TPSAB1 Copy Number Analysis by ddPCR

Last Literature Review: March 2024 Last Update:

TPSAB1 Copy Number Analysis 3017399

Methodology: Droplet Digital PCR (ddPCR)

Use to confirm a suspected diagnosis of hereditary alpha-tryptasemia (HαT), rule out HαT in individuals with symptoms of mast cell activation, or as an aid in the diagnosis of systemic mastocytosis.

Hereditary alpha-tryptasemia (HαT) is an autosomal dominant genetic trait that is caused by germline amplification of the alpha-tryptase isoform at the TPSAB1 locus on chromosome 16.  This test uses droplet digital PCR (ddPCR) to assess for the presence of TPSAB1 copy number variants (CNVs) and can be used in the diagnosis of HαT in individuals with consistent signs and symptoms.

Testing for TPSAB1 CNVs may also be considered in individuals with confirmed or suspected systemic mastocytosis, Ehlers-Danlos syndrome or other connective tissue abnormalities, recurrent anaphylaxis, severe postural orthostatic tachycardia syndrome, irritable bowel syndrome, systemic venom reactions, or other systemic immediate hypersensitivity reactions.

Disease Overview

Individuals with HαT may present with increased basal serum tryptase (≥8 ng/mL) and symptoms such as anaphylaxis, connective tissue abnormalities, dysautonomia, gastrointestinal problems, pain, and skin flushing and pruritus. , , ,  HαT may present on its own or in conjunction with mastocytosis. ,  Furthermore, HαT is enriched in individuals with mastocytosis. , 

An elevated serum tryptase level is a minor diagnostic criterion for systemic mastocytosis according to both the World Health Organization (WHO) and International Consensus Classification (ICC), however, the WHO recommends that measurements be adjusted in individuals with HαT to properly apply the criteria.  There are multiple suggested methods for this adjustment; refer to the National Comprehensive Cancer Network (NCCN) guidelines for more information. 

Genetics

Genes

TPSAB1, TPSB2

HαT is defined by a sum of five or more total TPSAB1/TPSB2 copies with at least two alpha-tryptase encoding copies of TPSAB1. Refer to the Possible TPSAB1/TPSB2 Genotypes section for possible genetic configurations.

Inheritance

Autosomal dominant

Structure/Function

The human tryptase locus maps to 16p13.3.  There are four paralogous tryptase genes (TPSG1, TPSB2, TPSAB1, and TPSD1) that code for tryptase, however, the primary secreted tryptase proteins are α-tryptase (encoded by TPSAB1) and β-tryptase (encoded by TPSAB1 and TPSB2). ,  Duplication and triplication of the TPSAB1 gene (as in HαT) can result in the elevation of basal serum tryptase levels and associated risk of severe mediator symptoms (eg, anaphylaxis). , 

Possible TPSAB1/TPSB2 Genotypes

Possible Genotypes in Hereditary α-tryptasemia
Allele 1 of TPSB2 and TPSAB1Allele 2 of TPSB2 and TPSAB1α-tryptase Copiesβ-tryptase CopiesTotal α + βRarityTryptase Concentration
β,ββ,β044CommonNormal
β,αβ,β134CommonNormal
β,αβ,α224CommonNormal
β,-*β,α123UncommonNormal
β,-*β,β033UncommonNormal
β,ββ,ββ055UncommonNormal
β,ααβ,β235CommonIncreased
β,ααβ,α325CommonIncreased
β,ααβ,βα336CommonIncreased
β,αα-,αα415UncommonIncreased
β,ααβ,αα426UncommonIncreased
β,αααβ,ααα628UncommonIncreased

*This assay quantifies the absolute copy numbers of α- and β-tryptase. It cannot determine whether a deletion is from the TPSB2 or TPSAB1 locus.

Sources: Greiner, 2021 

Test Interpretation

Sensitivity and Specificity

Clinical and Analytic Sensitivity and Specificity of TPSAB1 Copy Number Analysis by ddPCR
 SensitivitySpecificity
Clinical100% 90% in individuals with elevated basal serum tryptase 
Analytic>99%>99%
Source: Lyons, 2016 

Results

Result ReportedPossible CNVsResult Interpretation
Not increasedTPSAB1 (0)/TPSB2 (≤5)
TPSAB1 (1)/TPSB2 (≤3)
TPSAB1 (2)/TPSB2 (=2)
 		There is no increase in copy number of TPSAB1 (alpha-tryptase). Calculations are based on the allelic ratio of TPSAB1 to AP3B1 and TPSB2 to AP3B1 genes.
IncreasedTPSAB1 (≥2)/TPSB2 (≥3)
TPSAB1 (≥3)/TPSB2 (≥2)
TPSAB1 + TPSB2 (≥5) AND TPSAB1 (≥2)
 		There is an increase in copy number of TPSAB1 (alpha-tryptase), which is reported in HαT. Calculations are based on the allelic ratio of TPSAB1 to AP3B1 and TPSB2 to AP3B1 genes.

Limitations

  • Diagnostic errors may occur due to rare sequence and CNVs.
  • Single base pair substitutions, small deletions/duplications, and regulatory regions are not detected.
  • This test is unable to determine chromosomal phase of TPSAB1 and TPSB2 genes, ie:
    • Whether TPSAB1 copies are on the same or opposite chromosomes
    • Whether TPSAB1 and TPSB2 copies are on the same or opposite chromosomes
  • This assay detects only the total number of TPSAB1 and TPSB2 gene copies by normalizing to a reference gene (AP3B1). Therefore, rare CNVs that affect TPSAB1 allelic/chromosomal distribution are not detected by this assay.

References