Preimplantation Genetic Diagnosis
PGD for Sickle Cell Anemia
It is now possible for physicians to offer preimplantation genetic diagnosis (PGD) for sickle cell anemia to couples at risk of bearing a child with this disorder. The PGD procedure allows genetic testing to be performed on early embryos before implantation for the purpose of selecting only those embryos that are unaffected by abnormal sickle cell genes for uterine transfer.
Children’s Hospital of Wisconsin, Froedtert Hospital and the Medical College of Wisconsin offer PGD for sickle cell anemia.
Sickle Cell Anemia Disease and PGDSickle cell anemia is an autosomal recessive genetic disorder caused by a defect in the HBB gene, which codes for hemoglobin. Hemoglobin is the main substance in red blood cells and responsible for carrying oxygen from the air in the lungs to all parts of the body. Normal red blood cells contain a type of hemoglobin called hemoglobin A. These cells are flexible and can squeeze through tiny blood vessels and live about 120 days before they are replaced by new cells. People with sickle cell anemia make a different form of hemoglobin called hemoglobin S (S stands for sickle). Red blood cells containing mostly hemoglobin S live only about 10-20 days. Because the body cannot replace them fast enough, the blood is chronically short of red blood cells, a condition called anemia. These cells are stiff and their distorted shape prevents them from passing through tiny blood vessels, causing pain and allowing less blood and oxygen to reach parts of the body. Tissue and vital organs that do not receive normal blood and oxygen eventually become damaged.
In most cases both parents are carriers of the abnormal gene, having one normal gene and one abnormal gene. The presence of two abnormal genes is needed for sickle cell anemia. If each parent carries one sickle hemoglobin gene and one normal gene, each child has a 1 in 4 (25 %) chance of inheriting two defective genes and having sickle cell anemia, 1 in 4 (25 %) of having two normal genes and not having the disease, and a 50% chance of inheriting one normal and one abnormal gene and being an unaffected carrier like the parents. Some couples opt for prenatal diagnosis during pregnancy to determine whether a pregnancy is affected.
An alternative to prenatal testing for sickle cell anemia is now available using in vitro fertilization (IVF) with PGD to identify an unaffected embryo. In vitro fertilization (IVF) is the process of combining eggs and sperm to form embryos in the laboratory. IVF is often used to help couples with infertility have a child. Preimplantation genetic diagnosis is a procedure to analyze the genetic make-up of embryos formed through IVF for determination of disease status. Couples at risk for having a child with sickle cell anemia can use PGD to test for this disease. Based on this analysis, embryos that are not affected with sickle cell anemia can be selected for transfer to the uterus to establish a pregnancy.
PGD Process for the Purpose of Sickle Cell Anemia TestingParents and their affected child, when possible, will first undergo blood tests to determine whether the preimplantation genetic diagnosis will be feasible. If these tests show that PGD is possible, the parents will undergo an IVF cycle to form embryos.
The embryos created are sampled on the third day of development when they have approximately 6 to 8 cells. One or two cells are taken from each embryo for testing. The embryo is incubated until testing is complete. Each embryo has a 1 in 4 chance of being affected with sickle cell anemia. By testing these cells for abnormal sickle cell genes, only those embryos determined to be unaffected are selected for transfer back into a woman’s uterus. Thus, the chance to have an unaffected pregnancy is greatly increased.
Risks of PGDThe biopsy technique required to perform PGD has been in use since 1990. The likelihood of damage to an embryo during the removal of cells is low, but it can occur. This can result in a decreased chance of embryo survival. When compared to IVF without PGD, there is a somewhat decreased chance for embryo survival among embryos that have undergone PGD. IVF with PGD is not known to lead to an increased chance for congenital abnormalities, birth defects, mental retardation, or other abnormal development when compared to natural pregnancies. However, a normal pregnancy outcome is not guaranteed.
Overall, PGD is an accurate process for determining whether an embryo is affected with sickle cell anemia. There is a small risk of error or technical limitation that will prevent us from determining whether the sampled embryos have sickle cell anemia. Because of this and the fact that we do not test for other genetic conditions or chromosome problems, the option of testing early in pregnancy through prenatal diagnosis is available.
Preventive Measures During PGD CycleIt is important for couples undergoing PGD to alter their sexual activity during the PGD cycle. Couples must refrain from intercourse until the cycle is complete.
Cost of PGD
IVF with PGD is expensive. Insurance companies generally do not cover the cost of IVF with or without PGD. We will provide you with a detailed cost analysis for IVF and PGD at your request.
Requests for information regarding the Children’s Hospital of Wisconsin, Froedtert Hospital and the Medical College of Wisconsin PGD program and IVF can be obtained by contacting the Reproductive Medicine Clinic at 414-805-7370 or by using our Contact Us form. Information regarding PGD for the purpose of sickle cell anemia testing can be discussed in more detail with our geneticist or genetic counselor.
Date: Aug. 24, 2010
Last Review Date: June 27, 2011
Online Editor(s): Kimberly Cole