Loss of expression of BAP1 predicts longer survival in mesothelioma

SummaryBRCA1-associated protein 1 (BAP1) is a tumour suppressor gene frequently inactivated in mesothelioma, rarely also in association with germline mutation. BAP1 mutations have been associated with improved prognosis and distinct clin-icopathological features. We sought to determine the clin-icopathological significance of BAP1 immunohistochemistry (IHC) in mesothelioma. We performed IHC on a tissue microarray (TMA) cohort comprising all available thoracic mesotheliomas encountered during the period 1991-2014 at our institution (n = 229). All cases were independently reviewed to confirm the diagnosis and subclassify as epithe-lioid, sarcomatoid or biphasic. The median age at diagnosis was 72 years; 188 (82.1%) were male; 120 (52.4%) were epithelioid (median survival 13.0 months), 67 (29.3%) sar-comatoid (median survival 5.6 months) and 42 (18.3%) biphasic (median survival 10.6 months). Negative staining for BAP1 (defined as completely absent nuclear staining in the presence of positive internal controls in non-neoplastic cells) occurred in 106 (46.3%) mesotheliomas. There was complete interobserver concordance for BAP1 IHC status. BAP1 loss was strongly associated with younger age at onset (p < 0.01) and epithelioid differentiation (p < 0.01). BAP1 loss predicted an improved median survival of 16.11 months (95% CI 12.16–20.06) versus 6.34 months (95% CI 5.34–7.34), p < 0.01. In a multivariate model including age, gender and histological type, BAP1 loss, younger age and epithelioid differentiation remained protective (all p < 0.01). If our results are confirmed by others, BAP1 IHC may have a role to predict prolonged survival or triage formal genetic testing for germline BAP1 mutation in patients presenting with mesothelioma.