Uncovering Minimal Pathways in Melanoma Initiation

Abstract

Cutaneous melanomas are clinically and histologically heterogeneous. Most display activating mutations inBraforNrasand complete loss of function of one or more tumor suppressor genes. Mouse models that replicate such mutations produce fast-growing, pigmented tumors. However, mice that combineBrafactivation with only heterozygous loss ofPtenalso produce tumors and, as we show here, in an Albino background this occurs even withBrafactivation alone. Such tumors arise rarely, grow slowly, and express low levels of pigmentation genes. The timing of their appearance was consistent with a single step stochastic event, but no evidence could be found that it requiredde novomutation, suggesting instead the involvement of an epigenetic transition. Single-cell transcriptomic analysis revealed such tumors to be heterogeneous, including a minor cell type we term LNM (Low-pigment,Neural- and extracellularMatrix-signature) that displays gene expression resembling “neural crest”-like cell subsets detected in the fast-growing tumors of more heavily-mutated mice, as well as in human biopsy and xenograft samples. We provide evidence that LNM cells pre-exist in normal skin, are expanded by Braf activation, can transition into malignant cells, and persist with malignant cells through multiple rounds of transplantation. We discuss the possibility that LNM cells not only serve as a pre-malignant state in the production of some melanomas, but also as in important intermediate in the development of drug resistance.