Preimplantation Genetic Diagnosis (PGD)

In general, three types of conditions can be detected through PGD. Looking for abnormal numbers of chromosomes, or aneuploidy, is the first. The second type of PGD looks for single gene disorders, such as cystic fibrosis, Tay-Sachs disease, and sickle cell anemia. Finally, balanced translocations can be detected through PGD. A person who carries a balanced translocation has the correct number of chromosomes, but two of the chromosomes have exchanged pieces. A translocation carrier’s offspring are at increased risk for chromosomal abnormalities, such as Down Syndrome. In addition, balanced translocations are responsible for approximately 3-5% of repeat miscarriages.

To understand how PGD works, we need to describe some aspects of the fertilization and subsequent early embryo development. The process that results in sperm and eggs containing 23 chromosomes is called meiosis. In their resting state, eggs exist in a state of arrested meiosis, and still contain all 46 chromosomes. During ovulation, meiosis resumes, and the egg extrudes one copy of its 46 chromosomes in a small structure called the polar body. Soon after fertilization occurs, a second polar body, containing 23 maternal chromosomes, is expelled. On the day following an egg retrieval, these two polar bodies can be seen in the normally fertilized egg under the microscope.

Aneuploidy studies begin with polar body biopsies. On the day following egg retrieval, after normal fertilization has been detected, the two polar bodies are removed from the fertilized egg, without invading the cell itself, or harming the nuclei. Special stains, or probes, are applied to the biopsy material that attach to chromosomes 13, 16, 18, 21, and 22. These chromosomes were selected because they account for the vast majority of aneuploidies. Since the test relies on visualizing little colored spots under the microscope, it is not possible to study every chromosome. Rare abnormalities involving other chromosomes can be missed, but fortunately, these embryos almost never progress to the fetal stage. Since polar body biopsies only evaluate the genetic material from the egg, the sex of the embryo cannot be determined from this procedure. Also, aneuploidy resulting from a chromosomally abnormal sperm cannot be detected by analyzing the polar bodies. However, in about 80-90% of chromosomally abnormal embryos, the extra or missing chromosome came from the egg. Sometimes, for technical reasons, the polar body biopsy does not provide clear information regarding aneuploidy, and a blastomere biopsy may be performed to clarify the results.

Sex determination requires a blastomere biopsy. This slightly more invasive procedure requires removal of one or two of the early embryonic cells at the 6-8 cell stage, on day 3 after the egg retrieval. To identify the sex of the embryo, such as when needed to test for X-linked genetic disorders, probes for chromosomes 13, 18, 21, X and Y are applied to the cell(s).

The second type of PGD seeks out single gene disorders. Different expressions of a single gene are known as alleles. A large library of genetic probes has been created that can specifically identify different alleles at specific gene sites that cause many genetic disorders. In some conditions, there are many different possible mutations of the gene, and specific probes for each patient are created.

With recessive traits, like cystic fibrosis, for a couple that has had a previous child with the disorder, each parent possesses a normal and an abnormal allele. When these two people reproduce, on average 50% of their embryos will carry one copy of the abnormal allele and 25% will carry two copies of the abnormal allele and will be affected by the disease. For dominant traits, one copy of the abnormal allele is sufficient to cause the disease. In most cases, people who have one abnormal dominant allele have the condition in question. Therefore, each of their offspring has a 50% chance to inherit the gene, and thus the disease.

In theory, blastomere biopsies are superior to polar body biopsies, because they allow identification of both maternal and paternal genes. This allows differentiation of normal, carrier, and disease states. This approach uses the technology of polymerase chain reactions (PCR). DNA strands are cut into smaller pieces by enzymes, and millions of copies of the DNA are produced by the PCR. The strands of DNA are allowed to come back together, and abnormal genes cause the strands to align differently, thus allowing the identification of the abnormal gene.

Unfortunately, because of a technical problem, known as allele dropout (ADO), an abnormal allele can be hidden or not detectable by the testing, so that an embryo with a genetic abnormality could appear to be unaffected. To minimize the interference of ADO, if both parents have the same gene mutation, polar body biopsy is done first. Depending on how many copies of the abnormal allele are present in the polar bodies, we can determine whether the egg carries the normal allele or the abnormal allele. All eggs that carry the abnormal allele are eliminated. Because polar body biopsy only analyzes the maternal contribution of the DNA, this technique cannot determine whether the resulting embryo would be normal or a carrier of the disease. Therefore, blastomere biopsy may need to be performed as a second step. To further reduce the problems associated with ADO, and increase the accuracy of DNA tests, linked markers are also utilized. Linked markers are segments of DNA that lie very close to the gene being studied. By testing the linked markers, as well as the gene in question, we can confirm the results of the genetic testing and determine whether ADO has occurred.

The third group of patients who can benefit from PGD are those with balanced translocations. Carriers of balanced translocations are healthy individuals, but have high miscarriage rates and are at increased risk for having children with unbalanced chromosome translocations, which result in an extra amount of one chromosome and a missing piece of another chromosome. Probes can be developed to detect these unusual chromosomal abnormalities, and can be used on cells taken from a blastomere biopsy. This procedure would greatly reduce the risk of having a child with a severe chromosomal problem, resulting from the translocation, and would also greatly reduce the miscarriage rate.

To date, there are only a few hundred babies that have been born after performing PGD. At one of the world’s largest centers, Reproductive Genetics Institute (RGI) in Chicago, the overall pregnancy rate following PGD for single gene disease is about 20%. This is somewhat lower than expected, since most of these patients were not infertile to begin with. Hopefully, as the technology advances and the opportunity to help more couples rises, we will see an increase in these success rates. The pregnancy rate following PGD for aneuploidy is about 26%. At Huntington Reproductive Center (HRC), we have developed an efficient system to allow us to send embryo biopsy material to RGI for analysis. We have been working with RGI in this manner for about one year.

Many ethical questions surround PGD. Some people see the beginning of eugenics – the striving towards some hypothetical genetic perfection, along with intolerance of those who are less than perfect. At the present time, we are far away from this. As discussed above, there are definite limits to what can be tested for in the embryo. As the technology advances, the medical community, and society at large, will need to define the boundaries of how PGD should be used. If it becomes possible, do we want parents to be able to select for traits like eye color and height? Since it is already possible to select for sex, should couples be allowed to do this when a sex-linked genetic disorder is not involved? If the couple needs to undergo IVF for infertility, should they be denied this option?

Another concern expressed towards genetic medicine is that by attempting to eliminate individuals with genetic conditions, we could create a society intolerant of people with congenital disorders. If there exists a method to prevent these individuals from being conceived would society discriminate against parents for choosing to allow conception and delivery of these babies? Clearly, this potential problem has existed for over two decades because parents have the choice of terminating pregnancies due to chromosomal or genetic problems that are discovered with amniocentesis. For the most part, society has integrated this technology without stigmatizing parents or their children for being born with an inherited disease. There is no reason to believe that society will expect couples to undergo PGD, unless the couple desires it.

Finally, there is the omnipresent issue of cost. PGD adds $3,000 - $5,000 to an IVF cycle. Adding in the costs of medications and IVF, this becomes an expensive way to prevent genetic disease. However, to the couple who has a child with a severe problem, and cannot face the choice of pregnancy termination, this procedure is invaluable. These costs may be offset by the enormous cost to parents and society of caring for these often very sick children.

The marriage of IVF and genetics was inevitable. Much work needs to be done to make PGD a more comprehensive and available alternative. Society must watch the development of this technology and help define its value and determine reasonable boundaries, before the technology becomes more sophisticated. Despite these challenges, PGD is an exciting development that takes IVF beyond its already worthy goal of creating families, by impacting on public health through the prevention of severe diseases, before they occur.