Structural determinants of ligand response specificity in the mast cell activating GPCR, MRGPRX2

The mast cell-specific G-protein coupled receptor MRGPRX2 (Mas-Related G Protein-coupled Receptor X2) has roles in itch and pain, and it mediates clinically relevant allergy-like responses to a diverse assortment of drugs. The varied responses of individuals to MRGPRX2 agonists, leading to drug hypersensitivity reactions in some cases, suggests the presence of consequential variants in the population. However, genetic associations with drug responses are poorly understood. We used heterologously-expressed MRGPRX2 to investigate the effect of 18 naturally occurring non-synonymous single nucleotide polymorphisms on activation by representative compounds from several classes, including neuropeptides, opioid agonists, antibiotics, neuromuscular blocking agents, and polycationic aromatic compounds. The results demonstrate robust activation of wild-type MRGPRX2 by some, but not all, agonists of each class, and not by agonists of closely related MRGPRX1. Naturally occurring MRGPRX2 amino acid substitutions had a range of effects on the potency of different agonists, including loss of function and gain of function phenotypes. The MRGPRX family has diverged from other Class A GPCRs at sites canonically important for receptor activation. Deleterious mutations were found in canonical GPCR functional sites, including those where the MRGPRX family has divergent sequences, as well as in novel functional sites. Severe loss-of-function mutations were generally scored as likely to be deleterious by the Evolutionary Action algorithm while gain-of-function mutations had low to moderate scores. The results will inform future studies aimed at developing drugs targeting MRGPRX2 and uncovering disease-relevant mutations.