Insulinoma-associated protein-1: A novel marker for diagnosis of small cell carcinoma of the lung

Objectives: Insulinoma-associated protein-1 (INSM-1) has been identified as a novel, robust marker for neuroendocrine differentiation in normal and neoplastic human tissue. This is currently the solitary nuclear neuroendocrine marker in diagnostic immunohistochemistry (IHC). The objectives of the study were to ascertain the utility of INSM-1 in the identification of small cell lung carcinoma (SCLC) and compare its diagnostic efficacy with neuroendocrine markers, namely synaptophysin, chromogranin-A, and CD56 or Neural Cell Adhesion Molecule (NCAM). Materials and Methods: The current case series included 31 cases of SCLC collected retrospectively and prospectively. Immunohistochemical staining for pancytokeratin and thyroid transcription factor-1 (TTF-1) was performed in all cases along with staining for neuroendocrine markers, namely synaptophysin or chromogranin-A. The staining for the supplementary neuroendocrine marker, namely CD-56, was performed in 12 cases. INSM-1 immunohistochemistry was performed in all cases. Statistical analysis: The frequency of expression of INSM-1 was assessed and compared with the frequencies of expression of synaptophysin, chromogranin, and CD-56. A p <0.05 considered as a statistically significant. Results: The current study included 31 cases of SCLC in which INSM-1 immunostaining revealed positivity in 96.77% ( n = 30/31) cases. Positive immunostaining for synaptophysin was identified in 70.97% ( n = 22/31) cases, while chromogranin was present in 74.19% ( n = 23/31) cases. CD56 was positive in 58.33% ( n = 7/12) cases. Conclusions: INSM1 has been identified as a vital transcription factor for neuroendocrine differentiation. The sensitivity for INSM-1 in the identification of SCLC is higher than that for synaptophysin, chromogranin, and CD-56. The expression of this marker is nuclear which makes it slightly easier to interpret results, particularly in cases with necrosis and crushing. INSM-1 may be a novel immunohistochemistry marker that can be used as a standalone marker in the first line for the identification of SCLC.