A RAS-independent biomarker panel pedicts response to MEK-inhibitors in colorectal cancer.

e15524 Background: Biomarker discovery and development are essential for stratifying cancer patients in order to improve treatment outcomes. In colorectal cancer (CRC), mutations in the TGF-β/BMP pathway, especially in the SMAD4 gene have been correlated with decreased overall survival and are suspected to modulate drug sensitivity on the cellular level, hence SMAD4 mutations are worthwile targets for novel targeted therapy aproaches. Methods: In the present study, we uncover the mechanistic role of a loss-of-function mutation in SMAD4 in syngeneic patient-derived organoids (PDOs). CRISPR-engineered SMAD4 R361H PDOs were subjected to a comparative drug screening, RNA-Sequencing and multiplex protein profiling analysis (DigiWest®). We have confirmed the response towards MEK inhibition of the initial model in an additional set of 62 PDOs with known mutational status. Results: We show that acquisition of SMAD4 loss-of-function mutations renders PDOs sensitive to MEK-inhibitors. Further, an activation of the TGF-β/BMP signaling pathway, specifically of the BMP branch was observed in SMAD4 wt PDOs; indicating that BMP signaling is likely responsible for the resistance towards MEK inhibition. It is plausible that functional loss of SMAD4 and thus loss of BMP signaling renders SMAD4 mutated tumors more sensitive to MEK-inhibitors. By looking at additional genes involved in TGF-β/BMP signaling that are frequently mutated in CRC, we identified the novel gene mutational SFAB-signature ( SMAD4, FBXW7, ARID1A, or BMPR2), when at least one pathogenic mutation is present in these genes. The frequency of SFAB in CRC patient cohort (TCGA, n = 594) was comparable to the frequency of SFAB in our PDOs. For PDOs with SFAB-signature, we found up to 95% and 70% significant positive prediction for cobimetinib and selumetinib, respectively and also up to 70% positive prediction for trametinib. Thus, the SFAB-signature predicts response to MEK inhibition in PDOs with a very high confidence. We further investigated whether the RAS status of CRC PDOs does predict sensitivity to MEK inhibition. The RAS status alone and in combination with SFAB-signature failed to yield better prediction sensitivity to MEK-inhibitors. Conclusions: The present study is a significant step forward to more personalized treatment regimens for CRC patients by early inclusion of MEK-inhibitors. The SFAB-signature should be put to clinical testing as a RAS-independent biomarker for stratification of patients providing a valuable alternative treatment option against CRC, thus ensuring that all patients receive effective and specific therapies as early as possible.