Analysis of More than 16,000 Human Tumor and Normal Tissues Identifies Uroplakin 3B as a Useful Diagnostic Marker for Mesothelioma and Normal Mesothelial Cells

BAP1 Loss, Nuclear Grading, and Nonepithelioid Features in the Diagnosis of Mesothelioma in Italy: Nevermore without the Pathology Report

The pathologic diagnosis of pleural mesothelioma is generally based on international guidelines, but no compulsory points based on different drugs approvals in different European countries are required to be reported. According to the last (2021) edition of the World Health Organization classification of pleural tumors, the nuclear grade of epithelioid-type mesothelioma should be always inserted in the pathologic report, while the presence of BRCA-associated protein-1 (BAP1) (clone C4) loss and a statement on the presence of the sarcomatoid/nonepithelioid component are fundamental for both a screening of patients with suspected BAP1 tumor predisposition syndrome and the eligibility to perform first-line immunotherapy at least in some countries. Several Italian experts on pleural mesothelioma who are deeply involved in national scientific societies or dedicated working groups supported by patient associations agreed that the pathology report of mesothelioma of the pleura should always include the nuclear grade in the epithelioid histology, which is an overt statement on the presence of sarcomatoid components (at least 1%, in agreement with the last classification of pleural mesothelioma) and the presence of BAP1 loss (BAP1-deficient mesothelioma) or not (BAP1-retained mesothelioma) in order to screen patients possibly harboring BAP1 tumor predisposition syndrome. This review aims to summarize the most recent data on these three important elements to provide evidence regarding the possible precision needs for mesothelioma.

Uroplakin-IIIb as a novel immunohistochemical marker for mesothelioma

Distinguishing mesothelioma from non-small cell lung carcinoma often requires a battery of immunohistochemical stains, as many traditional markers used in mesothelioma lack sufficient specificity to allow them to be used alone. A recent large-scale TMA screen identified uroplakin-IIIb (UpIIIb; clone MSVA-736M) as a potentially specific marker for mesothelioma. We examined the performance of this antibody using tissue microarrays containing a panel of 48 epithelioid mesotheliomas, 26 sarcomatoid mesotheliomas, and 144 non-small cell lung carcinomas (NSCLCs). Here we show that UpIIIb has good sensitivity (37/47 evaluable cases positive, 79%) and excellent specificity for distinguishing epithelioid mesothelioma from NSCLC (0/140 evaluable cases positive). UPIIIb sensitivity for epithelioid mesotheliomas was only slightly inferior to the established highly specific mesothelioma marker HEG1 (41/46 evaluable cases positive on the same TMA, 89%). However, UpIIIb did not stain any sarcomatoid mesotheliomas (0/24 evaluable cases positive). We also found that UpIIIb stained a proportion of high-grade serous ovarian carcinomas, a perennial diagnostic confounder in the context of mesotheliomas. Taken together, our data suggest that UpIIIb can be used as a highly specific and sensitive mesothelial marker when the diagnostic question is epithelioid mesothelioma versus NSCLC; in particular, UpIIIb staining will pick up some number of epithelioid mesotheliomas that are HEG1 negative. Since UpIIIb is known to stain some proportion of urothelial carcinomas as well as gynecologic and a few pancreatic tumors, it should be used with caution in the peritoneal cavity or when the differential diagnosis includes carcinomas from these locations.