Aggressive Systemic Mastocytosis Is Associated with a High Kit D816V Allele Burden in Peripheral Blood.
Authors: Erben Philipp , Bolz Georg , Popa Juliana , Metzgeroth Georgia , Müller Martin C , Gosenca Darko , Ernst Thomas , Hofmann Wolf-Karsten , Hochhaus Andreas , Reiter Andreas 
Published: October 2019

Abstract
Abstract 2899
Poster Board II-875
According to 2008 WHO definitions, diagnosis of systemic mastocytosis (SM) is based on the presence of one major criteria (multifocal dense infiltrates of mast cells in bone marrow or organ biopsies) and at least one of the following minor criteria (i) >25% are atypical cells on bone marrow smears or are spindle-shaped in mast cell infiltrates of visceral organs, ii) KIT point mutation at codon 816 in the bone marrow or other extracutaneous organs, iii) mast cells express CD2 and/or CD25, iv) baseline serum tryptase concentration >20 ng/mL) or the presence of at least three minor criteria. Patients with aggressive SM (ASM) have to present with one or more C-findings (i) neutrophils <1,000/μL, Hb <10g/dL, or platelets <100,000/μL, ii) hepatomegaly with impaired liver function – elevated transaminases and/or bilirubin levels and/or hypoalbuminemia, iii) palpable splenomegaly with signs of hypersplenism, iv) malabsorption with significant hypoalbuminemia and/or significant loss. ASM and MCL are clearly associated with an inferior survival. The activating KIT D816V point mutation is thought to be pivotal for pathogenesis and potential targeted therapy with novel tyrosine kinase inhibitors. However, conventional sequencing (CS) of RNA/DNA extracted from bone marrow (BM) samples of SM patients reveals the mutation in less than 50-60% of patients while peripheral blood (PB) samples are frequently negative as consequence of low numbers of malignant cells and poor assay sensitivity. For better qualitative and additional quantitative assessment of the KIT D816V allele burden (expressed as ratio KIT D816V vs. KIT wildtype), we sought to establish (i) a D-HPLC (denaturing-high performance liquid chromatography) assay combined with direct sequencing in case of a positive D-HPLC signal and (ii) a LightCyclerTM based quantitative PCR (RQ-PCR). The assay sensitivities were calculated through serial cell (KIT D816V positive HMC-1 cells in NB4 cells) and RNA dilutions. The detection limit was estimated between 0.1 and 0.5% for both assays which was thus significantly improved in comparison to the detection limit of CS which was at 10–15%. Patient material was available from BM (n=134) and PB (n=93) from 173 patients (88 m, 85 f, median age 54 years) with diagnosis of SM including 10 patients with confirmed ASM (n=9) or aleukemic variant of MCL (n=1) according to WHO definitions. At diagnosis, D-HPLC, RQ-PCR and CS were positive in 78% (104/134), 84% (112/134) and 60% (80/134) of BM samples, respectively. Two patients had a KIT D816H and one patient had a KIT D816L mutation. In 54 cases with contemporaneously collected BM and PB samples, the KIT D816V mutation was found in PB of BM positive patients by D-HPLC, RQ-PCR and CS in 47% (22/46), 55% (27/49) and 38% (14/37) of cases, respectively. This information allowed the detection of the KIT D816V mutation in an additional 27 of 39 (69%) patients without available bone marrow biopsies. All ASM patients were KIT D816V positive in PB with a median KIT D816V allele burden of 36% (range 26–98%) vs. 7.8% (range 0.1–61%) in other stages of SM (p=0.0021). Eleven of 124 (8.9%) SM patients with a KIT D816V allele burden >20% are currently explored for the potential diagnosis of ASM or MCL. In conclusion, D-HPLC is a reliable and sensitive method for the screening of variable KIT mutations in BM and PB of SM patients and is clearly superior to CS. Application of RQ-PCR assays for the most common D816V mutation may overlook rare mutations but allows quantification of the KIT D816V allele burden which may be useful for diagnosis of ASM and monitoring of residual disease during treatment with novel tyrosine kinase inhibitors, e.g. midostaurin or dasatinib.

Disclosures:
No relevant conflicts of interest to declare.