PGD for Single Gene Disorders
Single gene disorders are genetic conditions caused by the alteration or mutation of a specific gene in the affected person’s DNA. Single gene disorders are heritable and often run in families. Individuals with a family history of a single gene disorder may be at risk for passing the condition onto their children. Examples of single gene disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, myotonic dystrophy, Duchenne and Becker muscular dystrophies, Fragile X syndrome and spinal muscular atrophy, to name a few.
What single gene disorders can be tested?
Below, we have listed the single gene disorders that PGD has been attempted or successfully completed by our Institute. If the disease you are interested in testing by PGD is not listed, please contact us or call 773-472-4900. In most cases, PGD is possible even for rare genetic conditions, if the gene and/or gene change involved is known.
In the Single Gene Disorders Tested listed below we provided, where possible, a link to a website with more information under "Disorder Tested", a link to a medical website under "Link", an "X" to indicate a "PGD Set up" has been done, an "X" to indicate a "PGD" cycle was complete, an "X" to indicate that a "Pregnancy" was achieved, and an "X" to indicate a "Healthy Baby" was born.
How can PGD for single gene disorders help my family?
PGD is able to distinguish between genetically normal and affected embryos. Currently, this is the only way to determine whether an embryo is affected with a genetic condition prior to pregnancy. PGD has been utilized by many couples wishing to greatly reduce the risk of having a child with a devastating genetic disorder.
How do we test for single gene disorders?
Generally, due to the limited amount of DNA that is available for study from (one cell) and due to the limited timeframe to obtain results, a special system for testing will be developed for each couple, before they begin their IVF cycle. This is called the PGD Setup. This often requires a blood sample from each partner and may require additional samples depending on the circumstances. Our genetic counselors will inform you of the samples required, the time required for the PGD setup, and the cost for the setup.
Once the setup is completed, PGD for single gene disorders can be performed by:
There are two basic types of preimplantation diagnosis - polar body biopsy analysis and embryo biopsy analysis. Both types of analysis have advantages and limitations. Polar body testing focuses on the maternal contribution, and is an earlier method of testing. Generally, polar body testing is performed in any case where the female partner of the couple carries the gene mutation of interest (she may have the condition or she may be a carrier of that condition). Embryo biopsy and analysis involves testing the embryo directly. Therefore, we can analyze both maternal and paternal genetic contributions. Embryo biopsy is performed in cases when the male partner carries the gene mutation of interest (he may have the condition or be a carrier of the condition). Embryo biopsy is performed after the embryo has formed and in some cases has a higher chance of misdiagnosis or error. In many cases, we analyze both polar bodies and a cell from the embryo (blastomere) in order to provide the highest level of accuracy possible. Our genetic counselors will review with you which approach our Institute believes to be the most beneficial to you.
The cell(s) that are biopsied (polar bodies and/or blastomeres) are analyzed using a technique called polymerase chain reaction or PCR. PCR allows the laboratory to use a small amount of DNA to obtain rapid and accurate results. In addition to testing for the gene change(s) or mutation(s) running in the family, our laboratory also will use DNA linked markers. Linked markers are used in a fashion similar to DNA fingerprinting and allow the laboratory several ways to determine which gene(s) have been inherited by each egg or embryo. We have found linked markers to greatly increase the accuracy of PGD as well as increase the chances to obtain a test result for each egg or embryo.
Intracytoplasmic sperm injection or ICSI is required when PGD is performed for single gene disorders. ICSI is a technique in which a single sperm is injected directly into each mature egg for the purpose of fertilization. ICSI was first introduced in 1990 as a fertility treatment for men with low sperm count or poor sperm quality. Prior to ICSI, each egg was flooded with many sperm in order for one sperm to fertilize the egg “naturally”. After fertilization, the remaining unused sperm (and sperm DNA) surrounded the egg and developing embryo. When performing PGD for single gene disorders, ICSI is required in order to greatly reduce the risk of sperm contamination of the cells tested. Without the use of ICSI, the laboratory may incorrectly test the excess sperm DNA, not the DNA from the egg or embryo, leading to misdiagnosis of the embryo.
What is the accuracy of the testing?
Typically, the accuracy for polar body analysis is between 95-98% and the accuracy of blastomere analysis (only) is between 90-98%. The accuracy of the testing will vary depending upon the genetic condition being studied, the method by which it is being studied, and the number of linked markers for the family. In addition, there will usually be a range of different accuracies for each case, as there are several different possible outcomes. It is important to keep in mind that the testing focuses in only on the condition being studied, and it is not possible to check for all genetic and/or chromosome problems at this early stage of embryo development.
Does PGD replace prenatal testing?
No, PGD does not replace prenatal testing, such as chorionic villus sampling or amniocentesis. PGD is a research-based test allowing for a similar diagnosis to those available by prenatal testing. However, prenatal testing is still recommended, as this is currently the standard-of-care. Our genetic counselors can discuss what prenatal testing options are available to you.
Next Steps
Please review our PGD information packet and pamphets and contact our
genetic counselors at 773-472-4900 or e-mail us with any questions or inquiries
regarding our PGD program.