The RUNX1 transcription factor mediates cell-type specific gene expression. RUNX1 suppression and perturbations are recurrently associated with breast tumor initiation and progression. However, the mechanisms governing the dual roles of RUNX1 in sustaining the mammary epithelial phenotype while epigenetically suppressing initiation of cancer-compromised gene expression are poorly understood. To address this, we used the power of degron-mediated acute, selective, and complete RUNX1 ablation in human mammary epithelial cells. RUNX1 mediates promoter and distal enhancer-driven expression of a gene cohort. Dynamic epigenomic responsiveness upon RUNX1 ablation reveals a rapid and selective decrease in chromatin accessibility and H3K27ac at RUNX1-bound enhancers, but not promoters. While differentially initiated and expressed genes contacted by RUNX1-bound enhancers are enriched in pathways involved in epithelial maintenance and stemness, genes with RUNX1-promoter occupancy support DNA damage responsiveness. Modified cell morphology, metabolic control, increased breast cancer stemness, plasticity, anchorage-independent survival, chemoresistance, and perturbed DNA damage reactivity are observed upon RUNX1 ablation. Together, these findings define RUNX1 as an epigenetic tumor suppressor that maintains epithelial cell state by preserving enhancer activity and preventing gene expression associated with hallmarks of cancer.

Lineage-defining transcription factors are key oncogenic drivers but remain difficult to target pharmacologically due to the absence of ligandable pockets. The molecular rules governing substrate recognition by large HECT ubiquitin ligases also remain incompletely understood, limiting efforts to exploit these enzymes for targeted protein degradation. Here we combine genome-wide CRISPR knockout screening with base editor tiling screens at amino acid resolution, both coupled to an endogenous knock-in reporter of the SCLC lineage oncogenic transcription factor ASCL1, to systematically interrogate the mechanisms governing its degradation. These complementary screens unbiasedly identify the HECT ubiquitin ligase HUWE1 as the dominant regulator of ASCL1 stability in small cell lung cancer (SCLC) and resolve a conserved C-terminal phospho-degron centered on Ser207 and terminal Trp/Phe residues that are required for HUWE1 docking and ubiquitin-mediated degradation. Unexpectedly, base editor screening further uncovers a previously unrecognized regulatory module within HUWE1: a short negatively charged helix that functions as an autoinhibitory gate controlling access of phospho-degron substrates to HUWE1. Charge-flipping mutations within this regulatory helix relieve autoinhibition and accelerate degradation of multiple HUWE1 phospho-degron substrates, including ASCL1 and the canonical HUWE1 substrate DDIT4. Stabilization of ASCL1 through degron disruption paradoxically impairs SCLC proliferation, revealing that dynamic proteasome-coupled turnover is required for transcription factor function. Together, these findings reveal molecular rules governing HUWE1 phospho-degron recognition and identify a regulatory gate controlling substrate engagement. They also illustrate a generalizable strategy for resolving degradation mechanisms of undruggable transcription factors in their endogenous cellular context.

Small cell lung cancer (SCLC) is a highly aggressive malignancy that lacks effective targeted therapies, in part due to frequent loss-of-function mutations in tumor suppressors and the absence of recurrent oncogenic drivers. Approximately 15% of SCLCs harbor inactivating mutations in NOTCH1 or NOTCH2, and most neuroendocrine-high SCLCs exhibit low NOTCH activity. Using CRISPR-Cas9 screening in primary cell lines derived from NOTCH1/2-isogenic SCLC genetically engineered mouse models, we identified TRIM28 as a synthetic lethal dependency in NOTCH2-inactivated SCLCs. Loss of TRIM28 in this context robustly induced expression of endogenous retroviruses (ERVs), activated viral sensing pathways, and triggered a type I interferon response. Mechanistically, NOTCH2 inactivation increased reliance on TRIM28-mediated ERV silencing, creating a hyperdependence on TRIM28 via the STING-MAVS-TBK1 axis. Notably, TRIM28 was essential for tumor growth only in the setting of NOTCH2 loss. These findings identify TRIM28 as a potential therapeutic target in NOTCH2-deficient or low-NOTCH2-expressing SCLC.

RNA-seq data of the SCLC adherent cells that are either Notch-WT or Notch-Mutant

GSEA Hallmarks analysis of RNA-seq of bulk lung tumors

Despite small cell lung cancers (SCLCs) having a high mutational burden, programmed death-ligand 1 (PD-L1) immunotherapy only modestly increases survival. A subset of SCLCs that lose their ASCL1 neuroendocrine phenotype and restore innate immune signaling (termed the "inflammatory" subtype) have durable responses to PD-L1. Some SCLCs are highly sensitive to Aurora kinase inhibitors, but early-phase trials show short-lived responses, suggesting effective therapeutic combinations are needed to increase their durability. Using immunocompetent SCLC genetically engineered mouse models (GEMMs) and syngeneic xenografts, we show durable efficacy with the combination of a highly specific Aurora A kinase inhibitor (LSN3321213) and PD-L1. LSN3321213 causes accumulation of tumor cells in mitosis with lower ASCL1 expression and higher expression of interferon target genes and antigen-presentation genes mimicking the inflammatory subtype in a cell-cycle-dependent manner. These data demonstrate that inflammatory gene expression is restored in mitosis in SCLC, which can be exploited by Aurora A kinase inhibition.

Loss of Function (LOF) mutations in the NOTCH receptors are found in several different cancers most notably in small cell lung cancer (SCLC) and squamous cell carcinomas (SCC). We previously developed genetically engineered mouse models (GEMM) of SCLC using CRISPR/Cas9 engineered to be NOTCH1-Mutant, NOTCH2-Mutant, and NOTCH-WT. Synthetic lethality provides a paradigm for targeting cancers with LOF mutations in tumor suppressor genes. In applying this paradigm, one looks for specific vulnerabilities that are created upon loss of the gene of interest. Using 6 cell lines developed from these CRISPR-based SCLC GEMMs which are isogenic to NOTCH, we performed CRISPR/Cas9 LOF negative selection screens (using an sgRNA library enriched in druggable enzymes) to identify synthetic lethal interactors with LOF NOTCH mutations. Our CRISPR/Cas9 screen identified TRIM28 (Tripartite Motif Containing 28, or KAP1) as a highly significant synthetic lethal interactor with NOTCH1 or NOTCH2. We validated the synthetic lethal interaction between NOTCH1/2 and TRIM28 using both human and mouse NOTCH-isogenic cell lines. Interestingly, RNA-sequencing of NOTCH2-Mutant or NOTCH-WT SCLC cell lines after TRIM28 CRISPR inactivation showed robust increases in endogenous retroviral (ERV) expression and the MDA5/RIG-I/MAVS RNA-sensing machinery leading to hyperactivation of the TBK1/IRF3/7 innate immune signaling pathway and the pro-inflammatory cytokine CXCL10 selectively in cells with NOTCH inactivated. Notably, inactivation of TBK1 activity or JAK-STAT activity using specific small molecule inhibitors of TBK1 or JAK completely reversed the synthetic lethality between NOTCH and TRIM28 suggesting that the synthetic lethality phenotype is a consequence of hyperactivation of innate immune signaling, which is known to be cytotoxic. Together our findings uncover a novel vulnerability in SCLCs with LOF NOTCH mutations and perhaps other cancers with LOF NOTCH mutations and suggest a strategy that could also potentiate anti-tumor immunity by increasing innate immune signaling in tumor cells. Citation Format: Deli Hong, Matthew A. Booker, Sidrah Anjum, Yixiang Li, Tran C. Thai, Michelle Y. Wang, David A. Barbie, Michael Y. Tolstorukov, Jun Qi, Matthew G. Oser. Identification of synthetic lethal vulnerabilities in cancers with loss of function mutations in NOTCH. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5727.

Small cell lung cancer (SCLC) exists broadly in four molecular subtypes: ASCL1, NEUROD1, POU2F3 and Inflammatory. Initially, SCLC subtypes were thought to be mutually exclusive, but recent evidence shows intra-tumoural subtype heterogeneity and plasticity between subtypes. Here, using a CRISPR-based autochthonous SCLC genetically engineered mouse model to study the consequences of KDM6A/UTX inactivation, we show that KDM6A inactivation induced plasticity from ASCL1 to NEUROD1 resulting in SCLC tumours that express both ASCL1 and NEUROD1. Mechanistically, KDM6A normally maintains an active chromatin state that favours the ASCL1 subtype with its loss decreasing H3K4me1 and increasing H3K27me3 at enhancers of neuroendocrine genes leading to a cell state that is primed for ASCL1-to-NEUROD1 subtype switching. This work identifies KDM6A as an epigenetic regulator that controls ASCL1 to NEUROD1 subtype plasticity and provides an autochthonous SCLC genetically engineered mouse model to model ASCL1 and NEUROD1 subtype heterogeneity and plasticity, which is found in 35–40% of human SCLCs.

Supplementary Figures S1-S11

GSEA analysis of RNA-seq of SCLC adherent cells

S1. RUNX1 mRNA is decreased during breast cancer progression. S2. RUNX1 does not change cell proliferation. S3. RUNX1 represses tumor growth in mammary fat pad. S4. Gate for MCF10AT1 sorting. S5. CD24high Cells have high RUNX1 expression in MCF10AT1 cells. S6. Loss of RUNX1 promotes stemness in MCF10A and MCF7 cells. S7. Overexpression RUNX1 in MCF10CA1a cells does not change BCSC population. S8. Zeb1 is expressed at low level in MCF10CA1a cells. S9. Schematic diagram of ChIP qPCR primers and amplicons over Zeb1 for ChIP-qPCR. S10. Knockdown Zeb1 in MCF10AT1 ns cells does not change cancer stem cell phenotypes.

RNA-seq data of bulk lung tumors from sgControl RPP, sgNotch1 RPP, sgNotch2 RPP, and sgAscl1 RPP CRISPR-based SCLC GEMMs

Abstract Small cell lung cancer (SCLC) is a high-grade neuroendocrine cancer that accounts for ~15% of lung cancers. While nearly all SCLCs are genetically driven by near universal loss of function (LOF) mutations in RB1 and TP53; several recent studies show that there are different phenotypic SCLC molecular subtypes characterized by expression of lineage transcription factors. These include the neuroendocrine ASCL1 and NEUROD1 subtypes which together comprise ~70-80% of SCLCs. Initially subtypes were thought to be mutually exclusive, but recent evidence shows intra-tumoral subtype heterogeneity and plasticity between subtypes. A recent study found that 35-40% of human SCLCs express both ASCL1 and NEUROD1, but the mechanisms driving ASCL1 and NEUROD1 intra-tumoral heterogeneity are not well understood. My laboratory previously developed an autochthonous CRISPR-based SCLC genetically-engineered mouse model (GEMM) generated by intratracheally injecting adenoviruses encoding Cre recombinase and sgRNAs targeting Rb1, Trp53, and Rbl2. Cre turns on Cas9 expression and allows for CRISPR/Cas9 editing of Rb1, Trp53, and Rbl2 in somatic cells in the lungs. The unique advantage of this model is that it allows the inclusion of sgRNAs targeting additional genes of interest in the same adenovirus. Using this CRISPR-based autochthonous SCLC GEMM approach, we studied the consequences of inactivating the epigenetic modifier KDM6A during SCLC tumorigenesis. KDM6A functions as an H3K27 histone demethylase and also exists in the COMPASS complex with KMT2C/D to promote H3K4 mono-/di-methylation at enhancers. KDM6A along with its protein binding partner KMT2D are mutated in SCLC and KDM6A has been implicated in controlling differentiation in other lineages. Strikingly, we found that KDM6A inactivation in SCLC GEMMs induced plasticity from ASCL1 to NEUROD1 resulting in SCLC tumors that expressed both ASCL1 and NEUROD1. ATAC-sequencing showed open chromatin at the promoters of NEUROD1 and NEUROD1 target genes in KDM6A inactivated tumors. Interestingly, KDM6A inactivated tumors showed a spectrum of ASCL1 to NEUROD1 heterogeneity where some KDM6A inactivated tumors completely lost ASCL1 and solely expressed NEUROD1, some tumors expressed ASCL1 and NEUROD1 in a mutually exclusive manner, while others primarily expressed ASCL1 with very few NEUROD1 positive cells. Mechanistically, KDM6A binds and maintains ASCL1 target genes in an active chromatin state with its loss increasing H3K27me3 near both promoters and enhancers, and decreasing H3K4me1/2 at enhancers together leading to a cell state primed for ASCL1 to NEUROD1 subtype switching. This work identifies KDM6A as an epigenetic regulator that controls ASCL1 to NEUROD1 subtype plasticity and provides an autochthonous SCLC GEMM to model ASCL1 and NEUROD1 subtype heterogeneity, which is found in 35-40% of human SCLCs. Citation Format: Leslie Duplaquet, Yixiang Li, Matthew A. Booker, Yingtian Xie, Radhika A. Patel, Deli Hong, Thomas Denize, Emily Walton, Yasmin N. Laimon, Roderick Bronson, Jackson Southard, Shuqiang Li, Sabina Signoretti, Michael Y. Tolstorukov, Paloma Cejas, Henry W. Long, Michael C. Haffner, Matthew G. Oser. Small cell lung cancer subtype plasticity is regulated by KDM6A. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5774.