G12DNRAS and kinase-dead BRAF cooperate to drive naevogenesis and melanomagenesis

Malin Pedersen1, Amaya Viros2, Martin Cook2, Richard Marais2

1The Institute of Cancer Research, London, UK, 2The Cancer Research UK Manchester Institute, Manchester, UK

Background

Melanoma is the most deadly form of skin cancer and is a disease that is rising in incidence. Q61 mutant NRAS and V600 mutant BRAF, the most common mutations in melanoma, are mutually exclusive unless the tumours are placed under the selective pressure of BRAF drugs, whereas some of the rare NRAS and BRAF mutants are coincident even without drug selection. Notably, where mutant RAS and BRAF are coincident, it involves codons G12 and G13 of NRAS, and codons such as G466, G469, D594 and G596 of BRAF, all of which impair or kill BRAF kinase activity.

Method

We used Cre-recombinase / LoxP technology to express G12DNRAS and kinase-dead BRAF (D594ABRAF) at physiological levels in adult mouse melanocytes.

Results

We here show that G12DNRAS and kinase-dead BRAF (D594ABRAF) cooperate to drive melanocyte proliferation, producing lesions of lightly pigmented dendritic melanocytes in the superficial reticular dermis and heavily pigmented epitheliod/ovoid melanocytes in the deep reticular dermis. The superficial paucicellular lesions resemble naevus of Ota, and the highly cellular deeper lesions resemble dermal melanocytosis with scattered foci of blue naevi. G12DNRAS and D594ABRAF also drive the development of hypo-pigmented melanomas composed of undifferentiated atypical spindle melanocytes extending from the upper dermis to the subcutaneous layers, and hyper-pigmented melanomas of predominantly heavily pigmented pleomorphic epithelioid melanocytes in the deep reticular dermis which resemble animal-type melanoma in humans. All lesions stain positive for melanoma markers and for Ki67, pERK and pAKT.

Conclusion

A small number of human melanomas harbour G12/G13 NRAS mutations that are coincident with inactivating mutations in BRAF. We express G12DNRAS and kinase-dead BRAF in mouse melanocytes and show that they cooperate to drive naevogenesis and melanomagenesis, producing lesions that resemble human melanoma with distinct histopathological characteristics. Thus, we have developed a tractable model to study human melanoma initiation and progression.

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