IA33 Mechanisms of Small-Cell Lineage Transformation in Resistance to Targeted Therapies

Abstract

EGFR tyrosine kinase inhibitors (TKIs) are highly effective for tumors with EGFR mutations. However, resistance to these compounds remains a major issue, with the most frequent mechanism including the acquisition of a secondary mutation in EGFR (T790M) (1), followed by amplification of the hepatocyte growth factor receptor (MET) gene (2) and mutations in BRAF and PIK3CA genes (3,4). Epithelial–mesenchymal transition (EMT) and lineage transformation are less frequent but also prevalent, with up to 15% of cases with acquired resistance to first- and second-generation EGFR TKIs demonstrating histologic change from lung adenocarcinoma (LUAD) to small-cell lung cancer (SCLC) (4). Histologic plasticity as a mechanism of resistance is becoming increasingly prominent as other resistant mechanisms can now be successfully targeted (5). Currently, as with de novo SCLC, conventional platinum doublet chemotherapy is the standard of care for patients with treatment-induced SCLC. Unfortunately, this treatment often produces an incomplete and nondurable response followed by inevitable relapse within months, leading to poor patient outcomes (6). Thus, this mechanism of resistance will represent a major barrier towards the success of third-generation TKIs, and new strategies to prevent this lineage shift or to treat SCLC transformed tumors are urgently needed. Despite the increasing clinical importance, the biologic pathways regulating LUAD to SCLC transformation are poorly understood. Assessment of clinical samples has revealed that EGFR-mutant tumors universally lose EGFR protein expression upon SCLC transformation, despite still harboring EGFR mutation (7). Furthermore, the mutation spectrum of these transformed cases includes inactivation of the tumor suppressors RB and p53 in nearly all cases, mirroring de novo SCLC (7). However, accumulating experimental evidence has demonstrated that while necessary, dual inactivation of RB and p53 is not sufficient to cause SCLC lineage transformation in EGFR-mutated LUAD, suggesting that additional factors are required (7). MYC amplification and PIK3CA mutation have been proposed to potentially cooperate with RB/p53 loss to facilitate transformation (8), and specific epigenetic regulators may also provide the appropriate context for lineage reprogramming to occur. Despite this, no in vitro or in vivo models of SCLC transformation in EGFR TKI resistance have been developed, making it difficult to comprehensively explore the molecular events driving this lineage shift. Interestingly, there are clear differences between LUAD and SCLC regarding EGFR expression and gene alterations in MAPK pathway including EGFR/KRAS mutations: EGFR is usually not expressed (9) and EGFR/KRAS mutations are extremely rare in SCLC (10); in contrast, EGFR/KRAS play crucial roles in LUAD biology, including regulating differentiation in addition to proliferation (11). To date, however, no clear explanation has been given for these differences. We have recently shown that activation of MAPK signaling in SCLC leads to suppression of the neuroendocrine phenotype—including downregulation of the transcription factors NEUROD1, INSM1, BRN2, and ASCL1—and transformation to an NSCLC-like state (12). Using this model system, we have begun to elucidate the key transcription factors and epigenetic changes that drive SCLC to NSCLC transformation in the hope that the same processes will also be involved in the clinically relevant scenario: SCLC transformation from EGFR mutant LUAD during TKI resistance. We suggest that only EGFR-mutant LUADs that do not reactivate MAPK signaling through secondary EGFR mutations or alterations in parallel kinase pathways (i.e., MET) during development of TKI resistance will be able to undergo SCLC lineage transformation, and that RB/p53 loss and epigenetic plasticity provide the permissive context in which this transformation can occur. Greater understanding of lineage transformation in LUAD will provide important insights in terms of managing outcomes of patients undergoing targeted therapy and offer new avenues towards treatment of TKI resistant tumors.