| Summary: | Background Mixed neuroendocrine and non-endocrine neoplasms (MiNENs) are challenging to diagnose and manage clinically. The current understanding of MiNENs’ pathobiology, molecular mechanisms, and management is incomplete. Though microsatellite instability (MSI) is known to impact carcinogenesis, reports examining MSI mechanisms for MiNENs are rare. Methods We report an unusual colonic MSI-MiNEN uncovered in an 89-year-old woman and the review of the literature. Results Pathologic inspection revealed a high-grade carcinoma composed of tumor cells with neuroendocrine histologic traits and immunophenotype intermixed with mucin-containing signet ring–like cells arranged in nested and micronodular patterns. Loss of MLH1 and PMS2 mismatch repair proteins was detected in tumor cells. INSM1 immunostaining highlighted about 50% of the tumour, further reinforcing the MiNEN diagnosis. Next-generation sequencing identified multiple carcinogenic mutations. Because of the advanced stage of the tumor and its adhesion to the adjacent organs, surgical resection was aborted; immunotherapy was initiated. The tumor is in remission 30 months following initiation of treatment, and the patient remains asymptomatic. Conclusion This unique MSI MiNEN was characterized by its immunohistochemical and molecular signatures and illustrated how correctly diagnosing MSI can strongly improve a patient’s outcomes.
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