Defining How Oncogenic and Developmental Mutations of PIK3R1 Alter the Regulation of Class IA Phosphoinositide 3-Kinases

The class I phosphoinositide 3-kinases (PI3Ks) are key signaling enzymes composed of a heterodimer of a p110 catalytic subunit and a p85 regulatory subunit, with PI3K mutations being causative of multiple human diseases including cancer, primary immunodeficiencies, and developmental disorders. Mutations in the p85α regulatory subunit encoded by PIK3R1 can both activate PI3K through oncogenic truncations in the iSH2 domain, or inhibit PI3K through developmental disorder mutations in the cSH2 domain. Using a combined biochemical and hydrogen deuterium exchange mass spectrometry approach we have defined the molecular basis for how these mutations alter the activity of p110α/p110δ catalytic subunits. We find that the oncogenic Q572∗ truncation of PIK3R1 disrupts all p85-inhibitory inputs, with p110α being hyper-activated compared with p110δ. In addition, we find that the R649W mutation in the cSH2 of PIK3R1 decreases sensitivity to activation by receptor tyrosine kinases. This work reveals unique insight into isoform-specific regulation of p110s by p85α. [Display omitted] •Oncogenic Q572∗ variant of p85α leads to hyper-activation of p110α compared with p110δ•HDX-MS revealed that Q572∗ leads to disruption of all inhibitory interfaces in p110α•An SHORT syndrome mutation in p85α leads to decreased sensitivity to RTKs for p110α/δ Dornan et al. use HDX-MS and activity assays to describe the molecular mechanisms of activating and loss-of-function mutations in the regulatory subunit p85α of the class IA PI3K complexes. These data further expand our understanding of how PI3Ks are inhibited and activated downstream of phosphorylated receptors, and how clinical mutations lead to PI3K misregulation leading to cancer and developmental disorders.