Principal findings
Considering growing clinical discomfort with the increasing utilization of PGS in IVF and recently arising questions about accuracy of the procedure, this study attempted to assess the clinical reproducibility of PGS results. To bring some clarity to the subject, we dissected 11 embryos previously reported to be aneuploid. To our surprise, a repeat analysis by a third laboratory was congruent in only 2/11 (18.2 %) embryos. Indeed, 4/11 embryos (36.4 %) were on repeat evaluation normal 46, XX and 46, XY, and an additional 2/11 embryos were mosaic, with at least one fragment reported as normal 46, XX or 46, XY.
Though here investigated embryo numbers were small and, therefore, do not allow for statistically valid prevalence assessments, combined, these numbers suggest a potential false-positive PGS rate of as high as almost 55 %. Moreover, intra-embryo discrepancies were observed in 50 % (5/10) of embryos suggesting much higher trophectoderm mosaicism in blastocyst-stage embryos than has previously reported.
Strength and limitations
Greco et al. recently reported only a 4.8 % mosaicism rate in 3802 blastocyst stage embryos they examined by single trophectoderm biopsy [
13]. That this represents a significant underestimate is suggested by an earlier study of Fragouli et al. who reported 32.4 % among 52 investigated blastocyst stage embryos to be mosaic, 30 % uniformly aneuploid and 42.3 % uniformly euploid [
17]. This discrepancy in mosaicism rates can, likely, be explained by Greco et al. performing only a single trophectoderm biopsy, while Fragoulis et al., like we in this study, dissected embryos.
A single trophectoderm biopsy usually includes only approximately 5–6 cells, while the whole trophectoderm contains at that stage a few hundred. This consideration alone demonstrates how low the statistical likelihood of detecting mosaicism (i.e., presence of chromosomally normal and abnormal cells) is within a single trophectoderm biopsy. The likelihood of detecting mosaicism will, however increase with increasing numbers of biopsies, both within individual biopsies but also between biopsies at different areas since those may represent euploid and/or aneuploid cell lineages of trophectoderm.
Bolton et al. recently demonstrated in a mouse model that the fate of aneuploid cells in early embryos depends on lineage: aneuploid cells in the fetal lineage (i.e., inner cell mass) are eliminated by apoptosis, whereas those in the placental lineage (i.e, trophectoderm) demonstrate severe proliferative defects. From blastocyst stage on aneuploid cells progressively deplete, and mosaic embryos have full developmental potential as long as they contain sufficient euploid cells [
18]. With aneuploid cells preferably seggregating into the trophectoderm, it appears reasonable to assume that mosaicism rates will increase with number of trophectoderm biopsies performed. This was also suggested by the recent study of Orvieto et al. [
12].
Under the assumption of a relatively high mosaicism rate in trophectoderm, the biological plausibility of PGS to reliably determine ploidy of embryos with only one trophectoderm biopsy, therefore, has to be questioned. The difference in mosaicism prevalence between Greco’s study on one [
13], and Fragouli’s [
17] and our study on the other hand, likely, offers a good estimate of how much mosaicism is missed by only a single, randomly chosen trophectoderm biopsy.
As the recent study by Bolton et al. [
18] and our study, however, suggest, Fragouli et al., erred when concluding that the significant embryo mosaicism they reported was clinically irrelevant for the accuracy of PGS since only 17 % of mosaic embryos contained also normal cells. They also appear incorrect when concluding that mosaic embryos, therefore, only unlikely would be able to lead to pregnancy and, if they did, pregnancies would, likely, end in miscarriages [
17]. As Bolton et al. demonstrated in the mouse [
18], and Greco et al. [
13] and the PGS Consortium here, mosaic embryos appear to exhibit rather surprising developmental potential.
Combining Greco’s data [
13] with here reported cases, so far 26 infertile women worldwide received allegedly aneuploid (and/or mosaic) embryos, resulting in 11 chromosomally normal live births and/or ongoing clinical pregnancies, for a rather remarkable rate of 42.3 % of likely live birth in absence of even a single miscarriage. These unexpectedly high live birth rates in both studies are that more noteworthy since, as demonstrated by small number of available embryos, they involved women with very LFOR. Such patients iusually produce extremely low live birth and very high miscarriage rates [
1].
The ultimate pregnancy potential from transfer of mosaic embryos has still to be determined. It, likely, will depend on patient age, a woman’s FOR and the specific aneuploidy/mosaicism pattern an embryo presents with. Studies to obtain this information appear of utmost importance. The International PGS Consortium, therefore, established a PGS Registry for Transfer of Aneuploid/Mosaic Embryos, which can be accessed at
www.centerforhumanreprod.com or by e-mailing questions to ykizawa@thechr.com.