Preimplantation Genetic Diagnosis – Definition: What is PGD?
Preimplantation genetic diagnosis is a genetic testing method. Reproductive physicians perform it at a very early stage of development on the genetic material of an artificially conceived embryo.
A PGD may be used in cases of suspected …
- … a severe monogenic hereditary disease (mutation on one gene)
- … a chromosomal disorder: structural (translocation) or numerical (aneuploidy screening: mono-, nullo- or trisomy)
- … a sex-linked severe hereditary disease
Preimplantation genetic diagnosis in Germany
For example, preimplantation genetic diagnosis is only approved if there are serious hereditary diseases in the family and severe damage is considered likely. Even if you already have a child with a hereditary disease, have suffered a stillbirth or miscarriage in the past, or have a fertility disorder, you are one of the high-risk couples who are eligible for preimplantation genetic diagnosis.
Prerequisites for preimplantation genetic diagnosis:
- application and approval of the ethics committee
- medical/human genetic and psychological counseling
- Execution in a specialized, certified center
How does PGD work?
Before preimplantation genetic diagnosis can start on the embryo, human geneticists must develop a separate genetic testing procedure for each couple. This requires blood and DNA samples from the man and woman, and possibly from the couple’s existing children.
Blastomere biopsy
After four days in the Petri dish, the fertilized egg cell has reached the so-called eight-cell stage. These eight cells (blastomeres) are toti-/omnipotent cells. This means that, in principle, a separate embryo could develop from each of these cells. According to the Embryo Protection Act, this early biopsy for PGD is prohibited in Germany – but it is used in other countries.
Blastocyst biopsy
The cells of the blastocyst are arranged in an outer and inner cell layer. From the outer cells (trophoblasts), one to two pieces are taken for preimplantation diagnostics.
Despite improved culture media, only about 50 percent of artificially fertilized eggs reach the blastocyst stage.
Polar body examination
Actually, this method, which is intended to improve the success rate of IVF, belongs to prefertilization diagnostics rather than to preimplantation diagnostics:
Since the egg and sperm have not yet fused at the time of the polar bodies, strictly speaking fertilization has not yet taken place. With the removal of the polar bodies, polar body diagnostics thus circumvents the Embryo Protection Act and does not require the approval of the ethics committee.
Preimplantation diagnostics: procedure for genetic testing.
For preimplantation genetic diagnosis, genetic information (DNA) must be extracted from the embryonic nucleus and examined. Chromosomal and genetic alterations can be identified using the following techniques:
- Polymerase chain reaction (PCR): Amplification of individual genes/gene segments.
- Fluorescence in situ hybridization (FISH): labeling of several selected genes of a chromosome
PGD: Pros and Cons
Opponents and proponents have been debating the pros and cons, and especially the ethical concerns, of using preimplantation genetic diagnosis for years.
Pro PGD
- Fulfillment of the desire to have children for high-risk couples
- Preimplantation genetic diagnosis is physically and emotionally less stressful than abortion in the case of a severely damaged embryo / fetus.
- Preimplantation genetic diagnosis remains a well-controlled exception for high-risk couples (since regulated by law).
- Artificial insemination absolutely necessary, with all the associated risks
- High error rate of PGD: sorting out potentially healthy embryos, additional careful prenatal diagnostics (e.g. amniocentesis) necessary
- Great ethical responsibility: which diseases are serious (life worth living vs. life not worth living)? Danger of misuse and first step towards “designer baby”.
- Discrimination against people with disabilities
PGD: risks and complications
The probability of becoming pregnant after artificial insemination is also lower than with natural conception. If a pregnancy has occurred, couples are nevertheless recommended to undergo careful prenatal diagnostics (ultrasound, amniocentesis, umbilical cord puncture) due to the relatively high error rate of preimplantation diagnostics, with all the associated risks and consequences.
