Abstract
Study question
Are aneuploid cells in human preimplantation embryos eliminated by apoptosis due to proteotoxic stress and autophagy-mediated apoptosis?
Summary answer
Proteotoxic stress, autophagy and apoptosis are differentially activated in aneuploid embryos, showing that aneuploid cells are eliminated by these mechanisms during early human embryogenesis.
What is known already
Aneuploidies are a common feature of human preimplantation embryos which could explain low success rates after in vitro fertilization (IVF). While most aneuploidies of meiotic origin are detrimental, transfer of euploid-aneuploid mosaic embryos can lead to healthy live-births. Moreover, the proportion of aneuploid cells are lower in blastocysts when compared to cleavage stage embryos. In the mouse, aneuploid cells are eliminated from the epiblast by autophagy-mediated apoptosis in a p53-dependent manner. We propose that in human embryos, aneuploidy causes chronic protein misfolding which leads to autophagy-induced apoptosis.
Study design, size, duration
Eighty-one blastocysts that were diagnosed by PGT as euploid (n = 49) or uniformly combined abnormal (CA, n = 32), i.e. 2 or more chromosomes were abnormal in every cell, were warmed. Sixty-seven were suitable for trophectoderm (TE) biopsy, 54 biopsies were successfully tubed and sent for RNA-sequencing while the remainder of the embryos was fixed for immunostaining. Thirty-three day-3 embryos were overnight incubated in 0.5µM reversine allowed to develop into blastocysts and treated as the PGT embryos.
Participants/materials, setting, methods
After TE biopsy, we live-stained the embryos with either Caspase-3/7 or 8 and subsequently fixed them. The biopsies underwent RNA-sequencing using the SMART-seqv4 and the fixed embryos were immunostained for LC3B, p62 (autophagy) and HSP70 (proteotoxic stress). Confocal imaging was performed using a Zeiss LSM800 confocal microscope and the presence of signal was quantified using the Zen Blue 2.0 and Arivis software.
Main results and the role of chance
Forty-two percent of the embryos in which we induced aneuploidies using reversine developed into blastocysts, which is comparable to untreated embryos. After immunostaining, we observed that CA and reversine-treated (RT) embryos contained less cells than euploid embryos (median number of nuclei: 43.5, 47, 90, respectively). This correlates with a higher expression of apoptotic markers Caspase-3/7 in CA embryos (p = 0.0199) and Caspase-8 in both aneuploid groups (CA: p = 0.0085 and RT: p = 0.0394). Aneuploid embryos showed significantly increased HSP70 levels (median intensity per cell: euploid=165, CA = 313, RT = 400), LC3B (median puncta per cell: euploid=3.07, CA = 10.10, RT = 19.62) and p62 (median puncta per cell: euploid=17.60, CA = 30.53), suggesting increased proteotoxic stress and autophagy. Preliminary analysis of the RNA-sequencing data reveals enrichment for pathways such as the p53-pathway, protein secretion, TNFA signaling via NFkB and apoptosis, supporting the hypothesis of a link between aneuploidy and apoptosis.
Limitations, reasons for caution
No functional tests e.g. with inhibitors of autophagy were carried out. RNA-sequencing was carried out on a small sample; we will expand this sample in the near future.
Wider implications of the findings
This study shows for the first time the mechanism by which aneuploid cells are eliminated from the human preimplantation embryo, explaining how mosaic embryos can still lead to a healthy and genetically normal live birth.
Trial registration number
not applicable
Expression of genes involved in early cell fate decisions in human embryos and their regulation by growth factors