Sofi, S. | bioRxiv Science

bioRxiv2023

Dominant CIZ1 fragments drive epigenetic instability and are expressed in early stage cancers

CIZ1 is a nuclear matrix protein that is part of the large RNA-dependent supramolecular assembly complexes (SMACs) that form at the inactive X chromosome (Xi) in female cells, and smaller assemblies throughout the nucleus in males and females. It plays a role in maintenance of epigenetic state and gene expression in differentiated cells, via stabilisation of histone post-translational modifications H2AK119ub1 and H3K27me3, added by polycomb repressive complexes (PRC) 1 and 2. Here, we show that expression of the N-terminal replication domain (RD) and C-terminal anchor domain (AD) of human CIZ1 transcript is uncoupled, with consistently elevated AD in early stage breast cancers, and sporadically elevated AD in other common solid tumours. At the protein level CIZ1-Xi SMACs are corrupted in female breast cancers cells, and this is accompanied by elevated AD-encoding transcripts. We modelled the effect of AD fragments in primary murine embryonic fibroblasts and observed dominant-negative interference with CIZ1 SMACs during their assembly in early G1 phase. Mutagenesis identified the matrin 3 homology domain as essential for self-interaction to form stable homodimers in vitro, and as a determinant of its dominant-negative effect in cells, implicating the dimerization interface in CIZ1 SMAC integrity. SMAC disruption was coincident with depletion of PRC1-dependent H2AK119ub1 from Xi chromatin, in a manner abrogated by the PR-deubiquitinase inhibitor PR619, suggesting that CIZ1 SMACs normally stabilise H2AK119ub1 by shielding Xi chromatin from attack by deubiquitinases. Moreover, SMAC disruption was accompanied by changes in gene expression within days. Together, the data suggest that inappropriate expression of CIZ1 AD fragments could drive epigenetic instability in early stage breast cancers by destabilizing the CIZ1 SMACs that normally protect repressed chromatin. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=80 SRC="FIGDIR/small/558821v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@13b1be2org.highwire.dtl.DTLVardef@1a3a16corg.highwire.dtl.DTLVardef@1bac386org.highwire.dtl.DTLVardef@7ac176_HPS_FORMAT_FIGEXP M_FIG C_FIG

cell biology↗

bioRxiv2020

A polyglutamine domain is required for de novo CIZ1 assembly formation at the inactive X chromosome

CIP1-interacting zinc finger protein 1 (CIZ1) forms large assemblies at the inactive X chromosome (Xi) in female fibroblasts in an Xist lncRNA-dependent manner. Here we address the requirements for assembly formation, and show that CIZ1 interacts directly with Xist via two independent domains in its N- and C-terminus. Interaction with Xist repeat E, assembly at Xi in cells, and the complexity of self-assemblies formed in vitro, are all modulated by alternatively-spliced exons that include two glutamine-rich prion-like domains (PLD1 and PLD2), both conditionally excluded from the N-terminal domain. Exclusion of PLD1 alone is sufficient to abrogate de novo establishment of new CIZ1 assemblies and Xi territories enriched for H3K27me3 in CIZ1-null fibroblasts. Together the data suggest that PLD1-driven CIZ1 assemblies form at Xi, are nucleated by interaction with Xist and amplified by multivalent interaction with RNA, so implicating a polyglutamine tract in the maintenance of epigenetic state.

cell biology↗