Ere overrepresented when compared with the overall KRAS-mutated tumor population. Additionally, KRAS codon 13 mutations might exhibit weaker in vitro transforming activity than codon 12 mutations [25], and some authors have indicated that KRAS codon 13 mutations may be associated with better outcomes after cetuximab treatment than with other KRAS mutations [26?9]. Nevertheless, the molecular mechanism leading to this outcome remains unknown. Recently, Tejpar et al. demonstrated that the addition of cetuximab to first-line chemotherapy appears to benefit patients with KRAS c.38G.A (p.G13D) tumors, and 22948146 the relative treatment effects were similar to those in patients with KRAS wildtype ZK-36374 site tumors but with lower absolute values [30]. A small number of available experimental data demonstrate that tumor clones carrying KRAS codon 13 mutations are less aggressive than thosecarrying codon 12 mutations. This is because KRAS codon 13 exhibit higher levels of apoptosis [31]. Several studies have suggested that there is a reduced transforming capacity of the codon 13 mutation compared with the codon 12 mutation tested in both in vitro and in vivo systems [32,33]. On the contrary, Tveit et al. [34] and Gajate et al. [35] did not observe any difference in the efficacy of cetuximab when comparing the codon 13 with codon 12 mutations. Moreover, KRAS codon13-mutated mCRCs were classified as poor prognostic markers and more aggressive in several studies [36?8]. Because of the lack of a consensus about whether KRAS mutated in codon 13 can confer different CRC phenotypes or responses to anti-EGFR therapies, there remains a need to clarify the molecular mechanisms underlying the changes occurring in the structure of the protein because of the different mutations. To address these questions, we employed a series of simulations to study the molecular mechanisms of c.35G.A (p.G12D), c.38G.A (p.G13D), and WT. We sought to test the hypothesis that a single residue substitution on codon 13 of KRAS could have effects on its dynamics, and a simple amino acid substitution might influence the structural dynamics of KRAS and hence its affinity to ligands and finally these changes would affect the patient’s response to the treatment.Materials and Methods Molecular ModelingA (PS)2 server [39,40] was used for building the homology-based models. The server uses effective consensus strategies, combining structural- and profile-based comparison methods, for both template selection and target-template alignment. For this study, the (PS)2 server selected the X-ray crystal structure of the KRASGTP complex (PDB ID: 3GFT) through a template consensus strategy [40] as the template structure. The models of WT KRAS and MT KRAS (c.35G.A (p.G12D) and c.38G.A (p.G13D)) were built using this template. Finally, the KRASGTP complexes were constructed by superimposing the predicted KRAS models on to the crystal structure of the KRAS-GTP complex.Molecular DockingThe modeling ITI 007 web structures were used as the initial coordinates for docking purposes. The binding site for virtual docking was ?determined by considering the protein residues located #8 A away from the GTP binding pocket. iGEMDOCK v2.1 
[41,42] was used to generate the docked conformation of ligands and to rank the conformations 
23115181 according to their docking scores. We used its molecular docking platform to dock the GTP to the active cavity of the KRAS models (WT and MT) with a population size of 300, a number of generations of 80, and the nu.Ere overrepresented when compared with the overall KRAS-mutated tumor population. Additionally, KRAS codon 13 mutations might exhibit weaker in vitro transforming activity than codon 12 mutations [25], and some authors have indicated that KRAS codon 13 mutations may be associated with better outcomes after cetuximab treatment than with other KRAS mutations [26?9]. Nevertheless, the molecular mechanism leading to this outcome remains unknown. Recently, Tejpar et al. demonstrated that the addition of cetuximab to first-line chemotherapy appears to benefit patients with KRAS c.38G.A (p.G13D) tumors, and 22948146 the relative treatment effects were similar to those in patients with KRAS wildtype tumors but with lower absolute values [30]. A small number of available experimental data demonstrate that tumor clones carrying KRAS codon 13 mutations are less aggressive than thosecarrying codon 12 mutations. This is because KRAS codon 13 exhibit higher levels of apoptosis [31]. Several studies have suggested that there is a reduced transforming capacity of the codon 13 mutation compared with the codon 12 mutation tested in both in vitro and in vivo systems [32,33]. On the contrary, Tveit et al. [34] and Gajate et al. [35] did not observe any difference in the efficacy of cetuximab when comparing the codon 13 with codon 12 mutations. Moreover, KRAS codon13-mutated mCRCs were classified as poor prognostic markers and more aggressive in several studies [36?8]. Because of the lack of a consensus about whether KRAS mutated in codon 13 can confer different CRC phenotypes or responses to anti-EGFR therapies, there remains a need to clarify the molecular mechanisms underlying the changes occurring in the structure of the protein because of the different mutations. To address these questions, we employed a series of simulations to study the molecular mechanisms of c.35G.A (p.G12D), c.38G.A (p.G13D), and WT. We sought to test the hypothesis that a single residue substitution on codon 13 of KRAS could have effects on its dynamics, and a simple amino acid substitution might influence the structural dynamics of KRAS and hence its affinity to ligands and finally these changes would affect the patient’s response to the treatment.Materials and Methods Molecular ModelingA (PS)2 server [39,40] was used for building the homology-based models. The server uses effective consensus strategies, combining structural- and profile-based comparison methods, for both template selection and target-template alignment. For this study, the (PS)2 server selected the X-ray crystal structure of the KRASGTP complex (PDB ID: 3GFT) through a template consensus strategy [40] as the template structure. The models of WT KRAS and MT KRAS (c.35G.A (p.G12D) and c.38G.A (p.G13D)) were built using this template. Finally, the KRASGTP complexes were constructed by superimposing the predicted KRAS models on to the crystal structure of the KRAS-GTP complex.Molecular DockingThe modeling structures were used as the initial coordinates for docking purposes. The binding site for virtual docking was ?determined by considering the protein residues located #8 A away from the GTP binding pocket. iGEMDOCK v2.1 [41,42] was used to generate the docked conformation of ligands and to rank the conformations 23115181 according to their docking scores. We used its molecular docking platform to dock the GTP to the active cavity of the KRAS models (WT and MT) with a population size of 300, a number of generations of 80, and the nu.