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YAP1, the main Hippo pathway effector, is a potent oncogene and is overexpressed in non‐small‐cell lung cancer (NSCLC); however, the YAP1 expression pattern in small‐cell lung cancer (SCLC) has not yet been elucidated in detail. We report that the loss of YAP1 is a special feature of high‐grade neuroendocrine lung tumors. A hierarchical cluster analysis of 15 high‐grade neuroendocrine tumor cell lines containing 14 SCLC cell lines that depended on the genes of Hippo pathway molecules and neuroendocrine markers clearly classified these lines into two groups: the YAP1‐negative and neuroendocrine marker‐positive group (n = 11), and the YAP1‐positive and neuroendocrine marker‐negative group (n = 4). Among the 41 NSCLC cell lines examined, the loss of YAP1 was only observed in one cell line showing the strong expression of neuroendocrine markers. Immunostaining for YAP1, using the sections of 189 NSCLC, 41 SCLC, and 30 large cell neuroendocrine carcinoma (LCNEC) cases, revealed that the loss of YAP1 was common in SCLC (40/41, 98%) and LCNEC (18/30, 60%), but was rare in NSCLC (6/189, 3%). Among the SCLC and LCNEC cases tested, the loss of YAP1 correlated with the expression of neuroendocrine markers, and a survival analysis revealed that YAP1‐negative cases were more chemosensitive than YAP1‐positive cases. Chemosensitivity test for cisplatin using YAP1‐positive/YAP1‐negative SCLC cell lines also showed compatible results. YAP1‐sh‐mediated knockdown induced the neuroendocrine marker RAB3a, which suggested the possible involvement of YAP1 in the regulation of neuroendocrine differentiation. Thus, we showed that the loss of YAP1 has potential as a clinical marker for predicting neuroendocrine features and chemosensitivity. YAP1 is possibly involved in the regulation of neuroendocrine differentiation of lung tumors. Loss of YAP1 correlated with strong expression of neuroendocrine markers. Loss of YAP1 is a predictor of chemothensitivity in high‐grade neuroendocrine tumors.

YAP 1 , the main Hippo pathway effector, is a potent oncogene and is overexpressed in non‐small‐cell lung cancer ( NSCLC ); however, the YAP 1 expression pattern in small‐cell lung cancer ( SCLC ) has not yet been elucidated in detail. We report that the loss of YAP 1 is a special feature of high‐grade neuroendocrine lung tumors. A hierarchical cluster analysis of 15 high‐grade neuroendocrine tumor cell lines containing 14 SCLC cell lines that depended on the genes of Hippo pathway molecules and neuroendocrine markers clearly classified these lines into two groups: the YAP 1 ‐negative and neuroendocrine marker‐positive group ( n = 11), and the YAP 1 ‐positive and neuroendocrine marker‐negative group ( n = 4). Among the 41 NSCLC cell lines examined, the loss of YAP 1 was only observed in one cell line showing the strong expression of neuroendocrine markers. Immunostaining for YAP 1, using the sections of 189 NSCLC , 41 SCLC , and 30 large cell neuroendocrine carcinoma ( LCNEC ) cases, revealed that the loss of YAP 1 was common in SCLC (40/41, 98%) and LCNEC (18/30, 60%), but was rare in NSCLC (6/189, 3%). Among the SCLC and LCNEC cases tested, the loss of YAP 1 correlated with the expression of neuroendocrine markers, and a survival analysis revealed that YAP 1‐negative cases were more chemosensitive than YAP 1‐positive cases. Chemosensitivity test for cisplatin using YAP 1‐positive/ YAP 1‐negative SCLC cell lines also showed compatible results. YAP 1 ‐sh‐mediated knockdown induced the neuroendocrine marker RAB 3a , which suggested the possible involvement of YAP 1 in the regulation of neuroendocrine differentiation. Thus, we showed that the loss of YAP 1 has potential as a clinical marker for predicting neuroendocrine features and chemosensitivity.