Ling Cai

DNA methyltransferase DNMT1 plays an essential role in maintaining DNA methylation homeostasis. While DNMT1-mediated DNA methylation mainly occurs outside the active chromatin domains, such as CpG islands, how DNMT1 interplays with hypomethylated nucleosome environment to restrict CpG island methylation remains unclear. Here we report the structure of DNMT1 with a nucleosome containing unmodified CpG sites. The interaction of the DNMT1 CXXC domain with the unmodified CpG sites within the nucleosome core or the linker strengthens the DNMT1-nucleosome association, which contributes to the enrichment of DNMT1 in CpG islands. On the other hand, the CXXC domain is positioned in a manner that blocks the target recognition domain of DNMT1 from accessing potential DNA substrates. Consistently, disrupting the CXXC-CpG interaction leads to DNMT1 hyperactivity on nucleosomes with unmodified CpG sites, or those with spatially proximate unmodified and hemimethylated CpG sites. Furthermore, our whole-genome bisulfite-sequencing analysis reveals that introducing the CXXC-CpG interaction-defective DNMT1 mutation leads to substantial increase in CpG island methylation but not so much in overall genomic methylation, causing severe defects in transcriptional programming and stem cell differentiation. Together, this study establishes a role for the CXXC domain in safeguarding the CpG island hypomethylation, vital to DNA methylation homeostasis during development.

The H3K27me-specific methyltransferase enhancer of zeste homologue 2 (EZH2) is the catalytic subunit of the repressive complex Polycomb repressive complex 2. EZH2 is typically implicated in transcriptional silencing, but it can also activate gene expression. Here, we show that EZH2 contains three adjacent transactivation domains (EZH2TAD) that are recognized by the TAZ2 domain of the transcriptional coactivator and acetyltransferase p300 (p300TAZ2). Binding interfaces identified by chemical shift perturbations in NMR experiments, measurements of binding affinities, and analysis of the complex formation by mass photometry demonstrate that each EZH2TAD can be concomitantly bound by a separate p300TAZ2. Interaction of EZH2TADs with p300TAZ2 stimulates H3K18- and H3K27-specific acetyltransferase activity of p300. We show that in 22Rv1 prostate cancer cells, EZH2 occupies a large set of gene loci lacking H3K27me3, and these non-canonical genomic sites are instead co-occupied by p300, RNA Polymerase II and BRD4 and are rich in histone marks associated with transcriptional activation. Our findings shed light on the potential basis for such a high degree of genetic co-localization through the direct association of p300TAZ2 with EZH2TADs.

Cancer-causing aberrations recurrently target the chromatic-regulatory factors, leading to epigenetic dysregulation. Almost all patients with synovial sarcoma (SS) carry a characteristic gene fusion, SS18::SSX, which produces a disease-specific oncoprotein that is incorporated into the switch/sucrose non-fermentable (SWI/SNF) chromatin-remodeling complexes and profoundly alters their functionalities. Targeting epigenetic dependency in cancers holds promise for improving current treatment. Leveraging on cancer cell dependency dataset, pharmacological tools, and genomic profiling, we find WDR5, a factor critical for depositing histone H3 lysine 4 (H3K4) methylation, to be an unexplored vulnerability in SS. Mechanistically, WDR5 and SS18::SSX interact and colocalize at oncogenes where WDR5 promotes H3K4 methylation and the chromatin association of SS18::SSX-containing chromatin-remodeling complexes. WDR5 degradation by proteolysis-targeting chimera (PROTAC) not only suppresses the SS18::SSX-related oncogenic programs but additionally causes the ribosomal protein deregulations leading to p53 activation. WDR5-targeted PROTAC suppresses SS growth in vitro and in vivo, providing a promising strategy for the SS treatment.

Breast cancer (BC) remains the second leading cause of cancer-related mortalities in women. Resistance to hormone therapies such as tamoxifen, an estrogen receptor (ER) inhibitor, is a major hurdle in the treatment of BC. Enhancer of zeste homolog 2 (EZH2), the methyltransferase component of the Polycomb repressive complex 2 (PRC2), has been implicated in tamoxifen resistance. Evidence suggests that EZH2 often functions noncanonically, in a methyltransferase-independent manner, as a transcription coactivator through interacting with oncogenic transcription factors. Unlike methyltransferase inhibitors, proteolysis targeting chimeras (PROTAC) can suppress both activating and repressive functions of EZH2. Here, we find that EZH2 PROTACs, MS177 and MS8815, effectively inhibited the growth of BC cells, including those with acquired tamoxifen resistance, to a much greater degree when compared to methyltransferase inhibitors. Mechanistically, EZH2 associates with forkhead box M1 (FOXM1) and binds to the promoters of FOXM1 target genes. EZH2 PROTACs induce degradation of both EZH2 and FOXM1, leading to reduced expression of target genes involved in cell cycle progression and tamoxifen resistance. Together, this study supports that EZH2-targeted PROTACs represent a promising avenue of research for the future treatment of BC, including in the setting of tamoxifen resistance.

Prostate cancer (PCa) is a heterogeneous disease exhibiting both genetic and epigenetic deregulations. Epigenetic alterations are defined as changes not based on DNA sequence, which include those of DNA methylation, histone modification, and chromatin remodeling. Androgen receptor (AR) is the main driver for PCa and androgen deprivation therapy (ADT) remains a backbone treatment for patients with PCa; however, ADT resistance almost inevitably occurs and advanced diseases develop termed castration-resistant PCa (CRPC), due to both genetic and epigenetic changes. Due to the reversible nature of epigenetic modifications, inhibitors targeting epigenetic factors have become promising anti-cancer agents. In this chapter, we focus on recent studies about the dysregulation of epigenetic regulators crucially involved in the initiation, development, and progression of PCa and discuss the potential use of inhibitors targeting epigenetic modifiers for treatment of advanced PCa.