The pre-implantation genetic diagnosis (PGD) consists in genetically analyze the embryo before it is transferred to the uterus. The analysis can be either of the chromosomes or of the gens, and it aims to prevent the transfer of genetic or chromosomal disease carrier embryos.
How is it done?
The process starts with a biopsy, which consists in the removal of some cells of the embryo, without damage to its development. This cells are sent to a genetic lab to be analyzed, while the embryo stays at the fertilization lab waiting to be transferred to the uterus, in case the result turns positive.
PGD and PGS are forms of genetics diagnosis realized before the embryo implantation in the uterus. That requires that the patient`s eggs have been obtained through IVF process, so the generated embryo can be submitted to a genetic analysis. The embryo can be either maintained in culture or frozen until the conclusion of the genetic test, it is only transferred to the uterus if it is healthy.
PGD and PGS Indications
We can divide patients with indication to pre implantation genetic test in two groups:
1) Couples that need IVF for marital infertility treatment and do the embryonic genetic analysis to avoid the risk of having genetically modified children.
2) Couples who already have the risk of transmitting any genetic disease to their children and need to realize IVF to select healthy embryos.
Chromossomal anomalies, also known as aneuploidy, are the result of an altered number of chromosomes at the moment of the fusion between spermatozoon and egg genetic material. A well known example is the Down Syndrome, result of an extra copy of the chromosome 21. The abnormalities can also occur due to gain or loss of parts of any chromosome or to translocations, when a material piece in any chromosome is swapped with a piece of another one.
Genetic alterations are the ones transmitted from parents to children, through genes mutations that have information to certain diseases. The genetic tests, in these cases, can indentify if that certain embryo had received the modified gene from the mother or father, depending on who has the specific disease.
PGD and PGS in infertility treatment by IVF
Approximately 50% of human embryos have abnormalities in the number of chromosomes, and this is the main reason for embryo implantation fail. In other words, genetic cause is one of the most important factors of fail in assisted reproduction treatments.
One of the factors of marital infertility is advanced age of both partners. However, female age has a much more evident component. That occurs because women are already born with a specific quantity of eggs, which she’ll use during her whole life and, at the end of her reproductive age, most of the genetically normal eggs have already been consumed during each menstrual period. Women over 37 years old usually have some trouble to get pregnant and, women over 40 years old are very exposed to have children with chromosomal abnormalities. In these cases, PGD can be a good alternative to low the probabilities of aneuploidy children.
Lab tests used for the genetic analyses of the biopsied cells:
- Complete chromosomal screening by Array CGH or NGS
- Karyomapping (genetic map)
Array CGH uses fluorescence attached to a software and compares the whole patient’s genome with a reference one (know as normal), and identifies the differences between them, with a high resolution scale. The method permits the analysis of 23 pairs of chromosomes.
As well as Array CGH, Next Generation sequencing (NGS) is capable of detecting alterations in all chromosomes. It is a last generation method that analyzes and sequences the whole DNA of the cells.
Normal Male embryo analysis
Male embryo with Down Syndrome analysis