Application of preimplantation genetic testing of embryo in assisted reproductive technology

Bachelor Do Thi Linh, MSc. Vo Nguyen Thuc

IVFMD, My Duc Hospital

What is embryo preimplantation genetic testing?

Preimplantation genetic testing of embryo is used to detect genetic abnormalities in embryos generated from in vitro fertilization (IVF) before these embryos are transferred to a patient. Genetic testing begins with a biopsy of a divided embryo on day 3 or of a blastocyst on day 5 or 6 to collect a few cells containing genetic material of that embryo. However, blastocyst biopsies are commonly used nowadays because more cells can be collected and embryos are less affected.

Previously, embryo pre-implantation genetic testing was known for its two techniques: preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). However, from September 2017, the European Society of Human Reproduction and Embryology (ESHRE) agreed to change those two common terms into preimplantation genetic testing (PGT). In particular, PGT is divided into 3 groups: (1) PGT for aneuploidy (PGT-A), (2) PGT for monogenic defects (PGT-M) and (3) PGT for chromosome structural rearrangement (PGT-SR).

What are the indications of preimplantation genetic testing?

Following ESHRE's practical guidelines,  preimplantation genetic testing is indicated as follows:

- PGT-A is often applied to elderly patients (> 35 years old), repeated miscarriage (> 2-3 times), repeated implantation failure or having a child with chromosome abnormalities  such as Down's syndrome, Turner, Edwards, Patau ...

- PGT-M is usually indicated for patients with monogenic disorder (such as Thalassemia anemia), sex-linked chromosome mutations (Hemophilia A coagulation disorder), repetitive syndrome, loss or addition of nitrogen base or HLA (Human Leukocyte Antigen) compatibility finding.

- PGT-SR is usually applied to subjects with  abnormal chromosome structures that are not listed in the case of PGT-A, including: deletion, insertion, unbalanced translocation ...

What is the effectiveness of assisted reproduction in combination with PGT-A?

Currently, PGT-A accounts for more than 90% of the IVF cases indicated with PGT, mainly to elderly patients who failed to implant many times in order to select the best potential embryos for embryo transfer. According to updated data in 2018, 40% of the IVF cycles in the United States performed PGT-A. Besides the indication of PGT-M and PGT-SR, PGT-A has been increasingly used. Therefore, the effectiveness of PGT-A attracts a growing interest.

A few randomized controlled clinical trials (RCT) show that PGT-A improves implantation and the progression of pregnancy. Studies show that PGT-A is not beneficial to patients under 35 years of age, as well as to patients under 38 years old performing IVF in the first cycle. This technique is beneficial to patients ³ 38 years of age in improving treatment efficacy. However, it should be noted that, in a group of patients ³ 38 years of age performing PGT-A, the culture of blastocysts from a limited number of embryos at the stage of dividing embryos increases the risk of patients without embryos to use. Therefore, the consultation for a patient to use PGT should be selectively performed on each patient group, and this should include both benefits and associated risks.

The increasing cost in the IVF cycle combining PGT is also a matter of consideration. A cost-effectiveness analysis of over 11,000 IVF cycles performing PGT-A showed that PGT-A was cost-effective when a patient has more than 2 blastocysts for biopsy. PGT-A is associated with increased costs for embryo biopsy, genetic testing and embryo storage. Meanwhile, the cycle without implementing PGT-A may increase the costs associated with multiple embryo transfers or treatment for multiple pregnancy or early fetal loss. In general, PGT-A can help shorten the time for patients to achieve baby delivery after IVF.

A major problem related to genetic testing on embryos today is the possibility of diagnostic errors caused by mosaic embryos. A mosaic embryo is an embryo with more than one cell line carrying different genetic material. Therefore, a biopsy collecting 3 to 5 embryonic cells as a representative for the whole embryos for genetic testing pose a risk of missing a normal embryo or selecting an abnormal embryo. A study published recently updated results from 151 IVF centers in the world to help better understand the use of embryos after PGT-A. It shows that there are 125 IVF centers (accounting for 83% of the surveyed centers) that perform PGT-A mainly on elderly patients and patients who fail to implant many times. In particular, 20% of the surveyed centers allow the transfer of embryos with abnormal PGT-A results to patients. In particular, 49.3% of abnormal PGT-A embryo transfer cycles result in progressive or alive pregnancy. This is a positive result to reinforce the consultation of patients using embryos with selective abnormal PGT-A results in the absence of other options. Thus, PGT-A can help guide embryo use to prioritize the potential of conception rather than to become a basis for complete exclusion of embryos for use in IVF.

What is the new trend of PGT?

PGT from embryo biopsy is the commonly used method today. However, there are still many concerns about the invasiveness of the technique that may affect embryo quality and transfer results. A new attractive approach at the moment is the Non invasive preimplantation genetic testing (NI-PGT).

NI-PGT from miRNA, embryo metabolism or proteomics has been studied. However, the application of this technique to clinical practice still faces many obstacles. Recently, NI-PGT using cell free DNA (cfDNA) in a culture medium or blastocyst fluid is a potential approach being studied by many authors. The hypothesis is that the presence of embryonic cfDNA in a culture medium is due to a release during correction, or a programed death during a normal embryo development. Most studies noted the presence of cfDNA in the medium or embryonic fluid with a high rate of successful DNA amplification of 80-100% and a corresponding rate of the trophoblast cell biopsy of 75-95%. Along with the perfection of the sample collection process and the advancement in analytical techniques in genetic labs, NI-PGT using cfDNA is a promising technology that could be applied in the future.

REFERENCES

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