Urothelium-specific expression of mutationally activated Pik3ca initiates early lesions of non-invasive bladder cancer.

Despite the fact that ∼70% of bladder cancers are non-invasive and have high recurrence rates, early stage disease is understudied. The relative lack of models to validate the contribution of molecular drivers of bladder tumorigenesis is a significant issue. While mutations in PIK3CA are frequent in human bladder cancer, an in vivo model for understanding their contribution to bladder tumorigenesis is unavailable. Therefore, a Upk2-Cre/Pik3caH1047R mouse model expressing one or two R26-Pik3caH1047R alleles in a urothelium-specific manner was created. Pik3caH1047R functionality was confirmed by quantifying Akt phosphorylation and mice were characterized by assessing urothelial thickness, nuclear atypia, and expression of luminal and basal markers at 6 and 12 months of age. At 6 months, Pik3caH1047R mice developed increased urothelial thickness and nuclear atypia, however, at 12 months, Pik3caH1047R mice did not exhibit progressive disease. Immunohistochemistry shows urothelium maintained luminal differentiation characterized by high Foxa1 and Pparγ expression. In addition, mice were subjected to low-dose carcinogen exposure (N-Butyl-N-(4-hydroxybutyl)nitrosamine (BBN)). Surprisingly, Pik3caH1047R mice exhibited no significant differences after exposure relative to mice without exposure. Furthermore, ssGSEA analysis of invasive human tumors showed those with mutant PIK3CA do not exhibit significantly increased PI3K/AKT pathway activity compared to wildtype PIK3CA tumors. Overall, these data suggest that Pik3caH1047R can elicit early tumorigenic changes in the urothelium, but progression to invasion may require additional genetic alterations.