Molecular profiling of aggressive variant urothelial carcinoma.

378Background: The WHO recognizes multiple variant histologies of urothelial carcinoma (vUC), many of which have been associated with poor outcomes compared with urothelial carcinoma (UC). We aimed to explore molecular differences between aggressive vUC and UC. Methods: 23 micropapillary (MP), 16 plasmacytoid (P), 23 sarcomatoid (S), 7 nested (N), 6 clear cell (CC), and 2 giant cell (GC) vUC specimens were tested between 2012 to 2018 via a multiplatform profiling service (Caris Life Sciences, Phoenix, AZ) consisting of gene sequencing (next generation sequencing [NGS]), gene amplification (CISH or NGS), and protein expression (immunohistochemistry [IHC]). Findings were compared to 435 control UC specimens using the Chi-square test. Results: 84% of samples were from primary tumor. Alterations identified are summarized in Table 1, and are notable for high rates of TP53 mutations across histologic subtypes, varying rates of RB1, ERBB2 and FGFR mutations, and overall low rates of DNA damage repair (DDR) mutat...