Clonal selection and transcriptional reprogramming (e.g., epithelial-mesenchymal transition or phenotype switching) are the predominant theories thought to underlie tumor progression.,However, a “division of labor” leading to cooperation among tumor-cell subpopulations could be an additional catalyst of progression.,Using a zebrafish-melanoma xenograft model, we found that in a heterogeneous setting, inherently invasive cells, which possess protease activity and deposit extracellular matrix (ECM), co-invade with subpopulations of poorly invasive cells, a phenomenon we term “cooperative invasion”.,Whereas the poorly invasive cells benefit from heterogeneity, the invasive cells switch from protease-independent to an MT1-MMP-dependent mode of invasion.,We did not observe changes in expression of the melanoma phenotype determinant MITF during cooperative invasion, thus ruling out the necessity for phenotype switching for invasion.,Altogether, our data suggest that cooperation can drive melanoma progression without the need for clonal selection or phenotype switching and can account for the preservation of heterogeneity seen throughout tumor progression.,•Inherently invasive and poorly invasive melanoma subpopulations co-invade•Leader cells are inherently invasive and provide MT1-MMP and deposit ECM•Follower cells affect the mode of invasion and thereby elicit protease dependency•Cooperative invasion bypasses phenotype switching and maintains tumor heterogeneity,Inherently invasive and poorly invasive melanoma subpopulations co-invade,Leader cells are inherently invasive and provide MT1-MMP and deposit ECM,Follower cells affect the mode of invasion and thereby elicit protease dependency,Cooperative invasion bypasses phenotype switching and maintains tumor heterogeneity,Tumor heterogeneity has been viewed as the outcome of adaptive competition or differential transcriptional reprogramming (phenotype switching).,Alternatively, heterogeneity may reflect a division of labor between tumor cell subpopulations with complementary acquired characteristics.,Chapman et al. now show reciprocal interactions between heterogeneous melanoma cell subpopulations that lead to cooperative invasion without competition and without phenotype switching.,Thus, cooperative behavior emerges as a property of heterogeneity relevant for tumor biology and worth targeting therapeutically.
Knowledge of tumor mutation status is becoming increasingly important for the treatment of cancer, as mutation-specific inhibitors are being developed for clinical use that target only sub-populations of patients with particular tumor genotypes.,Melanoma provides a recent example of this paradigm.,We report here development, validation, and implementation of an assay designed to simultaneously detect 43 common somatic point mutations in 6 genes (BRAF, NRAS, KIT, GNAQ, GNA11, and CTNNB1) potentially relevant to existing and emerging targeted therapies specifically in melanoma.,The test utilizes the SNaPshot method (multiplex PCR, multiplex primer extension, and capillary electrophoresis) and can be performed rapidly with high sensitivity (requiring 5-10% mutant allele frequency) and minimal amounts of DNA (10-20 nanograms).,The assay was validated using cell lines, fresh-frozen tissue, and formalin-fixed paraffin embedded tissue.,Clinical characteristics and the impact on clinical trial enrollment were then assessed for the first 150 melanoma patients whose tumors were genotyped in the Vanderbilt molecular diagnostics lab.,Directing this test to a single disease, 90 of 150 (60%) melanomas from sites throughout the body harbored a mutation tested, including 57, 23, 6, 3, and 2 mutations in BRAF, NRAS, GNAQ, KIT, and CTNNB1, respectively.,Among BRAF V600 mutations, 79%, 12%, 5%, and 4% were V600E, V600K, V600R, and V600M, respectively. 23 of 54 (43%) patients with mutation harboring metastatic disease were subsequently enrolled in genotype-driven trials.,We present development of a simple mutational profiling screen for clinically relevant mutations in melanoma.,Adoption of this genetically-informed approach to the treatment of melanoma has already had an impact on clinical trial enrollment and prioritization of therapy for patients with the disease.