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Role of Doublecortin-Like Kinase 1 (Dclk1) Positive Tuft Cells in Colitis-Associated Colorectal Cancer

The following abstract was presented as part of London Health Research Day 2018.

Research Areas: Cancer biology; Mechanisms of disease
First Author: Alice Shin
Supervisor(s): S. Asfaha

Colorectal cancer (CRC) is the second leading cause of cancer death in Canada, with the major risk factor being chronic inflammation. However, how inflammation leads to cancer is not well understood. Our recent work has focused on a colonic epithelial cell known as the tuft cell that uniquely expresses the protein doublecortin-like kinase 1 (Dclk1). Using Cre-dependent lineage tracing of Dclk1-expressing cells, we previously showed that Dclk1 labels long-lived quiescent cells in the colon that serve as a cellular origin of CRC upon dextran sulfate sodium (DSS)-induced inflammatory injury.  

The aim of this study was to determine the generalizability of inflammation-induced tumor promotion from genetically susceptible Dclk1+ cells and explore the mechanism by which inflammation induces tuft cell cancer initiation. We hypothesized that colonic inflammatory insults lead to dedifferentiation of Dclk1+ tuft cells to a stem cell state susceptible to tumor initiation.

Materials and Methods:
To investigate the various forms of injury or infection that can activate quiescent tuft cells, we generated tamoxifen-inducible Cre transgenic mice that allow for Dclk1+ cell lineage tracing and cell-specific knock-out of the tumor suppressor adenomatous polyposis coli (APC) (Dclk1-CreERT2/R26-tdTom/APCfl/fl). Following tamoxifen induction, mice were administered colitis-inducing agents dextran sodium sulfate (DSS), trinitrobenzene sulfonic acid (TNBS), oxazolone or Citrobacter rodentium. To examine the role of dedifferentiation in colonic tumor initiation, we ablated Lgr5+ intestinal stem cells (ISCs) post DSS-induced colitis in our Dclk1/APCfl/fl mice. To further investigate the mechanism by which inflammation contributes to tuft cell activation, we constitutively activated IKKβ, a positive regulator of NF-κB signaling, in APC-mutated Dclk1+ cells (Dclk1/APCfl/fl/R-IKKβca).

Treatment with DSS, TNBS, oxazolone, or C. rodentium induced colonic inflammation as detected by significantly increased myeloperoxidase (MPO) activity and histologic analysis. DSS administration led to Dclk1+ cell-derived colonic tumors as previously reported. Surprisingly, administration of TNBS, oxazolone, or C. rodentium in Dclk1/APCfl/fl mice did not lead to colonic tumorigenesis up to 52 weeks following induction of colitis. Interestingly, ablation of Lgr5+ ISCs post colitis significantly reduced colonic tumors in DSS-treated Dclk1/APCf/f mice. Furthermore, constitutive activation of IKKβ in Dclk1+ cells resulted in reduced epithelial injury in DSS-induced colitis and fewer colonic tumors. 

Discussion and Conclusions: 
Our data suggests that an inflammatory response unique to DSS-induced colitis, and not TNBS, oxazolone or C. rodentium infection, results in colonic tumor formation. Interestingly, the colonic transformation of Dclk1+ tuft cells in DSS colitis appears to be mediated through Lgr5-expressing cells. Dclk1+ cell-specific NF-κB signaling appears to affect the severity of colitis and tumorigenesis. These findings provide insight into the molecular mechanisms by which Dclk1-derived colonic tumors arise.