Systematic functional characterization of resistance to PI3K inhibition in breast cancer

PIK3CA (which encodes the phosphoinositide-3 kinase (PI3K) alpha isoform) is the most frequently mutated oncogene in breast cancer. Small-molecule PI3K inhibitors have shown promise in clinical trials; however, intrinsic and acquired resistance limits their utility. We used a systematic gain-of-function approach to identify genes whose upregulation confers resistance to the PI3K inhibitor BYL719 in breast cancer cells. Among the validated resistance genes, PIM kinases conferred resistance by maintaining common downstream PI3K effector activation in an AKT-independent manner. Concurrent pharmacological inhibition of PIM and PI3K overcame this resistance mechanism. We also observed upregulated PIM expression and activity in a subset of biopsies from breast cancers that developed clinical resistance to PI3K inhibitors. PIM1 overexpression is mutually exclusive with PIK3CA mutation in treatment-naïve breast cancers, suggesting downstream functional redundancy. Together, these results offer new insights into resistance to PI3K inhibitors and may support clinical studies of combined PI3K/PIM kinase inhibition in a subset of PIK3CA-mutant cancers