Preimplantation Genetic Diagnosis - Dr. Muzaffer Uçarer

There is not a long history of reconciling genetic technology with IVF applications. For the first time in the early 1990s, with the aim of preventing sexually transmitted diseases, the applications that started with the determination of the sex of the embryo and generally called preimplantation genetics showed a very rapid development and now allows the diagnosis of single gene diseases and tissue antigens at the embryo level. Preimplantation genetic applications are divided into two.

The first and most widely used are the applications called preimplantation genetic screening (PGS), which are used to reveal the structural and numerical disorders of the chromosomes. Preimplantation genetic screening is done using a technology called FISH (flourescent in-situ hybridization). Under the fluorescent microscope, normal or abnormal structures and numbers can be seen from the chromosomes paired with probes that give different color reflections. For example, the disease called Down syndrome, which is the most common cause of congenital retardation, occurs when there are three of the 21st chromosomes instead of two. When FISH is performed, 3 probes attached to the 21st chromosome will be seen. The embryo that has been diagnosed with Down syndrome will thus not be placed in the uterus.

PGS has three main uses. These are advanced female age (after 38 years of age), history of repeated miscarriage (3 or more miscarriages), and repeated IVF failure (no pregnancy despite a total of 10 embryo transfers in 3 or more IVF applications). However, PGS, in its current form, does not increase the rate of going home with a live baby in these 3 cases. The reason for the failure in its current state is 1) the evaluation of only a limited number of chromosomes with the current technology, 2) the false-negative and false-positive results of the FISH method; 3) the potential for embryo biopsy to damage the embryo. In fact, in a recent Dutch study, the pregnancy rate was found to be lower in the PGS arm in the PGS arm compared to those who did not undergo PGS due to advanced female age. In our Center, we only perform PGD in cases with structural or numerical chromosome problems in male or female chromosome examination. We do not prefer PGS.

Preimplantation genetic diagnosis (PGD), on the other hand, is based on the recognition of diseases known to exist and inherited from a single gene. With a different technology called PCR, the change in the gene causing the disease is detected at the embryo level and unhealthy embryos are not transferred. The number of single gene diseases that can be diagnosed with PGD is increasing day by day. Thalassemia (Mediterranean anemia), which is one of the most common single gene diseases that is especially relevant in our country, is the most common among PGD applications. Apart from thalassemia, the diagnosis of many single gene diseases such as sickle cell anemia, Tay Sachs disease, Fragile X syndrome and these is possible with PGD. With PGD, in couples with a diseased child, a tissue compatible sibling can be made with the aim of transplantation with stem cells taken from bone marrow or cord blood for this child. In this way, the couple can both have a healthy child and have stem cell transplantation for their diseased children.

With the rapid advances in genetic engineering, repair of faulty genes will be possible in the not too distant future. Although the determination of the entire genetic structure at the embryo level and the replacement of diseased genes may seem like fiction today, it seems likely that these will happen in the future. Of course, these practices also have an ethical dimension that cannot be ignored. Therefore, it is very important to carefully examine the ethical dimensions.

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