Multivalent tumor suppressor adenomatous polyposis coli promotes Axin biomolecular condensate formation and efficient ß-catenin degradation.

The tumor suppressor adenomatous polyposis coli (APC) is frequently mutated in colorectal cancers. APC and Axin are core components of a destruction complex that scaffolds GSK3ß and CK1 to earmark ß-catenin for proteosomal degradation. Disruption of APC results in pathologic stabilization of ß-catenin and oncogenesis. However, the molecular mechanism by which APC promotes ß-catenin degradation is unclear. Here, we find that the intrinsically disordered region (IDR) of APC, which contains multiple ß-catenin and Axin interacting sites, undergoes liquid-liquid phase separation (LLPS) in vitro. Expression of the APC IDR in colorectal cells promotes Axin puncta formation and ß-catenin degradation. Our results support the model that multivalent interactions between APC and Axin drives the ß-catenin destruction complex to form biomolecular condensates in cells, which concentrate key components to achieve high efficient degradation of ß-catenin.

Histone lysine methyltransferases in biology and disease.

The precise temporal and spatial coordination of histone lysine methylation dynamics across the epigenome regulates virtually all DNA-templated processes. A large number of histone lysine methyltransferase (KMT) enzymes catalyze the various lysine methylation events decorating the core histone proteins. Mutations, genetic translocations and altered gene expression involving these KMTs are frequently observed in cancer, developmental disorders and other pathologies. Therapeutic compounds targeting specific KMTs are currently being tested in the clinic, although overall drug discovery in the field is relatively underdeveloped. Here we review the biochemical and biological activities of histone KMTs and their connections to human diseases, focusing on cancer. We also discuss the scientific and clinical challenges and opportunities in studying KMTs.

Rapid and reversible epigenome editing by endogenous chromatin regulators.

Understanding the causal link between epigenetic marks and gene regulation remains a central question in chromatin biology. To edit the epigenome we developed the FIRE-Cas9 system for rapid and reversible recruitment of endogenous chromatin regulators to specific genomic loci. We enhanced the dCas9-MS2 anchor for genome targeting with Fkbp/Frb dimerizing fusion proteins to allow chemical-induced proximity of a desired chromatin regulator. We find that mSWI/SNF (BAF) complex recruitment is sufficient to oppose Polycomb within minutes, leading to activation of bivalent gene transcription in mouse embryonic stem cells. Furthermore, Hp1/Suv39h1 heterochromatin complex recruitment to active promoters deposits H3K9me3 domains, resulting in gene silencing that can be reversed upon washout of the chemical dimerizer. This inducible recruitment strategy provides precise kinetic information to model epigenetic memory and plasticity. It is broadly applicable to mechanistic studies of chromatin in mammalian cells and is particularly suited to the analysis of endogenous multi-subunit chromatin regulator complexes.Understanding the link between epigenetic marks and gene regulation requires the development of new tools to directly manipulate chromatin. Here the authors demonstrate a Cas9-based system to recruit chromatin remodelers to loci of interest, allowing rapid, reversible manipulation of epigenetic states.

Arabidopsis AtMORC4 and AtMORC7 Form Nuclear Bodies and Repress a Large Number of Protein-Coding Genes.

The MORC family of GHKL ATPases are an enigmatic class of proteins with diverse chromatin related functions. In Arabidopsis, AtMORC1, AtMORC2, and AtMORC6 act together in heterodimeric complexes to mediate transcriptional silencing of methylated DNA elements. Here, we studied Arabidopsis AtMORC4 and AtMORC7. We found that, in contrast to AtMORC1,2,6, they act to suppress a wide set of non-methylated protein-coding genes that are enriched for those involved in pathogen response. Furthermore, atmorc4 atmorc7 double mutants show a pathogen response phenotype. We found that AtMORC4 and AtMORC7 form homomeric complexes in vivo and are concentrated in discrete nuclear bodies adjacent to chromocenters. Analysis of an atmorc1,2,4,5,6,7 hextuple mutant demonstrates that transcriptional de-repression is largely uncoupled from changes in DNA methylation in plants devoid of MORC function. However, we also uncover a requirement for MORC in both DNA methylation and silencing at a small but distinct subset of RNA-directed DNA methylation target loci. These regions are characterized by poised transcriptional potential and a low density of sites for symmetric cytosine methylation. These results provide insight into the biological function of MORC proteins in higher eukaryotes.

Transcriptional gene silencing by Arabidopsis microrchidia homologues involves the formation of heteromers.

Epigenetic gene silencing is of central importance to maintain genome integrity and is mediated by an elaborate interplay between DNA methylation, histone posttranslational modifications, and chromatin remodeling complexes. DNA methylation and repressive histone marks usually correlate with transcriptionally silent heterochromatin, however there are exceptions to this relationship. In Arabidopsis, mutation of Morpheus Molecule 1 (MOM1) causes transcriptional derepression of heterochromatin independently of changes in DNA methylation. More recently, two Arabidopsis homologues of mouse microrchidia (MORC) genes have also been implicated in gene silencing and heterochromatin condensation without altering genome-wide DNA methylation patterns. In this study, we show that Arabidopsis microrchidia (AtMORC6) physically interacts with AtMORC1 and with its close homologue, AtMORC2, in two mutually exclusive protein complexes. RNA-sequencing analyses of high-order mutants indicate that AtMORC1 and AtMORC2 act redundantly to repress a common set of loci. We also examined genetic interactions between AtMORC6 and MOM1 pathways. Although AtMORC6 and MOM1 control the silencing of a very similar set of genomic loci, we observed synergistic transcriptional regulation in the mom1/atmorc6 double mutant, suggesting that these epigenetic regulators act mainly by different silencing mechanisms.

MORC family ATPases required for heterochromatin condensation and gene silencing.

Transposable elements (TEs) and DNA repeats are commonly targeted by DNA and histone methylation to achieve epigenetic gene silencing. We isolated mutations in two Arabidopsis genes, AtMORC1 and AtMORC6, which cause derepression of DNA-methylated genes and TEs but no losses of DNA or histone methylation. AtMORC1 and AtMORC6 are members of the conserved Microrchidia (MORC) adenosine triphosphatase (ATPase) family, which are predicted to catalyze alterations in chromosome superstructure. The atmorc1 and atmorc6 mutants show decondensation of pericentromeric heterochromatin, increased interaction of pericentromeric regions with the rest of the genome, and transcriptional defects that are largely restricted to loci residing in pericentromeric regions. Knockdown of the single MORC homolog in Caenorhabditis elegans also impairs transgene silencing. We propose that the MORC ATPases are conserved regulators of gene silencing in eukaryotes.