Preimplantation Genetic Diagnostics (PGD)
PGD is a relatively new, advanced method used to identify genetic defects of the embryo before implanting it into its prospective mother´s uterus. To this day, it is the only method that can prevent conceiving of a child with disability and, conversely, can assist in finding an embryo for a successful transfer without defects and diseases. PGD is the only option for families with poor genetic predispositions to prevent a transfer of serious genetic defects and diseases.
This method can be used only in conjunction with IVF (in-vitro fertilization) treatment within assisted reproduction. A high risk of congenital chromosomal aberration (Down syndrome, etc.) or other hereditary diseases of the couple´s child are both important indications for selecting this method. Chromosomal aberrations (Down syndrome, Edwards syndrome, Patau syndrome) or genetic mutations (mutations of single genes), such as mutations causing cystic fibrosis, thalassemia or fragile-X syndrome, sickle cell anemia, phenylketonuria, hemophilia, Alport syndrome , mutations of the tumor suppressor gene p53 or neurofibromatosis type I and similar defects are among the most regularly tested abnormalities. Sometimes, when a mother is a carrier of a disease that affects only boys (hemophilia A and B, daltonism, ectodermal dysplasia, fragile-X syndrome), the child’s gender needs to be determined. This also concerns couples that have undergone a long, unsuccessful assisted reproductive therapy (including unsuccessful IVF cycles) or repeated miscarriages. Current trend is to broaden the spectrum of the tested diseases. The PGD procedure requires the basic IVF protocol.
Material for the respective examination is acquired by biopsy of the following:
• pole element (typically in countries where direct invasion to the embryo is not legally allowed)
• blastomere (most frequent)
• blastocyst
This is followed by a moleculo-genetic and cytogenetic testing of the acquired material by the FISH or PCR method.
In the preimplantation genetic diagnosis (PGD), we are able to examine chromosomal abnormalities by the FISH method or genetic mutations by the PCR method.
The FISH method is the most frequently used method to determine the quantity of a chromosome or to show its sections. The embryo cell – blastomere – is fixated on a glass foundation for a microscopic examination after it has been safely released from the embryo. Then, the appropriate procedure (hybridization) is performed for simultaneous marking of several different types of chromosomes by fluorescent probes, which show the sections of a DNA characteristic for a given chromosome. Abnormalities of chromosomes 13, 16, 18, 21, 22, X, Y are usually examined but, lately, it has been possible to examine different chromosomes using the so called microchips.
In case of examining one particular gene with possible mutations, the Polymerase Chain Reaction (PCR) method is used, in which the DNA is isolated from the core of the blastomere and it is then thoroughly examined.
Injecting only the “healthy” embryos is the basic principle of the method. Embryos are considered “healthy” if no abnormalities are found by the PGD method. This does not exclude other, not tested abnormalities.
The PGD method is quite controversial and it is illegal in many states because of its direct invasion of an embryo.
Preimplantation genetic diagnosis is not covered by health insurance. Partial funding is available in some cases.