Insulinoma-associated protein 1 (INSM1) expression in breast carcinomas with neuroendocrine morphologies: application and future prospective

Herein, we describe the first cases of neuroendocrine (NE) phenotype mammary carcinomas in which the NE nature of the tumors was confirmed only by insulinoma-associated protein 1 (INSM1), without expressions of traditional “gold standard” NE indicators. This is also the first analysis to use INSM1, a promising antibody with high sensitivity and specificity, in the field of breast oncology. Three patients were, respectively, 42-, 58-, and 64-year-old Japanese women with breast tumors showing characteristic NE morphologies. Immunohistochemically, these malignancies revealed diffuse nuclear expressions of INSM1, whereas chromogranin A and synaptophysin did not show distinct NE features in their cytoplasm. Based on the identification of INSM1 as well as our present immunohistochemical results, the frequency of detecting NE differentiation in systemic neoplasms, including breast NE phenotype cancers, is anticipated to increase such that, ultimately, our observations might contribute to the development of novel treatments including molecular targeted drugs for these tumor entities.