Regulation of CHK1 inhibitor resistance by a c-Rel and USP1 dependent pathway

We previously discovered that deletion of c-Rel in the E-Myc mouse model of lymphoma results in earlier onset of disease, a finding that contrasted with the expected function of this NF-{kappa}B subunit in B-cell malignancies. Here we report that c-rel -/- E{micro}-Myc cells have an unexpected and major defect in the CHK1 pathway, with almost undetectable levels of CHK1 and CLSPN protein leading to therapeutic resistance to the highly specific CHK1 inhibitor (CHK1i) CCT244747. Similar downregulation of CHK1 levels was also seen in CCT244747 resistant U20S osteosarcoma cells. Further investigation revealed that downregulation of the deubiquitinase USP1 is responsible, at least in part, for these effects. Importantly, we demonstrate that c-rel -/- E{micro}-Myc lymphoma cells survive though upregulation of compensatory PI3K/AKT pathway activity. Moreover, targeting this pathway with Pictilisib (GDC-0941) effectively killed c-rel -/- E{micro}-Myc in vivo, while having no effect on wild type E-Myc cells. This data reveals an NF-{kappa}B regulated pathway controlling CHK1 activity in cancer cells and identifies a potential mechanism for both acquiring and overcoming CHK1i resistance in cancer patients.