Search

Histone acetyltransferases (HAT) catalyze the acylation of lysine side chains and are implicated in diverse human cancers as both oncogenes and non-oncogene dependencies1. Acetyl-CoA-competitive HAT inhibitors have garnered attention as potential cancer therapeutics and the first clinical trial for this class is ongoing (NCT04606446). Despite broad enthusiasm for these targets, notably including CBP/p300 and KAT6A/B2–5, the potential mechanisms of therapeutic response and evolved drug resistance remain poorly understood. Using comparative transcriptional genomics, we found that the direct gene regulatory consequences of CBP/p300 HAT inhibition are indistinguishable in models of intrinsically hypersensitive and insensitive acute myeloid leukemia (AML). We therefore modelled acquired drug resistance using a forward genetic selection and identified dysregulation of coenzyme A (CoA) metabolism as a facile driver of resistance to HAT inhibitors. Specifically, drug resistance selected for mutations in PANK3, a pantothenate kinase that controls the rate limiting step in CoA biosynthesis6. These mutations prevent negative feedback inhibition, resulting in drastically elevated concentrations of intracellular acetyl-CoA, which directly outcompetes drug-target engagement. This not only impacts the activity of structurally diverse CBP/p300 HAT inhibitors, but also agents related to an investigational KAT6A/B inhibitor that is currently in Phase-1 clinical trials. We further validated these results using a genome-scale CRISPR/Cas9 loss-of-function genetic modifier screen, which identified additional gene-drug interactions between HAT inhibitors and the CoA biosynthetic pathway. Top hits from the screen included the phosphatase, PANK4, which negatively regulates CoA production and therefore suppresses sensitivity to HAT inhibition upon knockout7, as well as the pantothenate transporter, SLC5A68, which enhances sensitivity. Altogether, this work uncovers CoA plasticity as an unexpected but potentially class-wide liability of anti-cancer HAT inhibitors and will therefore buoy future efforts to optimize the efficacy of this new form of targeted therapy.

Histone deacetylases (HDACs) have been widely pursued as targets for anti-cancer therapeutics. However, many of these targets are universally essential for cell survival, which may limit the therapeutic window that can be achieved by drug candidates. By examining large collections of CRISPR/Cas9-based essentiality screens, we discovered a genetic interaction between HDAC1 and HDAC2 wherein each paralog is synthetically lethal with hemizygous deletion of the other. This collateral synthetic lethality is caused by recurrent chromosomal translocations that occur in diverse solid and hematological malignancies, including neuroblastoma and multiple myeloma. Using genetic deletion or dTAG-mediated degradation, we show that HDAC2 disruption suppresses the growth of HDAC1-deficient neuroblastoma in vitro and in vivo. Mechanistically, we find that targeted degradation of HDAC2 in these cells prompts the degradation of several members of the nucleosome remodeling and deacetylase (NuRD) complex, leading to diminished chromatin accessibility at HDAC2/NuRD-bound sites of the genome and impaired control of enhancer-associated transcription. Furthermore, we reveal that several of the degraded NuRD complex subunits are dependencies in neuroblastoma and multiple myeloma, providing motivation to develop paralog-selective HDAC1 or HDAC2 degraders. Altogether, we identify HDAC1/2 collateral synthetic lethality as a new therapeutic target and reveal a novel mechanism for exploiting NuRD-associated cancer dependencies.

Transcriptional co-regulators, which mediate chromatin-dependent transcriptional signaling, represent tractable targets to modulate tumorigenic gene expression programs with small molecules. Genetic loss-of-function studies have recently implicated the transcriptional co-activator, ENL, as a selective requirement for the survival of acute leukemia and highlighted an essential role for its chromatin reader YEATS domain. Motivated by these discoveries, we executed a screen of nearly 300,000 small molecules and identified an amido-imidazopyridine inhibitor of the ENL YEATS domain (IC50 = 7 µM). Leveraging a SuFEx-based high-throughput approach to medicinal chemistry optimization, we discovered SR-0813 (IC50 = 25 nM), a potent and selective ENL/AF9 YEATS domain inhibitor that exclusively inhibits the growth of ENL-dependent leukemia cell lines. Armed with this tool and a first-in-class ENL PROTAC, SR-1114, we detailed the response of AML cells to pharmacological ENL disruption for the first time. Most notably, displacement of ENL from chromatin by SR-0813 elicited a strikingly selective suppression of ENL target genes, including HOXA9/10, MYB, MYC and a number of other leukemia proto-oncogenes. Our study reproduces a number of key observations previously made by CRISPR/Cas9 loss of function and dTAG-mediated degradation, and therefore, both reinforces ENL as an emerging leukemia target and validates SR-0813 as a high-quality chemical probe.

ENL is a transcriptional co-activator that recruits elongation machinery to active cis-regulatory elements upon binding of its YEATS domain – a chromatin reader module – to acylated lysine side chains. Discovery chemistry for the ENL YEATS domain is highly motivated by its significance in acute leukemia pathophysiology, but cell-based assays able to support large-scale screening or hit validation efforts do not presently exist. Here, we report on the discovery of a target engagement assay that allows for high-throughput ligand discovery in living cells. This assay is based on the cellular thermal shift assay (CETSA) but does not require exposing cells to elevated temperatures, as small-molecule ligands are able to stabilize the ENL YEATS domain at 37 °C. By eliminating temperature shifts, we developed a simplified target engagement assay that requires just two steps: drug treatment and luminescence detection. To demonstrate its value for higher throughput applications, we miniaturized the assay to a 1,536-well format and screened 37,120 small molecules, ultimately identifying an acyl-lysine-competitive ENL/AF9 YEATS domain inhibitor.

Upon binding of transcription factors to cis-regulatory DNA sequences, transcriptional co-regulators are required for activation or suppression of chromatin-dependent transcriptional signaling. These co-regulators are frequently implicated in oncogenesis via causal roles in dysregulated, malignant transcriptional control and have represented one of the fastest growing target classes in small-molecule drug discovery. However, challenges in targeting co-regulators include identifying evidence of a cancer-specific genetic dependency, matching the pharmacologically addressable protein fold to a functional role in disease pathology, and achieving the necessary selectivity to exploit a given genetic dependency. Here, we discuss how recent trends in cancer pharmacology have confronted these challenges, positioning co-regulators as tractable targets towards the development of emergent cancer therapies.