Correlation of alpha- and beta-specific tryptase gene expression with TPSAB1 and TPSB2 genotypes in humans

Abstract

Abstract Germline duplications or triplications of alpha-tryptase encoding sequence at TPSAB1 have been linked to elevated basal serum tryptase and are estimated to effect approximately 5% of the general population. However, no assay currently exists to quantify specific alpha- or beta-tryptase gene expression. Using a similar bioinformatic approach to what we used when designing the alpha- and beta-tryptase sequence-specific TPSAB1 genotyping assay, we developed a novel ddPCR assay able to specifically quantify alpha- and beta-tryptase gene expression. Using this assay, we then interrogated cultured primary human mast cells and isolated basophils from individuals with hereditary alpha tryptasemia syndrome – caused by increased TPSAB1 copy number – and from healthy controls. By correlating these results with individual germline TPSAB1 and TPSB2 genotypes we found that, as would be predicted, increasing alpha-tryptase encoding copy number at the TPSAB1 locus correlated with higher alpha-tryptase gene expression in healthy volunteers. Similarly volunteers lacking alpha-trytpase sequence at TPSAB1 were not detected to have alpha-tryptase gene expression, confirming the specificity of the assay. However, among individuals harboring extra TPSAB1 copies, alpha-tryptase gene expression levels were several orders of magnitude higher than beta tryptases, even when they were present in allelic balance (e.g. 3,3). These data suggest that elevated basal serum tryptase levels seen in hereditary alpha tryptasemia result predominantly from deregulated alpha-trypase gene expression, likely due to co-inheritance of as yet unidentified enhancer or promoter elements.