PGD: Who Is a Candidate for Preimplantation Genetic Diagnosis?

Preimplantation genetic diagnosis (PGD) is testing for single gene disorders. It is the genetic testing technique recommended when couples are at risk of transmitting a known genetic abnormality to their children. It is used in conjunction with in vitro fertilization (IVF) to test for a specified genetic disease or diseases. With PGD, embryos without the genetic defect are transferred to the mother's uterus, which reduces the chances of the child having an inherited genetic abnormality.

"Preimplantation genetic diagnosis is an option for couples who are at risk of having a child with a genetic condition that runs in their family," says Lauren Isley, MS, Genetic Counselor with Genesis Genetics Institute.in Detroit, MI. "Common examples of this would be a couple who are both carriers for Cystic Fibrosis or Sickle Cell Anemia, or an individual who is affected with Huntington disease or Marfan syndrome and does not wish to pass it on to their children."

A single gene disorder is a genetic disease caused by a change (mutation) in a particular gene that causes the gene to not function properly. There are thousands of different single gene disorders; however, many of them are very rare. There are typically three inheritance patterns for single gene disorders:

• Autosomal recessive. A mother and father must both be carriers of a mutation in the gene that causes that disorder, and they have a 25 percent chance in each pregnancy of having a child with the disorder. (Example: Cystic Fibrosis)
• Autosomal dominant. These diseases are caused by one non-working copy of a pair of genes, so only one parent has to carry the gene, and there is a 50 percent chance in each pregnancy of having a child with the disorder. (Example: Huntington's Disease)
• X-linked. These disorders involve genes that are located on the X-chromosome. Remember, women have two X chromosomes, and men have one X chromosome and one Y chromosome. Women who are carriers for an X-linked disorder have a 50 percent chance with each pregnancy of passing on the non-working copy of the gene, but usually only the male children will be severely affected. Men who have an X-linked disorder will ;pass the non-working copy of the gene to their daughters who will then be carriers for the disorder. (Example: Hemophilia A)

At Genesis, a couple going through PGD would first meet with one of the genetic counselors to discuss PGD testing in detail and decide whether it is the right option for their family. "Our laboratory would then collect DNA samples through buccal swabs from the family to build a customized genetic probe to test their embryos," Isley explains. "Once this is completed, the couples begins the IVF procss, and we coordinate with their fertility center to perform the genetic testing directly on the samples from their embryos."

PGD may be recommended for

• Couples who have had a previous birth of a child with a single-gene defect, such as cystic fibrosis, Tay-Sachs disease, sickle cell anemia and Huntington disease. Single gene disorders are the result of a single mutated gene.
• Couples who have a family history of X-linked diseases, which are passed to the male child through the mother. Sex-linked recessive disorders include hemophilia, fragile X syndrome and most neuromuscular dystrophies. Sex-linked dominant disorders include Rett syndrome, incontinentia pigmenti, pseudohyperparathyroidism, and vitamin D–resistant rickets.
• Couples in which both partners are "carriers" for a single gene disorder based on screening tests and are at risk for passing on inherited genetic disease to their offspring.

Some fertility clinics allow patients to use PGD for gender selection.