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604 Exploring POU2AF1 (BOB-1) Dependency and Transcription Addiction in Multiple Myeloma

Program: Oral and Poster Abstracts
Type: Oral
Session: 651. Myeloma: Biology and Pathophysiology, excluding Therapy I
Hematology Disease Topics & Pathways:
multiple myeloma, Diseases, cell regulation, Biological Processes, Plasma Cell Disorders, Lymphoid Malignancies
Monday, December 7, 2020: 9:30 AM

Zuzana Chyra, PhD1,2,3*, Mehmet Kemal Samur, PhD4*, Anil Aktas-Samur, PhD5*, Yan Xu, MD5*, Eugenio Morelli, MD6, Sophia Adamia, PhD7, Woojun D Park8*, Lin Charles, PhD9*, Masood A. Shammas, PhD10,11*, Roman Hajek, MD, PhD2,12, Mariateresa Fulciniti, PhD5 and Nikhil C. Munshi, MD13

1University Hospital Ostrava, Department of Haemato-Oncology, Ostrava, Czech Republic
2Faculty of Medicine, University of Ostrava, Ostrava, Czech Republic
3Department of Medical Oncology, Dana Farber Cancer Institute, Harvard Medical School, Boston
4Department of Medical Oncology, Dana Farber Cancer Institute, Boston, MA
5Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA
6Department of Medical Oncology, DANA FARBER CANCER INSTITUTE, Boston, MA
7Jerome Lipper Multiple Myeloma Center, LeBow Institute for Myeloma Therapeutics, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA
8Department of Molecular and Human Genetics, Baylor College of Medicine, Huston, TX
9Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX
10VA Boston Healthcare System, Boston, MA
11Dana-Farber Cancer Institute, Boston, MA
12Department of Haematooncology, University Hospital Ostrava, Ostrava, Czech Republic
13Department of Medical Oncology, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA

Multiple Myeloma (MM) is a complex disease driven by numerous genetic and epigenetic alterations. Recurrent IgH translocations, copy number abnormalities and somatic mutations all contribute to myelomagenesis, yet true drivers of the disease have yet to be identified.

In order to investigate the enhancer landscape and identify new dependencies and actionable therapeutic targets in MM, we have generated high-quality active enhancer landscape in MM cell lines as well as in large cohort of primary patient myeloma cells (n=70) and normal plasma cell. We integrated this data with an in-house curated atlas of 600+ active enhancer profile across a wide range of tumor types and normal tissues. Combining this data with gene expression and genetic dependency (CRISPR KO) enables a multidimensional integration of how transcriptional regulation intersects with tumor specific dependencies. We identified genes that are super-enhancer associated, expressed, and required specifically in MM versus other tumor types or normal tissues. Many of the specific and potent dependencies in MM are transcription factors, and especially those establishing plasma cell identity. Among others, the POU2AF1 gene, which encodes the OCA-B/BOB-1, a B cell transcriptional coactivator protein, represented the most striking dependency in MM. BOB-1 is a gene regulatory factor that regulates B-cell development, maturation and GC formation. Although BOB-1 is expressed throughout B-cell development, we found it to be highly expressed in CD138+ plasma cells from patients with precursor conditions (MGUS and SMM) as well and established MM compared to normal plasma cells, giving us the rationale to study the transcriptional program associated with BOB-1 in MM cells.

To confirm CRISPR data described above, we have performed loss-of-function (LOF) studies using shRNA, siRNAs as well as antisense GapMers specific for BOB-1. Downregulation of BOB-1 in a panel of MM cell lines caused MM cell growth inhibition in a time-dependent manner which was associated with G2/M cell cycle arrest and induction of MM cell apoptosis.

Transcriptomic analysis by RNA-sequencing revealed a set of 72 genes (adj p-value=0.2) commonly modulated in AMO-1 and NCI-929 MM cell lines upon BOB-1 depletion, as compared with scrambled cells. Among the top 5 downregulated genes in BOB-1 knockdown samples, we found AMPD1, KCNN3, BHLHA15, HID1 and XBP1. The modulation of BOB1 target genes was confirmed at the protein level by Western blot analysis in BOB-1 LOF and gain-of-function (GOF) cell systems. Analysis of patient expression datasets revealed that these genes are positively correlated with BOB-1 expression in primary MM cells and overexpressed in MM cells compared to normal plasma cells.

One of these 5 genes positively controlled by BOB-1, BHLHA15, is the gene coding for the transcription factor Mist1, which functions as a coregulator of the unfolded protein response (UPR) master transcription factor XBP1. Target genes of the XBP1- BHLHA15 axis are involved in lipid synthesis, protein folding and secretion, and ER-associated protein degradation. As a result, BOB-1 knockdown increased de novo protein synthesis in MM cells compared to control cells. Interestingly, Mist1 gene expression is induced during ER stress by XBP1, but as ER stress subsides, Mist1 serves as a feedback inhibitor. Moreover, we identified that LOF studies in two MM cell lines confirmed increased expression of BOB-1 and XBP1 upon KD of Mist1. However, XBP1 and/or Mist1 are dispensable for MM cell survival, as gene depletion did not affect MM cell viability. Among the genes most significantly upregulated by BOB-1 depletion was heme oxygenase 1 HMOX1 gene, whose expression is significantly lower in-patient MM cells compared to normal plasma cells. Moreover, its lower expression significantly correlated with poor clinical outcome; and siRNA depletion of HMOX1 increased MM cell growth in MM cell lines, suggesting important role in MM. Mechanistic studies aimed at investigating the functions of HMOX1 in MM and its relation with BOB-1 are ongoing and will be presented.

In conclusion, we here report BOB1 as an important transcriptional regulator in MM and its two down-stream novel target genes (BHLHA15 and HMOX1) with potential significant biological role in MM.

Disclosures: Hajek: BMS: Consultancy, Honoraria, Research Funding; Oncopeptides: Consultancy; PharmaMar: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Fulciniti: NIH: Research Funding. Munshi: Janssen: Consultancy; C4: Current equity holder in private company; Karyopharm: Consultancy; Amgen: Consultancy; OncoPep: Consultancy, Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; BMS: Consultancy; Legend: Consultancy; Adaptive: Consultancy; AbbVie: Consultancy; Takeda: Consultancy.

*signifies non-member of ASH