Can MUM1 Be a Useful Marker for Differentiating Malignant Melanoma From Benign Nevi?

Abstract: Reliable markers are needed to help differentiate malignant melanoma (MM) from benign melanocytic nevi (BMN). MUM1 (multiple myeloma oncogene 1)/IRF4, a transcription factor expressed in tumors such as MM, may be a potential diagnostic marker. This study evaluated MUM1 expression in MM and BMN using the Allred scoring system. This retrospective, single-center study included patients diagnosed with MM or BMN through excision biopsy at St. Marianna University Hospital between January 1, 2019 and December 31, 2021. Immunohistochemical staining for MUM1, S-100, HMB45, and Melan A was performed on paraffin sections. Two certified pathologists, masked to the diagnoses, independently evaluated the staining. MUM1 expression was scored using the Allred system, including the proportion score (PS), intensity score (IS), and total score (TS). This study included 32 patients with MM and 36 patients with BMN. S-100 was positive in all patients with MM, and HMB45 and MelanA were positive for 94% of patients with MM. MUM1 staining was positive in 97% of patients with MM and 100% of patients with BMN. The most common PS was 5, observed in 72% of patients with MM and in 50% of patients with BMN. An IS of 2 was the most frequent in 59.4% of the patients with MM and 66.7% of the patients with BMN. A TS of 7 was the most common in 62.5% of the patients with MM and 55.6% of the patients with BMN. No significant differences in the PS, IS, or TS were observed. These findings suggest that although MUM1 is a sensitive marker, it is not reliable for distinguishing MM from BMN.