Therapeutic Strategies for Targeting CDKN2A Loss in Melanoma.

Loss of the tumor suppressor gene CDKN2A, encoding p16 and p14, is a frequent event driving melanoma progression. Therefore, therapeutic strategies aimed at CDKN2A loss hold great potential to improve melanoma treatment. Pharmacological inhibition of the p16 targets CDK4/6 is a prime example of such a strategy. Other approaches exploit cell cycle deregulation, target metabolic rewiring, epigenetically restore expression, act on dependencies resulting from co-deleted genes, or are directed at the effects of CDKN2A loss on immune responses. This review explores these therapeutic strategies targeting CDKN2A loss, which potentially open up new avenues for precision medicine in melanoma.

Anti-tumour immunity induces aberrant peptide presentation in melanoma.

Extensive tumour inflammation, which is reflected by high levels of infiltrating T cells and interferon-γ (IFNγ) signalling, improves the response of patients with melanoma to checkpoint immunotherapy1,2. Many tumours, however, escape by activating cellular pathways that lead to immunosuppression. One such mechanism is the production of tryptophan metabolites along the kynurenine pathway by the enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which is induced by IFNγ3-5. However, clinical trials using inhibition of IDO1 in combination with blockade of the PD1 pathway in patients with melanoma did not improve the efficacy of treatment compared to PD1 pathway blockade alone6,7, pointing to an incomplete understanding of the role of IDO1 and the consequent degradation of tryptophan in mRNA translation and cancer progression. Here we used ribosome profiling in melanoma cells to investigate the effects of prolonged IFNγ treatment on mRNA translation. Notably, we observed accumulations of ribosomes downstream of tryptophan codons, along with their expected stalling at the tryptophan codon. This suggested that ribosomes bypass tryptophan codons in the absence of tryptophan. A detailed examination of these tryptophan-associated accumulations of ribosomes-which we term 'W-bumps'-showed that they were characterized by ribosomal frameshifting events. Consistently, reporter assays combined with proteomic and immunopeptidomic analyses demonstrated the induction of ribosomal frameshifting, and the generation and presentation of aberrant trans-frame peptides at the cell surface after treatment with IFNγ. Priming of naive T cells from healthy donors with aberrant peptides induced peptide-specific T cells. Together, our results suggest that IDO1-mediated depletion of tryptophan, which is induced by IFNγ, has a role in the immune recognition of melanoma cells by contributing to diversification of the peptidome landscape.