The emerging mutational landscape of G proteins and G-protein-coupled receptors in cancer

Abstract

Aberrant expression and activity of G proteins and G-protein-coupled receptors (GPCRs) are frequently associated with tumorigenesis. Deep sequencing studies show that 4.2% of tumours carry activating mutations in GNAS (encoding Gαs), and that oncogenic activating mutations in genes encoding Gαq family members (GNAQ or GNA11) are present in ~66% and ~6% of melanomas arising in the eye and skin, respectively. Furthermore, nearly 20% of human tumours harbour mutations in GPCRs. Many human cancer-associated viruses also express constitutively active viral GPCRs. These studies indicate that G proteins, GPCRs and their linked signalling circuitry represent novel therapeutic targets for cancer prevention and treatment.

Figure 1. The residue positions most frequently mutated in cancers in the context of different functional states of the G protein α-subunits

Agonist-occupied G protein coupled receptors (GPCRs) couple to heterotrimeric G proteins, thereby promoting the release of GDP from the Gα-subunit, followed by loading of GTP and dissociation from Gβγ (Receptor bound, nucleotide exchange). Then, GTP-bound active Gα stimulates its cognate effectors (GTP/effector bound, active) as long as the Gα-subunit remains loaded with GTP. Gα proteins then hydrolyze GTP to GDP, a process often accelerated by RGS proteins, thus turning off the switch represented by the active Gα-subunit. Eventually, GDP-bound Gα re-associates with Gβγ, returning the complex to an inactive state (GDP/ Gβγ bound, inactive). The newly reassembled inactive heterotrimer can couple again with available agonist-stimulated GPCRs. The mutation hot-spots are the conserved arginine (blue) and glutamine (orange) residues in conformational switch regions I and II, respectively. These residues are involved in the interaction with Gβγ subunits in the inactive, GDP-bound state of the Gα and in the nucleotide exchange in the receptor bound state (as observed in the ternary complex structure with a GPCR). In the GTP-bound state, the direct interaction of these residues with GTP positions the conformational switches optimally for engagement of the effector proteins. Finally, and most importantly, these residues are directly involved in GTP hydrolysis and consequent G protein inactivation. By interfering with GTP hydrolysis, the prevalent cancer driving mutations result in constitutive activation of the Gα-subunits and persistent stimulation of their downstream signaling pathways.

Figure 2. Cancer-related mutations in human thyroid stimulating hormone receptor, TSHR, projected onto a 3D model

The image shows a view along the membrane plane (A) and across the membrane plane from the intracellular side (B). The receptor is shown in ribbon form; the most frequently mutated positions are shown as spheres and colored from N- to C-terminus. The size of each sphere is proportional to the frequency of tumors with mutations in the corresponding position. The most frequent mutation cluster is located on the intracellular side of the sixth alpha helix of the transmembrane region (TM6) likely resulting in constitutive ligand-independent activity of the receptor.