POU2F3-expressing intraepithelial small-cell carcinoma with mixed small-cell carcinoma and conventional-type urothelial carcinoma of the urinary bladder.

Based on lineage-specific transcription factors, small-cell neuroendocrine carcinoma (SmCC) of the urinary bladder has recently been subtyped into three molecular subtypes: ASCL1, NEUROD1 and POU2F3. The latter is a master transcriptional regulator of tuft cells (TCs) which are rare solitary cells found in various mucosal epithelia such as the gastrointestinal tract, but which have not been reported in the bladder. The POU2F3 subtype shows low or absent neuroendocrine marker expression. A case of mixed SmCC and conventional-type urothelial carcinoma (CUC) of the urinary bladder with POU2F3-expressing intraepithelial small-cell carcinoma in keeping with a tuft cell phenotype, arising in association with intestinal metaplasia (IM) is described. The presence of POU2F3-expressing cells in normal urothelium, cystitis cystica glandularis and IM of the urinary bladder is demonstrated in separate cases of cystitis cystica glandularis with IM. Also, POU2F3 expression is identified in a subset of bladder SmCC.

Molecular and Pathologic Characterization of YAP1-Expressing Small Cell Lung Cancer Cell Lines Leads to Reclassification as SMARCA4-Deficient Malignancies.

PURPOSE: The classification of small cell lung cancer (SCLC) into distinct molecular subtypes defined by ASCL1, NEUROD1, POU2F3, or YAP1 (SCLC-A, -N, -P, or -Y) expression, paves the way for a personalized treatment approach. However, the existence of a distinct YAP1-expressing SCLC subtype remains controversial. EXPERIMENTAL DESIGN: To better understand YAP1-expressing SCLC, the mutational landscape of human SCLC cell lines was interrogated to identify pathogenic alterations unique to SCLC-Y. Xenograft tumors, generated from cell lines representing the four SCLC molecular subtypes, were evaluated by a panel of pathologists who routinely diagnose thoracic malignancies. Diagnoses were complemented by transcriptomic analysis of primary tumors and human cell line datasets. Protein expression profiles were validated in patient tumor tissue. RESULTS: Unexpectedly, pathogenic mutations in SMARCA4 were identified in six of eight SCLC-Y cell lines and correlated with reduced SMARCA4 mRNA and protein expression. Pathologist evaluations revealed that SMARCA4-deficient SCLC-Y tumors exhibited features consistent with thoracic SMARCA4-deficient undifferentiated tumors (SMARCA4-UT). Similarly, the transcriptional profile SMARCA4-mutant SCLC-Y lines more closely resembled primary SMARCA4-UT, or SMARCA4-deficient non-small cell carcinoma, than SCLC. Furthermore, SMARCA4-UT patient samples were associated with a YAP1 transcriptional signature and exhibited strong YAP1 protein expression. Together, we found little evidence to support a diagnosis of SCLC for any of the YAP1-expressing cell lines originally used to define the SCLC-Y subtype. CONCLUSIONS: SMARCA4-mutant SCLC-Y cell lines exhibit characteristics consistent with SMARCA4-deficient malignancies rather than SCLC. Our findings suggest that, unlike ASCL1, NEUROD1, and POU2F3, YAP1 is not a subtype defining transcription factor in SCLC. See related commentary by Rekhtman, p. 1708.