Preimplantation Genetic Diagnosis

Preimplantation Genetic Diagnosis: PGD (Preimplantation Genetic Diagnosis: PGT) is the procedure in which, during an IVF (In Vitro Fertilization) cycle, embryos are genetically tested for a specific disorder and only those found to be healthy are selected and transferred to the mother.

Preimplantation Genetic Screening (PGS): is the evaluation of embryos for chromosomal abnormalities called aneuploidy before they are placed into the uterus. With the PGS method, the aim is to identify embryos in which no anomaly is observed, transfer these embryos, and achieve the birth of a healthy baby.

Aneuploidy, which is known as having a higher or lower number of chromosomes than normal, is one of the most common chromosomal abnormalities. Preimplantation Genetic Screening (PGS) is the evaluation of embryos for aneuploidy before they are placed into the uterus. Aneuploidy may be present in both the egg and the sperm, or it may arise in the embryo at the stage of fertilization. Depending on the type of chromosomal abnormality present, aneuploidy may cause physical and/or mental developmental problems. With the PGS method, the aim is to identify embryos in which no anomaly is observed, transfer these embryos, and achieve the birth of a healthy baby.

Some of the embryos obtained in IVF cannot implant into the uterus, some stop developing at an early stage, and some, even if they pass through these stages, may not be able to continue their development until birth. As a result, 20% of pregnancies, as in natural pregnancies, may end in miscarriage. Among the causes of embryo developmental failure, chromosomal irregularities constitute the largest share. The most common chromosomal abnormalities seen in miscarriages are trisomy (presence of three copies of a chromosome) of chromosomes 13, 16, 18, 21 and 22, monosomy (presence of a single copy of a chromosome), and numerical abnormalities of the sex chromosomes (X and Y).

What is PGD?

Preimplantation Genetic Diagnosis (PGD) is applied to couples who carry a risk for cystic fibrosis, thalassemia or balanced structural translocation. Balanced structural translocation causes the formation of genetically unbalanced gametes (sperm or egg), and if these achieve fertilization, they may lead to deficiencies or excesses in genetic material and thus to a chromosomally abnormal embryo. Such a situation in the embryo may result in embryo death, miscarriage, or the birth of a child with serious medical problems.

How and When Are PGS/PGD Performed?

After fertilization, the embryo is monitored in the culture system for 3 days until it reaches the 8–10 cell stage. One or two cells are taken from these 3-day-old embryos by biopsy from their blastomeres, and chromosomal analysis is performed for specific chromosomes. PGS/PGD is applied to the chromosomes on which chromosomal anomalies are most frequently seen in miscarriages and live births (13, 16, 18, 21, 22, X and Y).

While PGS/PGD is being performed on chromosomes, special probes (small DNA fragments) are bound to specific regions to be analyzed using the Fluorescence In Situ Hybridization (FISH) method. Each probe is labeled with a different fluorescent dye. These fluorescent probes are brought into contact with the biopsied cells and are expected to hybridize with specific chromosomes. Using a fluorescent microscope, the colored signals (one, two or three) seen on a given chromosome are evaluated by the geneticist. A single signal is evaluated as monosomy, two signals as disomy (normal), and three signals as trisomy. In this way, aneuploidy in specific chromosomes can be identified.

What are the advantages of PGS/PGD?

· Selecting the best embryo to be transferred, thereby increasing the pregnancy rate, and deciding which embryos will be frozen or discarded

· Reducing the risk of miscarriage

· Providing better guidance for patients

· Encouraging patients who have chromosomally normal embryos but have not been able to conceive

Indications for PGS

· Advanced maternal age (over 35–37 years)

· Recurrent miscarriages

· Recurrent IVF failure

· Presence of chromosomal abnormality in a previous pregnancy

· Application of Intracytoplasmic Sperm Injection (ICSI) with abnormally shaped sperm

· Presence of aneuploidy in sperm

· Presence of aneuploid mosaicism in at least one of the partners

· At least one of the partners being a carrier of an X-linked disease

Indications for PGD

· At least one of the partners being a carrier of a rearranged structural chromosome (translocation, inversion, deletion)

· Aiming to achieve a healthy pregnancy by diagnosing a known genetic disease

PGD has some disadvantages:

· Certain technical problems may be encountered during the preparation or biopsy stage

· Even if a successful IVF and PGD procedure is performed, pregnancy may not occur after transfer.

· The analysis results of a single cell may be undiagnosable or limited due to mosaicism (because blastomeres of the embryo may have different contents). For this reason, the result should be confirmed by prenatal diagnosis.

· Since only a limited number of chromosomes can be checked, not all chromosomal and genetic abnormalities can be diagnosed with PGD.

· Consequently, only a specific test can be performed on the biopsied cell. With a single cell taken for testing, it is not possible to screen all genetic problems.

What Are the Benefits of PGD?

• In appropriate cases, it increases the success of IVF treatment.
• In selected cases, it increases the pregnancy rate.
• It reduces the risk of pregnancy ending in abortion (miscarriage).
• It reduces the need for medical termination of pregnancy.
• It reduces the rate of multiple pregnancies.
• It reduces the economic and psychological burden of repeated unsuccessful IVF attempts.

Who Should Undergo PGD?

• Women of advanced maternal age
• Couples who have not achieved pregnancy despite two or more IVF cycles
• Couples with recurrent early pregnancy losses (miscarriages) (for reasons other than translocation carrier status)
• Couples who are balanced translocation carriers
• Partners at risk for certain single-gene diseases that can be diagnosed, such as Familial Mediterranean Fever, Sickle Cell Anemia, Cystic Fibrosis, SMA
• Selection of an embryo that is HLA-compatible with family members
• Couples who have had a child with a genetic disease from previous pregnancies
• Mothers with a history of pregnancy with aneuploidy (chromosomal abnormality)
• Cases of gonadal mosaicism (cases in which, despite normal genetic test results in the partners, there are two or more offspring with the same abnormality)
• TESE cases (cases associated with severe male infertility)
• Poor responders (cases that respond inadequately to hyperstimulation protocols)
• In diseases with X-linked inheritance, if direct genetic diagnosis of the disease in question cannot be made, embryonic sex determination.

Is There a Margin of Error in Embryo Biopsy and PGT?
To perform PGT (Preimplantation Genetic Diagnosis), a cell sample must be taken from each of the patient’s embryos. During PGT, the likelihood of damage to embryos is extremely low, and the damage rate is reported as 0.3%.

Although it varies according to the method used, the probability that the PGT test will give an incorrect result ranges between 2–7%.

What Are “Single-Gene Diseases”?
Single-gene diseases are a group of genetic disorders that occur as a result of dysfunction of the units called “genes” encoded on DNA. These types of genetic diseases increase particularly in consanguineous marriages.

Because consanguineous marriages are common in our country, it is very important that such diseases are diagnosed in the prenatal period (prenatal diagnosis) and even at the embryo stage (preimplantation diagnosis). It is possible to perform preimplantation diagnosis for all single-gene diseases that can be diagnosed postnatally (after birth) or prenatally (before birth).

Preimplantation genetic diagnosis of single-gene diseases is based on DNA analysis of a single cell. Preimplantation genetic diagnoses have been performed for a long list of diseases such as Cystic Fibrosis, Hemophilia A and B, Alpha-1 antitrypsin deficiency, Tay-Sachs and Sickle Cell Anemia, Retinitis pigmentosa, Thalassemias (Mediterranean anemia), Alport syndrome, Gaucher’s disease, long-chain acyl-CoA dehydrogenase deficiency, Multiple epiphyseal dysplasia, Achondroplasia (dwarfism), Neurofibromatosis, Epidermolysis bullosa, Myotonic dystrophy, X-linked hydrocephalus, cancer predisposition and Fanconi anemia.