Genes of the Rat Sarcoma (RAS) family are frequently mutated in malignancies of various etiologies. Neuroblastoma-RAS (NRAS) mutations are particularly common in malignant melanoma and occur in 15-25% of all cases. Because direct targeting of the mutant protein has yet not been possible in a clinical setting, the majority of modern targeted treatment approaches aims at indirectly blocking NRAS by interfering with its downstream signaling effectors.
Studies indicate, that NRAS activates a variety of different downstream signaling cascades including, but not limited to the MAPK, PI3K/AKT/mTOR and cell cycle pathways. In the projects that led to this PhD thesis, we therefore evaluated the influence of various inhibitors within the above-mentioned pathways using in vitro and in vivo mouse xenograft models of NRAS mutant melanoma. Our results revealed a marked sensitivity of cells to targeted MEK inhibition. Results also suggest that PI3K/AKT/mTOR signaling and cell cycle regulation genes are critical for activated NRAS cell homeostasis. Importantly, combined blocking of MEK+PI3K/mTOR1,2 and MEK+Plk1 triggered marked tumor cell apoptosis and cell cycle arrest. These two small molecule composites were superior to MEK inhibition alone and a variety of different other inhibitor combinations. We also showed that combined MEK+PI3K/mTOR1,2 as well as MEK+Plk1 blockade is synergistic when specific inhibitor ratios are used.
In another project, we discovered new targets for indirect NRAS inhibition using genetically modified model systems of NRAS mutant cells and omic-scale, computer-assisted discovery tools. Our findings revealed that genectic alterations in the two mutational hotspots of NRAS located in codon 12 and 61, elicit different downstream signals that can be used for mutations-specific targeted blockade. PIM2 activity prevailed in NRAS(G12) cells and CK2a activity in NRAS(Q61) mutants.
Together, these results suggest that targeting both, the MEK and PI3K/AKT/mTOR pathways, as well as the MEK and Plk1/cell cycle pathways have anti-tumor activity. Pending further testing, such composites might serve as potential therapeutic options for patients with NRAS mutant melanoma in the future. Our data further imply that mutation-specific, personalized targeted inhibition may pave the way to individualized therapeutic strategy for a disease with currently very limited effective therapies.