CRACD-controlled cell plasticity and small cell lung cancer - PROJECT SUMMARY/ABSTRACT Small cell lung cancer (SCLC) is an extremely deadly cancer, and current chemo/radiation therapies lack durable effects. Immunotherapy with immune checkpoint blockades is effective for only ~13% of SCLC patients and rarely results in sustained responses in extensive-stage SCLC. Targeted therapy development for SCLC is challenging as SCLC tumors typically have few actionable drivers but instead are mainly driven by loss-of- function mutations in RB1 and TP53 (>90%) and frequent loss and inactivation of other potential tumor suppressors, including a set of epigenetic regulators. Nevertheless, recent discoveries from SCLC genomics provide a new framework for understanding the biology of SCLC and identifying the molecular vulnerabilities of the disease. Functional characterization of the recurrent mutations in putative tumor suppressor genes is crucial for defining the mechanisms of tumorigenesis, which will facilitate the discovery of biomarkers for tumor prevention and intervention. We recently identified a new tumor suppressor gene, CRACD (Capping protein inhibiting Regulator of ACtin Dynamics; KIAA1211/CRAD). CRACD encodes a protein that binds to and inhibits the capping proteins, thereby facilitating actin polymerization. CRACD is frequently mutated or transcriptionally downregulated in SCLC patient tumors. Cracd knockout (KO) promotes the transformation of preneoplastic precursor lung epithelial cells. Cracd KO also significantly accelerates SCLC development in an autochthonous mouse model in which tumor initiation is induced by the deletion of Rb1, Trp53, and Rbl2 triple KO mouse models. Cracd KO SCLC tumors harbor distinct root cell clusters with aberrant cell lineage trajectories and impaired antigen presentation. Single-cell transcriptome analysis has further stratified SCLC patients by CRACD inactivation and antigen presentation pathway impairment. Intriguingly, ablation of Cracd alone induces hyperplasia of neuroendocrine cells and aberrant cell lineage plasticity in the mouse lung. These results led to the hypothesis that CRACD inactivation induces aberrant neuroendocrine cell plasticity to initiate and promote SCLC tumorigenesis via dysregulated nuclear actin dynamics and subsequent epigenetic reprogramming. The proposed study will unveil the biology of SCLC initiation and progression and determine the cells-of-origin and their relevant cell lineages inducing neuroendocrine cell plasticity, providing novel insight into SCLC tumorigenesis. Additionally, this study will establish a new model system for SCLC initiation and progression and introduce a novel somatic engineering system, which is technologically innovative. Completing this study will also lay a solid foundation to determine whether CRACD loss is a molecular signature for specific SCLC patient stratification and immunotherapy response prediction, which will be conceptually and clinically innovative.
