CRISPR screens identify a novel combination treatment targeting BCL-X_L and WNT signaling for KRAS/BRAF-mutated colorectal cancers

Metastatic or recurrent colorectal cancer (CRC) patients require systemic chemotherapy, but the therapeutic options of targeted agents remain limited. CRC patients with KRAS or BRAF gene mutations exhibit a worse prognosis and are resistant to anti-EGFR treatment. Previous studies have shown that the expression of anti-apoptotic protein BCL-X_L is increased in CRC patients with KRAS/BRAF mutations, suggesting BCL-X_L as a therapeutic target for this subgroup. Here, we performed genome-wide CRISPR/Cas9 screens of cell lines with KRAS mutations to investigate the factors required for sensitivity to BCL-X_L inhibitor ABT-263 using single-guide RNAs (sgRNAs) that induce loss-of-function mutations. In the presence of ABT-263, sgRNAs targeting negative regulators of WNT signaling (resulting in WNT activation) were enriched, whereas sgRNAs targeting positive regulators of WNT signaling (resulting in WNT inhibition) were depleted in ABT-263-resistant cells. The activation of WNT signaling was highly associated with an increased expression ratio of anti- to pro-apoptotic BCL-2 family genes in CRC samples. Genetic and pharmacologic inhibition of WNT signaling using β-catenin short hairpin RNA or TNIK inhibitor NCB-0846, respectively, augmented ABT-263-induced cell death in KRAS/BRAF-mutated cells. Inhibition of WNT signaling resulted in transcriptional repression of the anti-apoptotic BCL-2 family member, MCL1, via the functional inhibition of the β-catenin-containing complex at the MCL1 promoter. In addition, the combination of ABT-263 and NCB-0846 exhibited synergistic effects in in vivo patient-derived xenograft (PDX) models with KRAS mutations. Our data provide a novel targeted combination treatment strategy for the CRC patient subgroup with KRAS or BRAF mutations.