Creating novel mouse models of lung cancer
Squamous cell carcinoma (SCC) of the lung is a subtype of NSCLC that leads to ~40,000 deaths each year in the US alone. Whereas numerous mouse models of adenocarcinoma have been created and used to study the underlying biology and treatment of adenocarcinoma, relatively less is known about SCC. In 2011, The Cancer Genome Atlas (TCGA) sequenced human squamous lung tumors and identified the most frequently altered genes. Sox2 is one of the most frequently altered genes in human SCC, amplified in ~21% and overexpressed in 60-90% of tumors. We used a lentiviral approach to deliver candidate squamous-associated oncogenes to the mouse lung along with Cre recombinase, which allows for conditional deletion of tumor suppressor genes. We discovered that Sox2 expression cooperates with loss of the tumor suppressor Lkb1 to promote SCC (Mukhopadhyay et al, Cell Reports, 2014).
Lkb1 is altered in 6-19% of human SCCs and its loss is associated with activation of the mTOR pathway. We observed that mTOR pathway activity frequently co-occurs with Sox2 expression in human SCC. Mouse SCCs highly resemble their human counterparts at the level of histopathology, biomarker expression and signaling pathway activation. More recently, we have improved our mouse models by decreasing the latency of tumor development by altering additional genes involved in human squamous lung cancer. Sox2/Lkb1/Nkx2-1 mice develop squamous tumors in 3–4 months that highly resemble the human disease (Mollaoglu et al., Immunity, 2018). We found that squamous tumors have an enrichment of neutrophils with features that suggest the neutrophils promote squamous lung cancer. We are using these models to identify:
- How the cell of origin impacts SCC development
- Cooperating genetic events associated with SCC
- Therapeutic targets for SCC
- Therapeutic vulnerabilities in the immune microenvironment
- The function of neutrophils in SCC
Identifying drug resistance mechanisms in different subtypes of lung cancer
Small cell lung cancer (SCLC) represents ~15% of all lung cancer cases. SCLC is a neuroendocrine lung tumor that is highly aggressive with high rates of metastases and acquired chemotherapy resistance. The average survival time of patients with SCLC is only 10 months, and treatment options have remained unchanged for almost 40 years. Mouse models of SCLC have been created based on conditional genetic loss of the tumor suppressor genes Rb1and p53, which are both lost in the vast majority of SCLCs. Amplification and overexpression of MYC family genes, including C-MYC, L-MYC, and N-MYC, are also common in SCLC. By combining Myc overexpression with Rb1 and p53 loss, we developed a new genetically-engineered mouse model that recapitulates a specific subtype of human SCLC (Mollaoglu et al, Cancer Cell, 2017). Tumors in these mice develop rapidly, as early as 5 weeks, are highly metastatic, and are sensitive to combined treatment with an Aurora Kinase inhibitor and chemotherapy. Using gene expression analysis, we found distinct molecular profiles associated with human and mouse SCLC tumors with high MYC levels. More recently, we have used unbiased approaches to better understand how MYC-high SCLC is distinct from MYC-low SCLC. Using metabolite profiling, we found that MYC-high SCLC is metabolically distinct and highly dependent on arginine for survival (Chalishazar et al., Clin Can Res, 2019). We are actively working with numerous clinicians to translate these findings to clinical trials.
Our work seeks to identify new therapeutic targets for SCLC using single-cell sequencing technologies, drug screening, and comparative analyses between mouse and human. Understanding how the genetic characteristics of a given tumor dictate therapeutic response will help tailor therapy to the individual.
Regulation of the Mdm2/p53 tumor suppressor network in lung cancer
The tumor suppressor p53 is the most commonly mutated gene in human cancer. Approximately 50% of tumors harbor point mutations in p53, but virtually all tumors have a defect in the p53 pathway. As a transcription factor, p53 responds to cellular stress by inducing target genes that promote cell cycle arrest, DNA repair, apoptosis or senescence. Because p53 plays a central role in determining cell fate decisions between life and death, elucidating the signaling circuitry that governs p53 function is critical for understanding tumorigenesis and manipulating p53 for therapeutic purposes.
The role of p53 in chemotherapy response is controversial. In some tissues, p53 promotes apoptosis, which would promote tumor cell death. In other tissues, p53 can promote cell cycle arrest, DNA damage repair and senescence—processes that would protect tumors from chemotherapy. Recently, it has become appreciated that in breast, bladder and lung cancer, wildtype p53 can promote chemotherapy resistance. We previously described a novel pathway that may contribute to wildtype p53-mediated chemotherapy resistance. We discovered that the Caspase-2-PIDDosome complex is responsible for cleavage and inhibition of Mdm2, a master regulator of p53. Cleavage of Mdm2 converts it from an inhibitor of p53 to an activator of p53. Although not a current area of focus, one of our lab goals is to determine the role of Mdm2 cleavage in p53 signaling and therapeutic resistance. These studies will contribute to our understanding of drug resistance mechanisms as well as p53 pathway regulation in normal development and cancer.
The Caspase-2-PIDDosome promotes p53 stability and activity. PIDD is a p53 target gene that is induced upon DNA damage. PIDD accumulation promotes assembly of the Caspase-2-PIDDosome complex, which activates the protease Caspase-2. Caspase-2 cleaves and removes the RING domain of Mdm2, converting Mdm2 from an inhibitor of p53 to an activator of p53 (Oliver et al, Mol Cell, 2011).