Abstract 142: HDAC3 is required for pathognomonic features of Langerhans cell histiocytes
Authors: Dimitrion Peter , Toor Jugmohit , Ge James , Wang Qiyan , Allen Carl , Zhou Li , Mi Qing-Sheng 
Published: March 2024

Abstract
Langerhans cell histiocytosis (LCH) is an inflammatory myeloid neoplasm that develops due to dysregulated dendritic cell (DC) development and most commonly affects the children with an incidence of 2.6 to 8.9 cases per million individuals per year. BRAFV600E, the most common disease-driving mutation, activates the mitogen-activated protein kinase (MAPK) pathway causing pathognomonic features that lead to the accumulation of LCH cells and formation of granulomatous lesions. These features include enhanced myelopoiesis, reduced tissue-egress and acquisition of an oncogene induced senescence-associated secretory phenotype (SASP) (marked by increased expression of anti-apoptotic proteins and inflammatory cytokines). Accumulation of LCH cells can occur in any organ causing a wide range of clinical sequelae, which is directly related to the originating cell bearing the oncogenic mutation. Frontline therapy for LCH involves combination chemotherapy and steroidal anti-inflammatories, or MAPK inhibitors, which have significant toxicity and fail to eliminate disease causing precursors. We have found that HDAC3 is required for epidermal LC homeostasis and hypothesized that LCH cells rely on similar epigenetic factors to LCs. CD11cCre LSL-BRAFV600E (BRAFV600ECD11c) mice develop severe multifocal LCH with pronounced lesion development in their livers and lungs, hepatosplenomegaly, and a reduced lifespan due to the accumulation of LCH cells. To test our hypothesis, we generated BRAFV600ECD11c HDAC3fl/fl (BRAFV600EHD3KO) mice, which produce LCH cells that harbor a conditional deletion in the deacetylase domain of HDAC3. Compared to BRAFV600ECD11c mice, BRAFV600EHD3KO mice exhibited significantly less hepatosplenomegaly, reduced lesional burden, and improved survival indicating reduced LCH disease burden. Compared to BRAFV600ECD11c, flow cytometry showed BRAFV600EHD3KO had reduced numbers of LCH cells in lungs and livers, which were more apoptotic. We also found decreased frequencies of circulating LCH cells and LCH precursors, indicating that a lack of HDAC3 activity prevents the development of LCH cells. Bone-marrow derived LCH-like cells provide a valuable drug screening tool. We treated Bone-marrow derived LCH-like cells with RGFP966, an HDAC3-specific inhibitor, increased apoptosis indicated by annexin-V and DAPI staining, reduced expression of Bcl-2, increased CCR7 expression, and decreased S6 phosphorylation (an indication of a loss of the SASP), thus inhibition of HDAC3 may prove therapeutically efficacious by abrogating pathognomonic features of LCH cells. Together, our findings identify HDAC3 as a critical epigenetic regulator LCH cell pathophysiology and HDAC3 blockade would address a great need in treatment of patients with LCH. Furthermore, our results indicate that LCH cells rely on similar epigenetic regulators as epidermal LCs despite coming from a different myeloid lineage.
Citation Format: Peter Dimitrion, Jugmohit Toor, James Ge, Qiyan Wang, Carl Allen, Li Zhou, Qing-Sheng Mi. HDAC3 is required for pathognomonic features of Langerhans cell histiocytes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 142.