Hereditary Alpha Tryptasemia: Genotyping and Associated Clinical Features

Abstract

Hereditary alpha tryptasemia is an autosomal dominant genetic trait caused by increased germline copies of TPSAB1 encoding alpha-tryptase. Individuals with this trait have elevated basal serum tryptase, and may present with associated multisystem complaints. Both basal serum tryptase levels and severity of clinical symptoms display a gene-dose relationship with TPSAB1, whereby higher tryptase levels and greater symptom severity are correlated with increasing numbers of alpha-encoding TPSAB1. As the functional effects of increased basal serum tryptase and/or altered tryptase gene expression are elucidated, greater insights will be gained into the symptoms associated with hereditary alpha tryptasemia and their potential therapy.

Keywords: Autosomal dominant; Genotyping; Hypertryptasemia; Mast cell activation.

Figure 1.. Schematic of tryptase secretion from human mast cells.

Pro-tryptases generated in mast cells undergo sequential proteolytic cleavage to become mature tetrameric tryptase, stabilized by heparin, and stored in secretory granules (top) awaiting appropriate stimuli to induce degranulation. Alternatively, pro-tryptases can be secreted constitutively into serum as enzymatically inactive pro-peptides (bottom). Adapted from Caughey GH. Tryptase genetics and anaphylaxis. J Allergy Clin Immunol 2006;117(6):1412; with permission.

Figure 2.. Identified tryptase genotypes encoded at TPSAB1 and TPSB2.

Canonical alpha- and beta-tryptase genotypes based upon conserved copy number (top) and those identified resulting from increased TPSAB1 copy number encoding one or two additional alpha-tryptase copies on single alleles (bottom). Additional genotypes are likely to be identified, and other variant beta isoforms that have already been identified (19) are excluded for simplicity.

Figure 3.. Strategies for tryptase genotyping.

(A) Two alleles in an individual with hereditary alpha tryptasemia – one allele with two beta-tryptase sequences at TPSAB1 and TPSB2 (top), and the second trait-associated allele with three alpha-tryptase encoding TPSAB1 copies and a single beta-tryptase encoding TPSB2 copy (bottom). Genomic DNA (gDNA) extracted from cells from this individual, containing these six different alpha- or beta-tryptase sequences are either amplified or restriction digested. The amplified gDNA can then either be (B) Sanger Sequenced, or (C) treated with the restriction enzyme EcoRV and Southern blotted to perform relative quantitation of alpha- and beta-tryptases. In both cases the ratio of alpha- to beta-tryptases calculated would be 1:1. (D) Unamplified digested gDNA is assayed by droplet digital PCR (ddPCR), which allows for absolute copy number detection of alpha- and beta-tryptase sequences, yielding the genotype determination 3α:3β.