Genetically defined individual reference ranges for tryptase limit unnecessary procedures and unmask myeloid neoplasms

Abstract

Serum tryptase is a biomarker used to aid in the identification of certain myeloid neoplasms, most notably systemic mastocytosis, where baseline (BST) levels >20 ng/mL are a minor criterion for diagnosis. Whereas clonal myeloid neoplasms are rare, the common cause for elevated BST is the genetic trait hereditary alpha-tryptasemia (HαT) caused by increased germline TPSAB1 copy number. To date, the precise structural variation and mechanism(s) underlying elevated BST in HαT and the general clinical utility of tryptase genotyping, remain undefined. Through cloning, long-read sequencing, and assembling of the human tryptase locus from an individual with HαT, and validating our findings in vitro and in silico, we demonstrate that BST elevations arise from over-expression of replicated TPSAB1 loci encoding canonical α-tryptase protein due to co-inheritance of a linked over-active promoter element. Modeling BST levels based upon TPSAB1 replication number we generate new individualized clinical reference values for the upper limit of 'normal'. Using this personalized laboratory medicine approach, we demonstrate the clinical utility of tryptase genotyping, finding that in the absence of HαT, BST levels >11.4 ng/mL frequently identify indolent clonal mast cell disease. Moreover, substantial BST elevations (e.g., >100 ng/mL) which would ordinarily prompt bone marrow biopsy, can result from TPSAB1 replications alone and thus be within 'normal' limits for certain individuals with HαT.

Detection of TPSAB1 copy number variation for the diagnosis of hereditary alpha‐tryptasemia by quantitative PCR

“…Lack of alpha tryptase-encoding sequences has been reported by others in approximately the same percentage. 3 The last column shows a comparison of the results obtained with the two methods, revealing a 96% consistency. Using only confirmed data from patients and a 5% prevalence in the general population, sensitivity of qPCR was 100%, specificity 87.50%, positive likelihood ratio…”

“…2 TPSAB1 CNV has been reported in approximately 5% of the normal population. 3 Several methods of TPSAB1 CNV analysis (i.e. by end-point PCR and Sanger sequencing 4,5 ) were initially promising but they did not stand the test of time.…”

Human germline heterozygous gain-of-functionSTAT6variants cause severe allergic disease

“…Based on local availability, research or clinical next-generation sequencing of the genomic DNA was performed using either whole exome or a targeted panel approach (as described previously; Béziat et al, 2021;Campbell et al, 2022;Chovanec et al, 2022;Hebert et al, 2022;Murrell et al, 2022;Tarailo-Graovac et al, 2016). After bioinformatic analysis, de novo and inherited STAT6 variants that were predicted to be damaging and that segregated with disease were identified in each family (Tables S4 and S5).…”

Clonal mast cell disorders and hereditary α‐tryptasemia as risk factors for anaphylaxis

“…The increase in BST levels among HαT carriers has raised interesting questions regarding commonly accepted cut‐off values for ‘normal’ BST levels, as well as the utility of non‐adjusted BST levels of >20 ng/ml and >200 ng/ml as a minor diagnostic criterion for SM and a B‐finding for SSM, respectively. These questions were addressed by the recent publication by Chovanec et al, which introduced a data‐driven model (the aptly named Basal Serum Tryptase Clinical cut‐off Assigned by Locus Copy number of UTR‐Linked element and Associated TPSAB1 Encoded Replication – BST CALCULATER) to generate prediction intervals for the upper limit of normal based on TPSAB1 copy numbers 88 . The model also generated an upper limit of normal (ULN) of 11.4 ng/ml for individuals without HαT, identical to the cut‐off value already in use by laboratories worldwide, which the researchers deemed valid for identifying a clinically abnormal BST level indicative of cMCD.…”