Role of CIZ1 in maintenance of epigenetic landscape in primary differentiated mouse embryonic fibroblasts

The inactive X chromosome (Xi) serves as a model for establishment and maintenance of repressed chromatin and the function of polycomb repressive complexes (PRC1/2). Here we show that Xi transiently relocates from the nuclear periphery towards the interior during its replication, in a process dependent on CIZ1. Compromised relocation of Xi in CIZ1-null primary mouse embryonic fibroblasts is accompanied by loss of PRC-mediated H2AK119Ub1 and H3K27me3, increased solubility of PRC2 catalytic subunit EZH2, and genome-wide deregulation of polycomb-regulated genes. Xi position in S phase is also corrupted in cells adapted to long-term culture (WT or CIZ1-null), and also accompanied by specific changes in EZH2 and its targets. The data are consistent with the idea that chromatin relocation during S phase contributes to maintenance of epigenetic landscape in primary cells, and that elevated soluble EZH2 is part of an error-prone mechanism by which modifying enzyme meets template when chromatin relocation is compromised. Transcriptome analysis of primary (before passage 5) and culture-adapted derivative cell lines of murine embryonic fibroblasts in female WT (6 replicates) and female CIZ1 null (6 replicates) by RNA-seq. Additional samples include 3 inducible CIZ1-null primary murine embryonic fibroblast, 2 of which were induced with doxycycline.