MITF links differentiation with cell cycle arrest in melanocytes by transcriptional activation of INK4A
Tumors often display unrestricted cell cycling attributable to a dysfunctional G1-S checkpoint. One of the mechanisms leading to such a defect is the inactivation of the cyclin-dependent kinase inhibitor p16. Although inactivation of p16 is observed in a wide range of tumors, including cutaneous melanoma, genetic alteration of p16 is reportedly uncommon in uveal melanoma. Here we show that the p16 promoter is hypermethylated in 6 of 12 uveal melanoma cell lines and in 7 of 22 primary uveal melanomas analyzed. Five of seven patients with a methylated primary tumor died of metastatic disease compared with 2 of 15 patients with a nonmethylated primary tumor. We also show that all uveal melanoma cell lines with a hypermethylated p16 promoter have lost p16 expression but have maintained the expression of p14. Treatment of uveal melanoma cell lines with 5-aza-2*-deoxycytidine results in demethylation of p16 and in reexpression of p16 mRNA, which is maintained upon withdrawal of the 5-aza-2*-deoxycytidine. In conclusion, p16 promoter methylation appears to be a common event in uveal melanoma and is accompanied by the loss of p16 expression.