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1610 Dysfunctional HDAC8 Impacts Genomic Integrity and Is a Novel Therapeutic Target in Multiple Myeloma

Program: Oral and Poster Abstracts
Session: 651. Multiple Myeloma and Plasma Cell Dyscrasias: Basic and Translational: Poster I
Hematology Disease Topics & Pathways:
Translational Research
Saturday, December 11, 2021, 5:30 PM-7:30 PM

Zuzana Chyra1,2,3*, Srikanth Talluri, PhD4,5*, Rao Prabhala, PhD4,5, Mehmet K. Samur, PhD6,7*, Anil Aktas-Samur, PhD8,9*, Yan Xu, MD10,11*, Aaron B. Beeler, PhD12*, Roman Hajek, MD, PhD13,14,15, Mariateresa Fulciniti, PhD10, Masood A. Shammas, PhD5,16* and Nikhil C. Munshi, MD, PhD17,18,19

1The LeBow Institute for Myeloma Therapeutics and Jerome Lipper Multiple Myeloma Center, Department of Medical Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA, Boston, MA
2Department of Hemato-Oncology,, University Hospital Ostrava, Ostrava, Czech Republic
3Department of Internal Studies, University of Ostrava, Ostrava, Czech Republic
4Dana-Farber Cancer Institute, Boston, MA
5VA Boston Healthcare System, Boston, MA
6Department of Data Science, Dana-Farber Cancer Institute, Boston, MA
7The Jerome Lipper Multiple Myeloma Center, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA
8Department of Data Science, Dana Farber Cancer Institute, Boston, MA
9Department of Medical Oncology, Dana Farber Cancer Institute, Boston, MA
10Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA
11Institute of Hematology & Blood Diseases Hospital, Chinese Academic Medical Science & Peking Union Medical College, Tianjin, China
12Department of Chemistry and Center for Molecular Discovery, Boston University, Boston
13Czech Myeloma Group, Brno, Czech Republic
14Department of Haematooncology, University Hospital Ostrava, Ostrava, Czech Republic
15Faculty of Medicine, Department of Clinical Studies, University of Ostrava, Ostrava, Czech Republic
16The Jerome Lipper Multiple Myeloma Center, Dana-Farber Cancer Institute, Harvard Medical School, Boston
17Dana-Farber Cancer Institute, Harvard Medical School, Boston
18Boston VA Healthcare System, West Roxbury, MA
19Department of Medical Oncology, Jerome Lipper Multiple Myeloma Center, Dana-Farber Cancer Institute, Boston, MA

The histone modifications and associated changes in chromatin structure and function have emerged as important epigenetic mechanisms impacting gene expression and have significant translational relevance in cancers, including multiple myeloma (MM). Epigenetic intervention with histone deacetylases (HDACs) inhibitors is emerging as a promising therapeutic strategy in combination with current anti-myeloma agents. Although pan-HDAC inhibitors have been shown to be effective both in preclinical and clinical setting, they seem to be associated with toxicity. It is, therefore, extremely important to understand the biological and molecular roles of individual HDACs to then selectively target them to limit toxicities observed with pan-HDAC inhibitors. Based on our observation that elevated HDAC8 expression correlates with poor overall survival in MM patients in three different datasets including one publicly available dataset (GSE39754), we evaluated its functional role in MM.

HDAC8, a member of class I HDAC isoenzymes, is responsible for the deacetylation of lysine residues on the N-terminal part of the core histones as well as non-histone proteins. We performed genetic modulation of HDAC8 by loss-of-function studies, using shRNA as well as siRNAs targeting HDAC8. Downregulation of HDAC8 in 3 different MM cell lines caused MM cell growth inhibition in a time-dependent manner which was associated with induction of cell apoptosis. Consistently, treatment with a selective and potent HDAC8 inhibitor (OJI-1) caused a significant inhibition of MM cell growth in a panel of 20 MM cell lines (IC50 = 80 nM) in a time- and dose-dependent manner, while having a minimal impact on six PBMC samples from healthy donors both in resting and activated state (IC50 = 150 nM).

The mechanism of cell death was apoptosis as demonstrated by annexin-labeling. Importantly, both the HDAC8 knockdown and OJI-1 treatment inhibited DNA breaks as evidenced from γH2AX expression or a single cell gel electrophoresis method to visualize and quantitate DNA breaks. HDAC8 inhibition also caused inhibition of RAD51 foci and HR activity, as measured by strand-exchange assay. Interestingly, non-homologous end joining in MM cells was not impacted by these treatments. Consistent with these data, the overexpression of HDAC8 in MM as well as in normal cells increased DNA breaks and HR activity. Furthermore, the inhibition of HDAC8 (by knockdown and OJI-1) inhibited, whereas its overexpression increased genomic instability, as assessed by micronucleus assay, in surviving MM cells. We also demonstrate that HDAC8 interacts with RAD51 and impacts its acetylation. The treatment of MM cells with HDAC8 inhibitor (OJI-1) increased RAD51 acetylation. Next, we examined the in vivo efficacy of the HDAC8 conditional knockdown in a human xenograft mouse model, using H929 cells injected subcutaneously in SCID mice. HDAC8 knockdown not only caused a significant reduction in tumor growth but also increased survival (p=0.0016) compared to mice injected with control cells. Evaluation of tumors from these mice confirmed in vivo inhibition of DNA breaks and HR activity, and induction of apoptosis following HDAC8-knockdown. HDAC8 inhibitor OJI-1 also synergistically increased the cytotoxicity of existing MM drugs including dexamethasone, bortezomib and lenalidomide.

In conclusion, our results demonstrate that elevated HDAC8 in MM cells is involved in inhibition of apoptosis but also contributes to increased DNA breaks and dysregulation of homologous recombination and genome stability. Therefore, HDAC8 is a novel target for therapeutic application in MM. Selective and potent HDAC8 inhibitor OJI-1 has shown a favorable therapeutic index with synergistic effect in combination with existing MM drugs.

Disclosures: Hajek: Pharma MAR: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; BMS: Consultancy, Honoraria, Research Funding. Munshi: Janssen: Consultancy; Bristol-Myers Squibb: Consultancy; Amgen: Consultancy; Takeda: Consultancy; Celgene: Consultancy; Karyopharm: Consultancy; Abbvie: Consultancy; Adaptive Biotechnology: Consultancy; Oncopep: Consultancy, Current equity holder in publicly-traded company, Other: scientific founder, Patents & Royalties; Novartis: Consultancy; Pfizer: Consultancy; Legend: Consultancy.

*signifies non-member of ASH