Dynamic evolution of EZHIP, an inhibitor of the Polycomb Repressive Complex 2 in mammals

The Polycomb Repressive Complex 2 (PRC2) is an ancient, conserved chromatin-interacting complex that controls gene expression, facilitating differentiation and cellular identity during development. Its regulation is critical in most eukaryotes. EZHIP was recently characterized as a PRC2 inhibitor and 'oncohistone mimic' in mammals. Although Ezhip expression is typically restricted to the germline, its aberrant expression in pediatric brain tumors inhibits PRC2-mediated H3K27 methylation and drives disease progression. To gain a deeper understanding of its normal functions, we systematically examined Ezhip evolution across 70 mammals using comparative genomics, synteny analysis, and motif discovery. Bolstering previous work, we find that Ezhip originated and has been strictly retained on the X chromosome in placental mammals. In addition to the highly conserved H3K27M-like histone mimic motif, our motif analysis reveals seven previously unidentified EZHIP motifs, including a putative nuclear localization signal, and tandem repeats that are largely well-conserved in placental mammals, except in some lineages. We hypothesize that these motifs are also critical to EZHIP's functions, including in PRC2 interaction and inhibition. We show that Ezhip has evolved under strong diversifying selection in primates and underwent dynamic expansions and losses across species. Some paralogs, such as Ezhip2 in primates, also evolved under positive selection. Based on its evolutionary attributes and germ-cell expression, we propose that Ezhip arose and evolved rapidly due to inter-parental conflict over fetal development in utero in placental mammals. Our work provides a foundation for further investigations into EZHIP's essential, potentially multifaceted roles in mammalian reproduction and disease.