Melanocyte transformation requires complete loss of all pocket protein function via a mechanism that mitigates the need for MAPK pathway activation

• Published in: Oncogene 2017-06, Vol.36 (26), p.3789-3795
• Main Authors: Tonks, I D, Mukhopadhyay, P, Schroder, W A, Sorolla, A, Mould, A W, Handoko, H Y, Ferguson, B, Muller, H K, Keith, P, Hayward, N K, Walker, G J, Kay, G F
• Format: Article
• Language: English
• Subjects: 45/70; 631/208; 64; 64/60; 692/53; Age; Animals; Apoptosis; Carcinogens; Care and treatment; Cell Biology; Cell culture; Cell Transformation, Neoplastic - metabolism; Cell Transformation, Neoplastic - pathology; Clonal deletion; Cyclin-dependent kinase; Cyclin-dependent kinase 4; Cyclins; Epigenetics; Genetic transformation; Homeostasis; Human Genetics; Humans; INK4a protein; Internal Medicine; MAP kinase; MAP Kinase Signaling System; Medicine; Medicine & Public Health; Melanocytes; Melanocytes - enzymology; Melanocytes - metabolism; Melanocytes - pathology; Melanoma; Mice; Mice, Knockout; Mutation; Oncology; p16 Protein; Phosphorylation; Proteins; Ras protein; short-communication; Tumor Suppressor Protein p53 - metabolism; Tumorigenesis
• ISSN: 0950-9232; 1476-5594
• DOI: 10.1038/onc.2016.511

Deregulation of p16INK4A is a critical event in melanoma susceptibility and progression. It is generally assumed that the major effect of loss of p16 function is mediated through the CDK−cyclin pathway via its influence on the pocket protein (PP) pRb. However, there are also two other PPs, p107 and p130, which, when phosphorylated by CDK–cyclin complexes, play a role in permitting cell progression. Cohorts of mice carrying melanocyte-specific knockouts (KOs) of various combinations of the three PPs were generated. Mice null for pRb, p107, p130 or any combination of double mutants did not develop melanoma. Surprisingly, melanocyte-specific loss of all three PPs facilitated melanoma development (median age of onset 308 days, penetrance 40% at 1 year). Tumorigenesis was exacerbated by Trp53 co-deletion (median age of onset 275 days, penetrance 82% at 1 year), with cell culture studies indicating that this difference may result from the apoptotic role of Trp53. Melanomas in PP;Trp53-deficient mice lacked either Ras or Braf mutations, and hence developed in the absence of constitutive MAPK pathway activation. The lag period between induction of total PP or PP/Trp53 KO and melanoma development indicates that additional genetic or epigenetic alterations may account for neoplastic progression. However, exome sequencing of PP;Trp53 KO melanomas failed to reveal any additional recurrent driver mutations. Analysis of the putative mutation signature of the PP;Trp53 KO melanomas suggests that melanocytes are primed for transformation via a mutagenic mechanism involving an excess of T>G substitutions, but not involving a preponderance of C>T substitutions at CpG sites, which is the case for most spontaneous cancers not driven by a specific carcinogen. In sum, deregulation of all three PPs appears central to neoplastic progression for melanoma, and the customary reference to the p16 INKA /CDK4/pRB pathway may no longer be accurate; all PPs are potentially critical targets of CDK-cyclins in melanoma.