PIK3CA C-terminal frameshift mutations are novel oncogenic events that sensitize tumors to PI3K-α inhibition.

Abstract

<i>PIK3CA</i> hotspot mutation is well established as an oncogenic driver event in cancer and its durable and efficacious inhibition is a focus in the development and testing of clinical cancer therapeutics. However, hundreds of cancer-associated <i>PIK3CA</i> mutations remain uncharacterized, their sensitivity to PI3K inhibitors unknown. Here, we describe a series of <i>PIK3CA</i> C-terminal mutations, primarily nucleotide insertions, that produce a frame-shifted protein product with an extended C terminus. We report that these mutations occur at a low frequency across multiple cancer subtypes, including breast, and are sufficient to drive oncogenic transformation in vitro and in vivo. We demonstrate that the oncogenicity of these mutant p110α proteins is dependent on p85 but not Ras association. P110α-selective pharmacologic inhibition blocks transformation in cells and mammary tumors characterized by <i>PIK3CA</i> C-terminal mutation. Taken together, these results suggest patients with breast and other tumors characterized by <i>PIK3CA</i> C-terminal frameshift mutations may derive benefit from p110α-selective inhibitors, including the recently FDA-approved alpelisib.