Genetic deficiency of human mast cell alpha-tryptase.

BACKGROUND: Human alpha- and beta-tryptases are proteases secreted by mast cells. Beta (but not alpha) tryptases are implicated in asthma. Genes encoding both types of tryptases cluster on chromosome 16p13.3. OBJECTIVE: This study examines the hypothesis, generated from mapping data, that alpha-alleles compete with some beta-alleles at one locus and that an adjacent locus contains beta-alleles exclusively. This hypothesis predicts that beta-alleles outnumber alpha and that some genomes lack alpha genes altogether. METHODS: To test this hypothesis, we developed PCR-based techniques to distinguish alpha from beta genes. We then genotyped genomic DNA from individuals and tryptase-expressing cell lines. RESULTS: In support of our hypothesis, we find that alpha-tryptase deficiency affects 80/274 (29%) of individuals surveyed. The genotype of the alpha-deficient individuals is betabetabetabeta, due to inheritance of four beta genes. The percentage of the population with the mixed genotypes alphaalphabetabeta and alphabetabetabeta is 21% and 50%, respectively. Accounting for all alpha- and beta-alleles at the tandem loci on 16p13.3, overall alpha-allele frequency is only 0.23, with beta-alleles considerably outnumbering alpha as hypothesized. In samples of defined ethnicity, alpha deficiency affects 45% of Caucasians, but a much lower percentage of other backgrounds, including African-Americans and Asians. Examination of cell lines reveals that HMC-1 and U-937 lack alpha-genes; thus, lack of alpha transcripts in these cells is due to absence of alpha-genes rather than beta-selective transcription. By contrast, alpha-transcribing Mono Mac 6 and KU812 cells contain alpha- and beta-genes. CONCLUSIONS: Genetic alpha-tryptase deficiency is common and varies strikingly between ethnic groups. Because beta-tryptases are implicated in allergic disorders, inherited differences in alpha/beta-genotype may affect disease susceptibility, severity and response to tryptase inhibitor therapy.

Dipeptidyl peptidase I is essential for activation of mast cell chymases, but not tryptases, in mice.

Dipeptidyl peptidase I (DPPI) is the sole activator in vivo of several granule-associated serine proteases of cytotoxic lymphocytes. In vitro, DPPI also activates mast cell chymases and tryptases. To determine whether DPPI is essential for their activation in vivo, we used enzyme histochemical and immunohistochemical approaches and solution-based activity assays to study these enzymes in tissues and bone marrow-derived mast cells (BMMCs) from DPPI +/+ and DPPI -/- mice. We find that DPPI -/- mast cells contain normal amounts of immunoreactive chymases but no chymase activity, indicating that DPPI is essential for chymase activation and suggesting that DPPI -/- mice are functional chymase knockouts. The absence of DPPI and chymase activity does not affect the growth, granularity, or staining characteristics of BMMCs and, despite prior predictions, does not alter IgE-mediated exocytosis of histamine. In contrast, the level of active tryptase (mMCP-6) in DPPI -/- BMMCs is 25% that of DPPI +/- BMMCs. These findings indicate that DPPI is not essential for mMCP-6 activation but does influence the total amount of active mMCP-6 in mast cells and therefore may be an important, but not exclusive mechanism for tryptase activation.