Molecular tumor heterogeneity in muscle invasive bladder cancer: Biomarkers, subtypes, and implications for therapy

Despite years of slow progress, muscle invasive bladder cancer (MIBC) is finally entering the era of molecularly guided targeted therapy. However, tumor heterogeneity is high in MIBC and may impact treatment response and resistance. The objective of this review is to dissect recent insights into inter- and intratumor heterogeneity (ITH) in MIBC, with emphasis on the clinical implications of this heterogeneity for biomarker-driven strategies and the development of new therapies.A nonsystematic review was performed in PubMed and EMBASE using the terms "tumor heterogeneity" and "bladder cancer."Intertumor heterogeneity, as reflected by different clinical phenotypes in different patients, has been partially explained with next generation sequencing and other molecular profiling technologies. RNA-based molecular subtyping, for example, provides a classification of MIBC into distinct categories that can be used for further molecular analysis, biomarker discovery, risk stratification, and treatment selection. Molecular subtyping and specific genomic alterations, especially in DNA damage repair genes, may help explain why some patients respond better to systemic chemotherapy and immunotherapy. Conversely, spatial and temporal ITH threaten to confound attempts to target specific molecular lesions since not all tumor cells within a patient may carry the relevant lesion. Improved understanding and management of ITH is required for the most effective use of biomarker-driven targeted therapies.Strategies to assess and overcome intertumor and ITH in MIBC will be critical steps toward realizing the objectives of precision oncology. Novel techniques such as analysis of circulating tumor DNA and single cell sequencing are likely to revolutionize our understanding of tumor heterogeneity.