 years), in men with abnormal sperm formation, in couples with repeated miscarriages or in couples with repeated implantation failures.

Current law allows the possibility of carrying out a PGD ​​cycle for the determination of histocompatibility antigens for therapeutic purposes for third parties. In other words, it can be applied in cases where a first-degree relative suffers from a serious illness that requires a transplant of compatible cord or bone marrow cells. For these cases, the express authorization of the corresponding health authority is required.

What is a chromosome like?

Chromosomes have thousands of tiny pieces of DNA that contain the genetic information, the so-called genes. As each chromosome has two copies, each gene also has two copies, each with information from each parent.

Chromosome abnormalities

Chromosomes they can be altered in number or shape. Numerical alterations consist of an abnormality in the number of copies of a chromosome, so instead of having two copies a chromosome can have one or three copies. The best known chromosomal disease is the one, in which there are three copies of chromosome 21 instead of two.

The structural abnormalities They are those in which a piece of chromosome is altered, it may well be absent as it has changed places (translocation).

What are monogenetic diseases?

Monogenic diseases are those caused by a failure (mutation) in a single gene. There are diseases dominantIn other words, the alteration of only one of the genes that encodes it (the one from the father or the mother) is enough to cause the disease. Dominant patients (the person who is ill has one healthy gene and one altered) can have healthy children if they pass on the healthy gene to them.

Other diseases are recessiveIn other words, they require the alteration of the two genes to develop. Recessive diseases can have healthy carriers, that is, people who they don't have the disease but one of the two chromosomes is altered. This people can transmit the disease to their offspring.