The KIT D816V expressed allele burden for diagnosis and disease monitoring of systemic mastocytosis

The activating KIT D816V mutation plays a central role in the pathogenesis, diagnosis, and targeted treatment of systemic mastocytosis (SM). For improved and reliable identification of KIT D816V, we have developed an allele-specific quantitative real-time PCR (RQ-PCR) with an enhanced sensitivity of 0.01-0.1 %, which was superior to denaturing high-performance liquid chromatography (0.5-1 %) or conventional sequencing (10-20 %). Overall, KIT D816 mutations were identified in 146/147 (99 %) of patients (D816V, n = 142; D816H, n = 2; D816Y, n = 2) with SM, including indolent SM (ISM, n = 63, 43 %), smoldering SM (n = 8, 5 %), SM with associated hematological non-mast cell lineage disease (SM-AHNMD, n = 16, 11 %), and aggressive SM/mast cell leukemia ± AHNMD (ASM/MCL, n = 60, 41 %). If positive in BM, the KIT D816V mutation was found in PB of all patients with advanced SM (SM-AHNMD, ASM, and MCL) and in 46 % (23/50) of patients with ISM. There was a strong correlation between the KIT D816V expressed allele burden (KIT D816V EAB) with results obtained from DNA by genomic allele-specific PCR and also with disease activity (e.g., serum tryptase level), disease subtype (e.g., indolent vs. advanced SM) and survival. In terms of monitoring of residual disease, qualitative and quantitative assessment of KIT D816V and KIT D816V EAB was successfully used for sequential analysis after chemotherapy or allogeneic stem cell transplantation. We therefore conclude that RQ-PCR assays for KIT D816V are useful complimentary tools for diagnosis, disease monitoring, and evaluation of prognosis in patients with SM.