Blog

PGT-A: A double edged sword?

pgta

The selection of the best quality embryos to transfer during in vitro fertilisation (IVF) is crucially important to maximise treatment success. Quality assessment is largely based on morphological features, such as the number and appearance of cells. However, these qualitative measurements are not the most reliable, as almost two-thirds of couples still do not get pregnant even after transferring “top grade” embryos. What could be going on underneath the perfect looking cover? Only more advanced technologies like preimplantation genetic testing for aneuploidy (PGT-A) can tell, or can it?

The use of PGT-A has a strong appeal to both patients and clinicians as it seems to offer a whole lot more information that enables a better prediction on the quality of an embryo. This will improve the probability of a successful pregnancy, while minimising the risk of failed implantation and miscarriages. Sounds like the game changer, doesn’t it?

Since last year, the opposing voice against PGT-A has been intensifying. If you are a patient about to embark on an IVF journey, knowing whether PGT-A is helpful for you is important. Now, let’s summarise the problems with PGT-A that have raised concerns in the past year.

The problem with wasted embryos

The most controversial concept around PGT-A that patients might not fully appreciate is that PGT-A does not improve the quality of embryos, rather, it eliminates “bad embryos”. What this means is that in an extremely unlucky situation, PGT-A may indicate that all of the fertilised embryos contain abnormal genetic materials, and therefore, none is suitable for transfer. While this may be an extreme example, the reality isn’t that far off as a large proportion of embryos are labelled “abnormal/aneuploid” and discarded.

But the problem is, are aneuploid embryos destined to fail? One 2019 study performed a worldwide survey of 151 IVF centres to find out if such “abnormal” embryos have indeed led to high treatment failures. Surprisingly, 49.3% cycles resulted in ongoing pregnancies or live births, and miscarriages are rare (9.3%). These data came as a shock - if the so-called “abnormal” embryos can lead to healthy babies, then are they really abnormal? For older patients who may only have less than a handful of embryos, even one lost embryo could make the difference between carrying home a baby or not.

In 2020, the debate around PGT-A has really intensified. First was one article published in the prestigious journal Human Reproduction titled “Hidden in plain sight: the overstated benefits and underestimated losses of potential implantations associated with advertised PGT-A success rates”. In this article, the author argued that PGT-A has become highly marketed as a mature and established diagnostic technique with overstated benefits. But in reality, the technology is still in its early days and the embryo wastage problem is hidden in plain sight.

The calculated efficacy of PGT-A is inflated as it does not take into account the loss rate, which was estimated to be as high as 52.8%. While PGT-A may result in increasing implantation rates, there are now fewer embryos left!

The problem with add-ons and embryo mosaicism

Following this paper, the International Do No Harm Group in IVF (IDNHG-IVF) also published similar concerns on the journal of Reproductive biology and endocrinology. The authors pointed out that PGT-A comes with cumulative add-ons including extended blastocyst culture, embryo cryopreservation, biopsy, and the disposal of what the procedure reports as abnormal embryos. In addition to the problem of embryo wastage, these increased add-ons could add to the potentially negative treatment outcome and financial burden. For example, not all embryos can survive the biopsy, freezing and thawing procedures.

Perhaps a bigger problem is embryo mosaicism, which is the concept that not all cells in the same embryo have the same genetic makeups. Since biopsy for PGT-A generally only takes a few cells out of the embryo, the result may not necessarily reflect what’s going on in the rest of the embryo. Interestingly, transfers of mosaic embryos have given rise to healthy babies. In fact, the majority of preimplantation human embryos have been found to be mosaic, containing both normal and abnormal cells.

In a landmark paper using mosaic mouse embryo models, the researchers found that preimplantation embryos have a certain level of self-correction. The mosaic embryos have full developmental potential, provided that they contain sufficient cells with normal chromosome numbers. Therefore, discarding an embryo just because one of its cells is deemed “genetically abnormal” seems short-sighted.

So when is the use of PGT-A justified?

If a patient has a specific chromosomal defect in the family, PGT-A may help avoid an unbalanced karyotype. In older patients whose eggs are known to have higher aneuploidy and miscarriage rates, PGT-A may be reasonable given the patients have a decent number of embryos available. But unfortunately, this is usually not the case given the sharp decline of ovarian reserve in patients older than 35 years old. Until there are clearer recommendations regarding the use of PGT-A, patients, clinicians and genetic counsellors are encouraged to discuss the potential risks and benefits of PGT-A in a personalised manner.

Sperm DNA damage/fragmentation: problems that evade the eyes

Male factor infertility accounts for half of the couples seeking fertility treatment, but not all causes can be explained. Within this group, 5-10% men have high amount of sperm DNA fragmentation despite having normal semen parameters. Studies have shown that sperm DNA fragmentation is correlated with poor reproductive outcomes, such as miscarriages, chromosomal abnormalities, congenital malformations, genetic disorders and neurological defects.

Today, DNA fragmentation tests are not yet considered as a routine analysis in fertility investigation, but should they be?

Why do DNA fragmentations occur?

There are many reasons as to why DNA fragmentations occur. Intrinsic factors are generally defects in the process of sperm production. For example, small breaks and nicks are induced in the DNA structure during normal maturation but these are later fixed by repair mechanisms. DNA damage checkpoints should also be in place to prevent damaged sperm from proceeding any further. However, defects in the repair and monitoring process could potentially result in permanent DNA fragmentation in ejaculated sperm.

Reactive oxidative species are the chief culprit in a process known as oxidative stress. They are extremely unstable particles that can damage genetic materials. Normally, our cells are equipped with antioxidant molecules that counteract such damage, but they could be overwhelmed by excessive oxidative stress. For example, the following could lead to the production of reactive oxidative species and consequently, DNA damage:

  • X rays and radiotherapy

  • Ultraviolet light (UV)

  • Cigarette smoking

  • herbicides

  • alcohol abuse

  • chronic stress

  • drugs (eg. acetaminophen)

  • chemotherapy

  • environmental pollution

Age is an important factor as some studies have suggested that men with advanced age are more prone to DNA fragmentation. Those under the age of 35 years were found to have considerably lower DNA damage as well as oxidative stress levels.

Higher DNA fragmentation levels are also observed in various medical conditions, including varicocele, microbial infections (such as Chlamydia) and leucocytospermia.

Can men with DNA fragmentation conceive naturally or through assisted reproductive technologies (ARTs)?

The probability of natural conception declines as the level of DNA damage increases. Studies have shown that if the level of DNA fragmentation is higher than 30%, then the chance of natural conception is almost zero. Therefore, if you or your partner have difficulty falling pregnant for more than 1 year of having regular, unprotected sex, diagnostic check-ups and ARTs should be considered.

Among the various ART techniques, intrauterine insemination has been shown to have low success if sperm DNA fragmentation is higher than 30%. In vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) are thought to be better approaches, but their success rates also depend on how much DNA damage exists in the sperm. Before ICSI, advanced sperm selection could help choose a sperm that appears the most promising. For example, under high magnification, scientists could see into the sperm heads where detrimental vacuoles may be present.

Once fertilised, DNA damage continues to affect embryo growing quality. While low level of DNA damage seems to be well tolerated, high fragmentation rate (>30%) is associated with lower blastocyst and pregnancy rates.

What treatment might help?

Sperm are delicate cells that are highly susceptible to oxidative stress. Previous reports have suggested that oral antioxidant treatment may reduce the elevated levels of sperm DNA fragmentation in ejaculated sperm. Adjusting the diet to include more antioxidant-rich foods may also be helpful to some extent. However, large clinical trials on this approach are yet to be conducted.

To date, in-depth research regarding the precise pathophysiological mechanisms underlying DNA damage is largely lacking. This limits the development of potential treatments for DNA repair. The true clinical value of using sperm DNA fragmentation as a prognostic predictor of treatment outcome and embryo quality are also areas that need further research.

Reference

Zini, Armand, & Agarwal, Ashok. (2018). A Clinician's Guide to Sperm DNA and Chromatin Damage. Springer International Publishing. https://doi.org/10.1007/978-3-319-71815-6