Luka Ozretić

Small cell lung cancer and extrapulmonary small cell carcinomas are the most aggressive type of neuroendocrine carcinomas. Clinical treatment relies on conventional chemotherapy and radiotherapy; relapses are frequent. The PD-1/PD-L1/PD-L2 pathway is a major target of anti-tumour immunotherapy. Aberrant PD-L1 or PD-L2 expression may cause local immune-suppression. Here we investigated expression of PD-1 and its ligands by immunohistochemistry and RNA-seq in small cell carcinomas. PD-L1 and PD-1 protein expression were analysed in 94 clinical cases of small cell carcinomas (61 pulmonary, 33 extrapulmonary) by immunohistochemistry using two different monoclonal antibodies (5H1, E1L3N). RNA expression was profiled by RNA-seq in 43 clinical cases. None of the small cell carcinomas showed PD-L1 protein expression in tumour cells. PD-L1 and PD-1 expression was noticed in the stroma: Using immunohistochemistry, 18.5% of cases (17/92) showed PD-L1 expression in tumour-infiltrating macrophages and 48% showed PD-1 positive lymphocytes (45/94). RNA-seq showed moderate PD-L1 gene expression in 37.2% (16/43). PD-L1 was correlated with macrophage and T-cell markers. The second PD-1 ligand PD-L2 was expressed in 27.9% (12/43) and showed similar correlations. Thus, the PD-1/PD-L1 pathway seems activated in a fraction of small cell carcinomas. The carcinoma cells were negative in all cases, PD-L1 was expressed in tumour-infiltrating macrophages and was correlated with tumour-infiltrating lymphocytes. Patients with stromal PD-L1/PD-L2 expression may respond to anti-PD-1 treatment. Thus, evaluation of the composition of the tumour microenvironment should be included in clinical trials. Besides conventional immunohistochemistry, RNA-seq seems suitable for detection of PD-L1/PD-L2 expression and might prove to be more sensitive.

Small cell lung cancers (SCLCs) and extrapulmonary small cell cancers (SCCs) are very aggressive tumors arising de novo as primary small cell cancer with characteristic genetic lesions in RB1 and TP53. Based on murine models, neuroendocrine stem cells of the terminal bronchioli have been postulated as the cellular origin of primary SCLC. However, both in lung and many other organs, combined small cell/non-small cell tumors and secondary transitions from non-small cell carcinomas upon cancer therapy to neuroendocrine and small cell tumors occur. We define features of "small cell-ness" based on neuroendocrine markers, characteristic RB1 and TP53 mutations and small cell morphology. Furthermore, here we identify a pathway driving the pathogenesis of secondary SCLC involving inactivating NOTCH mutations, activation of the NOTCH target ASCL1 and canonical WNT-signaling in the context of mutual bi-allelic RB1 and TP53 lesions. Additionally, we explored ASCL1 dependent RB inactivation by phosphorylation, which is reversible by CDK5 inhibition. We experimentally verify the NOTCH-ASCL1-RB-p53 signaling axis in vitro and validate its activation by genetic alterations in vivo. We analyzed clinical tumor samples including SCLC, SCC and pulmonary large cell neuroendocrine carcinomas and adenocarcinomas using amplicon-based Next Generation Sequencing, immunohistochemistry and fluorescence in situ hybridization. In conclusion, we identified a novel pathway underlying rare secondary SCLC which may drive small cell carcinomas in organs other than lung, as well.