Small Cell Lung Carcinoma Subtypes Defined by ASCL1, NEUROD1, POU2F3 and YAP1: Comprehensive Immunohistochemical and Histopathologic Characterization

ABSTRACT Introduction Recent studies have identified subtypes of small cell lung carcinoma (SCLC) defined by RNA expression of ASCL1, NEUROD1, POU2F3 and YAP1 transcriptional regulators. There are only limited data on distribution of these markers at the protein level and associated pathologic characteristics in clinical SCLC samples. Methods Expression of ASCL1, NEUROD1, POU2F3 and YAP1 was analyzed by immunohistochemistry in 174 SCLC patient samples. Subtypes defined by these markers were correlated with histologic characteristics, expression of neuroendocrine markers (synaptophysin, chromogranin A, CD56, INSM1) and other SCLC markers including neuroendocrine phenotype-associated marker DLL3. Results ASCL1 and NEUROD1 expression had the following distribution: ASCL1+/NEUROD1- 41%, ASCL1+/NEUROD1+ 37%, ASCL1-/NEUROD1+ 8% and ASCL1-/NEUROD1- 14%. Based on the relative expression, 69% of cases were ASCL1-dominant and 17% NEUROD1-dominant. POU2F3 was expressed in 7% of SCLC, and was mutually exclusive of ASCL1 and NEUROD1. YAP1 was expressed at low levels, primarily in combined SCLC, and was not exclusive of other subtypes. Both ASCL1-dominant and NEUROD1-dominant subtypes were associated with neuroendocrine markerhigh/DLL3high profile, whereas POU2F3 and other ASCL1/NEUROD1-double-negative tumors were neuroendocrine markerlow/DLL3low. Conclusions This is the first comprehensive immunohistochemical and histopathologic analysis of novel SCLC subtypes in patient samples. We confirm that ASCL1/NEUROD1-double-negative tumors represent a distinct neuroendocrine-low subtype of SCLC which is either uniquely associated with POU2F3 or lacks a known dominant regulator. Expression profiles of these markers appear more heterogeneous in native samples than in experimental models, particularly in regard to high prevalence of ASCL1/NEUROD1 co-expression. These findings may have prognostic and therapeutic implications and warrant further clinical investigation.