Interrogating roles for REST in small cell lung cancer therapy response and resistance

SUMMARY Small cell lung cancer (SCLC) is a lethal tumor type characterized by exquisite response to chemotherapy followed by rapid emergence of chemoresistance. Addition of PDL-1 inhibition to platinum/etoposide chemotherapy leads to improved clinical responses but only a small subset of SCLC patients benefit. Distinct

subsets of SCLC have been identified including high-neuroendocrine and low-neuroendocrine (NE) subtypes. High-NE SCLC, expressing high levels of ASCL1 (SCLC-A) or NEUROD1 (SCLC-N) is best understood, with genetically engineered mouse models available. However, our understanding of low-NE subtypes is poor, owing

in part to a lack of mouse models. A low-NE immune “inflamed” subset of SCLC (SCLC-I) exhibited increased response to immune checkpoint blockade and high expression of RE-1 Silencing Element (REST), a suppressor of neuronal and neuroendocrine gene expression. Aim 1 tests hypotheses that REST overexpression in a

mouse model of SCLC will result in low-NE SCLC and that REST increases immunogenicity. We will use gain and loss of function studies to dissect the contribution of REST to SCLC and to model low-NE SCLC. RNA-seq analyses will use data from mouse GEM models and isogenic cell lines with REST perturbation to identify genes

consistently regulated by REST while CUT&RUN will identify direct targets of REST. We will perform gain and loss of function studies to assess pathways through which REST alters the biology of SCLC, with an initial focus on how REST controls the expression of MHC-I. Aim 2 builds from our preliminary data that implicate REST

expression in driving chemotherapy resistance. We performed an in vivo functional CRISPR activation (CRISPRa) screen to identify genes that switch chemosensitive patient derived xenograft (PDX) models to become chemoresistant when overexpressed and among the top screen hits was REST. We will study the

impact of REST perturbation on chemotherapy response and will test our hypothesis that REST expression will cause a switch to chemoresistance. We will perform immunohistochemistry and molecular analyses to characterize the response of REST-perturbed PDX models to chemotherapy in vivo. With the increased

appreciation of low-NE SCLC but poor understanding of the underlying biology, it is critical that this important subset of SCLC be modelled and understood. This proposal provides new in vivo GEM and PDX models of low- NE SCLC, which are essential reagents to link biologically distinct SCLC subsets to the most promising

therapeutic approaches.