Some couples are at increased risk for transmitting genetic diseases to their children. The disorders include hemophilia, Tay-Sach’s disease, Sickle Cell Anemia, Cystic Fibrosis, Down’s syndrome, and others. Older women (>38) or those with
recurrent miscarriage may be at risk for chromosomal abnormalities called aneuploidies. PGD allows the embryologist to screen embryos for these and many other genetic abnormalities.
PGD patients undergo
in vitro fertilization to create embryos that will be transferred to the uterus. Once the embryos mature, the embryologist makes a small hole in the embryo’s outer membrane (zona pellucida) using a chemical (Acid Tyrode’s Solution), a microscopic needle, or a laser. At UAB, a laser is used for this procedure. A single cell is withdrawn for the PGD procedure(s) without damage to the embryo. Each cell contains the complete genetic makeup of the embryo.
The cell is examined for aneuploidy, which is an abnormal number of chromosomes, using fluorescent in situ hybridization (FISH). Using FISH, the chromosomes are stained which enables the embryologist to count them and often identify other abnormalities.
The polymerase chain reaction (PCR) is used when damage, or disease, is suspected on a particular chromosome segment. The PCR duplicates and amplifies certain chromosome sections so that disorders can be seen.
Some diseases, such as hemophilia, are sex-linked meaning they are transmitted on the X chromosome. Because males have only 1 X chromosome but females have 2, these sex-linked diseases are more apparent in males. Hemophilia is an example of a sex-linked disease, meaning that males are the ones usually affected. FISH can be used to separate male and female embryos, and in this case, only female embryos would be transferred to the mother. The same applies if a chromosomal abnormality is seen using PCR in that only normal embryos would be transferred.
There are many genetic diseases that can be identified using PGD and research continues to expand the list. No procedure is “100%”, however, PGD lowers the chances of genetic birth defects.