Abstract 410: ASXL3 is a potential biomarker of response of SCLC to targeted therapies

Abstract Small Cell Lung Cancer (SCLC) has been designated a recalcitrant cancer and a priority by National Cancer Institute. Despite recent insights regarding mechanisms mediating stemness, heterogeneity, and metastatic potential, these advances have yet to be fully translated to effective regimens for the treatment of SCLC. Previously, we identified Additional Sex Combs Like-3 (ASXL3) as a novel mediator of pluripotency in human respiratory epithelia and potential therapeutic target in SCLC. ASXL3 is one of three members of the ASXL family, whereas the functions of ASXL1 and ASXL2 as components of the PR-DUB complex have been extensively studied, the functions and significance of ASXL3 during malignant transformation remain elusive. To further investigate the relevance of ASXL3 activation in SCLC, we initially studied ASXL3 expression levels in various SCLC subtypes by qRT-PCR, digital RT-PCR, RNA-seq, and western blotting analysis. These studies demonstrated that ASCL (A) and ASCL and NeuroD1 (A+N) subtypes of SCLC cells contain thousand-folds more ASXL3 (ASXL3high) than Pou2f3 (P) and YAP1 (Y) SCLC subtypes (ASXL3low). ASXL3 levels were elevated when cancer cells were cultured under stemness-enriched conditions. Under normal culture conditions, ASXL3high SCLC cells showed higher intrinsic DNA damage, evidenced by a high percentage of γ-H2AX positive cells compared to ASXL3low SCLC cells. Additionally, ASXL3high SCLC cells displayed 38% γ-H2AX positive metaphase cells, and 42% of abnormal chromosome segregations during mitosis, including lagging chromosomes, DNA bridges, and unequal chromosome segregation indicative of profound genome instability. One reason for this extensive genomic instability is that ATM pathway activation in response to DNA replication stress is impaired in ASXL3high SCLC cells. Following exposures to 1mM hydroxyurea (HU) for 4, 8, and 24 hours, ATM autophosphorylation levels in ASXL3high SCLC cells were much lower than in ASXL3low SCLC cells. This insufficiency significantly affected the activation of downstream cascade targets including CHK2, KAP-1 and EZH2. Consistent with these findings, ASXL3high SCLC cells showed reduced sensitivities to ATM inhibitors. However, ASXL3high SCLC cells exhibited higher activity of PLK1 and were more sensitive to hydroxyurea. Importantly, low dose HU reduced the IC50 values of the PLK1 inhibitors, BI2536 or NMS-P937 by 80-100-fold in ASXL3high but not ASXL3low SCLC cells, suggesting a strong synergy between these two drugs in ASXL3high SCLC cells. Collectively, these findings suggest that ASXL3 functions to protect cells from lethality induced by genomic instability through monitoring ATM activation. Experiments are underway using SCLC PDX models to demonstrate the efficacy of this combined treatment in-vivo as a prelude to possible evaluation of this drug combination in SCLC patients. Citation Format: Ruihong Wang, Haitao Wang, Xinwei Wu, Keerti Mishra, Stephanie Shiffka, Frank Hernandez, Mary Zhang, Umay Tuana Tolunay, Sudheer Gara, Sichuan Xi, Shamus R. Carr, Choung D. Hoang, David S. Schrump. ASXL3 is a potential biomarker of response of SCLC to targeted therapies 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 (6Suppl): Abstract nr 410.