Originally described as interpatient variability, tumour heterogeneity has now been demonstrated to occur intrapatiently, within the same lesion, or in different lesions of the same patient.,Tumour heterogeneity involves both genetic and epigenetic changes.,Intrapatient heterogeneity is responsible for generating subpopulations of cancer cells which undergo clonal evolution with time.,Tumour heterogeneity develops also as a consequence of the selective pressure imposed by the immune system.,It has been demonstrated that tumour heterogeneity and different spatiotemporal interactions between all the cellular compontents within the tumour microenvironment lead to cancer adaptation and to therapeutic pressure.,In this context, the recent advent of single cell analysis approaches which are able to better study tumour heterogeneity from the genomic, transcriptomic and proteomic standpoint represent a major technological breakthrough.,In this review, using metastatic melanoma as a prototypical example, we will focus on applying single cell analyses to the study of clonal trajectories which guide the evolution of drug resistance to targeted therapy.
Targeted therapies as BRAF and MEK inhibitor combination have been approved as first-line treatment for BRAF-mutant melanoma.,However, disease progression occurs in most of the patients within few months of therapy.,Metabolic adaptations have been described in the context of acquired resistance to BRAF inhibitors (BRAFi).,BRAFi-resistant melanomas are characterized by an increase of mitochondrial oxidative phosphorylation and are more prone to cell death induced by mitochondrial-targeting drugs.,BRAFi-resistant melanomas also exhibit an enhancement of oxidative stress due to mitochondrial oxygen consumption increase.,To understand the mechanisms responsible for survival of BRAFi-resistant melanoma cells in the context of oxidative stress, we have established a preclinical murine model that accurately recapitulates in vivo the acquisition of resistance to MAPK inhibitors including several BRAF or MEK inhibitors alone and in combination.,Using mice model and melanoma cell lines generated from mice tumors, we have confirmed that the acquisition of resistance is associated with an increase in mitochondrial oxidative phosphorylation as well as the importance of glutamine metabolism.,Moreover, we have demonstrated that BRAFi-resistant melanoma can adapt mitochondrial metabolism to support glucose-derived glutamate synthesis leading to increase in glutathione content.,Besides, BRAFi-resistant melanoma exhibits a strong activation of NRF-2 pathway leading to increase in the pentose phosphate pathway, which is involved in the regeneration of reduced glutathione, and to increase in xCT expression, a component of the xc-amino acid transporter essential for the uptake of cystine required for intracellular glutathione synthesis.,All these metabolic modifications sustain glutathione level and contribute to the intracellular redox balance to allow survival of BRAFi-resistant melanoma cells.