Analysis of KIT expression and mutations in sinonasal, genital, and acrolentiginous melanomas.

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8563 Background: Melanoma is subdivided into molecular defined groups. One major subtype is characterized by a mutation in the BRAF gene. BRAF is less commonly mutated in acrolentiginous (ALM) and mucosal (MM) melanomas, instead these subtypes predominantly show aberrations in the KIT/CD117 gene. KIT aberrations predict the outcome of targeted therapies in ALM and MM patients, however the particular role of KIT expression in these melanoma subtypes remains controversial. To explore the relationship between KIT expression, mutation, and tumor stages, we investigated immunohistochemical KIT expression and mutation status in primary ALM/MM lesions and their metastases. We also compared the frequency of KIT mutations of MM in different anatomic locations. Methods: KIT immunoreactivity and mutation status was assessed in 41 ALM and 25 MM patients, using polyclonal rabbit anti-human KIT/CD117 antibody, and PCR was performed for KIT gene exons 9,11,13,17 and 18. Of these, 19 ALM and 15 MM patients had matched primary and metastatic lesions. Phosphorylated extracellular regulated kinase (P-ERK) was investigated in a subset of 8 ALM and 8 MM matched primary/metastatic pairs by immunohistochemistry. Results: Heterogeneous KIT immunoreactivity was observed in both primary and metastatic lesions. Mutations were present in four of 41 ALM (10%) and five of 25 MM (20%) patients. Only vulvar mucosal samples carried KIT mutations in contrast to sinonasal lesions (p= 0.0109). In KIT-mutated tumors, the mutations were present in KIT expressing as well as KIT negative cells, as shown by Laser Capture Microdissection (LCM). P-ERK expression was preferentially found in metastases. Conclusions: KIT expression is heterogeneous and shows no relationship with disease progression and mutation status, therefore KIT immunoreactivity is not a valuable marker for treatment decisions. In mucosal melanoma patients, KIT mutation frequency is significantly associated with anatomic location. Vulvar melanoma patients are prone to carry activating KIT mutations and thus may be more likely to benefit from KIT-targeted therapies than other subtypes. Therefore, especially vulvar melanoma patients should be screened for activating KIT mutations.