Expression of polo-like kinase (PLK1) in thin melanomas: a novel marker of metastatic disease.
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 (< or= 0.75 mm), which should have an excellent prognosis according to Breslow, can be lethal due to their ability to metastasize. In our study, thin malignant melanomas (< or= 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. 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). Our results suggest that PLK1 expression in thin melanomas is a reliable marker to identify patients at high risk for metastases.