Targeting Tumor-Driven Immune Tolerance to Overcome Resistance to Hedgehog Inhibition

SUMMARY Patients with basal cell nevus syndrome (BCNS) typically develop hundreds of basal cell carcinomas (BCCs) due to germline mutations that drive aberrant activation of the Hedgehog (Hh) signaling pathway. Affected individuals develop tumors even in early childhood and are at substantially increased risk of other potentially fatal malignancies such as medulloblastomas and rhabdomyosarcomas. Although Hh-targeted therapies using Smoothened (SMO) inhibitors are effective, their clinical utility is often limited by clinical resistance and tumor recurrence. Cemiplimab, a PD-1 blocking antibody approved for BCCs refractory to Hh inhibitors (HHi), also shows limited efficacy, with many patients experiencing disease progression. At present, there are no effective treatments for HHi-resistant BCCs. Given that many advanced BCCs are refractory to or recur after treatment with HHi, new intervention strategies targeting novel pathogenic pathways are urgently needed to achieve sustained remission. Our preliminary data indicate that HHi resistance is associated with dysregulation of BRD7 and BRD9, which are part of the SWItch/Sucrose Nonfermentable (SWI/SNF) chromatin remodeling complexes, and that tumoral BRD9 signaling promotes an immunosuppressive tumor microenvironment (TME). The proposed studies aim to elucidate BRD9-mediated immune suppression as a mechanism of HHi resistance. Utilizing CRISPR/Cas9-edited BRD9 knockout and BRD9-overexpressing BCC cells, along with BRD7KO/Ptch1+/- mice that develop HHi-resistant BCCs in response to UV exposure, Aim 1 will explore BRD9 regulation of chemokines and comprehensively define immune cell compositions and their functional states underlying HHi resistance. Aim 2 will determine the preclinical efficacy of combined targeting of BRD9 and immune checkpoints and test the hypothesis that BRD9 blockade potentiates anti-tumor immunity.