Abstract 107: EML4-ALK fusions drive lung adeno-to-squamous transdifferentiation through phase separation-mediated JAK-STAT activation
Authors: Qin Zhen , Ji Hongbin 
Published: April 2023

Abstract
Phenotypic plasticity is now recognized as a hallmark of cancer. EML4-ALK is the most common oncogenic fusion in non-small-cell lung cancer (NSCLC), accounting for approximately 5% of NSCLC patients. Different from other oncogenic drivers, e.g., EGFR or KRAS mutants, the ALK-rearranged tumors often show strong phenotypic plasticity and unique clinicopathologic features. Previous studies show that certain ALK-rearranged LUAD express the squamous biomarker p63 besides the adenomatous biomarker TTF1. Several studies also show that ALK-rearrangement is observed in certain lung tumors with mixed adenosquamous pathology, and quite rare in LUSC. Moreover, histological transformation such as adeno-to-squamous transdifferentiation (AST) are also observed in the relapsed ALK-rearranged LUAD patients failed from molecular targeted therapy. These clinical observations implicate a potential link between the oncogenic ALK fusions, and cancer plasticity, and even drug resistance. We established two lung cancer mouse models with wildtype EML4-ALK or L1196M mutant. H&E and IHC staining were conducted to pathologically define the LUAD and lung squamous carcinoma (LUSC). Single-cell RNA sequencing (scRNA-Seq) of mouse LUAD, LUSC and human ALK-rearranged LUAD were conducted to uncover the intratumorally heterogeneity and identify the cell subpopulations with squamous signature. Organoid culture system was established for AST process recapitulation and mechanism studies. Drug treatment in vitro and in vivo were conducted to study the TKI responses of LUAD and LUSC samples. Through analyses of RNA-seq and paired WGS data from 93 lung adenosquamous carcinoma specimens, we identified ALK-rearrangements were detectable at 7.5%. We then employ the EML4-ALK GEMM and find that transgenic expression of EML4-ALK fusion drives LUAD formation initially and squamous transformation at late stage, which mirrors the AST process in human ALK-rearranged lung adenosquamous carcinoma. We identify club cells as the main cell-of-origin for squamous transformation. Through recapitulating such lineage transition in organoids system, we identify the JAK-STAT signaling, activated by EML4-ALK phase separation, significantly promotes squamous transformation. Integrative study with scRNA-seq and immunostaining identify a plastic cell subpopulation in ALK-rearranged human LUAD showing squamous biomarker expression. Moreover, those also relapsed ALK-rearranged LUAD show notable upregulation of squamous biomarkers. Consistently, mouse squamous tumors or LUAD with squamous signature display certain resistance to ALK inhibitor, which can be overcome by combined JAK1/2 inhibitor treatment. Our study not only uncovers the link between EML4-ALK fusions and cancer plasticity as well as TKI resistance, but also provides a potentially effective therapeutic strategy. &lt;!–EndFragment–&gt;
Citation Format: Zhen Qin, Hongbin Ji. EML4-ALK fusions drive lung adeno-to-squamous transdifferentiation through phase separation-mediated JAK-STAT activation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 107.