Abstract
Cancer is a devastating disease defined by the uncontrolled proliferation of cells that can spread to distant sites throughout the body. Due to the everchanging nature of the disease, it is extremely difficult to treat as many of the cells become plastic and adapt to differing environments or even become resistant to therapeutic treatment. Small cell lung cancer (SCLC) has one of the lowest survival rates of all cancer types. This is due to no advancements in treatment for decades caused by a lack of understanding of what drives disease progression. Spurred by the dismal survival outcomes of the disease, SCLC was included in the 2012 Recalcitrant Cancer Act passed by Congress. Passing of this bill led to the formation of the NCI U24 SCLC Consortium that brings together scientists trying to tackle the unknowns of this disease. The dedication to collaboration and sharing information by members of the consortium, including our own lab, has led to innumerable advancements in our understanding of SCLC. As a field, it is becoming increasingly appreciated that despite high levels of heterogeneity among patient tumors, there are convergent malignant pathways that emerge during disease development. This includes the loss of differentiation status observed in more advanced tumors. It is crucial to understand the vulnerabilities that arise within these convergent pathways to increase the therapeutic options available to patients.
SCLC patient tumors are marked with a high frequency of genetic alterations. Many of these alterations include a high number loss-of-function mutations, including truncating mutations (nonsense, frameshift, and splice site mutations) and homozygous deletions. It is the ultimate goal of our lab to understand the role of recurrent alterations and how they impact tumor development through a combination of both in vitro and in vivo models. This includes frequent mutations in the genes encoding the chromatin modifiers, SMARCA4 and ARID1A, that are members of the SWI/SNF chromatin remodeling complex. SMARCA4 is the catalytic, core component while ARID1A is responsible for complex assembly. The work presented in this thesis is centered on understanding how alterations to SMARCA4 and ARID1A impact SCLC development. We uncovered a novel and intriguing role for SMARCA4 as a temporally specific tumor suppressor that is necessary for SCLC tumor initiation, but its expression is lost in the later stages to promote disease progression. Additionally, we uncovered alterations within the TME that arise when Smarca4 is lost. To determine if these phenotypes are specific to targeting SMARCA4 or if they are representative of complete SWI/SNF dysfunction, we additionally targeted Arid1a in SCLC genetically engineered mouse models (GEMM). We found that ARID1A is also crucial for SCLC tumor development. Intriguingly, despite being much smaller in size than their controls, the Arid1a-deficient tumors displayed late-stage phenotypes associated with losing their differentiation status. Further work should elucidate whether ARID1A also plays a context-dependent role in SCLC development.
Another recurrently mutated gene that we have explored as part of critical alterations driving SCLC development is ROBO1. Targeting Robo1 in precancerous cells enhanced their tumorigenic capacity. We tried to determine the in vivo impact of ROBO1 loss using an SCLC GEMM incorporating Robo1 knockout allele throughout the whole body. However, the GEMMs with homozygous Robo1 knockout showed growth defects postnatally and died prematurely. These outcomes precluded the study of ROBO1 in SCLC development in mice but instead uncovered a previously unknown role for ROBO1 regulating the growth hormone (GH)-insulin-like growth factor 1 (IGF1) axis during postnatal development. This study also presents the first model of Robo1-deficient mice that will be invaluable to study human genetic disorders related to the functional loss of ROBO1 and postnatal growth defects. Future studies should focus on the role of ROBO1 in SCLC using a model in which Robo1 mutation or deletion is done in a conditional manner.
The work in this thesis, particularly the work on SMARCA4, is significant in that it addresses an often overlooked yet fundamental aspect of cancer biology: the timing of alterations. It reveals a need to acknowledge how the models we use in experiments represent various timepoints of tumor development.
