Oncogenic Braf induces melanocyte senescence and melanoma in mice.

John Kyriakis

John Kyriakis

John Kyriakis

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This paper is important because the authors have developed a mouse model that closely recapitulates human melanoma. The approach involves the specific, inducible expression in melanocytes of BRafV600E, a mutant of B-Raf long implicated in melanoma, from the endogenous BRaf promoter. Although BRafV600E is the most common oncogenic mutation seen in human melanoma, a model that closely resembles the human disease was missing. The authors use a LoxP-stop-LoxP/Cre recombinase system for expression of the mutant. In the absence of Cre activity, the mutant Baf's expression is suppressed by the presence of a minigene encoding wild type B-Raf and the NeoR cassette used for stem cell selection. Expression of the mutant is incurred by induction of a tamoxifen-activated version of Cre which, in this model, is controlled by the melanocyte-specific tyrosinase enhancer/promoter. Accordingly, topical application of tamoxifen enables localized expression of the V600E mutant. The expression of the mutant is sufficient to produce melanomas that express all of the cardinal histologic features of melanoma, including aggressive tumorigenesis upon xenografting into nude mice. Using this system, the authors also find that, contrary to previous suggestions {1}, the tumor suppressor p16INK4a is not required for melanocyte suppression, and its loss does not promote tumorigenesis.

Frank de Gruijl

Frank de Gruijl

Frank de Gruijl

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In this study in transgenic mice, the Braf/V600E mutation –- common in malignant melanomas -- was shown to cause hyperpigmentation and formation of nevi (moles) with senescent melanocytes. Contrary to earlier inference, this senescent state did not require the tumor suppressor p16Ink4a. The Ras pathway appears to be the common “oncogenic driver” in cutaneous malignant melanoma: in a minority of melanomas Nras was found to be activated by mutation and, in a large majority, Braf was commonly found to be activated through a V600E mutation. Next to this oncogenic driver, the loss of expression of p16Ink4a appeared to be linked to tumor progression. Like the tumor suppressor p53, p16Ink4a was found to be activated in response to oncogenic Ras or Braf signalling and holds the cell in ‘replicative senescence’. Sensitive mutation-specific PCR techniques detected the presence of Braf mutations in nevi, potential benign precursor lesions of melanoma. Many of the senescent melanocytes in nevi appeared to over-express p16Ink4a which was inferred to cause the senescence {1}. But the mosaicism in p16Ink4a expression in senescent melanocytes already suggested that another mechanism contributed to the senescence {2}. Dhomen et al. show that transgenic but physiologic expression of Braf/V600E in melanocytes of mice yielded nevi with senescent melanocytes, but the same occurred in a p16Ink4a null background, i.e. p16Ink4a was not required for the formation of nevi and senescent melanocytes therein (confirmed by Haferkamp et al., 2009 {3}). The precise mechanism that causes this oncogenic Braf-related senescence needs to be established (autocrine IL6 or IGFBP7? {4,5}). The transgenic Braf/V600E expression in melanocytes gives rise to melanomas which do not metastasize. Like in human melanomas, the p16 expression is lost and some of the murine Braf/V600E melanomas show homozygous loss of the p16Ink4a/CDKN2A genes {6}. In p16Ink4a null mice, the Braf/V600E causes more melanomas earlier, i.e. p16Ink4a does enhance the tumor progression. Loss of tumor suppressor Pten appears to give rise to melanomas with a metastatic phenotype in the Braf/V600E transgenic mice {7}.