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Introduction to PGD for IVF

This website is dedicated to providing a greater understanding of the use of PGD for IVF treatments. We assume that the reader already has a good understanding of IVF. If not, the following links provide an excellent explanation of IVF and a summary of IVF history to date. Read them and then return here.

According to the Practice Committee of the Society for Assisted Reproductive Technology and the Practice Committee of the American Society for Reproductive Medicine, the term "preimplantation genetic testing" describes the process of removing cells from an egg or embryo for the purpose of conducting genetic or chromosomal testing before the embryo is returned to the uterus (embryo transfer). The same committees define the term PGD or preimplantation genetic diagnosis to apply when testing for specific genetic mutations in embryos is performed. The last term described by the committees is preimplantation genetic screening. This term is used when testing is performed to determine the number of chromosomes in an embryo. This is also called aneuploidy testing.

Preimplantation Genetic Diagnosis

PGD can be of value in a number of disease states that are caused by genetic mutations. If the mother carries a genetic mutation, then the cells which must be removed from an in vitro fertilized embryo are the first and second polar body. On the other hand, when the father carries a genetic mutation, one of the blastomeres from a dividing embryo must be removed.

PGD Biopsy technique
The embryo is surrounded by a hard protein shell called the zona pellucida. In order to remove cells from an egg or embryo, an opneing must be made in this shell. This opening can be made with a small laser, a sharpened glass needle or by spraying an acidic solution on the zonae pellucida.
The polar bodies or blastomeres are then removed from the embryo by inserting a small pippete into the newly created hole and applying gentle suction. The cells can be easily separated from the remainder of the embryo. The embryo itself will continue to grow and develop in the same manner as it would have if the PGD biopsy had not been performed. However, in the case of blastomere biopsy, there is one less cell present.
For example, if one blastomere is removed when the embryo is at the eight cell stage, there will be only seven cells remaining. The next time the cells of the embryo divide, there will be 14 cells instead of 16 and so on. Therefore, an embryo that has undergone a biopsy for preimplantation genetic diagnosis may not be as developed by the 5th day of embryo culture when the embrytos are typically transferred into the uterus.
Methods of testing in PGD
Due to the fact that the amound of DNA obtained from a single polar body or blastomere is very small, PGD scientists must first utilize techniques to amplify the amount of material before PGD testing. PGD employs a technique known as PCR to copy the segment of DNA that contains the gene of interest.
Enzymes are used to cut the double standed DNA molecule at specific points. The two strands of DNA are then separated. Each individual strand of DNA is a mirror image of the other. With the two strands separated, each can be coaxed to produce its own mirror image. when completed, there are not two double strands that are idnetical to the first. This process is then repeated thousands of times to generate an exponential amount of DNA double strands containing the gene of interest. Genes with mutations will produce fragments of differeent sizes than normal genes. Fragment of different sizes can be separated and determined by a technique known as gel elctrophoresis.
Heteroduplex analysis is a technique in which single DNA strands from the patient and another individual who is known to be normal are mixed together. The presence of a gene mutation in the patient will produce bending of the double strand hbrid and this can also be detected through gel elctrophoresis.
FISH is a technique is another technique that can be used in preimplantation genetic diagnosis to detect situations in which their is missing or extra chromosome material in an egg or embryo. Once the cells have been removed from the embryo, they are fized to a glass slide in the PGD laboratory. The cell membrane is disrupted allowing the contents of the cell inclduing the DNA to be spread out on the slide. Small bits of DNA which are mirror images of the areas of interest can then be sprinkled onto the slide. These bits of DNA are called probes and they are attached to a chemical which gives off light of a certain color. The slide is rinsed and then observed under ultraviolet light on a microscope. If a point of light remains, the PGD technician knows that the genetic abberation is present. If there is no color, then the mutation is not present. Different colors can be used to look at different areas and different mutations at the same time.

Preimplantation genetic screening

Aneuploidy
Human beings inherit one set of chromosomes from each of their parents. Each set contains 23 chromosomes for a total of 46 chromosomes. There are two copies of chromosome 1, two copies of chromosome 2 and so on up to chromosome 22. The 23rd chromosome set in humans is called the sex chromosomes. In females, there are two "X" chromosomes" In males, there is one "X" chromosome and one "Y" chromosome. In the process of egg or sperm creation, there are occasionally errors which arise. These errors may result in having an extra or missing chromosome. The toal amount of chromosomes, therefore, will be some number higher or lower than 46. This is known as aneuploidy. Aneuploidy is the most common type of error found in embryos by using preimplantation genetic screening.
Aneuploidy in IVF embryos
If an IVF embryo is aneuploid, there are three possible outcomes if the embryo is transferred into the uterus. Most of the embryos will not implant at all and no pregnancy will be established. If the embryo does implant, the majority of those pregnnacies will end in miscarriage. In a few cases, however, an aneuploid embryo may result in the birth of a live born baby. For example, if an embryo has only one "X" chromosome, this can result in a baby with a problem called Turner's syndrome. An extra copy of chromosome #21 causes Down's Syndrome. There are a handful of other syndromes that are identified which result from aneuploidy of various specific chromosomes.
PGD for Aneuploidy in IVF cycles
PGD for aneuploidy in IVF cycles is perhaps the most commonly performed type of preimplantation testing in the world. This reflects the fact that aneuploidy is such a common problem. Couples attempting IVF may use PGD in order to help increase the chance for pregnancy, reduce the risk for miscarriage or reduce the risk of having a baby with one of the associated syndromes. Virtually all studies agree that preimplantion testing for aneuploidy will lessen the chance for miscarriage or birth defects. However, there is some disagreement over whether preimplantation screening will improve the overall pregnnacy rate. Some studies have shown a higher pregnancy rate particularly in women who are 37 years of age and older. Some studies have failed to find an improvement, however.

PGD for other indications

PGD can be used for other indications. One of the most popular is for gender selection. The gender of a baby is determined by the sex chromosomes. PGD, therefore, can distinguish the embryos which are males from those that are females with extremely high reliability. Thus, PGD is the most accurate method to influence the gender of a baby.

PGD can also be used to identify babies that are tissue matches for other individuals that may be in need of transplants to cure current diseases. Although controversial, this method has been proven to be very successful in curing several types of diseases.

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