Capitalizing on tumor genotyping: towards the design of mutation specific inhibitors of phosphoinsitide-3-kinase

Abstract

PI3Ks catalyze the phosphorylation of the inositol hydroxyls of phosphoinositide membrane components. The changes in phosphorylation of the inositides recruit proteins to the plasma membrane that initiate important signaling cascades. PI3Kα, one of the class IA PI3Ks, is highly mutated in cancers. All mutations analyzed result in an increase in enzymatic activity. The structures of this enzyme determined by X-ray diffraction, provide a framework for analyzing the possible structural effect of these mutations and their effect on the enzymatic activity. Many of the mutations occur at domain interfaces where they can affect domain interactions and relieve the inhibition of the wild-type enzyme by the nSH2 domain of p85. This mechanism is analogous to the mechanism of physiological activation by activated tyrosine-kinase receptors in which the phosphorylated tyrosine of the receptor (or their substrates) dislodges the nSH2 from its inhibitory position in the complex by competing with its binding to a loop in the helical domain. Other mutations in the kinase domain can directly affect the conformation of the catalytic site. One mutation, His1047Arg, uses a completely different mechanism: it changes the conformation of the C-terminal loop in such a way that it increases the interaction of the enzyme with the membrane, granting increased access to the phosphoinositide substrates. Taking advantage of the reliance of some cancers on the increased activity of mutated PI3Kα, will require the development of isoform-specific, mutant-specific inhibitors. The structural, biochemical and physiological data that are becoming available for PI3Ks are an important first step in this direction.

Figure 1

Assembly of the heterodimer p110α /niSH2 p85α. The surface of the ABD, RBD, C2, and helical domains of p110α are colored grey with the kinase domain as green ribbons. The nSH2 and iSH2 domains of p85α are shown as orange ribbons. The arrow shows how the nSH2 domain can swing up to bind to phosphorylated receptors.

Figure 2

Recognition of phosphorylated peptides by the nSH2 domain. A. Surface of the nSH2 domain colored according to the electrostatic surface potential with the PDGF peptide as shown bound in PDB id 2IUI. B. Same surface with residues 539-545 of the helical domain as shown in the structure of 3HHM.

Figure 3

Oncogenic mutations in the p85α subunit. A. Some of the mutations along the iSH2 domain are shown in red. B. Mutation G376R of the nSH2 domain is at the interface with C2 domain. C. Scheme of the domains of p85.

Figure 4

Oncogenic mutations in p110α subunit shown in the structure of the p110α/niSH2. A. Some of the most common mutations shown as black dots are at the interfaces between domains. B. Scheme of the domains of p110α with the mutations observed in the structure in black.