Uncovering Minimal Pathways in Melanoma Initiation

Area of Science:

• Oncology
• Dermatology
• Molecular Biology

Background:

• Cutaneous melanomas exhibit significant clinical and histological heterogeneity.
• Common genetic alterations include activating mutations in BRAF or NRAS and loss of tumor suppressor genes.
• Existing mouse models with BRAF activation and tumor suppressor loss yield fast-growing, pigmented tumors.

Purpose of the Study:

• To investigate the development of slow-growing, hypopigmented melanomas in mice with BRAF activation alone or combined with heterozygous PTEN loss.
• To identify the cellular origins and molecular mechanisms underlying these distinct tumor types.
• To explore the role of specific cell populations in melanoma development and drug resistance.

Main Methods:

• Utilized genetically engineered mouse models with specific mutations (BRAF activation, PTEN heterozygous loss) on an albino background.
• Analyzed tumor growth kinetics, pigmentation levels, and timing of appearance.
• Performed single-cell transcriptomic analysis to identify distinct cell populations and their gene expression profiles.
• Investigated the pre-malignant potential and persistence of identified cell types through transplantation.

Main Results:

• BRAF activation alone on an albino background can induce rare, slow-growing, hypopigmented tumors, suggesting an epigenetic transition rather than de novo mutation.
• Single-cell transcriptomics revealed a heterogeneous tumor composition, including a minor cell type termed LNM (low-pigment, neural- and extracellular matrix-signature) cells.
• LNM cells exhibit gene expression similar to neural crest-like cells found in aggressive melanomas and appear to pre-exist in normal skin.
• LNM cells are expanded by BRAF activation, can transition to malignant cells, and persist through transplantation, indicating a pre-malignant role.

Conclusions:

• BRAF activation can initiate melanoma development through an epigenetic mechanism involving LNM cells, particularly in specific genetic backgrounds.
• LNM cells represent a pre-malignant state that can give rise to slow-growing melanomas.
• LNM cells may also play a crucial role as an intermediate in the development of melanoma drug resistance.