Assessing the true incidence of mosaicism in preimplantation embryos

Abstract

Modern technologies applied to the field of preimplantation genetic diagnosis for aneuploidy screening (PGD-A) have improved the ability to identify the presence of mosaicism. Consequently, new questions can now be addressed regarding the potential impact of embryo mosaicism on diagnosis accuracy and the feasibility of considering mosaic embryos for transfer. The frequency of chromosomal mosaicism in products of conception (POCs) of early miscarriages has been reported to be low. Mosaic embryos with an aneuploid inner cell mass are typically lost during the first trimester owing to spontaneous miscarriages. Most of the mosaics in established pregnancies would derive from placental mosaicism or placental aneuploidy, and mosaic embryos with aneuploid inner cell mass should be lost mainly due to first-trimester spontaneous miscarriages. The well described clinical outcomes of live births from mosaic embryos suggest a wide spectrum of phenotypes, from healthy to severely impaired. Therefore, there is a need to balance the risks of discarding a possibly viable embryo with that of transferring an embryo that may ultimately have a lower implantation potential.

Is cell-free DNA in spent embryo culture medium an alternative to embryo biopsy for preimplantation genetic testing? A systematic review

“…two or more cell populations with different genotypes in the same embryo, for chromosomal ploidy. Genetic discordance between inner cell mass and trophectoderm cells also was reported (Vera-Rodriguez and Rubio, 2017), indicating that comparison with whole-embryo data is preferable for evaluating SCM-PGT reliability. Only 40% (four out of 10) of the included studies, however, used whole embryos as DNA reference Ho et al, 2018;Kuznyetsov et al, 2018;Li et al, 2018).…”

“…This hypothesis is supported by the low molecular weight of cf-DNA, i.e. from 100 to 1000 bp, mean ≈ 400 bp (Stigliani et al, 2013) compared with the 3 million bp of genomic DNA; and the high rate of complementary and divergent aneuploidies between SCM and trophectoderm biopsy or whole embryo analysis (Vera-Rodriguez and Rubio, 2017). Therefore, cf-DNA detected in SCM should mostly reflect the genetic constitution of discarded cells, potentially diverging from the growing embryo genetic status.…”

“…Individual embryo culture in separate medium droplets is required for SCM-PGT. We propose to collect SCM from day 3 to day 5 embryos for four reasons: day-3 embryos can be extensively washed and the IVF medium changed before the start of the incubation for SCM collection, thus decreasing the risk of maternal contamination owing to residual corona radiata and cumulus cells; SCM associated with blastocysts displays higher cf-DNA detection and amplification rates and higher proportions of embryo cf-DNA compared with earlier stages; the incubation time should be between 24 h and 48 h to optimize cf-DNA release, while preventing its degradation; blastocysts are associated with higher implantation potential, improving pregnancy rates while decreasing PGT workload and cost, and present a lower risk of mosaicism compared with cleavage embryos (McCoy, 2017;Vera-Rodriguez and Rubio, 2017).…”

“…Second, embryo biopsy involves the removal of only few cells from the whole embryo, and this might lead to genetic misdiagnosis, i.e. false positive-negative, in the case of embryo mosaicism McCoy, 2017;Sachdev et al, 2017;Vera-Rodriguez and Rubio, 2017). Third, invasive procedures could be detrimental to the embryo reproductive potential.…”

Causes and consequences of chromosome segregation error in preimplantation embryos

“…However, the introduction and development of improved culture methods and assisted reproduction PROOF ONLY technologies such as in vitro fertilisation (IVF) in the last four decades provided an opportunity to study the chromosomal status of cells during early human development, and led to the realisation that whilst meiotically derived whole-embryo aneuploidies occur in some embryos, 'mosaic' aneuploidy, where only a subset of blastomeres within an embryo are aneuploid, is more prevalent (Taylor et al 2014). This phenomenon was first reported in 1993 using simple Fluorescent in situ hybridisation (FISH) to label and count the copies of a limited number of chromosomes, and subsequent studies using whole-genome hybridisation and modern sequencing approaches have revealed single chromosome gains or losses as the predominant genetic anomaly in mosaic embryos, occurring in up to 90% of embryos, depending upon the study (Delhanty et al 1993, Munné et al 1993, Echten-Arends et al 2011, Vera-Rodriguez & Rubio 2017. Additionally, polyploid and segmental mosaic aneuploidies are observed, albeit at much lower incidences (EchtenArends et al 2011) (Fig.…”