Preimplantation Genetic Diagnosis (PGD)
Some couples are at high risk for transmitting genetic diseases to their offspring. These “single gene” diseases include Tay Sach’s, Sickle Cell Anemia, Fragile X Syndrome, Cystic Fibrosis and others. An affected embryo from one of these couples will have the genetic abnormality present in its cells.
In addition, women over the age of 38 and women who have had recurrent miscarriages may be at a higher risk for chromosomal abnormalities called aneuploidies. This means that the embryo has an incorrect number of chromosomes. Embryos are analyzed following biopsy using preimplantation genetic diagnosis (PGD). As the name implies, PGD is “diagnosing the embryo prior to placing it into the uterus”.
When the embryo is 6-8 cells (approximately 72 hours after fertilization) a small hole is made in the zona pellucida (outer membrane) using a needle and one cell (a blastomere) is removed. Every cell in the body (or embryo) contains an individual’s entire genetic blueprint and the removal l of a single cell does not impede the further development of the embryo.
Analyses of the chromosomes from the blastomere are performed using fluorescent in situ hybridization (FISH) for aneuploidy detection and polymerase chain reaction (PCR) for single-gene defects. FISH allows the embryologist to count the number of chromosomes and identify certain specific abnormalities. The polymerase chain reaction allows examination of specific chromosome segments.
Some diseases, such as hemophilia, are “sex linked” disorders. Hemophilia is transmitted on the X chromosome (female) so a male child (XY) of a female carrier would have a high probability of having the disease. FISH can be used to separate the male and female embryos in these cases.
When an embryo is identified as abnormal it is not placed into the uterus. Much research is being conducted on the future uses of PGD and additional diseases that can be screened for will be discovered. Conditions such as recurrent miscarriage and multiple IVF failures may have a genetic basis that can some day be identified.
The Center for Reproductive Medicine is the only IVF laboratory in the Carolinas that can offer PGD services on-site.