Abstract A116: HRASG12S mutant mediates resistance to a PI3Kα inhibitor CYH33 in esophageal squamous cell carcinomas
Authors: Wang Yuxiang , Zhang Xu , Ma Qingyang , Zhang Xi , Yang Chunhao , Tan Cun , Hu Landing , Ding Jian , Meng Linghua 
Published: March 2020

Abstract
Esophageal cancer is the seventh most common cancer and the sixth leading cause of cancer-related mortality in the world. More than 80% of esophageal cancers are esophageal squamous cell carcinomas (ESCCs), and no molecularly targeted therapy is currently available. Aberrant activation of PI3Kα occurs frequently in ESCC, making it a promising target for the treatment of ESCC. CYH33 is a novel PI3Kα-selective inhibitor and displays promising efficacy against ESCC in preclinical settings, which is currently in phase I clinical trial for the treatment of ESCC. Acquired resistance is likely to develop after continuous administration of PI3Kα-selective inhibitor in breast cancer patients. It is of great importance to study the mechanism conferring resistance to CYH33 in ESCC. We established a CYH33-resistant cell line, namely KYSE180C, by incubating ESCC KYSE180 cells with increasing concentrations of CYH33. Though CYH33 was able to inhibit Akt phosphorylation, it was much less potent to inhibit the cell cycle progression and proliferation in KYSE180C cells compared to its activity in parental cells. To dissect the mechanism rendering resistance to CYH33, we conducted whole-genome sequencing in KYSE180 and KYSE180C cells. 29 new somatic mutations were detected in KYSE180C cells, including HRASG12S mutant. Accordingly, heterozygous mutation of HRASG12S and hyper-activated ERK signaling were detected in monoclonal cells derived from KYSE180C cells. Similar to KYSE180C cells, these monoclonal cells were resistant to CYH33. Overexpression of HRASG12S conferred KYSE180 cells tolerant to CYH33, which was accompanied with enhanced phosphorylated ERK and S6. Moreover, overexpression of HRASG12S also rendered other ESCC cells, including KYSE70, KYSE410, and KYSE510 cells resistant to CYH33. By contrast, down-regulation of HRAS with SiRNA restored the sensitivity of KYSE180C monoclonal cells to CYH33, which was associated with the induction of apoptosis. Combination of CYH33 with MEK inhibitor MEK162 or ERK inhibitor GDC09994 but not RAF inhibitor GSK2118436 inhibited phosphorylation of ERK and S6 in KYSE180C cells, which was consistent with the enhanced inhibition on cell proliferation. In summary, continuous treatment of CYH33 might induce genomic instability and HRASG12S mutant mediated resistance to CYH33 in ESCC cells. Combination of MAPK inhibitors with CYH33 are able to overcome resistance mediated by HRAS hyper-activation.
Citation Format: Yuxiang Wang, Xu Zhang, Qingyang Ma, Xi Zhang, Chunhao Yang, Cun Tan, Landing Hu, Jian Ding, Linghua Meng. HRASG12S mutant mediates resistance to a PI3Kα inhibitor CYH33 in esophageal squamous cell carcinomas [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A116. doi:10.1158/1535-7163.TARG-19-A116