Transcriptomic analysis and epigenetic regulators in human oocytes at different stages of oocyte meiotic maturation

Human oocytes are highly specialized cells with the capacity to store and regulate mRNAs during oocyte maturation, in preparation for post-fertilization steps. Here we performed single-oocyte transcriptomic analysis of human oocytes in three meitoic maturation stages - Germinal Vesicle (GV; n = 6), Metaphase I (MI; n = 6) and Metaphase II (MII; n = 7). Single-oocyte transcriptomic analysis revealed that the total number of expressed genes progressively decreased from GV to MII stages, with 9660 genes being transcribed in GV, 8734 in MI and 5889 in MII. The same tendency was observed for the number of uniquely expressed genes, with 1328 uniquely expressed genes in GV, 401 in MI and 72 in MII. GO analysis of the uniquely expressed genes showed distinct terms in GV oocytes such as transferase activity, organonitrogen compound metabolic process and ncRNA processing. Analysis of Differentially Expressed Genes (DEGs) between the three maturation stages revealed 1165 DEGs between GV and MII oocytes, with 635 being upregulated and 528 downregulated, 42 DEGs between GV and MI, with 38 being upregulated and 4 downregulated, and no significant changes in gene expression between MI and MII oocytes. Comprehensive analysis of epigenetic regulators showed high expression of several histone-modifying enzymes, namely deacetylases, acetylases, lysine demethylases and methyltransferases, and DNA methylation regulators, namely the maintenance methyltransferase DNMT1 and its co-regulators DPPA3 and UHRF1. Some of these epigenetic regulators were differentially expressed between maturation stages, namely SIRT3, SIRT6, KDM3AP1, KMT2E, DNMT1, DPPA3 and the MEST and RASGRF1 imprinted genes. Our study contributes with important information on the transcriptional landscape of human oocytes in different stages of meiotic maturation, providing important insights into candidate biomarkers of human oocyte quality.