Serum tryptase correlates with the KIT D816V mutation burden in adults with indolent systemic mastocytosis

Systemic mastocytosis (SM) is characterized by the growth of neoplastic mast cells (MCs). Most adults with indolent SM carry the KIT D816V mutation. We recently introduced the D816V+ allele fraction as a disease marker in SM using a sensitive and quantitative KIT D816V mutation analysis that consistently allows mutation detection in peripheral blood (PB) and bone marrow (BM). The D816V+ allele fraction represents a quantitative measure which allows KIT D816V-positivity to be analyzed as a continuous variable instead of a categorical variable (negative/positive) as previously described. Serum tryptase represents an established disease marker in SM, and it remains to be tested whether tryptase and the D816V+ allele fraction are associated or represent independent disease markers. In this study, correlation analysis between serum tryptase, the D816V+ allele fraction in PB and BM, and the MC fraction was performed in 57 indolent systemic mastocytosis (ISM) patients. We detected significant correlations between the D816V+ allele fraction, the mature neoplastic MC fraction, and serum tryptase which represent three different biological measures of disease burden. Mutation analysis was performed in two or more PB samples in 39 patients, and the results demonstrated high stability with no overall tendency to increasing D816V+ allele fractions over time. Considerable variation was nevertheless observed in the correlation analyses. Serum tryptase reflects the mature MC load, whereas the D816V+ allele fraction includes cells other than mature MCs to a variable extent. We conclude that tryptase and the D816V+ allele fraction represent different, although correlated, measures of disease status in SM.