Three Is Enough

Colorectal cancer is a prime example of the multiple hit hypothesis, a cancer that is driven by the accumulation of sequential mutations in key genes.  Here is a study in mice asserting that only three mutations in key pathways were sufficient to drive a metastatic form of this cancer in the animal model.  Abstract:

Colorectal cancer (CRC) is driven by the accumulation of driver mutations, but the contributions of specific mutations to different steps in malignant progression is not fully understood. In this study, we generated mouse models harboring different combinations of key CRC driver mutations (Apc, Kras, Tgfbr2, Trp53, Fbxw7) in intestinal epithelial cells to comprehensively investigate their roles in the development of primary tumors and metastases. Apc∆716 mutation caused intestinal adenomas and combination with Trp53R270H mutation or Tgfbr2 deletion induced submucosal invasion. The addition of KrasG12D mutation yielded EMT-like morphology and lymph vessel intravasation of the invasive tumors. In contrast, combinations of Apc∆716 with KrasG12D and Fbxw7 mutation was insufficient for submucosal invasion but still induced EMT-like histology. Studies using tumor-derived organoids showed that KrasG12D was critical for liver metastasis following splenic transplantation, when this mutation was combined with either Apc∆716 plus Trp53R270H or Tgfbr2 deletion, with the highest incidence of metastasis displayed by tumors with a Apc∆716 KrasG12D Tgfbr2-/- genotype. RNAseq analysis of tumor organoids defined distinct gene expression profiles characteristic for the respective combinations of driver mutations, with upregulated genes in Apc∆716 KrasG12D Tgfbr2-/- tumors found to be similarly upregulated in specimens of human metastatic CRC. Our results show how activation of Wnt and Kras with suppression of TGF-β signaling in intestinal epithelial cells is sufficient for CRC metastasis, with possible implications for the development of metastasis prevention strategies.