For some, the birth of a child with significant birth defects or a serious medical condition is the first indication that one or both of the parents may carry a genetic mutation that may result in a genetic disorder for their children. Other individuals may know they carry genetic mutations because of other affected family members. Some couples have been tested for common genetic mutations such as cystic fibrosis or sickle cell anemia, and are aware prior to conceiving a child that they are at risk of having a child with serious medical problems.
PGD is indicated for couples at risk for transmitting a specific genetic abnormality to their children. For carriers of autosomal (non-sex chromosome) dominant disorders, the risk that any given embryo may be affected is 50%, and for carriers of autosomal recessive disorders, the risk is 25%. For female carriers of X-linked disorders, the risk of having an affected embryo is 25% (half of male embryos).
For individuals who carry a known balanced chromosomal translocation, inversion, or other structural chromosomal rearrangement, there is increased risk that their eggs or sperm will have an unbalanced genetic composition resulting in excess or missing genetic material. This individual will appear “normal,” but an embryo derived after fertilization of an unbalanced sperm or egg with a partner’s normal one will have an unbalanced genetic composition. This can be identified in the egg or embryo using “telomeric probes” specifically designed for that individual patient, according to their unique abnormality. In this process, the couple undergoes In Vitro Fertilization (IVF) to create several early embryos. A single cell is removed from the embryo once it has achieved an 6 – 8-cell stage (blastomere). The nuclear material containing the DNA from this cell is then put through the DNA amplification process to make millions of copies of the gene that may contain the mutation. The amplified DNA is tested to determine whether or not the embryo from which the DNA was obtained carries the genetic mutation. In this manner, only unaffected embryos will be selected for implantation.
To remove cells from an oocyte or embryo, an opening in its shell-like exterior is created using a laser or acid solution. The cells are then extracted using a small suction pipette or by gently compressing the oocyte or embryo to extrude material through the opening. For maternally inherited mutations, genetic analysis can be performed on the oocyte cells removed three days after fertilization. The embryo then may remain in culture up to the blastocyst stage or be cryopreserved until the results of the genetic analysis are known.
Important things to keep in mind:
• Before PGD is performed, we encourage genetic counseling to ensure that you fully understand the risk for having an affected child, the impact of the disease on an affected child, and the limitations of available options that may help to avoid the birth of an affected child.
• PGD can reduce the risk for conceiving a child with a genetic abnormality carried by one or both parents if that abnormality can be identified with tests performed on a single cell.
• Prenatal diagnostic testing to confirm the results of PGD is strongly encouraged because the methods used for PGD have technical limitations that include the possibility for a false negative result.
PGS has been mostly used for women of advanced reproductive age undergoing IVF in order to increase implantation rates and reduce spontaneous abortions or trisomy. Aneuploidy (abnormal copy numbers of chromosomes) is the most common cause for early pregnancy failure which increases with maternal age. It can also be increased in chromosomally normal couples with recurrent early pregnancy loss or repeated failed IVF cycles despite the transfer of “high-quality” embryos.
PGS aneuploidy can be performed on the polar body removed from an oocyte or on one or more cells removed from an embryo. Only one or two blastomeres can be removed from an embryo without adversely affecting its developmental potential and the number of chromosome pairs from each nucleus that can be evaluated is limited (usually 9-11 at present). The specific chromosomes evaluated are chosen according to the patient’s prior history and include those involved in the most common aneuploidies identified in spontaneous miscarriages. Alternatively, the entire genome (all 23 chromosome pairs) can be amplified using a different technique which takes two to three days. Image-processing software is used to analyze the relative amounts of signals to determine the chromosome numbers and identify any structural chromosomal abnormalities. There are no specific indications for PGS since, by definition, it is performed in patients having no known chromosomal anomaly, mutation or other genetic abnormality. PGS has been proposed for patients at risk for having an increased prevalence of aneuploid embryos, including women of advanced maternal age and those with a history of recurrent early pregnancy loss, repeated IVF failure, or severe male factor infertility. PGS has been advocated for use in conjunction with IVF in efforts to identify and transfer only embryos with normal chromosomal arrangements.
Important things to keep in mind:
• Before PGS is performed, thorough education and counseling must be provided to ensure that patients fully understand the limitations of the technique, the risk of error, and the lack of evidence that PGS improves live-birth rates.
• Available evidence does not support the use of PGS as currently performed to improve live-birth rates in patients with advanced maternal age, previous implantation failure or in patients with recurrent pregnancy loss.
• Because the prevalence of aneuploidy is high in the embryos of patients with recurrent implantation failure, decisions concerning future treatment should not be based on the results of PGS in one or more cycles.
• Evidence does not support the use of PGS as currently performed to reduce miscarriage rates in patients with recurrent pregnancy loss related to aneuploidy.
What is preimplantation genetic diagnosis (PGD)?
PGD is a screening test that helps your doctor select the embryos that are most likely to result in a successful pregnancy and a healthy child. In PGD, embryos are screened for certain chromosome abnormalities prior to transfer into a woman’s uterus. Chromosome abnormalities may cause implantation failure, miscarriage or mental and physical problems in a child.
Who might be a candidate for PGD?
PGD may be helpful for: (1.) Women 35 years or older, (2.) Couples who have experienced recurrent pregnancy loss or IVF failure, (3.) Couples with one partner known to carry a balanced chromosome translocation. Translocations are chromosome abnormalities, which occur when chromosomes break and exchange fragments.
How is the sample for PGD obtained?
PGD can be performed on two sample types – blastomeres and polar bodies. Approximately three days after fertilization an embryo contains approximately six to eight cells, also known as blastomeres. A specially trained embryologist microsurgically removes one blastomere, places it on a slide and sends it to a specialized laboratory for testing. The results usually take 24 to 48 hours, allowing a day-five or day-six transfer of the embryo into the uterus. Alternatively, a sample from the egg, before it is fertilized by the sperm and starts to divide, called the polar body can be sent for PGD. Polar body testing can be done earlier than blastomere testing, but is used only when the mother is the carrier of a chromosome translocation. The polar body is a small bit of cytoplasm that is extruded from the egg prior to fertilization. Its release is part of the normal development of the egg and it contains a complementary set of chromosomes similar to that present in the egg. Polar body results tell whether or not the corresponding egg cell contains the chromosome translocation known to pose a risk to the embryo.
Does removing a single cell harm the future fetus?
Every cell of the embryo is totipotent until the fourth day after fertilization. Totipotency refers to a cell’s ability to become any type of cell, and eventually any type of tissue. Blastomere extraction is performed three days after fertilization or earlier, so at this point, the remaining cells can form a whole fetus. As current data indicates, removing a single cell from the day-three embryo does not appear to harm the fetus. However, sometimes additional cells must be removed in or to obtain an intact nucleus for analysis. Removal of additional cells may reduce the embryo’s ability to continue to develop.
How are the chromosomes tested?
A laboratory technique called fluorescence in situ hybridization (FISH) screens for numerical abnormalities of specific chromosomes. Commonly the laboratory tests chromosomes X, Y, 13, 15, 16, 17, 18, 21 and 22. This is based on trisomies arriving to term (X,Y,13,18,21), common in spontaneous abortions (16,22,15,21) and most common aneuploidies found in day 3 embryos (22,16,21,15,17). If the number is abnormal, this is called aneuploidy. In couples with known chromosome translocations, FISH may also be used to look for unbalanced translocations in the embryos. Aneuploidies and unbalanced translocations may result in pregnancy loss or the birth of a child with birth defects.
Does PGD detect all chromosome abnormalities?
PGD is approximately 90% accurate for detecting the most common aneuploidies. However, one-third of all chromosome abnormalities that can be found by chorionic villi sampling and amniocentesis are not detectable by PGD. In addition, PGD cannot be used to test all known genes or to pick certain traits such as blue eyes.