CRACD, a gatekeeper restricting proliferation, heterogeneity, 1 and immune evasion of small cell lung cancer 2

Small cell lung carcinoma (SCLC) is a lethal neuroendocrine type of lung cancer with limited therapeutic options. Despite recent advances in cancer immunotherapy, the efficacy of immunotherapy is limited to a subset of patients with SCLC. However, the mechanisms responsible for refractoriness to immunotherapy remain elusive. CRACD (capping protein inhibiting regulator of actin dynamics; KIAA1211/CRAD) is frequently mutated and transcriptionally downregulated in SCLC. Here we show that Cracd knockout (KO) enhances transformation of preneoplastic neuroendocrine cells and significantly accelerates SCLC development initiated by loss of Rb1, Trp53, and Rbl2 in the lung epithelium of mice. Cracd KO increases tumor cell heterogeneity in SCLC tumors. Notably, the Cracd-deficient SCLC tumors display exclusion of CD8+ T cells, which coincides with epigenetic suppression of the MHC-I pathway. Single-cell transcriptomic analysis identifies SCLC patient tumors with concomitant inactivation of CRACD and impairment of tumor antigen presentation. These findings define CRACD as a novel tumor suppressor that regulates the proliferation and immune recognition of SCLC cells, providing new insight into the mechanisms by which SCLC evades immune surveillance. Significance statement Although cancer immunotherapy shows promising outcomes, immunotherapy resistance is a major hurdle to overcome. Small cell lung cancer (SCLC) is a lethal neuroendocrine type of lung cancer with limited therapeutic options. We herein investigated role of the CRACD gene, which is frequently inactivated in SCLC. Cracd knockout (KO) significantly accelerated SCLC development in a mouse model. Strikingly, Cracd KO SCLC tumors displayed the complete loss of CD8+ T cells via the epigenetic suppression of the MHC-I pathway. Furthermore, single-cell transcriptomics stratified SCLC patients by concurrent features: CRACD inactivation and tumor antigen presentation impairment. This study identifies CRACD as a tumor suppressor that restricts the proliferation and immune recognition of SCLC cells, providing novel insight into how SCLC evades immune surveillance.

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