IVF Laboratory Procedures PGD Genetic Diagnosis
In IVF applications, embryos are selected between 2nd and 5th days and transferred to the expectant mother. Usually, 1 to 2 embryos of the highest quality according to their appearance (morphology) are selected among the others and transferred. Although the embryos with best appearances were transferred, implantation and clinical pregnancy rates still have not reached the desired levels. Miscarriage is seen at rates of up to 15-20% within the first 3 months after pregnancy occurs. Recent studies have shown that chromosomal disorders play an important role in the non-adherence of embryos.
Aneuploidy is seen in 30-50% of embryos obtained during IVF treatment. It has been revealed that although the transferred embryos are of good quality, there still may be a genetic defect. Embryos that are not genetically healthy have little or no chance of conceiving (autosomal monosomies). Even If pregnancy occurs, a significant portion results in miscarriage, and while it results in stillbirth or baby with congenital anomaly at lower rates.
Pre-implantation genetic diagnosis (PGD) has entered in vitro fertilization practices in recent years. Genetically normal and abnormal embryos can be detected with the PGD method. Numerical chromosomal disorders in embryos can be distinguished by performing aneuploidy screening. Aneuploidy screening is recommended for approximately 10% of patients presenting for IVF. With the application of PGD to these 10% of patients, this negative course can be significantly corrected by transferring normal embryos. The importance of PGD in vitro fertilization is gradually increasing and more and more patients are advised to undergo PGD. In this way, it is aimed to increase the pregnancy chances of couples applying for IVF and to reduce the possibility of miscarriage.
Although numerical chromosomal problems can be detected in embryos with aneuploidy screening, since the diagnosis is made according to 1 to 2 cells, it does not exclude possible mosaicism in embryos. Recent publications demonstrate that 20-30% of developing embryos may have differences in the number of chromosomes (mosaicism) in their cells (blastomeres). We know that a certain degree of chromosomally mosaic embryos can be considered normal and even these embryos can normalize themselves (self-correction) in the early stages of embryonic development. Therefore, aneuploidy screening cannot be applied to every IVF patient, it is advised especially to certain patient groups with recurrent pregnancy losses or chromosomal mosaicism in the mother or father.
Apart from aneuploidy screening, PGD should also be considered in cases where the mother or father or both are carriers of a balanced translocation. These couples usually have a history of recurrent miscarriage or not being able to get pregnant at all. Structural changes in the chromosome, which has two different types as reciprocal and Robertsonian translocations, can be detected in embryos by molecular genetic techniques such as FISH and cGH. Pregnancy can be achieved by transferring embryos found to be normal following PGD.
With the introduction of PGD into in vitro fertilization practices, it has been partially revealed why implantation and pregnancy rates are low. With this method, genetic defects were detected at rates of 30-40% even in embryos of the best quality, and up to 90% in embryos of poor quality. Therefore, as embryos appearing to be of good quality but are genetically abnormal may have a very low chance of conceiving, although pregnancy does occur, there is a possibility of resulting in miscarriage.
Couples who do not have a problem of conceiving but are carriers of genetic diseases are also offered the opportunity to have a healthy baby thanks to the PGD method. Such patients usually admit to our clinic with a history of recurrent miscarriage, stillbirth, and baby with anomaly. Some of the couples, who have already known that they are carriers of such a genetic disease, apply for help before attempting any pregnancy. Achieving a healthy pregnancy with PGD in carriers of a single-gene disease can now be achieved without leaving it to chance.
Besides, couples, whose previous child developed a disease requiring bone marrow transplantation, apply to our center for PGD application in order to have a tissue compatible but healthy (HLA typing-HLA determination) child with the sick child using IVF, and to heal the sick child with the cord blood or bone marrow to be taken from the healthy baby to be born afterwards. With the HLA compatible embryo transfer after PGD, both the sick child can be healed and a baby without genetic disease can be born.
WHO SHOULD UNDERGO PGD?
Preimplantation genetic diagnosis (PGD) is a method used to investigate whether the embryo is normal in terms of the chromosomes examined and genetic diseases. Investigating the embryo before transfer with the embryo biopsy procedure is an important method providing information about whether the embryo carries any genetic disease or whether it is chromosomally normal. The cell (blastomere) taken from the embryo by biopsy is forwarded to the genetics department. Embryos identified to be normal as a result of genetic analysis are transferred.
In the embryology laboratory, embryo biopsy is performed in 3 different periods of the embryo depending on the indication.
Polar body biopsy in the zygote stage, the first PB biopsy is performed just before ICSI/IVF (approximately 4 hours after the OPU procedure). The second PB (18-20 hours after ICSI/IVF) is aspirated by entering the section where the first PB was aspirated. It is especially applied in cases with single-gene and HLA diseases and in cases where total chromosomal abnormalities were found in the embryos in the previous application.
Blastomer biopsy (embryo biopsy) on the 3rd day,
Trpophoectoderm biopsy (TB) on the 4th day. TB procedure is applied to the 5th day embryos that reached the blastocyst stage.
Biopsy is a difficult procedure that can be performed by experienced embryologists with very special tools. Firstly, a hole is drilled (hatching) in the zona pellucida, which is the protective layer covering the egg or embryo. This procedure is carried out using mechanical, laser or acid thyroid methods. Laser hatching method which is the safest one, is used in our center. Then, by entering the embryo with a biopsy pipette through the hole, the target cell is taken out without damaging other cells. The obtained cell is delivered to the genetics laboratory. FISH (fluorescent in situ hybridization), PCR (polymerase chain reaction), microarray or cGH (comparative genomic hybridization) methods are used depending on the type of genetic research to be performed. FISH and cGH method are used for couples whose sex determination will be made due to aneuploidy screening, translocation carrier or single-gene disease, and PCR method is used for other single-gene diseases. Sex determination with PGD is prohibited in Turkey unless there is a genetic reason.
Currently, all genetic analysis methods are successfully applied in our center. Results usually take 1 to 2 days and normal embryos are transferred on the 4th or 5th day.
In which situations is pre-implantation genetic diagnosis recommended?
- Recurrent pregnancy losses
- Fetal chromosomal abnormality as a reason for miscarriage
- Carrier of translocation in the mother or father, or both
- Having a single-gene disease inherited by sex in the mother or father
- Having a single-gene disease or carrier in the mother or father
- Obtaining HLA-compatible-tissue-matched embryos for the treatment of the diseased child (conditions requiring marrow transplantation).
- Some examples of single-gene diseases with Preimplantation Genetic Diagnosis are;
– Marfan X-linked