RASSF6 exhibits promoter hypermethylation in metastatic melanoma and inhibits invasion in melanoma cells

Brain metastasisis a major contributor to cancer mortality, yet, the genetic changes underlying the development of this capacity remain poorly understood. RASSF proteins are a family of tumor suppressors that often suffer epigenetic inactivation during tumorigenesis. However, their epigenetic status in brain metastases has not been well characterized. We have examined the promoter methylation of the classical RASSF members (RASSF1A-RASSF6) in a panel of metastatic brain tumor samples. RASSF1A and RASSF2 have been shown to undergo promoter methylation at high frequency in primary lung and breast tumors and in brain metastases. Other members exhibited little or no methylation in these tumors. In examining melanomametastases, however, we found that RASSF6 exhibits the highest frequency of inactivation in melanomaand in melanomabrain metastases. Most melanomasare driven by an activating mutation in B-Raf. Introduction of RASSF6 into a B-RafV600E-containing metastatic melanomacell line inhibited its ability to invade through collagen and suppressed MAPK pathway activation and AKT. RASSF6 also appears to increase the association of mutant B-Raf and MST1, providing a potential mechanism by which RASSF6 is able to suppress MAPK activation. Thus, we have identified a novel potential role for RASSF6 in melanomadevelopment. Promoter methylation leading to reduced expression of RASSF6 may play an important role in melanomadevelopment and may contribute to brain metastases.