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The molecules of the cell are compartmentalized into membrane- and non-membrane-bound organelles. Many non-membrane-bound organelles are phase-separated biomolecular condensates with distinct physicochemical properties that can absorb and concentrate specific proteins and nucleic acids involved in discrete biochemical processes. We reasoned that selective condensate partitioning might also occur with small molecule drugs whose targets occur within condensates, and that the therapeutic index and efficacy of such compounds might therefore relate to their ability to partition into condensates. To test this idea, we focused our study on nuclear condensates reported in cell lines, demonstrated they occur in normal human and malignant breast cancer, and developed assays to test clinically active antineoplastic small molecule drugs relative to these condensates.To study the behavior of drugs within condensates, these were modeled in vitro with purified proteins and visualized by fluorescent confocal microscopy. We found that cisplatin, tamoxifen, JQ1, THZ1, and mitoxantrone are concentrated in specific protein condensates in vitro, and that this occurs through physicochemical properties independent of the drug target. For each drug, the small molecule partitioned into the same condensate in vitro in which its established target resides in vivo. A screen of a chemically diverse fluorescent probes and mutant-protein condensates demonstrated that pi-system interactions between aromatic moieties in the protein and small molecule govern concentration in condensates. These results show that clinically important drugs partition into specific protein condensates in vitro by virtue of defined chemical properties, thereby altering their local concentration.Alkylating agents are a class of commonly used antineoplastic compounds, of which cisplatin is a prominent example. In vitro droplet assays revealed that cisplatin is selectively concentrated in transcriptional condensates, and that this ability is required for efficient platination of target DNA. In cell studies revealed that cisplatin preferentially targets DNA contained within MED1 condensates, and disrupts the genetic regulatory elements that compose phase-separated transcriptional condensates. Live cell imaging demonstrated that transcriptional condensates are dissolved by cisplatin, whereas other condensates remain intact. Thus, we conclude that cisplatin preferentially modifies transcriptional condensate-associated DNA in cells, and that this causes selective condensate disruption. The mechanisms that produce drug resistance can provide clues to drug activity in the clinical setting. Investigating the behavior of tamoxifen within ER transcriptional condensates demonstrated that it disrupts these condensates in vitro and on oncogenes in cells; hormonal therapy resistant ESR1 mutations render these condensates resistant. MED1 overexpression, a poorly understood mechanism of tamoxifen resistance, increased the size of ER-MED1 condensates, thereby rending tamoxifen more dilute and ineffective when concentrated therein. This suggest that altering the size and nature of transcription condensates in breast cancer can mediate drug resistance in the clinical setting.Our results show that antineoplastic drugs partition selectively into condensates, that this can occur through physicochemical properties independent of their molecular targets, and that resistance to drugs may occur through condensate altering mechanisms. These results have implications for development of efficacious cancer therapeutics; effective target engagement will depend on factors such as drug partitioning in condensates. Assays of the type described here may thus help optimize condensate partitioning, target engagement, and the therapeutic index of drugs for cancer treatment. Citation Format: Isaac Klein, Ann Boija, Lena Afeyan, Susana Wilson Hawken, Mengyang Fan, Alessandra Dall'Agnese, Ozgur Oksuz, Jonathan Henninger, Krishna Shrinivas, Benjamin Sabari, Ido Sagi, Victoria Clark, Jesse Platt, Mrityunjoy Kar, Patrick McCall, Alicia Zamudio, John Mantiega, Eliot Coffey, Charles Li, Nancy Hannett, Yang Guo, Tim-Michael Decker, Tong Lee, Tinghu Zhang, Jing-Ke Weng, Dylan Taatjes, Arup Chakraborty, Phillip Sharp, Young Tae Chang, Anthony Hyman, Nathanael Gray, Richard Young. Partitioning of cancer therapeutics in nuclear condensates [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr GS3-10.

Key oncogenes can be misregulated in tumor cells by the acquisition of tumor-specific enhancers that direct gene expression through physical contacts (1,2). Identifying the full complement of enhancers regulating each oncogene could identify additional vulnerabilities in tumor expression programs, but remains challenging, because a given enhancer can regulate multiple genes and each of its targets may be genomically distant. Here we use DNA interaction data to construct gene regulatory landscapes for all expressed genes in a tumor cell and demonstrate that sets of enhancers can be assigned to these oncogenes using direct looping and insulated neighborhood data. A subset of these landscapes contains an exceptional amount of transcriptional apparatus, reminiscent of SEs (3), so we term this subset 3D-SEs. Well-characterized oncogenes, including c-Myc, acquire 3D-SEs in tumor cells where a c-Myc¬-associated linear SE was not identified, suggesting SE acquisition by c-Myc and other oncogenes has been underestimated. Each gene's regulatory network can also surprisingly extend beyond the confines of insulated neighborhoods and incorporate additional distal enhancers. Visualizing instances of 3D-SEs with microscopy demonstrates that they are components of liquid-liquid phase-separated bodies in cells, suggesting gene regulatory landscapes underpin these transcriptional condensates. The interactions comprising gene regulatory networks and their target genes thus extend previous interpretations of the targets of enhancers, signaling pathways, and disease-associated enhancer variants. References: 1. Hnisz, Abraham, Lee, et al. and Young, Cell 2013. 2. Mansour, Abraham, et al. and Young and Look, Science 2014. 3. Whyte, Orlando, Hnisz, Abraham, et al. and Young, Cell 2013. Citation Format: Brian J. Abraham, Yang Eric Guo, Denes Hnisz, Charles H. Li, Abraham S. Weintraub, Daniel S. Day, Jurian Schuijers, Tong Ihn Lee, Richard A. Young. Three-dimensional gene regulatory landscapes in normal and cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 971.

Transcriptional super-enhancers drive expression of oncogenes in many cancers and are being targeted with novel transcriptional and epigenetic therapeutics[1,2,3,4]. Super-enhancers are acquired by cancers through multiple mechanisms, including DNA translocation of an extant super-enhancer and focal amplification. We recently discovered a novel mechanism by which super-enhancers are nucleated in T cell acute lymphoblastic leukemias (T-ALLs)[5]. In this case, a small, monoallelic insertion creates a DNA binding site for a master transcription factor protein, which binds and recruits additional factors to nucleate the super-enhancer, which in turn drives high levels of the TAL1 oncogene. We describe here a method for unbiased identification of similar genomic insertions that nucleate potentially oncogenic regulatory elements in cancers. This approach uses data from genome-wide ChIP-Seq studies that map locations of enhancer-binding proteins to identify short DNA sequences missing from reference genomes. We have found and catalogued many additional genomic insertions in 80 additional cancers. An additional insertion has been functionally characterized and determined to be a bona fide enhancer-nucleating insertion that exists in patient genomes. I will describe new insights into the regulation of cancer that occur due to nucleation of novel regulatory elements. [1] Hnisz, Abraham, Lee, et al., Cell 2013 [2] Loven, Hoke, Lin, et al., Cell 2013 [3] Kwiatkowski, et al., Nature, 2014 [4] Wang, et al., Cell, 2015 [5] Mansour et al., Science 2014 Citation Format: Brian J. Abraham, Denes Hnisz, Abraham S. Weintraub, Nicholas Kwiatkowski, Charles H. Li, Sunniyat Rahman, Zhaodong Li, Tong Ihn Lee, A Thomas Look, Marc Mansour, Richard A. Young. Nucleation of transcriptional super-enhancers at tumor oncogenes by small insertions. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2004.