Microphthalmia transcription factor in malignant melanoma predicts occult sentinel lymph node metastases and survival

Microphthalmia transcription factor (Mitf) is involved in melanocyte development and differentiation. We previously reported that Mitf expression, as detected by immunohistochemical analysis, is an independent prognostic marker in patients with intermediate-thickness melanoma. However, the clinical significance of Mitf expression in melanoma is not well delineated. In this prospective study, we attempted to demonstrate the correlation between Mitf expression in primary melanoma and the sentinel lymph node status and prognosis. We prospectively examined primary cutaneous melanomas from 94 patients undergoing nodal staging by sentinel lymph node biopsy. We quantified the percentage of tumor cells whose nuclei stained with the Mitf antibody visually. Survival curves were generated using the Kaplan-Meier method. The correlation between Mitf expression and nodal status was evaluated using the Mann-Whitney U-test. Here we demonstrate that Mitf expression is directly correlated with both disease-free survival (DFS) and overall survival (OS) over a median follow-up of 28.5 months. The mean DFS and OS in the eight patients whose melanomas did not stain positive for Mitf were 15.75±3.36 months (median, 12 months) and 38.17±5.18 months (median, 29 months), respectively. These results are significantly lower than those for patients who showed evidence of Mitf expression, in whom the mean DFS and OS were 66.1±4.03 months (median, not reached, P=0.0001) and 66.75±38.17 months (median, not reached, P=0.0001), respectively. The mean DFS and OS with greater than 25% (67 patients) of the melanoma cells staining positive for Mitf expression were 78.37±2.78 and 82.38±1.6 months, respectively, compared with 26.37±3.2 months (P=0.0001) and 44.53±4.5 months (P=0.0001), respectively, with up to 25% (27 patients) of cells stained positive for Mitf expression. In addition, there was a significant relationship between Mitf expression and nodal status, as evaluated by sentinel node biopsy. For example, none of the melanomas with greater than 50% Mitf expression had a positive sentinel node biopsy. Our study shows that expression of the molecular marker Mitf in primary cutaneous melanomas is a useful tool in assessing lymph node status. Mitf immunostaining in the primary tumor serves as a reliable predictor of occult lymph node metastases, as well as a favorable prognosticator of DFS and OS in melanoma patients.