Loss of BAP1 expression is very rare in peritoneal and gynecologic serous adenocarcinomas and can be useful in the differential diagnosis with abdominal mesothelioma.

Gynecologic and primary peritoneal serous carcinoma may be difficult to distinguish from abdominal mesotheliomas clinically, morphologically, and immunohistochemically. BAP1 double-hit inactivation and subsequent loss of protein expression have been reported in more than half of all abdominal mesotheliomas. We therefore sought to investigate the expression of BAP1 in serous carcinoma and explore its potential utility as a marker in the differential diagnosis with mesothelioma. We searched the computerized database of the Department of Anatomical Pathology, Royal North Shore Hospital, Australia, for all cases of gynecologic and peritoneal serous carcinomas and mesotheliomas diagnosed between 1998 and 2014. Immunohistochemistry for BAP1 was then performed on tissue microarray sections. Cases with completely absent nuclear staining in the presence of a positive internal control in nonneoplastic cells were considered negative. If staining was equivocal (eg, absent nuclear staining but no internal control), staining was repeated on whole sections. Loss of BAP1 expression was found in only 1 of 395 (0.3%) serous carcinomas but in 6 of 9 (67%) abdominal mesotheliomas (P < .001) and 131 of 277 (47%) thoracic mesotheliomas (P < .001). We conclude that BAP1 loss occurs extremely infrequently in gynecologic and peritoneal serous adenocarcinomas, whereas it is very common in mesotheliomas including abdominal mesothelioma. Therefore, although positive staining for BAP1 cannot be used to exclude a diagnosis of mesothelioma, loss of BAP1 expression can be used to very strongly support a pathological diagnosis of abdominal mesothelioma over serous carcinoma.

Loss of expression of BAP1 is very rare in non-small cell lung carcinoma.

Germline mutations of the BAP1 gene have been implicated in a cancer predisposition syndrome which includes mesothelioma, uveal melanoma, cutaneous melanocytic lesions, renal cell carcinoma, and possibly other malignancies. Double hit inactivation of BAP1 with subsequent loss of expression of the BAP1 protein also occurs in approximately 50% of mesotheliomas. The link between BAP1 mutation and lung cancer is yet to be fully explored. We sought to assess BAP1 expression in a large cohort of lung cancers undergoing surgery with curative intent. We searched the Anatomical Pathology database of our institution for lung cancer patients undergoing surgery with curative intent between 2000 and 2010. Immunohistochemistry for BAP1 was then performed in tissue microarray format. Our cohort included 257 lung cancer patients, of which 155 (60%) were adenocarcinomas and 72 (28%) were squamous cell carcinomas, with no other subtype comprising more than 3%. BAP1 loss of expression was found in only one lung cancer. We conclude that BAP1 mutation occurs very infrequently (0.4%) in non-small cell lung cancer. Given that the pathological differential diagnosis between lung carcinoma and mesothelioma may sometimes be difficult, this finding increases the specificity of loss of expression for BAP1 for the diagnosis of mesothelioma.