Understanding the molecular underpinnings of cancer is of critical importance to developing targeted intervention strategies. in gene networks downstream of oncogenic gain and loss-of-function mutations. To identify genes regulated synergistically by cooperating oncogenic mutations at genomic scale, we compared mRNA expression profiles of young adult murine colon (YAMC) cells with those of YAMC cells expressing mutant p53175H (mp53), activated H-Ras12V (Ras) or both mutant proteins together (mp53/Ras)17 using Affymetrix microarrays. Using a step-wise procedure, we first identified 538 genes differentially expressed between mp53/Ras and YAMC control cells with a statistical cut off at p < 0.01 (N-test, Westfall-Young adjusted). A further subset of 95 annotated genes that respond synergistically (28 up/67 down) to the combination of mutant p53 and Ras proteins, termed cooperation response genes (CRG) was then determined using a synergy score, as described in methods (Figure 1, Supplementary Table 1, Supplementary File 1). Expression values and synergy scores for the CRGs derived from TaqMan low-density QPCR array (TLDA) data showed strong positive correlation with the values for the same genes obtained from microarray analysis (Supplementary Figures 1 and 2, Supplementary Table 2 and Supplementary File 2). Thus CRG identification was confirmed by independent methods, with final CRG selection based on microarray data, due to higher sample replication in this data set. Figure 1 Identification and characterization of cooperation response genes (CRGs) CRGs encode proteins involved in the regulation of cell signaling, transcription, apoptosis, metabolism, transport or adhesion (Figure 2A, B and Supplementary Table (R)-(+)-Corypalmine supplier 1), and in large proportion appear misexpressed in human cancer. For 47 of 75 CRGs tested co-regulation is found in primary human colon cancer and our murine colon cancer cell model (Figure 2C, Supplementary Figure 3). Furthermore, altered expression of 29 CRGs has been reported in a variety of human cancer (R)-(+)-Corypalmine supplier types, consistent with the direction of the change in gene expression observed in our experiments (Figure 2C, Supplementary Table 1 and references therein). Thus, modulation of CRG expression has common features in malignant cell transformation of both murine and human cells. Figure 2 Differential expression and synergy scores of CRGs in mp53/Ras cells and CRG co-regulation in human colon cancer The relevance of differentially expressed genes for malignant cell (R)-(+)-Corypalmine supplier transformation was assessed by genetic perturbation of a series of 24 CRGs (R)-(+)-Corypalmine supplier and 14 genes responding to p53175H and/or activated H-Ras12V in a noncooperative manner (non-CRGs). Perturbed ARHGEF2 genes were chosen across a broad range of biological functions, levels of differential expression and synergy scores (Figure 2, Supplementary Figure 4, Supplementary File 3). Gene perturbations were carried out in mp53/Ras cells with the goal to re-establish mRNA expression of the manipulated genes to levels relatively close to those found in YAMC control cells, and to monitor subsequent tumor formation following sub-cutaneous injection of these cells into immuno-compromised mice. Of the perturbed genes, 18 were up- and 20 down-regulated in mp53/Ras cells, relative to YAMC. Reversal of the changes in CRG expression significantly reduced tumor formation by mp53/Ras cells in 14 out of 24 cases (Figure 3A, left panel; Figure 4A, C; Supplementary Figure 5A and Supplementary Table 3), indicating a critical role in malignant transformation for a surprisingly large fraction of these genes. Perturbation of Plac8, Jag2 and HoxC13 gene expression had the strongest effects. We also combined perturbations of two CRGs, Fas and Rprm, that alone produced significant yet milder changes in tumor formation. This yielded significantly increased efficacy in tumor inhibition as compared with the respective single perturbations (Figure 4E, Supplementary Figure 5B and Supplementary Table 4). Thus, even genetic perturbations of CRGs with relatively smaller effects when examined on their own show evidence of being essential when analyzed in combination. Figure 3 Synergistic response of downstream genes to oncogenic mutations is a strong predictor for critical role in malignant transformation Figure 4 CRG perturbations reduce tumor formation of both mp53/Ras and human cancer cells In contrast to the multitude of CRG-related effects on tumor inhibition, out of the 14 non-CRG perturbations, only one showed a significant reduction in tumor.