PAX8 ‐positive conventional urothelial carcinomas of the urinary bladder and their distinct molecular profiles – A clinicopathologic study of 101 consecutive cases with next‐generation sequencing in 20 cases

Aims PAX8 immunohistochemistry (IHC) is often used to distinguish urothelial carcinomas (UCs) from tumours of renal and Mullerian origin. However, some UCs have been shown to be PAX8‐positive. This study investigates the frequency of PAX8‐positive conventional UCs of the urinary bladder without subtype morphology and/or divergent differentiation and their molecular profiles. Methods One hundred and one consecutive transurethral resections of the urinary bladder from 2019 to 2022 with a diagnosis of conventional urothelial carcinoma (UC) without subtype morphology and/or divergent differentiation were retrospectively reviewed. Representative sections were selected for whole‐slide PAX8 IHC (10336‐1‐AP; polyclonal). Next‐generation sequencing (NGS) was performed on all PAX8‐positive cases and on a subset of PAX8‐negative cases. Results PAX8 IHC was positive in 10% (10/101) of cases. Twenty cases underwent NGS, including all 10 PAX8‐positive UCs. All PAX8‐positive UCs had TERT promoter mutations. TSC1 alterations, NOTCH1 loss and WT1 loss were more frequent in the PAX8‐positive cases compared with the PAX8‐negative cases. RB1 loss was not seen in the PAX8‐positive UCs, while it was present in 40% of the PAX8‐negative UCs. Conclusions A subset of conventional UCs of the urinary bladder without subtype morphology and/or divergent differentiation are PAX8‐positive and have frequent alterations in TERT promoter, TSC1 , NOTCH1 , and WT1 , with an absence of RB1 loss. PAX8 positivity should be interpreted with caution if UC is in the differential, and NGS could be helpful in diagnostic workups as this study shows PAX8‐positive UCs may have a distinct molecular profile.