CHK1 Inhibitor Blocks Phosphorylation of FAM122A and Promotes Replication Stress

While effective anti-cancer drugs targeting the Chk1 kinase are advancing in the clinic, drug resistance is rapidly emerging. Here, we demonstrate that CRISPR-mediated knockout of the little-known gene FAM122A/PABIR1 confers cellular resistance to Chk1 inhibitors (CHK1is) and cross-resistance to ATR inhibitors. Knockout of FAM122A results in activation of PP2A-B55α, a Phosphatase that dephosphorylates the Wee1 protein and rescues Wee1 from ubiquitin-mediated degradation. The resulting increase in Wee1 protein expression reduces replication stress, activates the G2/M checkpoint, and confers cellular resistance to CHK1is. Interestingly, in tumor cells with oncogene-driven replication stress, Chk1 can directly phosphorylate FAM122A, leading to activation of the PP2A-B55α Phosphatase and increased Wee1 expression. A combination of a CHK1i plus a Wee1 Inhibitor can overcome CHK1i resistance of these tumor cells, thereby enhancing anti-cancer activity. The FAM122A expression level in a tumor cell can serve as a useful biomarker for predicting CHK1i sensitivity or resistance.