Uroplakins (UPs) are a family of cell membrane glycoproteins that links with each other and create plaques on the apical surface of the urothelium. These plaques form a membrane with a permeability barrier protecting the bladder surface by preventing the entry of urine from the urinary tract lumen and in addition secure a mechanical stability of the urothelium. At present four main types of UPs have been identified – UPIa, UPIb, UPII and UPIII. UP II and III are mainly expressed at the membrane surface of terminally differentiated urothelial cells (umbrella cells), but due to dimerization with UPI in the Golgi-apparatus cytoplasmic localization can be observed by IHC.
IHC for UP II and III is primarily used to determine the origin of a cancer of unknown primary site (CUP). Protein and mRNA analysis have indicated that UP II and III primarily is expressed in urothelial cells and as such enabling IHC for UP to be used to differentiate urothelial carcinomas from other neoplasia. Initially publications focused on IHC for UP III showing a high analytical specificity but only a moderate analytical sensitivity. The first paper by Kaufmann et al reported an overall sensitivity of 57% for primary and metastatic urothelial carcinomas. Subsequent publications e.g. by Aaron M. et al have not been able to confirm this and in the diagnostic work-up of CUP, IHC for UP III have shown a low analytical sensitivity in the range of 25-50% for invasive/metastatic urothelial carcinomas. IHC for UP II have been reported to provide an increased analytical sensitivity for urothelial carcinomas compared to IHC for UP III. Recent publications indicate a range of 70-80%. Both UPII and UPIII show a higher expression levels in grade I and II urothelial carcinomas compared to grade III.
At present and according to publications and preliminary data generated in one NordiQC reference, urethra and appendix/colon are recommended as positive and negative tissue controls for UP II/III. In the Urethra, protocols must be calibrated to provide a moderate to strong, distinct predominantly membranous staining reaction in virtually all umbrella cells. A weak cytoplasmic staining can be seen in basal and intermediate urothelial cells. In the appendix/colon no staining reaction should be seen, especially epithelial cells being negative. No data available on low expressing normal tissues/cells and thus important to secure an “as strong as possible reaction” for UP II/III in umbrella cells without reaction in the negative tissue controls.