Analysis of papillary urothelial carcinomas of the bladder with grade heterogeneity: supportive evidence for an early role of CDKN2A deletions in the FGFR3 pathway

Aims: The dual pathway model of urothelial carcinogenesis does not fully explain grade and stage progression in patients with initial low-grade, nonmuscle invasive urothelial carcinomas. Fibroblast growth factor receptor 3 (FGFR3) mutations are a hallmark of the low-grade pathway, with subsequent progression to muscle invasion occurring when FGFR3 mutant tumours exhibit a homozygous CDKN2A deletion. We hypothesized that grade heterogeneity represents the morphological manifestation of molecular changes associated with disease progression. Methods and results: We identified retrospectively 29 non-muscle invasive papillary urothelial carcinomas with grade heterogeneity (< 20% high grade). Nineteen had sufficient material for immunohistochemistry, CDKN2A fluorescence in-situ hybridization and FGFR3 mutation analysis. Eight pure low-grade urothelial carcinomas (PLGUC) were also analysed. FGFR3 mutation was seen in 10 of 19 cases. A homozygous CDKN2A deletion was identified in the low-grade areas of eight of nine (88%) technically suitable FGFR3 mutant cases (including five pTa cancers), in five of nine FGFR3 wild-type carcinomas and in none of the PLGUC. Increased MIB-1 expression was seen in low-grade areas of 12 of 19, in high-grade areas of 17 of 19 cases with grade heterogeneity and in none of the PLGUC. p53 staining was increased in one of 19 low-grade and seven of 19 high-grade areas. Conclusion: Our findings show that grade heterogeneity in urothelial carcinoma is characterized by increased MIB-1 labelling, and particularly in the FGFR3 mutant pathway, with homozygous deletions of CDKN2A in low-and high-grade areas. This would suggest that CDKN2A deletion occurs prior to grade progression and supports the current convention to assign the highest grade to urothelial carcinomas with grade heterogeneity.