Methods: Individuals with mastocytosis, physical urticarias, and idiopathic anaphylaxis were genotyped by ddPCR. Basal serum tryptase levels, KIT D816V status, and bone marrow biopsies were obtained. HMC1 cells were cultured in the presence of recombinant or patient-derived pro-tryptases, and cell proliferation was measured. KIT-expressing cells were purified from peripheral blood and genotyped for somatic TPSAB1 copy number differences.
Results: Prevalence of germline α-tryptase duplications was 10% (n=17/170), compared to 4.5% (4/83) of unselected volunteers. Duplications were not significantly enriched in physical urticarias (n=0/6), urticaria pigmentosa (n=0/27), mastocytoma (n=0/2) or diffuse cutaneous mastocytosis (n=1/8). Systemic mastocytosis patients harboring duplications (n=8/80) were significantly more likely to have D816V-negative disease (3/8 vs. 2/72; P=0.0062). Over 17% (8/47) of individuals with idiopathic anaphylaxis had α-tryptase duplications (p=0.028), accounting for all basal tryptase elevations in those without concomitant monoclonal mast cell activation (MMAS; n=4); two MMAS patients also had α-tryptase duplications. Pro-tryptases increased HMC1 proliferation at concentrations seen in α-tryptasemia.
Conclusions: Individuals with D816V-negative systemic mastocytosis or idiopathic anaphylaxis are significantly more likely to have hereditary α-tryptasemia. Elevated basal serum tryptase in patients with idiopathic anaphylaxis is exclusively associated with α-tryptasemia and/or a clonal mast cell population. Pro-tryptases promote proliferation of HMC1 cells in vitro, suggesting a putative role in mast cell homeostasis in vivo.