Preimplantation Genetic Diagnosis (PGD)

The PGD program evaluates known carriers of specific single-gene defects, such as Cystic Fibrosis, and parents whose children to be are at increased risk for selected chromosomal abnormalities such as Trisomy 21/Down Syndrome, Turner Syndrome, and specific unbalanced translocations. By diagnosing and transferring embryos shown unaffected with the studied gene, this technology has enabled couples at a genetic risk to have healthy children.

PGD has been made possible through in vitro fertilization (IVF), where multiple eggs (oocytes) are matured and retrieved by ultrasound-directed follicle aspiration. The oocyte is inseminated with a single sperm by intracytoplasmic sperm injection (ICSI), then grown in culture until the six-to eight-cell stage. One or two cells of the developing pre-embryo are removed (biopsied). The DNA is analyzed by making multiple copies of the gene of interest by a gene amplification technique known as the polymerase chain reaction (PCR). Unaffected embryos are transferred to the uterus in hopes of initiating a pregnancy with a child free from the at-risk disease.

PGD for Chromosomal Abnormalities - Flourescent In Situ Hybridization (FISH)

The field of PGD is expanding to evaluate the cells of pre-embryos for abnormal numbers of specific chromosomes (aneuploidy). A normal pre-embryonic cell will have 23 chromosomes from the mother and 23 chromosomes from the father for a total of 46 chromosomes. Commonly, the early dividing cells will not equally divide the chromosomal complement. This occurs more often as the age of the mother increases and is one of the most important reasons why fertility declines with increasing age. This is why the incidence of chromosomal and congenital anomalies increases with age.

By using the process of fluorescent in situ hybridization (FISH), we can count the number of specific chromosomes, generally the chromosomes more commonly involved in abnormalities such as chromosome 21, chromosome 18, or the number of X and Y chromosomes. In the photomicrograph shown here, you can see the nucleus of a biopsied cell (blue 'globe') with the fluorescent red, green and yellow spots within the nucleus demonstrating chromosomes 13/21 (red), 18 (aqua), X (green) & Y (yellow).

The FISH technique can also be used to evaluate specific chromosome structural rearrangements known as translocations. When a person carries a balanced translocation, their children are at risk of having an unbalanced translocation resulting in either extra or missing pieces of the chromosomes involved.

Another important application of FISH to determine of the status of pre-embryos prior to embryo transfer. By only transferring embryos without specific chromosomal defects, the total number of embryos transferred can be reduced, which in turn reduces the risk of triplet or quadruplet pregnancies.