Dual Inactivation of RB and p53 Pathways in RAS-Induced Melanomas

Abstract

The frequent loss of both INK4a and ARF in melanoma raises the question of which INK4a-ARF gene product functions to suppress melanoma genesis in vivo. Moreover, the high incidence of INK4a-ARF inactivation in transformed melanocytes, along with the lack of p53 mutation, implies a cell type-specific role for INK4a-ARF that may not be complemented by other lesions of the RB and p53 pathways. A mouse model of cutaneous melanoma has been generated previously through the combined effects of INK4a^Δ2/3 deficiency (null for INK4a and ARF) and melanocyte-specific expression of activated RAS (tyrosinase-driven H-RAS^V12G, Tyr-RAS). In this study, we made use of this Tyr-RAS allele to determine whether activated RAS can cooperate withp53 loss in melanoma genesis, whether such melanomas are biologically comparable to those arising inINK4a ^Δ2/3−/− mice, and whether tumor-associated mutations emerge in the p16^INK4a-RB pathway in such melanomas. Here, we report that p53inactivation can cooperate with activated RAS to promote the development of cutaneous melanomas that are clinically indistinguishable from those arisen on theINK4a ^Δ2/3 null background. Genomewide analysis of RAS-induced p53 mutant melanomas by comparative genomic hybridization and candidate gene surveys revealed alterations of key components governing RB-regulated G₁/S transition, including c-Myc, cyclin D1, cdc25a, and p21^CIP1. Consistent with the profile of c-Myc dysregulation, the reintroduction of p16^INK4a profoundly reduced the growth of Tyr-RASINK4a ^Δ2/3−/− tumor cells but had no effect on tumor cells derived from Tyr-RAS p53 ^−/−melanomas. Together, these data validate a role for p53inactivation in melanomagenesis and suggest that both the RB and p53 pathways function to suppress melanocyte transformation in vivo in the mouse.