Degree of Tissue Differentiation Dictates Susceptibility to BRAF-driven Colorectal Cancer

In murine models, we find that oncogenic BRAF paradoxically suppresses stem cell renewal and instead promotes differentiation and senescence. This corresponds to inefficient tumor formation in oncogenic BRAF mouse models of colon cancer. By reducing levels of differentiation in the gut via genetic manipulation of either of two distinct differentiation-promoting factors (Smad4 or Cdx2), stem cell activity is restored in BRAFV600E intestines, and the oncogenic capacity of mutant BRAF is amplified. In human patients, we observe that reduced levels of differentiation in normal tissue is associated with increased susceptibility to serrated colon tumors. Together, these findings help resolve the conditions necessary for BRAF-driven colon cancer initiation. RNAs were extracted from crypt jejunal epithelia of the indicated genotypes (BRAF-V600Ef/+;Villin-CreER(T2), Smad4f/f BRAF-V600Ef/+;Villin-CreER(T2) and Cdx2f/f BRAF-V600Ef/+Villin-CreER(T2)) one day following 4 consecutive days of tamoxifen injection. Uninjected mice served as control. After flushing the freshly harvested jejunum with cold PBS, the epithelia was dissociated from underlying mesenchyme by incubating with 3 mM EDTA/PBS at 4°C as described previously (Perekatt, 2014), and filtered through 70 micron filter to isolate the crypts. The crypts were washed with PBS, and pelleted to remove excess PBS prior to addition of TRIzol for RNA extraction according to manufacturer’s protocols.