PGD – Preimplantation Genetic Diagnosis
- Preimplantation testing: A general term which describes the removal of cells from an egg or embryo and subsequent testing.
- Preimplantation genetic diagnosis: This term applies when one or both genetic parents carry a gene mutation or a balanced chromosomal rearrangement and testing is performed to determine whether that specific mutation or an unbalanced chromosome has been transmitted to the egg or embryo.
- Preimplantation screening: This term applies when the genetic parents are known or presumed to be chromosomally normal and their embryos are screened for aneuploidy (having too many or too few chromosomes).
How does PGD work?
The day after the ICSI is performed; the injected eggs are inspected under the microscope to determine which have fertilized normally. The embryologist looks for two features of a normally fertilized egg: two pronuclei and two polar bodies. The pronuclei represent the chromosomes that came from the sperm and the egg and which now make up the chromosomes of the newly formed embryo. The polar bodies contain extra chromosomes that the egg got rid of.
Again, the embryo is returned to the incubator for further growth and development. By the fifth or sixth day after fertilization, the embryo should have reached a stage of development called the blastocyst stage. A blastocyst is an embryo which has divided into about 100 cells. The cells have divided into two groups: the inner cell mass is a small clump of cells which go on to the form the fetus and the trophoblast which makes up the majority of the sphere that comprises the blastocyst. The trophoblast cells may develop into non-fetal tissue such as the placenta or amnionic sac. Trophoblast cells may be removed in a process called a trophoblast biopsy. This represents the third opportunity to obtain genetic information from the embryo.At any of the above stages, the cells that were removed from the developing embryo and specifically the genetic material inside of the cells can be tested for various abnormalities or characteristics. This information can be used to select which embryos to place into the uterus.The most common type of preimplantation testing we do is to look at the number of each type of chromosome present. This is called aneuploidy testing. Any couple that is having in vitro fertilization is a potential candidate for aneuploidy testing. In all women, some percentages of the embryos are going to be chromosomally abnormal. In the past, the method used to test embryos for chromosome abnormalities was known as FISH or flourescent in-situ hybridization. While FISH is still used today for some indications, it is not the best method to assess for numeric chromosome abnormalities. The primary reason is that the number of chromosomes that can be tested using FISH is limited.
More recently, new technologies have been developed that allow testing of all 24 chromosomes in an embryo. One method is called CGH or comparative genomic hybridization. CGH can by performed on a specially designed chip and analyzed with a computer to interpret far more information than could be accomplished with the human eye alone.We can improve the chance for pregnancy and reduce the risk for miscarriage with this type of preimplantation testing.A small percentage of couples who have a problem with recurrent miscarriage may themselves have a chromosome abnormality known as a translocation. This is a structural abnormality that occurs between two chromosomes. Preimplantation testing can also be used to identify embryos with translocations.
There are other types of problems that can be detected in embryos also. We can perform true preimplantation genetic diagnosis. That is, identify embryos with certain genes or genetic mutations.
One of the more controversial procedures we have performed is testing embryos to determine whether they are tissue matched to siblings that may be suffering from diseases that could be cured with a bone marrow or stem cell transplant. Another controversial procedure is testing embryos to determine their gender so that a couple can have a child of a particular sex. This is known as gender selection.
Frequently Asked Questions About PGD
When FISH was the technology used to determine the number of each chromosome in the embryo, we were only able to look at a maximum of nine chromosomes. Since an embryo has 23 pairs of chromosomes, it means that we weren’t testing the remaining 14. Even though we chose the nine that we did test very carefully we were missing chromosome abnormalities about 10-15% of the time.
With CGH microarrays, we are able to test all of the chromosomes.
The timing of the embryo biopsy is also an important determinant of whether an abnormality may be missed. Polar body biopsies are highly accurate for diagnosing abnormalities that came from the egg which make up the vast majority of the numeric chromosome abnormalities but will miss the few that come from the sperm.
Blastomere biopsies can miss abnormalities becomes of a problem known as mosaicism. Mosaicism is when some of the cells of an embryo have different chromosomes than other cells. For exampe, if you remove a blastomere from an 8 cell embryo which is different from the other 7 cells, you may miss an abnormality.
Trophoblast biopsies remove several cells which can be compared. Mosaicism can therefore be detected and accounted for. The amount of abnormalities that are missed with trophoblast biopsy and CGH microarray is very very small.
When an embryo starts dividing, each of the daughter cells is supposed to be identical to the parent cell. Sometimes, however, the embryo can make a mistake. One cell from an eight cell embryo may be slightly different than the remaining seven cells. This is called mosaicism. Mosaicism can affect the results of PGD or PGS especially when blastomere biopsy is performed.
Remember, during a blastomere biopsy, one cell in an eight cell embryo is removed and tested. It is assumed that this cell is going to be representative of the entire embryo. If it is not, then a misdiagnosis can result. Trophoblast biopsy will reduce the chance for misdiagnosis due to mosaicism.
Preimplantation genetic diagnosis using FISH has been shown in our studies and in studies from other medical groups to increase the chance for pregnancy and reduce the risk of miscarriage in women who are 37 or older. Several larger studies using FISH, however, failed to find a benefit.
Don’t look for this to change any time soon. Although we have a law in Illinois which requires most employers to cover infertility, it took a great deal of effort to get that law passed. Even then, it was passed with some major loopholes that allow some employers to deny coverage to infertile couples.
PGD is a much more controversial technology than IVF. It can be used for things such as gender selection and selection of embryos for tissue typing. Many people do not believe that these technologies should be allowed. Because of this, there are not likely to be politicians that are going to be willing to back a measure that will require employers to cover PGD.
- Abnormalities in chromosome number to improve the IVF pregnancy rate and decrease miscarriage risk
- Abnormalities in chromosome structure to reduce the risk for recurrent miscarriage
- Genetic mutations
- Hereditary Cancer
- PGD for miscarriage
- Tissue matching (HLA typing)
- Gender Selection
Preimplantation Genetic Diagnosis: PGD Laboratory Techniques
- Flourescent in-situ hybridization – FISH
- Polymerase Chain Reaction – PCR
- Comparative Genomic Hybridization – CGH
Preimplantation Genetic Diagnosis: PGD Pictures of normal embryos
Preimplantation Genetic Diagnosis: PGD Pictures of abnormal embryos
- Blastomere: FISH: Trisomy 21 (Missing)