MOSAICISM AND ANEUPLOIDY LEAD TO GRADUAL LOSS OF BIOCHEMICAL IDENTITY AND INDUCE APOPTOSIS IN HUMAN BLASTOCYSTS

To investigate the transcriptomic consequences of mosaicism and aneuploidy in the inner cell mass (ICM) and the trophectoderm (TE) of human blastocysts. Prospective study comparing the transcriptomic identities of the ICM and the TE of blastocysts classified as euploid (n=5), mosaic (n=8) and aneuploid (n=6) by preimplantation genetic testing for aneuploidy. Mosaicism was defined in the range 30%-<70%. For RNA-seq, differentially expressed genes were calculated with DESeq2 package [Benjamini-Hochberg (BH)-adjusted p<0.01 & abs(log2FoldChange)>2 significant]. Fgsea algorithm was used for enrichment analysis on Kyoto Encyclopedia of Genes and Genomes pathways and Gene Ontology (GO) terms (BH-adjusted p<0.01 significant). ICM and TE of euploid blastocysts displayed expected lineage-specific transcriptomic differences. Cell cycle and 9 GO processes related to embryogenesis, including organogenesis and apoptosis involved in anatomical structure development, were significantly upregulated in the ICM. In contrast, telomere lengthening and 67 GO processes involved in energy production and biosynthesis, metabolism and transport of nucleic acids, carbohydrates, proteins and steroids were significantly upregulated in the TE. In mosaic and aneuploid blastocysts, several developmental and metabolic processes were also upregulated in the ICM and the TE, respectively. However, aneuploidy led to a stepwise impairment of cellular functions in both lineages. In the ICM, mosaic and aneuploid blastocysts significantly downregulated cell growth and upregulated inflammatory and extrinsic apoptotic pathways. This anti-survival effect was exacerbated in aneuploid blastocysts, which upregulated other stress response pathways in the ICM, including cell death due to oxidative stress and intrinsic apoptosis. In the TE, the number of significantly-upregulated metabolic processes decreased in mosaic and aneuploid blastocysts (48 and 5 GO processes, respectively), suggesting a gradual loss of biochemical features in response to aneuploidy. Transcriptomic identities of ICM and TE reflect their different roles in normal embryo development. While the ICM proliferates under a relatively quiescent metabolism, the TE serves as an active provider of energy and metabolites. Aneuploidy gradually disrupts TE biochemical functions and activates specific cell death programs in the ICM. This is consistent with the known dual role of apoptosis as an effector of both physiological and stress-driven pathological programs during embryogenesis.