Dr Melihan Bechir Managing Director, Fertilia Medlife, Romania
PGD is a way to determine whether an embryo is carrying the genetic material associated with a specific genetic disease. Patients with a family history of breast cancer, Tay-Sachs, cystic fibrosis or other kinds of inheritable genetic diseases are good candidates for this procedure.
This test looks at a particular, identifiable portion of DNA that makes up the gene for a particular disease. In order to do this test a patient, or the family member with the disease needs to have had genetic testing done in order for a “probe” to be made that will identify the specific section of DNA with the problem. This takes approximately 6 weeks. Therefore, the decision to do PGD must be made several months before the start of an IVF cycle. The biopsy is done either on the Polar Body/Polar Bodies or at cleavage stage and the analysis is traditionally done with FISH (Fluorescence In-Situ Hybridisation).
PGS can tell how many of each chromosome are in the cells of an embryo. Patients with a history of miscarriage, patients who are older and at higher risk for chromosomal problems, patients with a history of poor quality embryos during a previous IVF cycle, patients who are interested in a baby of a particular sex, or patients who have had a child with some type of chromosomal problem are good candidates for PGS. Down’s Syndrome, Turner’s Syndrome and Kleinfelter’s Syndrome are all problems associated with too many, or too few chromosomes. This testing is done using a method called CGH (Comparative Genomic Hybridization) array. It is not as specific as PGD and does not require advanced preparation of a probe.
The ability to test for these kinds of problems is a significant benefit for patients who have been through miscarriages, failed IVF cycles, or had family members affected by a serious genetic disease.
PGS and PGD can be combined, and will help determine genetic disease or chromosomal abnormalities. However, occasionally, there is also a chance that the procedure itself may have an adverse effect on the embryos.
It should be noted that the use of PGS and PGD may be restricted depending on legislation within a region, eg selecting embryos on the basis of gender for ‘family balancing’ reasons is illegal in the UK.
An embryo contains two polar bodies. The first polar body is within an oocyte and can be removed and tested.
Upon penetration of the egg by the sperm (fertilisation), but prior to the joining of the sperm’s genetic material with the egg’s genetic material, the second polar body is extruded. Although polar bodies should contain replica genetic information to the egg, they have no known function except to assist in cell division. They are simply “by-products” of the egg’s division. Once implantation occurs, the polar bodies disintegrate and are not part of the developing foetus.
Removal and genetic analysis of the polar bodies occurs on the first and second day after aspiration. In some instances, it is necessary to confirm a diagnosis made on polar body analysis by performing a blastomere biopsy. It is also possible that one or more polar bodies fail to provide a conclusive result. Polar body biopsies only provide the detection of maternally derived abnormalities.
Blastomere biopsy (for PGD/PGS) is a technique that is performed at the six to eight cell stage (about 72 hours or day 3 of embryo culture).
One or two cells, or blastomeres, are separated from the rest of the embryo for testing.
A hole is made, often with a laser, eg the Saturn 5™ Laser, in the zona pellucida which is the shell surrounding the developing embryo and the cell/s are gently removed. The developing embryo is then placed back into the culture media and returned to the incubator where it can resume its normal growth and development.
Trophectoderm biopsy is a relatively new technique often used for the genetic screening of embryos.
The technique involves taking cells from the outer layer of an embryo (that will form the placenta not the fetus) at day 5/6 of development. A blastocyst has 200-300 cells.
Some advantages are that, compared to the traditional method for biopsy (at the cleavage stage – day 3) more cells can be taken from the embryo to be analysed for the genetic condition, leading to a more reliable diagnosis and a smaller percentage of the embryonic mass taken from the cells which are not destined to form the fetus.