A review on Preimplantation Genetic Testing (PGT) for chromosomal abnormalities diagnosis.
A review on Preimplantation Genetic Testing (PGT) for chromosomal abnormalities diagnosis.
Reyhaneh Khodadadi,1,*Mohadese Farahani,2
1. Department of Biology, Faculty of Sciences, Arak University, Arak, Iran 2. Department of Biology, Faculty of Sciences, Arak University, Arak, Iran
Introduction: It's important to recognize how chromosomal abnormalities impact human reproduction. Researches show that natural fertility in humans follows an inverse U-curve during maternal reproductive years. This indicates that embryonic chromosomal abnormalities caused by meiotic errors during oocyte formation are the primary reason for reduced potential at both ends of the curve. This phenomenon is unique to humans. On average, even at the peak of a woman's fertility, chromosomal abnormalities affect around 20% of oocytes. It has been observed that around 50% of preimplantation human embryos possess chromosomal abnormalities. This is due to some level of error-proneness behavior during chromosome segregation in gametogenesis, which is actually beneficial for our species. It is important to understand that embryonic chromosomal abnormalities are not an anomaly but rather a planned and fundamental aspect of the natural reproduction process in humans. Chromosomal abnormalities, including aneuploidy (which is the most common genetic abnormality found in humans. Their high incidence in embryos is the main cause of failed implantation, pregnancy loss, and congenital birth defects. Typical examples are monosomy or trisomy, respectively resulting in 45 or 47 chromosomes.), chromosomal mosaicism (which occurs when cells have different chromosomal constitutions. The relevant type for PGT-A is a mix of euploid and aneuploid cells.), segmental abnormalities (refer to changes that occur in specific sections of chromosomes, and these changes often result in regional losses or gains.), structural rearrangements (abnormalities that alter the natural order of chromosomal segments but do not affect copy numbers. Include Balanced translocations, robertsonian translocations, insertions, and inversions.) Preimplantation Genetic Testing (PGT) helps detect chromosomal abnormalities and conditions that affect embryo health and viability.
Methods: In this review study, we analyzed articles related to modern fertility from 2015 to 2023, obtained from the PubMed and Google Scholar databases.using keywords such as preimplantation genetic testing; PGT; preimplantation genetic diagnosis; PGD; genetic screening; aneuploidy; genetic testing methods; comprehensive chromosome screening; next-generation sequencing; next-generation screening; NGS; comparative genomic hybridization; CGH; array comparative genomic hybridization; aCGH; single nucleotide polymorphism; SNP polymerase chain reaction PCR; quantitative polymerase chain reaction; quantitative real-time polymerase chain reaction; etc.
Results: In the past, PGT mainly relied on the Fluorescent In Situ Hybridization (FISH) technique. But today, modern methods such as array, Next-generation sequencing (NGS), SNP, array CGH, and quantitative PCR (qPCR) are now more commonly used to examine all 24 chromosomes. Among these methods, NGS is generally considered the most suitable for PGT-A, as it can diagnose mosaicism effectively. While Array CGH has many advantages, it does have a limitation in detecting polyploidy. It is particularly useful in detecting subchromosomal disorders (segmental aneuploidy), but it's important to consider the size of such disorders, especially if there is a history of that disorder in the family tree, especially in the parents.
After using the FISH technique to examine the first polar body resulting from meiosis I, it was discovered that up to 24% of cases showed aneuploidy. This percentage was influenced by the number and type of chromosomes examined, as well as the clinic conducting the examination. However, an array CGH studies that examined all chromosomes reported an even higher percentage of aneuploidy. As for meiosis II, examinations of the second polar body showed approximately 37% aneuploidy with FISH and 46% with CGH.
Part of the differences between FISH results and 24-chromosome methods is due to the limitation of the number of chromosomes that can be examined in the FISH method. What should be noted in the meantime is that meiotic aneuploidies are expected to result in embryos with the same aneuploidy in all cells.
Women over 35 years old who have aneuploidy in their eggs before implantation may be directly affected by their age. However, examining the polar body of the egg is not a reliable method for predicting the fetus's genetic status. Reports on aneuploidy during the third embryonic day vary depending on the technique used. Using the FISH technique, 25 to 83% of embryos are reported to be abnormal at this stage. This stage of development appears to have the highest incidence of division disorders. Clinical studies on PGT based on blastocyst biopsy show nearly 60% aneuploidy, with much of it being mosaicism. Examining aneuploidy during the blastocyst stage has an advantage since several trophoectoderm cells are biopsied, which increases the chances of detecting mosaicism and reduces the possibility of not getting an answer.
Conclusion: Although PGD is a widely accepted procedure, there is still ongoing debate regarding the acceptance of PGT-A. This will depend on advancements in our understanding of human embryo development and the success of biopsy strategies when combined with genetic analysis. It appears that there is a consensus that mosaicism is less problematic when performing trophoectoderm biopsy compared to embryo biopsy at the cleavage stage.