The mechanisms linking chronic
inflammation of the gut (IBD) and increased
colorectal cancer susceptibility are poorly understood. IBD risk is influenced by genetic factors, including the IBD5 locus (human 5q31), that harbors the IRF1 gene. A cause-to-effect relationship between chronic
inflammation and
colorectal cancer, and a possible role of IRF1 were studied in Irf1-/- mice in a model of
colitis-associated colorectal cancer (CA-CRC) induced by
azoxymethane and
dextran sulfate. Loss of Irf1 causes hyper-susceptibility to CA-CRC, with early onset and increased number of
tumors leading to rapid lethality. Transcript profiling (
RNA-seq) and immunostaining of colons shows heightened
inflammation and enhanced enterocyte proliferation in Irf1-/- mutants, prior to appearance of
tumors. Considerable infiltration of leukocytes is seen in Irf1-/- colons at this early stage, and is composed primarily of proinflammatory Gr1+ Cd11b+ myeloid cells and other granulocytes, as well as CD4+ lymphoid cells. Differential susceptibility to CA-CRC of Irf1-/- vs. B6 controls is fully transferable through hematopoietic cells as observed in bone marrow chimera studies. Transcript signatures seen in Irf1-/- mice in response to AOM/DSS are enriched in clinical specimens from patients with IBD and with
colorectal cancer. In addition, IRF1 expression in the colon is significantly decreased in late stage
colorectal cancer (stages 3, 4) and is associated with poorer prognosis. This suggests that partial or complete loss of IRF1 expression alters the type, number, and function of immune cells in situ during chronic
inflammation, possibly via the creation of a
tumor-promoting environment.