INSM1 immunostaining in solid papillary carcinoma of the breast

Solid papillary carcinoma (SPC) is a histological subtype of breast carcinomas. At least 50% of SPC show neuroendocrine differentiation. Insulinoma-associated protein 1 (INSM1) is a transcription factor now employed as a useful neuroendocrine marker. It is suppressed by the Notch signaling pathway in other neuroendocrine tumors. However, the usefulness of INSM1 as a neuroendocrine marker and the relationships between INSM1 and NOTCH receptors in SPC of the breast currently remain unclear. To clarify the usefulness of INSM1 as a neuroendocrine marker and the relationships between INSM1 and NOTCH receptors in SPC, we performed immunohistochemistry using 19 tissue specimens of SPC of the breast. We complementarily analyzed public RNA sequencing data on breast carcinomas. Immunohistochemical examinations revealed that the staining intensity of INSM1 was significantly higher in the neuroendocrine group than in the non-neuroendocrine group. Positive correlations were observed between INSM1 and synaptophysin (SYP), or chromogranin-A (CHGA). In all cases, NOTCH 2 and 3 were positive, while NOTCH 1 and 4 were negative. According to public RNA data analyses, there were positive correlations between INSM1 and SYP, or CHGA, and negative correlations between INSM1 and NOTCH1-3. INSM1 is useful as a diagnostic marker for SPC with neuroendocrine differentiation in the breast.