Regulatory T Cell Modulation by CBP/EP300 Bromodomain Inhibition

Abstract

Covalent modification of histones is a fundamental mechanism of regulated gene expression in eukaryotes, and interpretation of histone modifications is an essential feature of epigenetic control. Bromodomains are specialized binding modules that interact with acetylated histones, linking chromatin recognition to gene transcription. Because of their ability to function in a domain-specific fashion, selective disruption of bromodomain:acetylated histone interactions with chemical probes serves as a powerful means for understanding biological processes regulated by these chromatin adaptors. Here we describe the discovery and characterization of potent and selective small molecule inhibitors for the bromodomains of CREBBP/EP300 that engage their target in cellular assays. We use these tools to demonstrate a critical role for CREBBP/EP300 bromodomains in regulatory T cell biology. Because regulatory T cell recruitment to tumors is a major mechanism of immune evasion by cancer cells, our data highlight the importance of CREBBP/ EP300 bromodomain inhibition as a novel, small moleculebased approach for cancer immunotherapy.

Treg programming and therapeutic reprogramming in cancer

“…Post‐transcriptional acetylation of Foxp3 by the histone acetyltransferase (HAT) EP300 enhances Foxp3 stability and activity. EP300 inhibition selectively reduces the frequency and suppressive function of Tregs within tumours by reducing acetylation of Foxp3 itself, as well as reducing histone acetylation (a stimulating transcriptional mark) at key Treg‐activated genes, leading to decreased expression of Foxp3, LAG‐3, CTLA‐4 and TIM‐3 . In addition, pharmacological inhibitors that block the interaction of other bromodomains with acetylated histones, such as JQ1, can selectively disrupt the function of Tregs in tumours while leaving antitumour effector T‐cells fully functional …”

“…EP300 inhibition selectively reduces the frequency and suppressive function of Tregs within tumours by reducing acetylation of Foxp3 itself, as well as reducing histone acetylation (a stimulating transcriptional mark) at key Treg-activated genes, leading to decreased expression of Foxp3, LAG-3, CTLA-4 and TIM-3. 86,87 In addition, pharmacological inhibitors that block the interaction of other bromodomains with acetylated histones, such as JQ1, can selectively disrupt the function of Tregs in tumours while leaving antitumour effector T-cells fully functional. [86][87][88] Foxp3 transcription is also silenced epigenetically via DNA methylation of its locus at key conserved non-coding sites.…”

“…86,87 In addition, pharmacological inhibitors that block the interaction of other bromodomains with acetylated histones, such as JQ1, can selectively disrupt the function of Tregs in tumours while leaving antitumour effector T-cells fully functional. [86][87][88] Foxp3 transcription is also silenced epigenetically via DNA methylation of its locus at key conserved non-coding sites. Importantly, evidence from animal models and humans indicates that these loci are stably unmethylated in TI-Tregs, lending credence to the hypothesis that the TME naturally supports the immunosuppressive Treg programme.…”

Chemical probes targeting epigenetic proteins: Applications beyond oncology

“…Transcriptional profiling of human T cells treated with SGC-CBP30 showed a much more restricted effect on gene expression than that observed with (+)-JQ1, 60 but further studies will be necessary to fully explore the potential of CBP antagonism in autoimmune diseases. Independently, Genentech (GNE) and Constellation Pharmaceuticals (CPI) found a role for CBP inhibitors in the biology of regulatory T cells, which play important roles in cancer immunotherapy 61 . The p300/CBP associated factor, PCAF, another bromodomaining containing HAT has also been implicated in inflammatory diseases, and specific antagonists to the bromodomain of this protein have just been reported 62,63 …”

CREBBP/EP300 Bromodomain Inhibition Affects the Proliferation of AR-Positive Breast Cancer Cell Lines

“…However, these early compounds, although promising, lacked potency or specificity (6,8). Prompted by the preclinical success of BET bromodomains inhibitors, dual inhibitors of the bromodomains of CREBBP and EP300 have been recently developed (9)(10)(11)(12)(13)(14)(15). These inhibitors mediate several biological responses including antiproliferative effects in hematologic cancer cell lines, such as leukemia (11,13,16) and multiple myeloma (17) cell lines, and AR-positive prostate cancer cell lines (15).…”