Molecular characterization of neuroendocrine bladder cancer.

561 Background: Neuroendocrine bladder cancer (NEBC) is a rare and aggressive variant that is associated with poor survival outcomes. Because NEBC is rare, the molecular features of NEBC remain poorly characterized. Therefore, we characterized NEBC at the molecular level to understand the underlying biology and identify novel therapeutic targets. Methods: Whole transcriptome RNAseq was performed on FFPE cores from 24 NEBCs and 51 conventional muscle-invasive bladder cancers (MIBCs) from Tenon Hospital in Paris. Results: Unsupervised cluster analysis of 75 tumors generated 2 distinct clusters that separated NEBCs from MIBCs. The NEBC tumors were strongly enriched with biomarkers for the characteristic of neuroendocrine or small cell malignancies, including DLL3, SOX2, and EZH2. In addition, E2F1 pathway is significantly enriched due to the impair of RB/p53 pathways. Further, the NEBCs were enriched with the TCGA’s neuronal differentiation genes that were associated with high response rates in patients treated in atezolizumab (anti-PDL1) within the context of the ImVigor 210 trial. Nevertheless, the NEBCs were characterized by suppressed immune pathway gene expression signatures, such as the Th1 pathway, effector T cell lymphocyte, and IFNg that are usually highly enriched in tumors that are sensitive to immune checkpoint blockade. Of candidate mechanisms, the suppressed TGFbpathway activity observed in the NEBCs was the most obvious explanation for sensitivity to checkpoint blockade. Conclusions: NEBCs are distinct from conventional MIBCs by gene expression signature. They are also characterized by overexpression of canonical neuroendocrine markers and inhibition of TGFb pathway activity.