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.