Abstract PR002: Insights into the histone code recognition by the ATAD2B bromodomain
Authors: Phillips Margaret , Quinn Kathleen , Montgomery Cameron , Boyson Samuel P. , Chang Sunsik , Nix Jay C. , Frietze Seth E. , Glass Karen C. 
Published: December 2022

Abstract
The ATPase family, AAA domain-containing protein 2 (ATAD2/ANCCA), is an AAA nuclear co-regulator protein containing two ATPase domains and a bromodomain. Its closely related paralog, ATAD2B (KIAA1240), also contains two ATPase domains and a bromodomain. The AAA ATPase domains are broadly associated with ATP-driven molecular remodeling reactions while the bromodomains are evolutionarily conserved, chromatin ‘reader’ domains known to regulate gene expression by binding to acetyllysine on histones. Extensive studies on ATAD2 discuss its upregulation and correlation with poor prognosis in various cancers. Despite sharing high structural and sequence similarity with ATAD2, ATAD2B remains a poorly understood paralog. While previous immunostaining studies detected high expression levels of ATAD2B in breast carcinoma, there is minimal information on its role in oncogenesis. Previously, we have shown that the ATAD2B BRD can recognize mono- and di-acetyllysine modifications on N-terminal tails of histones H4 and H2A. However, since initiation and progression of cancer are characterized by multivalent histone marks, our goal is to understand how crosstalk between combinatorial histone modifications affects the acetyllysine recognition activity of the ATAD2B BRD. We hypothesize that the bromodomain recognizes the acetyllysine mark on multivalent histones and interacts with the chromatin and the ATPase domain. We expect that our results will explain the function of the ATAD2B bromodomain in the context of the overall role of ATAD2B in normal biological and cellular processes. Since bromodomains are excellent drug targets, blocking their activity will also suggest how to disrupt their possible role in oncogenesis. In this study, we have combined various innovative and interdisciplinary approaches ranging from biochemistry and structural biology to functional genomics and cellular biology to gain insights into how crosstalk between different histone H4 PTMs can modulate the chromatin ‘reader’ activity of the ATAD2B BRD. Binding affinities obtained from ITC indicate that the ATAD2B BRD can recognize the acetyllysine mark amongst multiple histone H4 PTMs. Most of these adjacent PTMs are permissible and show high affinities, while some combinations weaken the acetyllysine interaction. Our novel X-ray structures demonstrate a unique binding mode of ATAD2B BRD compared to ATAD2. While the difference in ligand selectivity could suggest that ATAD2B may regulate different biological functions, our cellular assays, including western blot and functional genomic studies, indicate that ATAD2B is conserved in its ability to bind the chromatin. Finally, we discuss how known ‘onco’ mutations within the histone H4 tail region, occurring at or around sites of PTMs, can affect the ATAD2B BRD - acetyllysine coordination. Overall, our study highlights how crosstalk between various histone PTMs regulates the ‘reader’ domain of ATAD2B and affects its recruitment to the chromatin, thus providing new insights for developing therapeutic interventions for treating various diseases.
Citation Format: Margaret Phillips, Kathleen Quinn, Cameron Montgomery, Samuel P. Boyson, Sunsik Chang, Jay C. Nix, Seth E. Frietze, Karen C. Glass. Insights into the histone code recognition by the ATAD2B bromodomain. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr PR002.