title: Melanocyte transformation requires complete loss of all pocket protein function via a mechanism that mitigates the need for MAPK pathway activation.
abstract: 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 p16INKA/CDK4/pRB pathway may no longer be accurate; all PPs are potentially critical targets of CDK-cyclins in melanoma.
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: I D
authors.initials: ID
authors.lastname: Tonks
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: P
authors.initials: P
authors.lastname: Mukhopadhyay
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: W A
authors.initials: WA
authors.lastname: Schroder
authors.affiliation: School of Anatomy, Physiology and Human Biology, The University of Western Australia, Crawley, Western Australia, Australia.
authors.forename: A
authors.initials: A
authors.lastname: Sorolla
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.|Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford, UK.
authors.forename: A W
authors.initials: AW
authors.lastname: Mould
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: H Y
authors.initials: HY
authors.lastname: Handoko
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: B
authors.initials: B
authors.lastname: Ferguson
authors.affiliation: Department of Pathology, University of Tasmania School of Medicine, Hobart, Tasmania, Australia.
authors.forename: H K
authors.initials: HK
authors.lastname: Muller
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.|Diamantina Institute for Cancer, Immunology and Metabolic Medicine, University of Queensland, Brisbane, Australia.
authors.forename: P
authors.initials: P
authors.lastname: Keith
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: N K
authors.initials: NK
authors.lastname: Hayward
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: G J
authors.initials: GJ
authors.lastname: Walker
authors.affiliation: Cell and molecular Biology Department, QIMR Berghofer Medical Research Institute, Herston, Queensland, Australia.
authors.forename: G F
authors.initials: GF
authors.lastname: Kay
mesh_terms: D000818:Animals; D002471:Cell Transformation, Neoplastic / Q000378:metabolism* / Q000473:pathology*; D006801:Humans; D020935:MAP Kinase Signaling System; D008544:Melanocytes / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology*; D051379:Mice; D018345:Mice, Knockout; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D016159:Tumor Suppressor Protein p53