MAD1 upregulation sensitizes to inflammation-mediated tumor formation.

Mitotic Arrest Deficient 1 (gene name MAD1L1), an essential component of the mitotic spindle assembly checkpoint, is frequently overexpressed in colon cancer, which correlates with poor disease-free survival. MAD1 upregulation induces two phenotypes associated with tumor promotion in tissue culture cells–low rates of chromosomal instability (CIN) and destabilization of the tumor suppressor p53. Using CRISPR/Cas9 gene editing, we generated a novel mouse model by inserting a doxycycline (dox)-inducible promoter and HA tag into the endogenous mouse Mad1l1 gene, enabling inducible expression of HA-MAD1 following exposure to dox in the presence of the reverse tet transactivator (rtTA). A modest 2-fold overexpression of MAD1 in murine colon resulted in decreased p53 expression and increased mitotic defects consistent with CIN. After exposure to the colon-specific inflammatory agent dextran sulfate sodium (DSS), 31% of mice developed colon lesions, including a mucinous adenocarcinoma, while none formed in control animals. Lesion incidence was particularly high in male mice, 57% of which developed at least one hyperplastic polyp, adenoma or adenocarcinoma in the colon. Notably, mice expressing HA-MAD1 also developed lesions in tissues in which DSS is not expected to induce inflammation. These findings demonstrate that MAD1 upregulation is sufficient to promote colon tumorigenesis in the context of inflammation in immune-competent mice. Author summary: Worldwide, colorectal cancer is the third most commonly diagnosed cancer and the second-highest cause of cancer-related deaths. A better understanding of the molecular causes of colorectal cancer could provide novel therapeutic targets for this disease. Mitotic Arrest Deficient-1 (MAD1) is commonly overexpressed in colorectal cancers, and elevated expression of MAD1 is associated with worse prognosis. Here we generated a new mutant mouse with inducible, modest overexpression of MAD1. First identified for its role in mitosis, MAD1 is required to ensure proper chromosome segregation. Elevated expression of MAD1 causes chromosome mis-segregation in the colons of these mice. During interphase, overexpressed MAD1 destabilizes the well-known tumor suppressor protein, p53. Consistent with this, mouse colons modestly overexpressing MAD1 show decreased expression of p53. Since inflammation is a risk factor for colorectal cancer, we induced inflammation in the colon using dextran sulfate sodium (DSS) and found that MAD1 overexpression significantly increased the incidence of colon tumors. Like inflammation-mediated colon tumors in humans, in the mice these tumors were more common in males than females. This work shows that MAD1 overexpression can promote colon cancer and suggests that MAD1 may be a novel drug target for this disease. [ABSTRACT FROM AUTHOR]