The evolution of bladder cancer genomics: What have we learned and how can we use it?

Abstract

Background: With advancements in molecular biology techniques, great progress has been made in the understanding of urothelial carcinoma pathogenesis.

Objective: To examine the historic description of molecular alterations in bladder cancer and their evolution towards our current comprehension of the biology of the disease.

Results: Historically, a two-pathway model was described from histological and cytogenetic studies: low-grade papillary non-muscle invasive bladder cancers (NMIBC) were described to arise from epithelial hyperplasia with loss of chromosome 9 as an early event, whereas muscle-invasive bladder cancers (MIBC) were considered to develop from dysplasia, associated with genetic instability. Although there could be connections between the 2 pathways, NMIBC and MIBC were largely believed to develop secondary to different molecular alterations. Next-generation sequencing has allowed important insights into cancer biology and a better understanding of the pathways involved in bladder cancer pathogenesis and heterogeneity. Urothelial carcinoma has been found to have a high frequency of somatic mutations compared to other solid tumors, including several mutations in multiple signaling pathways, such as cell cycle regulators (TP53, RB1), RTK/RAS/RAF pathway, PI3K/AKT/mTOR pathway and TERT gene promoter. Epigenetic changes and mutations in chromatin remodeling genes are especially frequent in bladder cancer. Mutations in FGFR3 and KDM6A are more common in NMIBC than in MIBC, whereas mutations in TP53 and KMT2D are more common in MIBC, suggesting the previously hypothesized 2 different pathways, with a subset of tumors progressing from NMIBC to MIBC. Using comprehensive RNA expression profiling studies, at least 5 subtypes of bladder cancer have been identified, the most fundamental division being Basal/Squamous-like and Luminal. These subtypes have different prognoses, natural histories and responses to systemic treatments: Luminal subtypes are enriched with papillary histology and have a better prognosis, while Basal/Squamous-like subtypes are enriched with squamous features, are associated with advanced stage at presentation, and portend a worse prognosis.

Conclusion: This new understanding of bladder cancer will optimistically translate into better understanding of this heterogeneous disease and lead to improvement in patient outcome and quality of life through better tailored treatments.

Keywords: Bladder cancer; Genetics; Molecular subtypes; Pathways; Urothelial carcinoma.