In this webinar, Dr Estefanía Montoya, PhD, Geneticist at UR Vistahermosa, Alicante, Spain, has explained what Non-invasive preimplantation genetic testing (niPGT) is, how it works and if it is the next revolution in reproductive genetics.
In vitro fertilization (IVF) offers two methods—conventional IVF and intracytoplasmic sperm injection (ICSI). After fertilization, the blastocyst undergoes assessment based on morphology, including cell count and size, before being transferred to the uterus. Relying solely on morphology assessment has limitations, leading to the risk of aneuploid embryos—those with abnormal chromosome numbers. Aneuploid embryos have a higher likelihood of miscarriage and implantation failure, with maternal age being a significant factor. To address this, pre-implantation genetic testing (PGT) is crucial. PGT determines the embryo’s aneuploidy status, aiding in the selection of euploid embryos during pre-implantation, significantly improving the chances of successful implantation and a healthy pregnancy.
In this webinar, three specific types of PGT are discussed: PGT-A, designed to screen for chromosomal aneuploidy; PGT-M, utilized for couples with a monogenic disorder in their family and PGT-SR, applicable to couples with existing structural rearrangements. These PGT techniques play a pivotal role in refining the embryo selection process, addressing specific genetic concerns, and optimizing the chances of a successful IVF outcome.
The initial and critical stage of PGT is obtaining an embryo biopsy, a delicate process aimed at preserving the embryo’s integrity. While it’s feasible to perform a biopsy three days post-fertilization when the embryo is at the 6-8 cell stage, the optimal time is at the blastocyst stage on day 5. This strategic timing not only enhances sensitivity to genetic conditions but is also less detrimental to the embryo. Upon obtaining the biopsy, the embryo’s DNA undergoes analysis using Next-Generation Sequencing. This analysis identifies the chromosomal load, enabling the selection of embryos with a higher likelihood of successful pregnancy. Subsequently, these carefully chosen embryos are transferred to the uterus one at a time.
PGT-A offers numerous advantages, elevating the IVF procedure by minimizing the risks of miscarriage, chromosomal abnormalities, and implantation failures. However, it is essential to acknowledge its limitations. Firstly, PGT-A is an invasive procedure demanding the expertise of highly trained embryologists, particularly during the embryo biopsy. Additionally, the potential for misdiagnosis does exist, as the cells extracted during the biopsy may not fully represent the entire embryo.
In 2013, the discovery of embryo cell-free DNA (cfDNA) in the blastocyst fluid during a natural pregnancy prompted a groundbreaking realization as the newfound knowledge transitioned into in vitro applications, demonstrating that cfDNA is released into the culture medium throughout in vitro embryo development. Therefore, the extraction of DNA from the culture medium allows for the implementation of non-invasive pre-implantation genetic testing (PGT), offering a method that prevents any risk to the embryo’s development.
Non-invasive pre-implantation genetic testing (niPGT) has an informative success rate ranging from 80% to 100%, where there is a successful extraction and interpretation of DNA from the culture medium. The timing of this extraction significantly influences the success rate, with studies revealing the peak success occurring on day 6 of in vitro embryo development. Comparative studies between niPGT and traditional biopsies indicate higher informative rates for niPGT, suggesting that this method is more representative of the entire embryo than the conventional biopsy approach. These studies have not only highlighted the advantages of niPGT but have also contributed to method optimization. Issues such as maternal DNA contamination in the culture medium have been addressed, allowing for enhanced reliability. Additionally, studies indicate that niPGT maintains a concordant success rate even with lower volumes of culture medium, further refining and validating the non-invasive genetic testing methodology.
NiPGT is a safer alternative, steering clear of potential harm to embryos. Comparative to embryo biopsy, it has an inherent simplicity reducing the dependency on highly trained embryologists, making it a more accessible option for a broader patient population. This approach not only incurs lower expenses but also exhibits the potential to provide a more accurate representation of chromosomal load compared to traditional biopsies, as evidenced by various studies.
In contrast, some studies introduce some uncertainty around the method as they suggest a lower informative success rate and diminished representation of the chromosomal composition. The understanding of cell-free DNA (cfDNA) remains uncertain, casting doubt on its true representation of chromosomal ploidy without false positives. To solidify its standing, the niPGT protocol requires optimization and validation in each laboratory. This includes refining culture conditions to maximize cfDNA yield while minimizing the risk of maternal contamination.
While niPGT is implemented in numerous clinics worldwide, the contentious nature of its clinical application persists, fuelled by conflicting research studies. Despite this ongoing debate, a noteworthy number of successful pregnancies and healthy live births have already been documented following niPGT procedures. Hence, it stands as a promising method with the potential to complement, or perhaps even replace, existing PGT methods in the near future.
PGT- SR stands for Preimplantation Genetic Testing for Chromosomal Structural Rearrangements) meaning structural rearrangements in the karyotype, e.g., Translocations, leading to chromosomal defects in the offspring. PGT will analyze embryos to determine their inheritance.
Regarding PGD, this is a term that is no longer used in reference to this. Regarding PGT and NGS, these terms are different aspects of genetic testing. PGT (Pre-Implantation Genetic Testing) is a broad term for the whole process of extracting the DNA from the embryo, performing genetic tests, determining the presence of aneuploidy, and selecting a healthy embryo for implantation. While NGS (Next Generation Sequencing) is a genetic testing technique that can be used to test the DNA in that embryo. It is a technique used within the field of genetic testing and can be applied to other applications other than PGT.
Mosaicism can be complex, involving varying proportions of normal and abnormal chromosome load among different cells within one embryo. Detection of mosaicism using niPGT would be a challenge as cfDNA represents an accumulation of genetic material from various cells within the developing embryo.
Depending on the mosaicism you are concerned about, each case could be evaluated differently with various methods considered.
Yes, some mosaic conditions can be ‘self-corrected’, for example, trisomy 18 is more likely to result in a viable pregnancy. Ongoing research and advancements in reproductive genetics may contribute to answering this question, as there is still a lot that is unknown about the phenomenon.
While different studies yield diverse findings, the general consensus is a 50:50 ratio of aneuploid to euploid embryos for women over 40. Individual variations are inevitable, and the likelihood of achieving successful live births is dependent on factors like overall health, reproductive history, and unique characteristics specific to each couple.
Both are very important, the degree of fragmentation, number as well as the rhythm of cell division.
