Hereditary α tryptasemia is a valid genetic biomarker for severe mediator-related symptoms in mastocytosis

Abstract

Mastocytosis is a hematopoietic neoplasm characterized by expansion of KIT D816V-mutated clonal mast cells in various organs and severe or even life-threatening anaphylactic reactions. Recently, hereditary α-tryptasemia (HαT) has been described as a common genetic trait with increased copy numbers of the α-tryptase encoding gene, TPSAB1, and associated with an increased basal serum tryptase level and a risk of mast cell activation. The purpose of our study was to elucidate the clinical relevance of HαT in patients with mastocytosis. TPSAB1 germline copy number variants were assessed by digital polymerase chain reaction in 180 mastocytosis patients, 180 sex-matched control subjects, 720 patients with other myeloid neoplasms, and 61 additional mastocytosis patients of an independent validation cohort. α-Tryptase encoding TPSAB1 copy number gains, compatible with HαT, were identified in 17.2% of mastocytosis patients and 4.4% of the control population (P < .001). Patients with HαT exhibited higher tryptase levels than patients without HαT (median tryptase in HαT+ cases: 49.6 ng/mL vs HαT- cases: 34.5 ng/mL, P = .004) independent of the mast cell burden. Hymenoptera venom hypersensitivity reactions and severe cardiovascular mediator-related symptoms/anaphylaxis were by far more frequently observed in mastocytosis patients with HαT than in those without HαT. Results were confirmed in an independent validation cohort. The high prevalence of HαT in mastocytosis hints at a potential pathogenic role of germline α-tryptase encoding TPSAB1 copy number gains in disease evolution. Together, our data suggest that HαT is a novel emerging robust biomarker in mastocytosis that is useful for determining the individual patient´s risk of developing severe anaphylaxis.

Figure 1. Prevalence of hereditary α tryptasemia and association of TPSAB1 genotype with biomarkers of disease burden in mastocytosis.

(A) Prevalence of α-tryptase encoding TPSAB1 copy number gain in mastocytosis compared with the control cohort and other myeloid neoplasms. (B) Serum tryptase in patients with (blue) or without (green) α-tryptase encoding TPSAB1 gain; circles indicate mastocytosis and squares other myeloid neoplasms. (C) Serum tryptase in mastocytosis patients with (blue) or without (green) α-tryptase encoding TPSAB1 gain stratified according to the tryptase genotype; squares indicate 2α:3β, triangles 3α:anyβ, and hexagons ≥4α:anyβ. The dotted line indicates 20 ng/mL as minor diagnostic criterion for systemic mastocytosis. (D-F) KIT D816V mutant allele burden (VAF) in peripheral blood or bone marrow aspirate samples (VAF liquid, D) or formalin-fixed paraffin-embedded bone marrow biopsy sections (VAF tissue, E), and histologically determined bone marrow mast cell infiltration (F) in patients with (blue) or without (green) α-tryptase encoding TPSAB1 gain. Vertical black lines indicate mean ± standard error of the mean. (G-H) Linear regression for the relation of serum tryptase and bone marrow mast cell infiltration (G) or KIT D816V mutant allele burden (VAF tissue, H) stratified to presence (blue) or absence (green) of TPSAB1 gain. ρ indicates Spearman’s correlation coefficient. **P<.01; ***P<.001.

Figure 2. Hereditary α tryptasemia is associated with indolent systemic mastocytosis.

(A) Distribution of the mastocytosis disease subtype in patients without α-tryptase encoding TPSAB1 gain (upper bar), any a allele TPSAB1 gain (middle bar), and ≥3 α-allelic copies (lower bar). Serum tryptase (B) and bone marrow mast cell infiltration (C) in ISM/SSM patients with (blue) and without (green) α-tryptase encoding TPSAB1 gain. Samples of SSM patients are shown in orange. Vertical black lines indicate mean 6 standard error of the mean. The dotted black line indicates 20 ng/mL serum tryptase as minor diagnostic criterion for SM, and the dotted orange lines indicate B-finding defining cutoffs (200 ng/mL serum tryptase and 30% mast cell infiltration, respectively). *P<.05.

Figure 3. Hereditary α tryptasemia predisposes to hymenoptera venom allergy and severe mediator symptoms in patients with mastocytosis.

(A) Frequency of allergy in patients with or without α-tryptase encoding TPSAB1 gain. Hymenoptera venom allergy is depicted in red; other types of allergy are in blue. (B-C) Frequency of any mediator-related symptoms (B) and cardiovascular mediator-related symptoms (C) in patients with or without α-tryptase encoding TPSAB1 gain. Mild symptoms (grade 1-2) are depicted in blue; severe symptoms (grade 3-4) are in red. (D) Severity grade of mediator-related symptoms in patients without α-tryptase encoding TPSAB1 gain (green), 2α:3β (blue squares), 3α: anyβ (blue triangles), or ≥4α:anyβ (blue hexagons) tryptase genotype. Horizontal red lines indicate the median severity grade. (E-F) Frequency of hymenoptera venom allergy (E) and severe cardiovascular mediator-related symptoms (F) in patients with or without α-tryptase encoding TPSAB1 gain of an independent validation cohort.