Abstract
Pre-implantation genetic testing (PGT) is a specialized technique that combines in vitro fertilization (IVF) technology with genetic testing. This paper provides a comprehensive overview of the different types of PGT, including PGT-A, PGT-M, and PGT-SR, highlighting their roles in reducing miscarriage rates, optimizing embryo selection, and minimizing the transmission of genetic conditions. Additionally, it discusses the technical aspects of embryo biopsy, the challenges associated with mosaicism, and ethical considerations, and patient counselling.
Introduction
Pre-implantation genetic testing (PGT) is a specialized technique that combines in vitro fertilization (IVF) with genetic testing of embryos before implantation. In 2017, the World Health Organization (WHO) adopted new nomenclature for pre-implantation genetic diagnosis (PGD), classifying it into PGT-M for monogenic diseases, PGT-SR for structural rearrangements, and PGT-A for aneuploidy. PGT assists in reducing the risk of genetic disorders, improving embryo selection, and minimizing the likelihood of transmitting genetic conditions to offspring.
In clinical practice, PGT is performed for individuals or couples with a known risk of transmitting genetic disorders. The PGT-A technology has also been applied as an additional tool for selecting embryos for transfer in IVF settings to improve reproductive outcomes.
Pre-implantation genetic testing
There are four primary types of PGT, each with a distinct purpose ( Table 1 ).
| Type | Indication | Risk |
|---|---|---|
| PGT-M (Monogenic disorders) | Carrier of specific monogenic diseases (e.g., cystic fibrosis, Huntington’s) | Misdiagnosis due to allele drop-out (ADO) |
| PGT-SR (Structural rearrangements) | Balanced chromosomal rearrangements (e.g., translocations, inversions) | Risk of unbalanced chromosomes, miscarriage |
| PGT-A (Aneuploidy screening) | Advanced maternal age, recurrent miscarriages, failed IVF cycles | False positives/negatives, embryonic biopsy risks |
| Pre-implantation tissue typing (PTT) | Tissue matching for sibling treatment | Ethical considerations, regulatory requirements |
PGT-A (Aneuploidy screening)
This technique screens embryos for numerical chromosomal abnormalities, which are a leading cause of miscarriages and impact the success of IVF attempts. By selecting euploid embryos, those with the correct chromosomal complement, PGT-A aims to enhance implantation rates, decrease the risk of miscarriage, and improve the probability of achieving a live birth.
Some studies have demonstrated improved ongoing pregnancy rates with PGT-A, particularly in patients with advanced maternal age or recurrent pregnancy loss. However, other trials indicate that PGT-A does not significantly improve cumulative live birth rates compared to conventional IVF, highlighting the need for individualized patient evaluation.
PGT-A may potentially reduce time to pregnancy in IVF. Current evidence does not indicate a significant improvement in live birth rates for most general IVF patients. It is therefore important for patients to discuss their individual circumstances with the fertility specialist to determine whether PGT-A is appropriate for them.
PGT-M (Monogenic disorders)
PGT-M allows for screening of embryos for specific genetic mutations linked to monogenic diseases, such as cystic fibrosis, Huntington’s disease, or sickle cell anaemia. This testing is useful for individuals and couples who are carriers of autosomal recessive or dominant genetic conditions, as it can prevent the transmission of these conditions to their children. Reproductive options for these couples include proceeding with natural conception without genetic testing, undergoing prenatal or pre-implantation genetic diagnosis, using gamete donation, or considering adoption. For those couples identified with a genetic risk, IVF is often needed primarly as a means to facilitate PGT.
PGT-M is performed using polymerase chain reaction (PCR) or next-generation sequencing (NGS) to detect the specific mutations, and it is frequently combined with linkage analysis to minimize the risk of misdiagnosis due to allele drop-out (ADO). ADO refers to the failure of one allele to amplify during the genetic testing process, which can result in inaccurate conclusions about the genetic status of an embryo. Linkage analysis helps to identify and verify both alleles of a gene, reducing the impact of ADO and increasing the diagnostic accuracy.
PGT-SR (Structural rearrangements)
This test is used for those who carry balanced chromosomal rearrangements, such as translocations or inversions. These chromosomal rearrangements can increase the risk of producing embryos with unbalanced chromosomes, which may lead to implantation failure, recurrent miscarriage, or chromosomal abnormalities in children. By selecting embryos that are free of unbalanced chromosomal rearrangements, PGT-SR improves the likelihood of a successful pregnancy and reduces the risk of genetic disorders in children.
Techniques commonly used for PGT-SR include array comparative genomic hybridization (aCGH) and next-generation sequencing (NGS). These methods allow for a comprehensive assessment of the chromosomal structure to identify imbalances, which is crucial for couples with known rearrangements.
Pre-implantation Tissue Typing (PTT)
Pre-implantation Tissue Typing (PTT) is a specialized type of embryo testing used to treat children with life-limiting blood disorders who require an exact tissue match for treatment. When a suitable close relative is not available, parents may opt to conceive another child and use PTT to select embryos that are an exact tissue match for their affected sibling. Due to this unique application, PTT is often referred to as ‘saviour sibling’ technology. The use of PTT requires additional regulatory approval on a case-by-case basis, ensuring that it is only performed when medically and ethically justified. In the UK, the Human Fertilisation and Embryology Authority (HFEA) strictly regulates PTT under its licensing framework. Clinics must apply for a specific license, demonstrating that the procedure is necessary and ethically sound.
Clinical implications and limitations
Despite its benefits, PGT has several limitations that should be considered and discussed with the patients. One of the significant limitations is that PGT does not guarantee a successful pregnancy or a baby free of all genetic abnormalities. It can only screen for specific genetic conditions that are known and testable.
Mosaicism is one of the most challenging aspects of PGT. Mosaicism refers to the presence of two or more different chromosomal complements within an embryo. This phenomenon can complicate genetic analysis because the cells biopsied may not accurately represent the chromosomal status of the entire embryo. Mosaicism can occur at any stage of embryo development and is estimated to be lower in blastocyst stage of embryo development. The clinical significance of mosaicism is still being studied, but it can lead to false-positive or false-negative results, making it difficult to determine the true viability of an embryo. Some mosaic embryos can still result in healthy pregnancies, while others may not be viable. It is crucial for healthcare professionals to provide detailed counselling to patients about the possibility of mosaic results and the uncertainty they may bring to the decision-making process.
Ethical aspects
Although regulations in the UK and other countries restrict the use of PGT to medical purposes, ethical debates continue. The concept of “designer babies”—where genetic testing is used to select for non-medical traits—poses a significant ethical dilemma, and it is crucial that patients understand the intended medical use of PGT.
The high cost of genetic testing and the associated IVF procedures may limit its access. Ensuring equitable access to PGT and ART services is an important ethical consideration that policymakers and healthcare providers must address.
Technical aspects of embryo biopsy and genetic analysis
The accuracy and reliability of PGT depend heavily on the techniques used for embryo biopsy and genetic analysis. PGT can be performed at different developmental stages, each with advantages and limitations:
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Polar body biopsy : Performed on oocytes before fertilization, primarily to detect maternal genetic abnormalities, but does not assess paternal contributions. This method does not interfere with embryo development but is less commonly used today due to its limited diagnostic scope.
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Cleavage-stage biopsy : While it provides more genetic information than polar body biopsy, it has largely fallen out of favour due to concerns about its impact on embryo viability and the higher risk of mosaicism.
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Blastocyst biopsy : The current standard for PGT, involving the removal of 5-10 trophectoderm cells (which later form the placenta), avoiding disruption to the inner cell mass (future fetus) on day 5 or 6. This method allows for a larger sample size, improving diagnostic accuracy. The introduction of vitrification, which effectively cryopreserves blastocysts with a high success rate for thawing, has addressed earlier time constraints, making blastocyst biopsy the preferred option. An experimental alternative to biopsy, non-invasive PGT (niPGT) analyzes cell-free DNA from embryo culture media, avoiding the need for direct embryo sampling. Early studies suggest that niPGT may replace invasive biopsy in the future, but current limitations include lower DNA yield and potential contamination. Further validation is needed before it becomes a standard clinical tool.
UK regulation of PGT
In the UK, PGT is regulated by the Human Fertilization and Embryology Authority (HFEA). The HFEA has categorized PGT-A as a “red” treatment add-on, indicating that there is limited evidence supporting its routine use for increasing live birth rates in general IVF population. PGT-A may still be considered for patient with recurrent miscarriages. The HFEA mandates that clinics provide patients with transparent information regarding the effectiveness and risks of PGT procedures, ensuring informed decision-making.
Prospective parents identified as having a genetic risk are often referred to a genetic specialist for comprehensive counselling and assessment. In the UK, funding for PGT-M is available through the NHS for eligible patients, depending on the specific genetic condition and clinical circumstances. According to the HFEA, pre-implantation genetic testing for monogenic or single-gene disorders (PGT-M) can currently be used to avoid over 1700 genetic conditions. New applications are continuously processed to add genetic conditions to the list from UK centres licensed to perform PGT-M, allowing for a targeted approach to minimize the risk of transmitting these conditions.
