PGD - Preimplantation Genetic Diagnosis
PGD or preimplantation genetic diagnosis is sometimes also referred to as preimplantation testing. This is probably a more accurate description since we do many other types of testing besides genetic testing. Recently, the Society for Assisted Reproductive Technologies and the American Society for Reproductive Medicine recommended the use of the following terms:
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.
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?
PGD is made possible through the use of IVF (in vitro fertilization). In short, a woman is first given fertility drugs to stimulate the development of multiple eggs in her ovaries. She is monitored during this time with blood tests and ultrasounds. At the appropriate time, the eggs are removed in a process known as an (oocyte) egg retrieval. Once the eggs are removed, they are inspected under the microscope to determine which eggs are mature and normal appearing. Each of these eggs will then have a single sperm injected into them. This process is called ICSI (intracytoplasmic sperm injection) .
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.
At this time, the embryologist can perform a polar body biopsy or removal of the polar bodies from the embryo. The fertilized eggs (without the polar bodies) are then placed back into the incubator and allowed to develop. Two days later, the embryos are removed and inspected. We hope to see embryos have reached the 8 cell stage. These cells are called blastomeres. In a normal embryo, each one of these blastomeres should contain identical chromosomal information. This is the second point at which genetic material can be removed from the embryo. Removal of a single cell at this point is in the process is called a blastomere biopsy.
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 gentic 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
Q) Does performing an embryo biopsy for PGD damage an embryo?
A) In the right hands...No. We have now studied thousands of embryos. Compared to IVF without PGD, embryos that have a polar body biopsy or embryo biopsy develop in a similar way. For example, when we compare the percentage of eggs that achieve normal fertilization, the PGD embryos which had a polar body biopsy had a normal fertilization rate of 78%. The eggs that did not have a PGD biopsy had a normal fertilization rate of 76%. Cleavage rate is a measure of the percentage of fertilized eggs that go on to start dividing. PGD embryos have a cleavage rate of 96%, compared to embryos without PGD which divide 95% of the time.
A very important quality measure is how often a fertilized egg will become a blastocyst. PGD embryos will develop into blastocysts about 40% of the time. Non-PGD embryos become blastocysts slightly more often at 47%, a difference that is not statistically different.
Rarely, an embryo can be damaged by the biopsy procedure itself. If this occurs, it can be identified right away by viewing it under a microscope.
Q) I'm under age 35, so my embryos won't have chromosome abnormalities, right?
A) Wrong. We and others have studied women of different age groups and have found that even younger women have abnormal embryos. We could identify that in women under 35, 40% of the embryos tested were abnormal (too many or too few chromosomes). This does not mean that all women will have 40% of their embryos abnormal. Some might have a higher abnormality rate and some lower. Overall, it averages out to 40% at that age. We have seen some younger women with recurrent IVF failure have abnormality rates over 90%
Q) What are the chances that an abnormal embryo is going to be missed by PGD?
A) Remember that an embryo can have many different types of abnormalities. Preimplantation genetic diagnosis is only going to test for a specific type of abnormality. For instance, testing to determine if an embryo will produce a baby with Down's syndrome (caused by three copies of chromosome 21) will not rule out the possibility that the embryo also has a gene mutation that would cause the baby to have cystic fibrosis.
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.
Q) What are the chances that an embryo will be diagnosed as abnormal when it is really ok?
A) 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.
Studies which used different technologies to test the same embryos have on occasion shown different results. The percentage of these discrepancies is low.
Q) Will PGD increase my chances of having a baby?
A) The answer is as yet unknown.
Ordinarily, older women have a lower pregnancy rate and a higher miscarriage rate. This is true even when performing fertility treatments such as IVF. With each year, the pregnancy rate declines and miscarriage rate rises. Both problems are primarily due to the higher rate of chromosomally abnormal embryos that occur in older women. It makes sense that by finding and placing the normal embryos into the uterus, the chances are better that a delivery will occur.
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.
Several recent small studies have shown a benefit to PGD with CGH. These studies showed that using 23 chromosome analysis of embryos eliminated the decrease in live birth rates associated with increased age. To date, there are no large scale studies which have compared live birth rates rates using 23 chromosome testing versus no testing.
It is possible that younger women may also benefit from PGD but since they have a lower percentage of abnormal embryos, the benefit is likely to be smaller. Therefore, a much larger number of women need to be studied in order to statistically prove an effect.
Q) Is it likely that my insurance will cover the cost of PGD?
A) It is very unlikely that PGD will be covered by your insurance. Most insurance companies still consider PGD to be experimental even though we have been doing PGD since the early 1990s..
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.
Polar Body Biopsy
The polar bodies can be removed after fertilization and used for preimplantation testing.
After the embryo has divided into eight cells (blastomeres), one of the cells can be removed and used for preimplantation testing.
After the embryo has reached the blastocyst stage, several cells that are not part of the fetus can be removed and used for preimplantation testing.
Types of Preimplantation Testing
Recent PGD - preimplantation genetic diagnosis developments
03/01/2008 PGD may reduce miscarriage rate by 50%
05/21/2007 PGD analysis was used to find abnormalities in couples with recurrent implantation failure
01/26/2005 PGD is used to prevent Rh sensitization
|Last Updated ( Sunday, 03 November 2013 )|