Expression of polo-like kinase (PLK1) in thin melanomas: a novel marker of metastatic disease

Background: The maximum thickness of a primary malignant melanoma as measured by Breslow's method is currently the most important prognostic factor. However, some thin melanomas (less than or equal to 0.75 mm), which should have an excellent prognosis according to Breslow, can be lethal due to their ability to metastasize. Methods: In our study, thin malignant melanomas (less than or equal to 0.75 mm) from 36 patients were analyzed with immunohistochemical techniques using monoclonal antibodies directed against PLK1 and Ki-67. The immunoreactivity of 22 melanomas which developed metastases within 5 years of follow-up was compared with a group of 14 non-metastasized melanomas. Two independent investigators evaluated stained sections. Differences of PLK1 and Ki-67 indices between melanomas with and without metastases were tested statistically using the Mann-Whitney U -test. Results: Malignant melanomas with metastases expressed PLK1 at markedly elevated levels compared to melanomas without metastases (median, 60.00% vs. 37.98%; p = 0.000053). The difference of the Ki-67 index between both groups was not significant (median, 6.35% vs. 4.53%; p = 0.150473). Conclusions: Our results suggest that PLK1 expression in thin melanomas is a reliable marker to identify patients at high risk for metastases.