BAP1 loss is unusual in well-differentiated papillary mesothelioma and may predict development of malignant mesothelioma.

Literature on BRCA1-associated protein 1 (BAP1) expression status in well-differentiated papillary mesothelioma (WDPM) is limited. In the present study, we examined the prevalence of BAP1 loss in WDPM by immunohistochemistry with clinical correlation, along with CDKN2A deletion status by fluorescence in situ hybridization (FISH). Eight patients diagnosed as having WDPM were identified from the surgical pathology file. Adenomatoid tumors (n = 8) and malignant mesothelioma (MM) (n = 39) were included for comparison. BAP1 immunohistochemistry was performed on representative block(s) from each case. CDKN2A FISH was also performed in the WDPMs and adenomatoid tumors. Clinical information was obtained from the medical records. Three of 8 WDPM patients showed synchronous or metachronous MM. All 3 cases showed BAP1 loss in both WDPM and the matched MM. Single-nucleotide polymorphism genomic microarray (n = 3) demonstrated a similar genetic profile in the WDPM and MM components, which supports their clonal relationship. The remaining 5 WDPM cases had intact BAP1 expression and had no evidence of disease on follow-up imaging studies at 1 to 71 months (median, 35 months). All 8 adenomatoid tumors had intact BAP1 expression, whereas 17 of 39 MM had BAP1 loss. CDKN2A FISH was negative for deletion in 4 WDPMs tested (including the case that developed MM) and all 8 adenomatoid tumors. In our study, WDPM did not show CDKN2A deletion in any case. BAP1 loss was also absent in all pure WDPM cases but was identified in all WDPM with synchronous or metachronous MM. Similar genetic landscape in WDPM and MM components suggested their clonal relationship.