Article

Human subjects are protected from mast cell tryptase deficiency despite frequent inheritance of loss-of-function mutations

Cardiovascular Research Institute, University of California at San Francisco, San Francisco, Calif, USA.
The Journal of allergy and clinical immunology (Impact Factor: 11.48). 09/2009; 124(5):1099-105.e1-4. DOI: 10.1016/j.jaci.2009.07.026
Source: PubMed

ABSTRACT

Mast cell tryptases have proposed roles in allergic inflammation and host defense against infection. Tryptase gene loci TPSAB1 and TPSB2 are known to be polymorphic, but the nature and extent of diversity at these loci have not been fully explored.
We sought to compare functional and nonfunctional tryptase allele frequencies and establish haplotypes in human populations.
Tryptase allele frequencies were determined by means of direct sequencing in 270 individuals from HapMap populations of European, African, Chinese, and Japanese ancestry. Haplotypes were predicted, validated in parent-child trios, and compared between populations.
We identify a new frame-shifted tryptase allele (betaIII(FS)) carried by 23% and 19% of individuals of European and African ancestry but 0% of Asian subjects. Homology models predict that betaIII(FS) is functionless. Our genotyping assay shows that allele and haplotype distributions in each population are unique. Strong linkage disequilibrium between TPSAB1 and TPSB2 (r(2)=0.83, D'=0.85) yields 2 major and 5 minor tryptase haplotypes.
Tryptase deficiency alleles (alpha and the newly discovered betaIII(FS)) are common, causing the number of inherited active genes to range from a minimum of 2 to a maximum of 4, with major differences between populations in the proportion of individuals inheriting 2 versus 4 active alleles. African and Asian populations are especially enriched in genes encoding functional and nonfunctional tryptases, respectively. Strong linkage of TPSAB1 and TPSB2 and pairing of deficiency alleles with functional alleles in observed haplotypes protect human subjects from "knockout" genomes and indeed from inheritance of fewer than 2 active alleles.

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    • "Indeed, ␣-tryptase deletion was reported in 30% of a study population of 274 individuals (Soto et al., 2002), and up to 57% in a UK-based population (Abdelmotelb et al., 2013). ␣-and ␤-tryptase gene polymorphisms are numerous, and the number of functional tryptase alleles an individual may carry varies from two to four (Trivedi et al., 2009). Although it seems that complete tryptase deficiency is not seen in human populations, the number and type of functional alleles carried by an individual may alter the baseline systemic tryptase levels. "
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