Alveolar rhabdomyosarcoma is a life-threatening myogenic cancer of children and adolescent young adults, driven primarily by the chimeric transcription factor PAX3-FOXO1. The mechanisms by which PAX3-FOXO1 dysregulates chromatin are unknown. We fi nd PAX3-FOXO1 reprograms the cis -regulatory landscape by inducing de novo super enhancers. PAX3-FOXO1 uses super enhancers to set up autoregulatory loops in collaboration with the master transcription factors MYOG, MYOD, and MYCN. This myogenic super enhancer circuitry is consistent across cell lines and primary tumors. Cells harboring the fusion gene are selectively sensitive to small-molecule inhibition of protein targets induced by, or bound to, PAX3-FOXO1-occupied super enhancers. Furthermore, PAX3-FOXO1 recruits and requires the BET bromodomain protein BRD4 to function at super enhancers, resulting in a complete dependence on BRD4 and a signifi cant susceptibility to BRD inhibition. These results yield insights into the epigenetic functions of PAX3-FOXO1 and reveal a specifi c vulnerability that can be exploited for precision therapy.SIGNIFICANCE: PAX3-FOXO1 drives pediatric fusion-positive rhabdomyosarcoma, and its chromatinlevel functions are critical to understanding its oncogenic activity. We fi nd that PAX3-FOXO1 establishes a myoblastic super enhancer landscape and creates a profound subtype-unique dependence on BET bromodomains, the inhibition of which ablates PAX3-FOXO1 function, providing a mechanistic rationale for exploring BET inhibitors for patients bearing PAX-fusion rhabdomyosarcoma. Cancer Discov; 7(8);

Metabolic stress results in p53 activation, which can trigger cell-cycle arrest, ROS clearance, or apoptosis. However, what determines the p53-mediated cell fate decision upon metabolic stress is not very well understood. We show here that PGC-1α binds to p53 and modulates its transactivation function, resulting in preferential transactivation of proarrest and metabolic target genes. Thus glucose starvation results in p53-dependent cell-cycle arrest and ROS clearance, but abrogation of PGC-1α expression results in extensive apoptosis. Additionally, prolonged starvation results in PGC-1α degradation concomitant with induction of apoptosis. We have also identified RNF2, a Polycomb group (PcG) protein, as the cognate E3 ubiquitin ligase. Starvation of mice where PGC-1α expression is abrogated results in loss of p53-mediated ROS clearance, enhanced p53-dependent apoptosis, and consequent severe liver atrophy. These findings provide key insights into the role of PGC-1α in regulating p53-mediated cell fate decisions in response to metabolic stress.

The MTA1 protein contributes to the process of cancer progression and metastasis through multiple genes and protein targets and interacting proteins with roles in transformation, anchorage-independent growth, invasion, survival, DNA-repair, angiogenesis, hormone-independence, metastasis and therapeutic resistance. MTA proteins control a spectrum of cancer promoting processes by modulating the expression of target genes and/or the activity of MTA-interacting proteins. In the case of MTA1, these functions are manifested through post-translational modifications of MTA1 in response to upstream signals, MTA1 interaction with binding proteins and the expression of target gene products. The MTA1 coregulator interacts with nucleosomes through modified histones and is an integrator of extracellular signaling and gene activator. Studies delineating the molecular basis of dual functionality of MTA1 reveal that the functions of MTA1-chromatin modifying complexes in the context of target gene regulation are dynamic in nature. The nature and targets of MTA1-chromatin modifying complexes are also governed by the dynamic plasticity of the nucleosome landscape as well as kinetics of activation and inactivation of enzymes responsible for post-translational modifications on the MTA1 protein. These broadly applicable functions also explain why MTA1 may be a ‘hub’ gene, whose current understanding is limited to selective influences on gene with roles in cancer but further research may reveal a more global influence. Because the deregulation of enzymes and their substrates with roles in MTA1-biology is not necessarily limited to cancer, we speculate that the lessons from MTA1 as a prototype dual master coregulator will be relevant for other human diseases. In this context, the concept of the dynamic nature of corepressor versus coactivator complexes and the MTA1 proteome as a function of time to signal is likely to be generally applicable to other multi-proteins regulatory complexes in living systems.

Despite being one of the most well-studied transcription factors, the temporal regulation of p53-mediated transcription is not very well understood. Recent data suggest that target specificity of p53-mediated transactivation is achieved by posttranslational modifications of p53. K120 acetylation is a modification critical for recruitment of p53 to proapoptotic targets. Our data reveal that histone deacetylase 5 (HDAC5) binds to p53 and abrogates K120 acetylation, resulting in preferential recruitment of p53 to proarrest and antioxidant targets at early phases of stress. However, upon prolonged genotoxic stress, HDAC5 undergoes nuclear export. Concomitantly, p53 is acetylated at the K120 residue and selectively transactivates proapoptotic target genes, leading to onset of apoptosis. Furthermore, upon genotoxic stress in mice where HDAC5 expression is downregulated, the onset of apoptosis is accelerated in the highly vulnerable tissues. These findings suggest that HDAC5 is a key determinant of p53-mediated cell fate decisions in response to genotoxic stress.

Summary Ewing sarcoma cells depend on the EWS-FLI1 fusion transcription factor for cell survival. Using an assay of EWS-FLI1 activity and genome-wide RNAi screening, we have identified proteins required for the processing of the EWS-FLI1 pre-mRNA. We show Ewing sarcoma cells harboring a genomic breakpoint that retains exon 8 of EWSR1 require the RNA-binding protein HNRNPH1 to express in-frame EWS-FLI1. We also demonstrate the sensitivity of EWS-FLI1 fusion transcripts to the loss-of-function of the U2 snRNP component, SF3B1. Disrupted splicing of the EWS-FLI1 transcript alters EWS-FLI1 protein expression and EWS-FLI1 driven expression. Our results show that the processing of the EWS-FLI1 fusion RNA is a potentially targetable vulnerability in Ewing sarcoma cells.