Frequent inactivation of the BAP1 gene in epithelioid‐type malignant mesothelioma

In the present study, we analyzed genomic alterations of BRCA 1‐associated protein 1 ( BAP1 ) in 23 malignant mesotheliomas ( MM s), 16 epithelioid and seven non‐epithelioid, consisting of 18 clinical specimens and five established cell lines. In examining these samples for homozygous deletions and sequence‐level mutations, we found biallelic BAP1 gene alterations in 14 of 23 MM s (61%). Seven of these 14 MM s had homozygous deletions of the partial or entire BAP1 gene, another five had sequence‐level mutations, including small deletions, a nonsense mutation, and missense mutations with additional monoallelic deletions, and the remaining two had homozygous mutations without allelic loss. All but one of the 14 BAP1 gene mutations were found in the epithelioid‐type MM s; BAP1 mutations were found in 13 of 16 epithelioid‐type MM s, but in only one of seven non‐epithelioid‐type MM s (13/16 vs 1/7; P   = 0.005). There was no BAP1 m RNA expression in MM s with biallelic deletion and repressed expression was confirmed in MM specimens with deletion/mutation as compared with M et5a, SV 40‐transformed normal mesothelial cells. W estern blot showed that seven of eight epithelioid MM s analyzed were BAP 1 negative. Immunostaining with anti‐ BAP 1 antibody in normal lung tissues revealed clear nuclear staining of normal mesothelial cells. No nuclear staining was observed among BAP 1 mutation‐positive MM tumors, whereas nuclear staining was observed among BAP 1 mutation‐negative MM tumors. These results suggest that the lack of the tumor suppressor BAP 1 may be more specifically involved in the pathogenesis of epithelioid MM rather than non‐epithelioid MM , and would be useful for diagnosis of epithelioid‐type MM . ( Cancer Sci 2012; 103: 868–874)