Molecular Subtypes of Primary SCLC Tumors and Their Associations With Neuroendocrine and Therapeutic Markers.

Abstract Introduction A new molecular subtype classification was recently proposed for small cell lung cancer (SCLC). It is necessary to validate it in primary SCLC tumors by immunohistochemical (IHC) staining and define its clinical relevance. Methods We used IHC to assess four subtype markers (ASCL1, NEUROD1, POU2F3, and YAP1) in 194 cores from 146 primary SCLC tumors. The profiles of tumor-associated CD3+ and CD8+ T-cells, MYC paralogs, schlafen-11, and synaptophysin were compared between different subtypes. Validation was performed using publicly available RNA sequencing data of SCLC. Results ASCL1, NEUROD1, POU2F3, and YAP1 were the dominant molecular subtypes in 78.2%, 5.6%, 7%, and 2.8% of the tumors, respectively; and 6.3% of the tumors were negative for all four subtype markers. Notably, three cases were uniquely positive for YAP1. Significant intratumoral heterogeneity was observed, with 17.6% and 2.8% of the tumors being positive for two and three subtype markers, respectively. The non-ASCL1/NEUROD1 tumors had more CD8+ T-cells and manifested more frequently an ‘inflamed’ immunophenotype. L-MYC and MYC were more commonly associated with ASCL1/NEUROD1 subtypes and non-ASCL1/NEUROD1 subtypes, respectively. Schlafen-11 expression was absent in 40% of the tumors, especially those negative for the four subtype markers. Synaptophysin was commonly expressed in the ASCL1 and NEUROD1 subtypes and was associated with less tumor-associated CD8+ T-cells and a ‘desert’ immunophenotype. Conclusions We validated the new molecular subtype classification in primary SCLC tumors by IHC and identified several intriguing associations between subtypes and therapeutic markers. The new subtype classification may potentially assist treatment decisions in SCLC.