INSM1 Expression in Peripheral Neuroblastic Tumors and Other Embryonal Neoplasms.

Insulinoma-associated protein 1 (INSM1) is a transcription factor that functions in neuroepithelial tissue development and shows expression in neuroendocrine neoplasms. Given the role of INSM1 in controlling differentiation of the sympatho-adrenal lineage, we hypothesized that INSM1 expression would define a subset of neuroblastic tumors. This study aimed to characterize the immunohistochemical profile of INSM1 in a cohort of peripheral neuroblastic tumors and compare INSM1 expression in these tumors to that seen in other embryonal neoplasms, using both tissue microarrays and whole-slide histologic sections. INSM1 showed nuclear expression in 39/50 (78%) peripheral neuroblastic tumors, including 27/32 (84%) neuroblastomas, 9/9 (100%) ganglioneuroblastomas, and 3/9 (33%) ganglioneuromas. Altogether, 70% of peripheral neuroblastic tumors showed anti-INSM1 immunoreactivity in more than 20% of tumor nuclei. Although no non-neuroblastic tumors in this study exhibited INSM1 expression in more than 20% of nuclei, focal or patchy staining was identified in 7/14 (50%) rhabdomyosarcomas, 7/22 (32%) nephroblastomas, and 4/20 (20%) Ewing sarcomas. The absence of INSM1 expression in peripheral neuroblastic tumors was restricted to undifferentiated and poorly differentiated neuroblastomas, as well as mature ganglioneuromas, mimicking the transient INSM1 expression seen in sympatho-adrenal differentiation during normal development. No significant association between MYCN amplification status and INSM1 expression was observed. We found that all 3 INSM1-negative neuroblastoma patients with available follow-up were alive at a median of 15 years, in comparison to 9 of 13 INSM1-positive neuroblastoma patients living at a median of 5 years. Additional studies are needed to determine whether INSM1 expression is indicative of a clinically significant differentiation state in neuroblastoma.