Overview: Small cell lung cancer (SCLC) is a recalcitrant cancer with no approved targeted therapies. This is partly because SCLC is driven by loss of function (LOF) mutations in tumor suppressor genes and lacks mutations in druggable oncogenes. Whole exome sequencing studies have demonstrated that SCLCs harbor near universal LOF mutations in the tumor suppressor genes RB1 and TP53, and ~25% of SCLCs have mutually exclusive LOF mutations in NOTCH. More than 70% of SCLCs express the neural/neuroendocrine lineage specific transcriptional activator ASCL1, which has been shown to be required for SCLC tumorigenesis, but is a transcription factor and is thought to be "undruggable". Our laboratory uses functional genomic approaches including CRISPR/Cas9 screening to uncover novel approaches to indirectly target LOF tumor suppressor genes and "undruggable" transcription factors in SCLC. We then study how inactivation of these new targets impacts SCLC tumorigenesis using a CRISPR/Cas9 genetically-engineered mouse model (GEMM).
Synthetic Lethality Approaches to Target Loss of Function Mutations in SCLC Tumor Suppressor Genes: Synthetic lethality provides a paradigm for targeting cancers that have LOF mutations in tumor suppressor genes. In applying this paradigm, one looks for specific vulnerabilities that are created upon loss of the gene of interest. We recently utilized a CRISPR/Cas9 based screening approach in SCLC and discovered that RB1 loss is synthetic lethal with Aurora kinases (A or B), which helped inspire new clinical trials treating SCLC patients with Aurora kinase inhibitors. We are now focused on better understanding whether synthetic lethal interactors with RB1 loss in SCLC may be targeted in combination or will synergize with other effective therapies in SCLC. Furthermore, we are exploring whether the approaches we developed to identify synthetic lethal interactors with RB1 can be applied to other SCLC tumor suppressor genes. Ultimately, we hope to identify synthetic lethal targets with LOF tumor suppressors in SCLC.
Identification of Mechanisms that Sustain SCLC Neuroendocrine Differentiation: Our previous work and work from other laboratories have shown that the histone demethylases KDM5A and LSD1 can function to repress NOTCH, which is required to sustain ASCL1 expression in SCLC. ASCL1 is a dependency in some SCLCs and required for SCLC tumorigenesis. Our laboratory is interested in understanding mechanisms that SCLCs utilize to sustain high expression of neuroendocrine transcription factors, such as ASCL1. We utilize both hypothesis-driven approaches based on our previous work on KDM5A, and unbiased approaches using CRISPR/Cas9 positive selection screening to interrogate the SCLC neuroendocrine differentiation state and what is required to maintain it. Ultimately, we hope to identify new targeted therapies that control neuroendocrine differentiation in SCLC.
CRISPR/Cas9 Genetically-Engineered Mouse Models of SCLC to Study Candidate Target Genes: Based on a previous established genetically-engineered mouse model (GEMM) that is generated by conditionally deleting Rb1, Trp53, and Rbl2 in the lung using Cre-Lox recombination (referred to hereafter as “Traditional RPP GEMM”), my laboratory developed a new SCLC GEMM entirely using CRISPR/Cas9. This was done by intratracheally injecting an adenovirus that encodes sgRNAs targeting Rb1, Tp53, and Rbl2 (RPP) and Cre-recombinase into lox-stop-lox-Cas9 (LSL-Cas9) mice (referred to hereafter as “CRISPR RPP GEMM”). A major advantage of our CRISPR RPP GEMM is that our CRISPR RPP GEMM allows for genetic inactivation of candidate target genes (“T”) using CRISPR by introducing “T” sgRNAs along with the RPP sgRNAs. We initially used this approach to study the consequences of inactivating the candidate dependency Kdm5a during SCLC tumorigenesis and demonstrated that Kdm5a functions to promote SCLC tumorigenesis and metastasis. Our laboratory utilizes this model to study how inactivation of new candidate target genes (either novel candidate oncoproteins identified using the CRISPR/Cas9 screening approaches described above or tumor suppressor genes that are mutated in human SCLC) impacts SCLC tumorigenesis. Additionally, our model is generated on a pure immunocompetent background and can be used for syngeneic transplant experiments to study combination approaches with immunotherapy.