Membrane mesothelin expression positivity is associated with poor clinical outcome of luminal-type breast cancer.

Mesothelin is expressed in various types of malignant tumors. The present study immunohistochemically investigated mesothelin expression and its clinicopathological significance in each subtype of breast cancer, with special reference to its cellular localization, in particular, membrane mesothelin expression. Using tissue specimens from 482 patients with breast cancer, immunohistochemistry was used to study mesothelin expression and help classify its localization as membrane or cytoplasmic expression. Mesothelin expression was detected in 77 (16.0%) cases and was the highest in triple-negative breast cancer (31/75; 41.3%), followed by human epithelial growth factor receptor type 2 type (6/33, 18.2%) and luminal type (36/374; 9.6%). Among the 482 cases, membrane mesothelin expression was detected in 73 cases and was significantly associated with a negative hormone receptor status, higher Ki-67 labeling index, nuclear grade 3 and a lower relapse-free survival rate. Cytoplasmic mesothelin expression was not significantly associated with a lower relapse-free survival rate (P=0.058). In the 343 cases of luminal type, the membrane mesothelin expression-positive group had significantly worse prognosis than the membrane mesothelin-expression-negative group (P=0.042). There was no significant difference in the relapse-free survival rate according to the membrane mesothelin expression status in the triple-negative type and other types. It was suggested that membrane mesothelin expression in luminal type breast cancer is associated with a lower rate of relapse-free survival.