KIT receptor is expressed in more than 50% of early-stage malignant melanoma: a retrospective study of 261 patients

The c-kit gene encodes a transmembrane receptor (KIT) with tyrosine kinase activity which is a specific target for anti-cancer therapy. We investigated KIT expression in a group of patients with early-stage malignant melanoma. Primary tumour specimens obtained from 261 radically resected patients with stage I and 11 malignant melanoma were examined for KIT expression. Formalin-fixed, paraffin embedded tissues were stained with the polyclonal rabbit anti-human anti-KIT antibody (Dako Cytomation Inc., Carpenteria, California, USA). Patients were classified into four groups according to the level of expression (0%, < 30%, 30-60% and > 60%). Univariate and multivariate analyses examining the impact of KIT expression, Breslow thickness, Clark level and microscopic ulceration on disease-free survival were performed. Within the population of 261 patients with early-stage melanoma with 62 recurrences during a follow-up of 64 months, KIT expression was found in 144 cases (55%). KIT was expressed in more than 60% of cells in 20 patients (8%), in 30-60% of cells in 64 patients (24%) and in less than 30% of cells in 60 patients (23%). KIT expression was not found in 117 patients (45%). In univariate analyses, the influence of KIT expression on disease-free survival was not proven (P=0.4956; log-rank test). Increasing Breslow thickness, a higher Clark level, the presence of microscopic ulceration and a higher stage were significantly associated with a shorter disease-free survival (P < 0.0001; log-rank test in all cases). In multivariate analysis, Breslow thickness, stage and KIT expression were significant negative prognostic factors for a shorter disease-free survival (P < 0.0001, P=0.0028, P=0.0488, respectively; stepwise Cox regression model). It can be concluded that KIT is expressed in more than one-half of early-stage malignant melanoma. KIT may serve as an additive prognostic factor to Breslow thickness and stage within the tested population. The therapeutic impact of KIT expression in malignant melanoma is uncertain. Results of ongoing pilot phase 11 studies may validate the efficacy of imatinib mesylate in malignant melanoma expressing KIT.