Super-Enhancer Dysregulation in Rhabdoid Tumor Cells Is Regulated by the SWI/SNF ATPase BRG1.

Mutations in the SWI/SNF chromatin remodeling complex occur in ~20% of cancers. In rhabdoid tumors defined by loss of the SWI/SNF subunit SMARCB1, dysregulation of enhancer-mediated gene expression is pivotal in driving oncogenesis. Enhancer dysregulation in this setting is tied to retention of the SWI/SNF ATPase BRG1-which becomes essential in the absence of SMARCB1-but precisely how BRG1 contributes to this process remains unknown. To characterize how BRG1 participates in chromatin remodeling and gene expression in SMARCB1-deficient cells, we performed a genome-wide characterization of the impact of BRG1 depletion in multiple rhabdoid tumor cell lines. We find that although BRG1-regulated open chromatin sites are distinct at the locus level, the biological characteristics of the loci are very similar, converging on a set of thematically related genes and pointing to the involvement of the AP-1 transcription factor. The open chromatin sites regulated by BRG1 colocalize with histone-marked enhancers and intriguingly include almost all super-enhancers, revealing that BRG1 plays a critical role in maintaining super-enhancer function in this setting. These studies can explain the essentiality of BRG1 to rhabdoid tumor cell identity and survival and implicate the involvement of AP-1 as a critical downstream effector of rhabdoid tumor cell transcriptional programs.

WDR5 facilitates recruitment of N-MYC to conserved WDR5 gene targets in neuroblastoma cell lines.

Collectively, the MYC family of oncoprotein transcription factors is overexpressed in more than half of all malignancies. The ability of MYC proteins to access chromatin is fundamental to their role in promoting oncogenic gene expression programs in cancer and this function depends on MYC-cofactor interactions. One such cofactor is the chromatin regulator WDR5, which in models of Burkitt lymphoma facilitates recruitment of the c-MYC protein to chromatin at genes associated with protein synthesis, allowing for tumor progression and maintenance. However, beyond Burkitt lymphoma, it is unknown whether these observations extend to other cancers or MYC family members, and whether WDR5 can be deemed as a "universal" MYC recruiter. Here, we focus on N-MYC amplified neuroblastoma to determine the extent of colocalization between N-MYC and WDR5 on chromatin while also demonstrating that like c-MYC, WDR5 can facilitate the recruitment of N-MYC to conserved WDR5-bound genes. We conclude based on this analysis that N-MYC and WDR5 colocalize invariantly across cell lines at predicted sites of facilitated recruitment associated with protein synthesis genes. Surprisingly, we also identify N-MYC-WDR5 cobound genes that are associated with DNA repair and cell cycle processes. Dissection of chromatin binding characteristics for N-MYC and WDR5 at all cobound genes reveals that sites of facilitated recruitment are inherently different than most N-MYC-WDR5 cobound sites. Our data reveals that WDR5 acts as a universal MYC recruiter at a small cohort of previously identified genes and highlights novel biological functions that may be coregulated by N-MYC and WDR5 to sustain the neuroblastoma state.

Emerging Themes in Mechanisms of Tumorigenesis by SWI/SNF Subunit Mutation.

The SWI/SNF chromatin remodeling complex uses the energy of ATP hydrolysis to alter contacts between DNA and nucleosomes, allowing regions of the genome to become accessible for biological processes such as transcription. The SWI/SNF chromatin remodeler is also one of the most frequently altered protein complexes in cancer, with upwards of 20% of all cancers carrying mutations in a SWI/SNF subunit. Intense studies over the last decade have probed the molecular events associated with SWI/SNF dysfunction in cancer and common themes are beginning to emerge in how tumor-associated SWI/SNF mutations promote malignancy. In this review, we summarize current understanding of SWI/SNF complexes, their alterations in cancer, and what is known about the impact of these mutations on tumor-relevant transcriptional events. We discuss how enhancer dysregulation is a common theme in SWI/SNF mutant cancers and describe how resultant alterations in enhancer and super-enhancer activity conspire to block development and differentiation while promoting stemness and self-renewal. We also identify a second emerging theme in which SWI/SNF perturbations intersect with potent oncoprotein transcription factors AP-1 and MYC to drive malignant transcriptional programs.

The SWI/SNF ATPase BRG1 facilitates multiple pro-tumorigenic gene expression programs in SMARCB1-deficient cancer cells.

Malignant rhabdoid tumor (MRT) is driven by the loss of the SNF5 subunit of the SWI/SNF chromatin remodeling complex and then thought to be maintained by residual SWI/SNF (rSWI/SNF) complexes that remain present in the absence of SNF5. rSWI/SNF subunits colocalize extensively on chromatin with the transcription factor MYC, an oncogene identified as a novel driver of MRT. Currently, the role of rSWI/SNF in modulating MYC activity has neither been delineated nor has a direct link between rSWI/SNF and other oncogenes been uncovered. Here, we expose the connection between rSWI/SNF and oncogenic processes using a well-characterized chemical degrader to deplete the SWI/SNF ATPase, BRG1. Using a combination of gene expression and chromatin accessibility assays we show that rSWI/SNF complexes facilitate MYC target gene expression. We also find that rSWI/SNF maintains open chromatin at sites associated with hallmark cancer genes linked to the AP-1 transcription factor, suggesting that AP-1 may drive oncogenesis in MRT. Interestingly, changes in MYC target gene expression are not overtly connected to the chromatin remodeling function of rSWI/SNF, revealing multiple mechanisms used by rSWI/SNF to control transcription. This work provides an understanding of how residual SWI/SNF complexes may converge on multiple oncogenic processes when normal SWI/SNF function is impaired.