Molecular profiling of aggressive variant urothelial carcinoma.

Abstract

378 Background: 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) mutations (29 genes reported) except in S. There were more ARID1A mutations detected in MP than UC (100% [3 specimens] v. 41.3%, p=0.044), and more CDH1 mutations in P than UC (50% [4 specimens] v. 2%, p<0.001). CISH ERBB2 (HER2) amplification was seen in 27.3% MP compared with only 10.4% in UC (p=0.005). Compared to UC, PD-L1 IHC (SP142 assay) was positive (>5%) in a high proportion of S (55.6%, p=0.002) but in a lower proportion of other vUC (e.g. absent in P). Tumor mutational burden (TMB) was high in a lower proportion of vUC: 18.4% UC vs. 14.3% MP, 0% P, 16.7% S. Conclusions: Aggressive variant histology UCs have a differential profile of molecular aberrations compared to UC, with notable differences in potential targets such as HER2 and DDR genes as well as immunotherapy biomarkers. Further studies are needed to confirm these findings, and may support therapy development for these rare, aggressive UC subtypes. Aberrations (%) in Variant Histology UC. [Table: see text]