Abstract SY24-03: Chromatin regulation in cancer
Authors: Crabtree Gerald 
Published: May 2015

Abstract
Recent exom sequencing studies have discovered that a wide variety of human malignancies appear to have driving mutations in genes encoding subunits of chromatin regulatory complexes. In particular mutations in the subunits of the BAF (mSWI/SNF) complex (Khavari, P. et al. Nature 1993; Wu, J.I. et al. Cell 2009) are most common and found in over 20% of all human malignancies (Kadoch, C et al. Nature Genetics 2013). Mutations in subunits of the polycomb repressive complex 2 (PRC2) are also frequent and are found most commonly in the catalytic EZH2 subunit (Garroway and Lander Cell 2013; Dawson MA, and Kouzarides T. Cell. 2012). These two activities oppose one another and appear to form a dynamic balance that controls expression of genes involved in cell proliferation, genome protection and development. BAF complexes can function as either onocgenes or as tumor suppressors (Kadoch, C. and Crabtree Cell 2013; Kadoch et al Nature Genetics 2013, Dunaief JL et al Cell 1994). (see also the Abstract by Cigall Kadoch). In general these subunits behave as tumor suppressors. We find that BAF complexes are necessary for the binding and function of TopoIIa over the genome (Dykhuzien et al Nature 2013). Deletion of subunits such as Brg, BAF250a, BAF47 or BAF57 leads to the presence of anaphase bridges reflecting a failure of decatenation during mitosis. Deletion of several BAF subunits also evokes the decatenation checkpoint with cell cycle arrest that can be specifically repressed with the TopoIIa S1524A mutation that is unable to initiate the decatenation checkpoint. TopoIIa directly interacts with BAF250a through a region that contains several dozen mutations in human cancer. A small molecule screen for inhibitors that block the ability of BAF complexes to suppress the PRC1 subunit, BMI revealed several TopoIIa inhibitors (Dykhuzien et al Nature 2013). Surprisingly these TopoIIa inhibitors mimic both the repression of genes such as Bmi1, Ring1a and other polycomb subunits and also the activation of genes such as FGF4. These studies suggest that TopoIIa and BAF function together to both control transcription and also to resolve decatenated DNA during mitosis. The later may contribute to the role of BAF complexes as tumor suppressors.
Interestingly, the most frequently mutated subunits in human disease, BAF250a, SS18, Bcl7 and others are not required for in vitro chromatin remodeling, ATP activity or complex assembly and have no yeast homologues. In addition, we find that deletion of subunits of the complexes do not change nucleosome positioning over the mammalian genome. These unexpected findings underline the importance of developing new techniques to study chromatin regulation. With this goal in mind we have developed strains of mice that allow one to direct a specific chromatin regulatory activity to one allele of a marked gene using small molecule inducers of proximity. Using this approach we have explored the kinetics and stability of heterchromatin formation (Hathaway, Bell et al Cell 2012; Hodges et al PNAS 2013) revealing that H3K9Me3 induced heterochromatin is limited by intrinsic reaction rates rather than borders (Hodges et al PNAS 2012). Additional studies directed at the mechanism of action of the oncogenic MLL-fusion proteins and the ATP-dependent remodeling complexes will be discussed.
Citation Format: Gerald Crabtree. Chromatin regulation in cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr SY24-03. doi:10.1158/1538-7445.AM2014-SY24-03