Abstract 3792: Imetelstat inhibits RNA-editing mediated myeloproliferative neoplasm stem cell self-renewal
Authors: Ma Wenxue , Balaian Larisa , Mason Cayla , Diep Raymond , Pham Jessica , Jiang Qingfei , Lee Jeremy , Morris Sheldon , Mondala Phoebe , Chen Ping , Whisenant Thomas , Donohoe Mary , Heyman Benjamin , Ball Edward , Huang Fei , Jamieson Catriona 
Published: September 2020

Abstract
Introduction: Myeloproliferative neoplasms (MPN) including blast crisis (BC) CML and myelofibrosis (MF) are characterized by the clonal proliferation of hematopoietic cell lineages, mostly in the marrow. BC CML gives rise to TKI resistant myeloid progenitors that activate Wnt/β-catenin signaling pathway. However, BCR-ABL TKI resistant BC CML exhibits a robust telomerase activity and presents at very low or undetectable level in normal cells, and telomerase plays a pivotal role in cancer cell growth and may serve as an ideal target for anticancer therapeutics. In addition, β-catenin had been reported to transcriptionally regulate human telomerase reverse transcriptase (TERT). Telomerase is composed of the enzymatic reverse transcriptase protein TERT and the RNA template TERC. Thus, we investigated the capacity of imetelstat, a telomerase inhibitor, which binds to the TERC subunit with high affinity to prevent self-renewal of malignant progenitors. Recent clinical trials showed early signs of efficacy in MF. But, it's role in selectively inhibiting leukemia stem cell (LSC) self-renewing in CML has not been elucidated.
Methods and Results: Progenitors from BC CML were compared with chronic phase (CP) CML and primary normal samples, RNA-seq results revealed upregulation of TERT suggesting a role for TERT activation in BC CML progenitor transformation. Human MPN progenitor co-culture experiments revealed that combined treatment with dasatinib at 1 nM, and imetelstat at 1 or 5 uM significantly inhibited (p &lt; 0.001, ANOVA) in vitro replating of BC CML compared with aged bone marrow progenitors. Also, imetelstat (5 mM) significantly reduced replating of MF compared with normal progenitor samples. Treatment of humanized MPN mouse models, established with 5 different BC CML and 4 different MF samples, at 30 mg/kg of imetelstat, three times a week for 4 weeks resulted in a significant reduction of malignant progenitors and human CD45+ cells (p &lt; 0.001, t test) in BC CML, and a significant reduction of human CD45+ cells (p&lt;0.01, t test) in MF in both marrow and spleen, when compared with vehicle controls. FACS analysis revealed a significant reduction of activated β-catenin protein in BC CML engrafted human progenitors after imetelstat treatment (p &lt; 0.01, t test) compared with vehicle control. In addition, a significant inhibition of TERT (p = 0.03) and TERC (p = 0.02) was observed in BC CML LSC isolated from imetelstat treated mouse marrow when compared with vehicle control. Notably, imetelstat treatment spares normal human cells in humanized normal stem cell mouse models. Recent RNA-sequencing analysis showed an increase in ADAR1 expression during MPN progression, and our RNA-seq analysis demonstrated a decreased ADAR1 gene expression (p &lt; 0.005) as well as overall adenosine to inosine editing rates as measured by edits per million reads (p &lt;0.005) following imetelstat treatment of BC CML engrafted mice when compared to mismatch control.
Conclusions: Niche responsive interactions between the telomerase complex and the Wnt/β-catenin self-renewal pathway sensitize β-catenin activated MPN progenitors to imetelstat treatment in both the in vitro co-culture and in vivo humanized MPN mouse models thereby providing a strong rationale for studies assessing eradication of malignant progenitors using imetelstat.
Citation Format: Wenxue Ma, Larisa Balaian, Cayla Mason, Raymond Diep, Jessica Pham, Qingfei Jiang, Jeremy Lee, Sheldon Morris, Phoebe Mondala, Ping Chen, Thomas Whisenant, Mary Donohoe, Benjamin Heyman, Edward Ball, Fei Huang, Catriona Jamieson. Imetelstat inhibits RNA-editing mediated myeloproliferative neoplasm stem cell self-renewal [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3792.