PGD PGS: Preimplantation Genetic Diagnosis & Screening detects hereditary disease.
Both preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are laboratory procedures designed to allow testing of embryos prior to implantation so that couples can significantly lower their chance of having a child affected by certain genetic conditions. PGD applies when one or both of the genetic parents carry a genetic mutation (an abnormal change in a gene) and testing is performed to determine if a specific hereditary disease (from that gene mutation) has been transmitted to their embryo. PGS applies to couples who are known or presumed to be genetically normal but wish to screen their embryos for a genetic abnormality known as aneuploidy.
At Fertility Associates of Memphis and Memphis Fertility Lab, we perform PGD of human embryos to detect hereditary disease in couples with a known risk.
Under certain circumstances, we perform PGS to detect aneuploidy (abnormalities in chromosome material) in couples at risk. The first reports of preimplantation testing used successfully in conjunction with human in vitro fertilization (IVF) appeared in the early 1990’s. Since then, thousands of normal infants have been born through the use of advanced reproductive technology.
How is PGD PGS performed?
PGD/PGS requires patients to undergo a cycle of in vitro fertilization (IVF) in order to have embryos available for testing. Actively growing embryos have one or two of their cells, called blastomeres, safely removed (biopsied) in a microsurgical procedure. These blastomeres undergo genetic tests with the results known in one to two days. Only embryos with normal testing are transferred to the patient’s uterus to establish a pregnancy.
PGD PGS can be used to identify cystic fibrosis, Tay-Sachs disease, Duchenne muscular dystrophy, and sickle cell anemia, as well as X-chromosome-linked disorders.
PGD PGS can be used for identifying the mutation of specific genes. Examples include cystic fibrosis, Tay-Sachs disease, Duchenne muscular dystrophy, and sickle cell anemia, as well as X-chromosome-linked disorders. The method of analysis for gene disorders involves a technique that greatly multiplies the genetic code contained in a single blastomere, so that the specific gene(s) of interest may be detected. This technique is called polymerase chain reaction (PCR).
Aneuploidy occurs when there are an incorrect number of chromosomes. In humans we have 46 chromosomes arranged in 23 pairs. Some chromosomes are more likely than others to become “unpaired,” which can lead to early embryonic demise (chromosomes 13, 16, 18, and 22) or affect live births, such as Down syndrome (trisomy 21). Numerical mistakes in the sex chromosomes can lead to Turner’s syndrome (XO), or Klinefelter’s syndrome (XXY).
Checking for normal chromosome pairing is done using a technique called fluorescent in situ hybridization (FISH). Sometimes, chromosomes can break apart and attach to other chromosomes, a condition known as translocation.
Who can benefit from PGD, Preimplantation Genetic Diagnosis?
PGD testing is recommended for couples that have a family history of inherited disease. In these high-risk parents, testing may have to performed so that the specific gene mutation can be identified. PGD for aneuploidy is also offered for genetic parents who may have a rearrangement of their own chromosomes (translocation).
If we are a healthy couple, can PGS improve the chance of conceiving through IVF?
In human reproduction, even normal-appearing embryos frequently have aneuploidy. The transfer of such an embryo would result in a failure of implantation, miscarriage or the birth of a child with a chromosomal syndrome. Theoretically, PGS would greatly assist the embryologist in selecting the healthiest embryo(s) for transfer and therefore improve the success rate of IVF. Furthermore, the ability to identify “normal” embryos could allow transfer of just a single embryo avoiding the risk of multiple pregnancy.
However, PGS has been shown in studies to have some limitations. The technology does not allow the testing of all 23 pairs of chromosomes so it does not eliminate the aneuploidy risk. Second, the it appears that some of the cells in an embryo can be different from its’ neighboring cell (mosaicism). This situation would give an unreliable test result for that embryo. Lastly, removing cells can damage an embryo thus potentially decreasing the chance of implantation even in a healthy embryo.
For these reasons, most physicians only recommend PGS in couples with a unique risk for aneuploidy. For most healthy infertile couples, use of PGS is not currently recommended.
Fertility Associates of Memphis is dedicated, through state-of-the-art care, to providing the most advanced medical and surgical care to patients suffering from complex problems with infertility, recurrent pregnancy loss, gynecologic endocrine disorders, abnormal menstrual function, reproductive tract anomalies, and menopause. We offer a comprehensive and highly successful program for the care of the infertile couple. Our physicians are board-certified in reproductive endocrinology and infertility, the only Mid-South physicians who have achieved this qualification. For more information or to make an appointment, call 901-747-2229.