Introduction to preimplantation genetic screening
Singapore’s present total fertility rate of 1.20 is well below the population replacement rate of 2.1. Increasing birth rates is a national priority. One of the governmental incentives that has been introduced to improve birth rates is co-funding for assisted reproduction. However, on average, two-thirds of in-vitro fertilisation (IVF) cycles will not be successful. Although there are multiple factors behind unsuccessful IVF, the single biggest impact is thought to be from chromosomal abnormality.
The risk of chromosome aneuploidy increases with maternal age, as the oocytes are older and therefore more prone to meiotic disorders. More than half of the embryos from women under age 35 are chromosomally abnormal, rising to over 80% in those above the age of 40 (Ata et al 2012). Transferring an aneuploid embryo may result in failure to implant, miscarriage, or birth with disabilities that can be profound. The conventional method of selecting the best embryo for transfer is based on morphology, that is, their form and structure at defined stages, usually on Day 2, 3 or 5 after fertilization. However, there is poor correlation between morphology, embryo development and having the correct number of chromosomes (Fragouli et al 2014).
Morphological assessment of embryos cannot be relied on to ensure transfer of chromosomally normal embryos. Hence there is interest in finding better methods of embryo selection. Preimplantation genetic screening is a technique of selecting embryos for transfer. After IVF, an embryo biopsy is performed, and the biopsied cells analysed for their chromosome content. Only embryos that are euploid, i.e. contain the correct number of chromosomes, are selected for transfer. Earlier attempts to perform preimplantation genetic screening using fluorescence in-situ hybridization (FISH) technology were not shown to be beneficial. A maximum of 15 chromosomes could be analysed, therefore the aneuploidy status of the unanalysed chromosomes was unknown.
Around 2010, a microarray-based technology that analyses all 24 chromosomes called array comparative genomic hybridization (aCGH) was introduced as an alternative to FISH for preimplantation genetic screening (Harper et al 2010). aCGH is associated with relatively high test costs due to the nature of the one cell/ sample per microarray format. More recently, rapid developments in next generation sequencing (NGS) technology have tremendously reduced the cost of sequencing-based genomic analysis (Fiorentino et al 2014). NGS platforms allow multiple samples to be analysed simultaneously. This leads to improved workflow, higher throughput, and lower test costs.
Potential benefits of Preimplantation genetic screening
Preimplantation genetic screening could significantly increase overall IVF embryo implantation, clinical pregnancy, and healthy live-birth rates across most if not all prognostic groups. This may in turn translate into fewer IVF cycles that women have to undergo in order to bring home a healthy baby. This will reduce medical costs, physical pain, psychological and mental stress for the patient and family. In the long term, preimplantation genetic screening has the potential to reduce the number of IVF treatments needed to produce a healthy baby, to reduce the risk of miscarriage, and to avoid babies born with chromosomal aneuploidy syndromes, translating into a significant reduction in Singapore’s overall healthcare burden.
The evidence for PGS
Three randomised controlled trials of comprehensive chromosome screening had shown improvements in outcomes. (Yang et al 2012; Forman et al 2013; Scott et al 2013). More recently though, a multicentre randomised controlled trial (Munne et al 2017) of 588 women aged 25 to 40 years found that preimplantation genetic screening did not show benefit for all patients. There was, however, an improvement in ongoing pregnancy rate in women above the age of 35 who had preimplantation genetic screening (50.8%) versus women who did not (37.2%). The authors of the study suggest that standardization of clinical and laboratory protocols is essential for future studies, as there were 34 clinical sites and 9 laboratories across 4 countries performing the preimplantation genetic screening analysis.
Controversies regarding PGS
Mosaicism is a phenomenon whereby the chromosome content of dieerent cells varies. Presently, most embryo biopsies are performed at the blastocyst stage. About 5-10 cells from the trophectoderm (which eventually develops into the placenta) are removed and sent for analysis. Using NGS techniques, mosaicism may be detected. Although some practitioners may choose not to recommend transfer of embryos with a mosaic karyotype, others have reported successful live birth with normal chromosome content following transfer of mosaic embryos. However, the pregnancy and live birth rates following transfer of mosaic embryos is lower than following transfer of euploid embryos (Fragouli et al 2017). As this is an evolving situation, more research is required.
Preimplantation Genetic Screening in Singapore
Preimplantation genetic screening has not been permitted in Singapore by the authorities. However, the Ministry of Health has approved a study that is now underway at the ART centres in the public healthcare institutions. The study is exploring whether preimplantation genetic screening using NGS techniques can improve live birth rates in women of advanced maternal age, or in those with recurrent implantation failure, or those with recurrent pregnancy loss. Patient recruitment began in September 2017, and the outcome of this study will be eagerly awaited.