Insulinoma-associated protein 1 is a novel sensitive and specific marker for small cell carcinoma of the prostate

The correct diagnosis of small cell carcinoma (SCC) of the prostate is critical because of its aggressive behavior and poor prognosis. The histopathologic diagnosis could be challenging without neuroendocrine markers, which currently has limitations. Insulinoma-associated protein 1 (INSM1), a zinc-finger transcription factor, is considered to play an important role in the development of several neuroendocrine precursor cells. Its diagnosis value has only recently been evaluated. In this study, we analyzed the expression of INSM in three high-throughput RNA sequencing data sets and performed INSM1 immunohistochemistry on a large series of prostatic SCCs and non-neuroendocrine prostate tissues. To validate its possible utility as a diagnostic marker, the performance of chromogranin and synaptophysin was used for comparison. We found INSM1 mRNA is up-regulated in neuroendocrine prostate carcinoma samples from the published data sets. The results were verified by the immunohistochemistry performance. INSM1 was positive in 92.3% of prostatic SCCs, compared with 53.8% positive for chromogranin, and 84.6% positive for synaptophysin. INSM1 was also stained all of the mixed SCC-acinar adenocarcinomas and metastatic SCCs progression from acinar adenocarcinoma. Only 3.4% of benign prostate tissues and 4.0% of prostate adenocarcinomas were INSM1 positive. Our data suggest that INSM1 is a novel sensitive and specific marker for detection of SCC of the prostate. Application of INSM1 in clinical pathologic diagnosis will be valuable.