The Role of Immunohistochemistry as a Surrogate Marker in Molecular Subtyping and Classification of Bladder Cancer

Abstract

Bladder cancer (BC) is a highly heterogeneous disease, presenting clinical challenges, particularly in predicting patient outcomes and selecting effective treatments. Molecular subtyping has emerged as an essential tool for understanding the biological diversity of BC; however, its implementation in clinical practice remains limited due to the high costs and complexity of genomic techniques. This review examines the role of immunohistochemistry (IHC) as a surrogate marker for molecular subtyping in BC, highlighting its potential to bridge the gap between advanced molecular classifications and routine clinical application; Methods: We explore the evolution of taxonomic classification in BC, with a particular focus on cytokeratin (KRT) expression patterns in normal urothelium, which are key to identifying basal and luminal subtypes. Furthermore, we emphasise the need for consensus on IHC markers to reliably define these subtypes, facilitating wider and standardised clinical use. The review also analyses the application of IHC in both muscle-invasive (MIBC) and non-muscle-invasive bladder cancer (NMIBC), with particular attention to the less extensively studied NMIBC cases. We discuss the practical advantages of IHC for subtyping, including its cost effectiveness and feasibility in standard pathology laboratories, alongside ongoing challenges such as the requirement for standardised protocols and external validation across diverse clinical settings; Conclusions: While IHC has limitations, it offers a viable alternative for laboratories lacking access to advanced molecular techniques. Further research is required to determine the optimal combination of markers, establish a consensus diagnostic algorithm, and validate IHC through large-scale trials. This will ultimately enhance diagnostic accuracy, guide treatment decisions, and improve patient outcomes.

Keywords: bladder cancer; immunohistochemistry; molecular classification; molecular subtypes.

Figure 1

Flowchart of the narrative review process for identifying relevant studies on the molecular subtyping and classification of bladder cancer (BC) using immunohistochemistry (IHC) between 2016 and 2024.

Figure 2

Cellular composition of the urothelium and associated keratin expression. The figure illustrates the three main layers of the urothelium: basal, intermediate, and superficial. Each layer is characterised by the expression of specific keratins and other markers. The basal layer expresses KRT 5/6, KRT 14, integrin β4, and CD44; the intermediate layer expresses KRT 18 with low levels of KRT 5/6 and CD44; and the superficial layer is marked by KRT 20 and uroplakins, which are associated with terminal urothelial differentiation.

Figure 3

Evolution of molecular taxonomy in bladder cancer. This figure shows the progression from early microarray studies, which did not identify intrinsic subtypes, to the more sophisticated molecular classifications developed over time by institutions such as the University of North Carolina (UNC), MD Anderson Cancer Center (MDA), The Cancer Genome Atlas (TCGA), and (Lund University) Lund, culminating in the consensus classification (CMC). The timeline highlights the expansion from basal and luminal subtypes to more refined categories. The adoption of NanoString technology in 2023 marks a key advancement for clinical implementation, providing a more precise and efficient approach to molecular subtyping and therapy optimisation. The color coding within the figure reflects the categorization of molecular subtypes across studies: the red tones represent basal-like subtypes, which display basal characteristics across different classification systems; the blue tones correspond to luminal subtypes, associated with urothelial differentiation; gray indicates urothelial-like or p53-like subtypes, depending on the classification; green represents stromal-rich or infiltrated subtypes, which are distinguished by immune or stromal signatures; and orange and pink denote neuroendocrine and mesenchymal-like categories, introduced in more recent classifications.

Figure 4

Overview of the main studies on molecular subtyping of bladder cancer (BC) using immunohistochemical (IHC) markers. The figure highlights the various markers used across different studies from 2016 to 2023, with a focus on muscle-invasive bladder cancer (MIBC) and non-muscle-invasive bladder cancer (NMIBC). Each study is represented with corresponding markers (squares), the number of patients analysed, and the subtypes identified in each study (circles). A key is provided at the bottom to decode the colours and markers used for classification [20,28,37,38,44,48,49,50,51,54,55,56,57,58,59,61,63,70,72,74,77,79,80,87,88,89,90].