Approximately 2% of colorectal cancer is linked to pre-existing inflammation known as colitis-associated cancer, but most develops in patients without underlying inflammatory bowel disease. Colorectal cancer often follows a genetic pathway whereby loss of the adenomatous polyposis coli (APC) tumour suppressor and activation of β-catenin are followed by mutations in K-Ras, PIK3CA and TP53, as the tumour emerges and progresses1,2. Curiously, however, ‘inflammatory signature’ genes characteristic of colitis-associated cancer are also upregulated in colorectal cancer3,4. Further, like most solid tumours, colorectal cancer exhibits immune/inflammatory infiltrates5, referred to as ‘tumour elicited inflammation’6. Although infiltrating CD4+ TH1 cells and CD8+ cytotoxic T cells constitute a positive prognostic sign in colorectal cancer7,8, myeloid cells and T-helper interleukin (IL)-17-producing (TH17) cells promote tumorigenesis5,6, and a ‘TH17 expression signature’ in stage I/II colorectal cancer is associated with a drastic decrease in disease-free survival9. Despite its pathogenic importance, the mechanisms responsible for the appearance of tumour-elicited inflammation are poorly understood. Many epithelial cancers develop proximally to microbial communities, which are physically separated from immune cells by an epithelial barrier10. We investigated mechanisms responsible for tumour-elicited inflammation in a mouse model of colorectal tumorigenesis, which, like human colorectal cancer, exhibits upregulation of IL-23 and IL-17. Here we show that IL-23 signalling promotes tumour growth and progression, and development of a tumoural IL-17 response. IL-23 is mainly produced by tumour-associated myeloid cells that are likely to be activated by microbial products, which penetrate the tumours but not adjacent tissue. Both early and late colorectal neoplasms exhibit defective expression of several barrier proteins. We propose that barrier deterioration induced by colorectal-cancer-initiating genetic lesions results in adenoma invasion by microbial products that trigger tumour-elicited inflammation, which in turn drives tumour growth.

Higher order chromatin structure presents a barrier to the recognition and repair of DNA damage. Double-strand breaks (DSBs) induce histone H2AX phosphorylation, which is associated with the recruitment of repair factors to damaged DNA. To help clarify the physiological role of H2AX, we targeted H2AX in mice. Although H2AX is not essential for irradiation-induced cell-cycle checkpoints, H2AX −/− mice were radiation sensitive, growth retarded, and immune deficient, and mutant males were infertile. These pleiotropic phenotypes were associated with chromosomal instability, repair defects, and impaired recruitment of Nbs1, 53bp1, and Brca1, but not Rad51, to irradiation-induced foci. Thus, H2AX is critical for facilitating the assembly of specific DNArepair complexes on damaged DNA.The first 120 amino acids of the H2AX and the H2A1/2 bulk isoprotein species exhibit a high degree of similarity, but H2AX carries a unique COOH-terminal tail that contains the * To whom correspondence should be addressed. andre_nussenzweig@nih.gov. HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript consensus phosphatidyl inositol 3-kinase (PI-3 kinase) motif that is activated by DSBs (1, 2). Phosphorylation of H2AX (γ-H2AX) is induced by external genotoxic agents (2, 3) and is activated at physiological sites of recombination in lymphocytes (4, 5) and germ cells (6). Several essential DNA-repair factors implicated in homologous recombination (HR) (e.g., Brca1, Brca2, and Rad51) or that participate in both HR and nonhomologous end-joining (NHEJ) (e.g., Rad50, Mre11, Nbs1) form immunofluorescent foci that colocalize with γ-H2AX (7). However, the precise relation between focus formation and DNA repair is not understood.To determine the physiological role of H2AX in mammalian cells, we produced a targeted disruption of mouse H2AX (Web fig. 1A) (5,8). H2AX −/− mice were born at the expected frequency, and absence of H2AX protein was confirmed by two-dimensional gel electrophoresis and Western blotting (Web fig. 1, B to E) (8). Despite the loss of H2AX, treatment with γ-irradiation resulted in normal phosphorylation of Nbs1 (Web fig. 1E) (8).We conclude that H2AX is not essential for survival, or for irradiation-induced phosphorylation of Nbs1.H2AX −/− mice were growth retarded (Web fig. 2) (8), and H2AX −/− mouse embryo fibroblasts (MEFs) proliferated poorly in vitro (Fig. 1A). The difference in the growth of MEFs was partly due to a decrease in the number of dividing cells in H2AX −/− cultures as determined by incorporation of bromodeoxyuridine (BrdU) into DNA. During a 24-hour labeling period, only 44% of passage 1 H2AX −/− MEFs were actively cycling, compared with 72% for the controls, and the mitotic index of H2AX −/− MEFs was at least 50% lower than in wild-type cultures (see below; Fig. 1, D and F). By passage 4, H2AX −/− MEFS accumulated nondividing giant cells, suggesting premature entry into senescence. With continual passage, both H2AX −/− and wild-type MEFs went through crisis, after wh...

Summary The c-Myc HLH-bZIP protein has been implicated in physiological or pathological growth, proliferation, apoptosis, metabolism and differentiation at the cellular, tissue or organismal levels via regulation of numerous target genes. No principle yet unifies Myc action due partly to an incomplete inventory and functional accounting of Myc’s targets. To observe Myc target expression and function in a system where Myc is temporally and physiologically regulated, the transcriptomes and the genome-wide distributions of Myc, RNA polymerase II and chromatin modifications were compared during lymphocyte activation and in ES cells as well. A remarkably simple rule emerged from this quantitative analysis: Myc is not an on-off specifier of gene activity, but is a non-linear amplifier of expression, acting universally at active genes, except for immediate early genes that are strongly induced before Myc. This rule of Myc action explains the vast majority of Myc biology observed in literature.