Identification of Bivalent Domains in Baboon Induced Pluripotent Cells and Pachytene Spermatocytes and Round Spermatids

Bivalent domains in genes are defined as having simultaneously both an active histone modification such as trimethylation on histone H3 Lysine 4 (H3K4me3) and a repressive modification such as trimethylation of histone H3 Lysine 27 (H3K27me3) within the same region of an individual gene. Bivalent domains have been identified in embryonic stem cells, induced pluripotent stem cells (iPSCs) and germ lines of several species and are believed to silence developmental regulators enabling the maintenance of pluripotency and provide for initiating differentiation more quickly. The olive baboon (Papio anubis) is an outstanding model for stem cell derived therapies owing to its morphological, physiological, immunological and genetic homology with the human. The presence or absence of bivalent marks on particular developmental genes has not been studied in this animal model. Here we examined bivalent domains of 27 genes from different protein class and functionality which are poised in pachytene spermatocytes (PSs) and round spermatids (RSs) of five mammalian species (human, rhesus, bulls, mouse and opossum) in baboon iPSCs, PSs and RSs. Chromatin immunoprecipitation-quantitative PCR was performed to detect H3K4me3 and H3K27me3 enrichment on a region spanning --1000 bp of and including the transcription start site. We found that 24 of 27 genes have bivalent marks in baboon iPSC and 23 of 27 genes are bivalently marked in both baboon iPSC and human iPSC. We also found that 20 of 27 genes have bivalent marks in both PS and RS while four genes (SLITRK5, SOBP, HOXD10 and KCNK3) are not poised in either spermatocytes or spermatids. We find that there is significant gene bivalency in baboon pluripotent and germ cells similar to that seen in humans and other mammalian species. Although there is a high percentage of bivalent genes in both human and baboon PSs and RSs, the rhesus and other mammalians poised genes in PSs and RSs have a complete overlapping among these 27 genes with human. This result could be explained by different methodology employed, the different definition criteria of poised genes and bona fide differences from other species. Further research should be done to gain a broad view of poised genes in baboon models.