Preimplantation Genetic Diagnosis (PGD) and Screening (PGS)
Preimplantation genetic diagnosis (PGD) and Preimplantation Genetic Screening (PGS) allow screening of embryos for specific genetic diseases/chromosomes before the embryos are placed in the uterus. Transferring "screened embryos" lacking the genetic defect makes it extremely unlikely that the condition could be passed to the child.
PGD is generally used to mean that one or both members of the couple carries or has a known genetic mutation such as for cystic fibrosis, muscular dystrophy or other genetic diseases, or for an inherited chromosome condition such as a chromosome translocation or structural abnormality. On the other hand, PGS is used for couples who they themselves have normal or presumed normal chromosomes, but wish to prevent chromosomal abnormalities in their transferred embryos. Another term CCS, comprehensive chromosome screening, may be used as well for PGS.
Once the embryo reaches the six to eight cell stage, one or two cells can be removed removed (biopsied). Presently, state-of-the-art procedure calls for culturing embryos to day 5 (blastocyst stage), perform the biopsy of the trophectoderm, and vitrify the embryos. For single gene defects, the DNA is analyzed by making multiple copies of the gene of interest by a technique known as the polymerase chain reaction (PCR) as shown in the gel photo. Unaffected embryos are selected to be transferred to the uterus and/or cryopreserved for later use.
The PGD analysis that resulted in the first live birth from PGD for Tay-Sachs disease was performed at the Jones Institute. A healthy girl was born to a family who had previously lost a child to the disease.
PGS is used to evaluate the embryo cells for abnormal numbers of any chromosome (aneuploidy).
Sometimes the dividing cells do not equally distribute their chromosomal complement. This occurs more often as the age of the mother increases and is one of the reasons why fertility declines with increasing female age. While previous techniques could only screen for select chromosomes, newer technologies can provide a more complete chromosome assessment.
The above graph shows an example of the results for all chromosomes in a cell from a biopsied embryo. Normal results should be between the red/green lines. The two arrows indicate that this cell has extra chromosome material for Chromosome 7 and is missing one chromosome from the pair for Chromosome 10. Transfer of an embryo with these chromosome errors would not have resulted in a pregnancy/live birth.
Patients should discuss these options with their physicians. Patients who might be candidates for PGD/PGS include:
- Carriers of known genetic (inherited) diseases such as Cystic Fibrosis, hemophilia, Muscular Dystrophy, and others. If we know the genetic cause of the condition, then there is a high likelihood we can detect that in the embryo.
- Women over the age of 38
- Women who have had recurrent miscarriages
- Couples who have had previous conceptions with a chromosomal abnormality
- Couples who have had more than three IVF failures.
For more information on PGD/PGS, please discuss with your doctor or contact us at 757.446.7100