Heritable risk for severe anaphylaxis associated with increased α-tryptase-encoding germline copy number at TPSAB1

Background: An elevated basal serum tryptase level is associated with severe systemic anaphylaxis, most notably caused by Hymenoptera envenomation. Although clonal mast cell disease is the culprit in some individuals, it does not fully explain this clinical association. Objective: Our aim was to determine the prevalence and associated impact of tryptase genotypes on anaphylaxis in humans. Methods: Cohorts with systemic mastocytosis (SM) and venom as well as idiopathic anaphylaxis from referral centers in Italy, Slovenia, and the United States, underwent tryptase genotyping by droplet digital PCR. Associated anaphylaxis severity (Mueller scale) was subsequently examined. Healthy volunteers and controls with nonatopic disease were recruited and tryptase was genotyped by droplet digital PCR and in silico analysis of genome sequence, respectively. The effects of pooled and recombinant human tryptases, protease activated receptor 2 agonist and antagonist peptides, and a tryptase-neutralizing mAb on human umbilical vein endothelial cell permeability were assayed using a Transwell system. Results: Hereditary alpha-tryptasemia (H alpha T)-a genetic trait caused by increased alpha-tryptase-encoding Tryptase-alpha/beta 1 (TPSAB1) copy number resulting in elevated BST level-was common in healthy individuals (5.6% [n = 7 of 125]) and controls with nonatopic disease (5.3% [n = 21 of 398]). H alpha T-was associated with grade IV venom anaphylaxis (relative risk 5 2.0; P<.05) and more prevalent in both idiopathic anaphylaxis (n = 8 of 47; [17%; P = .006]) and SM (n = 10 of 82 [12.2%; P = .03]) relative to the controls. Among patients with SM, concomitant HaT was associated with increased risk for systemic anaphylaxis (relative risk = 9.5; P = .007). In vitro, protease-activated receptor-2-dependent vascular permeability was induced by pooled mature tryptases but not alpha- or beta-tryptase homotetramers. Conclusions: Risk for severe anaphylaxis in humans is associated with inherited differences in alpha-tryptase-encoding copies at TPSAB1.

Automated summary

This study revealed that genotypes have a significant impact on the severity of anaphylaxis in humans, particularly in association with H alpha T. The prevalence of H alpha T was higher in healthy individuals and controls with nonatopic disease, and it was particularly prominent in systemic mastocytosis and idiopathic anaphylaxis, suggesting it as a potential risk factor for severe allergic reactions.