INVESTIGATING THE ROLE OF NOTCH1 IN IMMUNOGENICITY AND LSD1 INHIBITOR THERAPY IN SMALL CELL LUNG CANCER - PROJECT SUMMARY SMALL CELL LUNG CANCER (SCLC) IS A NEUROENDOCRINE CANCER WITH EXCEPTIONALLY POOR PROGNOSIS. STANDARD TREATMENT COMBINES CHEMOTHERAPY WITH ANTI-PD-L1 IMMUNE CHECKPOINT BLOCKADE (ICB), BUT ONLY A SMALL SUBSET OF PATIENTS DEVELOP DURABLE RESPONSES. THIS IS DUE IN PART TO LOW TUMOR INFILTRATING LYMPHOCYTES AND REPRESSED TUMOR ANTIGEN PRESENTATION ON MAJOR HISTOCOMPATIBILITY COMPLEX CLASS 1 (MHC-1). RETROSPECTIVE ANALYSIS OF CLINICAL TRIAL DATA IN ONE STUDY REVEALED THE GREATEST CORRELATE OF RESPONSE TO ICB IN SCLC WAS ACTIVATION OF NOTCH SIGNALING. NOTCH SIGNALING SUPPRESSES NEUROENDOCRINE TRANSCRIPTION FACTORS, AND REDUCED NEUROENDOCRINE STATE IS ASSOCIATED WITH HIGHER IMMUNOGENICITY AND BETTER OUTCOMES WITH ICB TREATMENT IN SCLC. RECENTLY, THE MACPHERSON LAB SHOWED THAT INHIBITION OF LYSINE-SPECIFIC DEMETHYLASE 1 (LSD1) IN PATIENT-DERIVED XENOGRAFT (PDX) MODELS OF SCLC ACTIVATES NOTCH SIGNALING AND SUPPRESSES NEUROENDOCRINE GENE EXPRESSION. MOREOVER, OUR GROUP AND THE RUDIN LAB SHOWED THAT IN IMMUNOCOMPETENT MOUSE SCLC MODELS, LSD1 INHIBITION INCREASES CELL SURFACE EXPRESSION OF MHC-1, INCREASES CD8+ T-CELL INFILTRATION, AND BOOSTS RESPONSE TO ICB, LEADING TO CLINICAL EFFORTS TO COMBINE LSD1 AND PD-L1 INHIBITION IN SCLC PATIENTS. GIVEN THESE FINDINGS, I AIM TO UNDERSTAND HOW NOTCH1 SIGNALING IMPACTS ICB RESPONSE AND WHETHER NOTCH1 ACTIVATION IS REQUIRED FOR THE THERAPEUTIC BENEFIT OF LSD1 INHIBITION IN BOTH TUMOR-INTRINSIC DEATH AND IMMUNE CELL-MEDIATED KILLING. TO ACCOMPLISH THIS, IN AIM 1 I WILL WORK WITH SYNGENEIC MOUSE SCLC MODELS TO TEST MY HYPOTHESIS THAT NOTCH1 ACTIVATION PROMOTES IMMUNOGENICITY AND ICB RESPONSE. I HAVE DEVELOPED MOUSE SCLC LINES WITH INDUCIBLE EXPRESSION OF THE TRANSCRIPTIONALLY ACTIVE DOMAIN OF NOTCH1, N1ICD. I WILL INJECT CELLS INTO THE FLANK OF MICE TO TEST IF N1ICD INDUCTION PROMOTES ANTIGEN PRESENTATION, MODULATES THE IMMUNE MICROENVIRONMENT, AND BOOSTS RESPONSE TO ICB. IN AIM 2, I WILL USE NOTCH1 DELETED MODELS BOTH IN PDX AND MOUSE MODELS OF SCLC TO UNDERSTAND TUMOR-INTRINSIC AND IMMUNE-RELATED MECHANISMS OF LSD1I THERAPEUTIC RESPONSE THAT REQUIRE NOTCH1 ACTIVATION. I HAVE DEVELOPED NOTCH1-DELETED PDX MODELS AND FOUND THAT NOTCH1- DELETION CONFERS RESISTANCE TO LSD1I IN VIVO. THROUGH MOLECULAR ANALYSIS OF LSD1I-TREATED TUMORS, I HOPE TO UNCOVER THE MECHANISMS DRIVING LSD1I RESISTANCE IN NOTCH1-DELETED PDX MODELS. I HAVE ALSO GENERATED NOTCH1-DELETED MOUSE SCLC LINES AND WILL TREAT THESE MODELS WITH LSD1I AND ICB TO DETERMINE WHICH LSD1I- MEDIATED THERAPEUTIC BENEFITS ARE DRIVEN BY NOTCH1 INDUCTION. FINDINGS FROM BOTH AIMS WILL HELP IDENTIFY VULNERABILITIES IN SCLC RELATED TO NOTCH SIGNALING AND BIOMARKERS OF LSD1I RESPONSE AND RESISTANCE IN SCLC PATIENTS.