UK Single Institution Experience of Mastocytosis: Guy's and St Thomas' NHS Foundation Trust
Authors: Oni Clare , Moonim Mufaddal , Desterro Joana , Grattan Clive , Curto-Garcia Natalia , Woodley Claire , Ciesielska Monika , Drasutyte Deimante , Francis Yvonne , O'Sullivan Jennifer , Olaoni Thompson , Green Anna , Yurina Miki , Ong Mark , Cross Nick C.P. , McLornan Donal P. , Radia Deepti 
Published: November 2019

Introduction: Systemic mastocytosis (SM) is a heterogeneous disorder of neoplastic mast cells ranging from an indolent to aggressive multi-system disease. We registered our UK Mastocytosis Centre of Excellence with the European Competence Network of Mastocytosis (ECNM) in 2005. Our multidisciplinary team (MDT) includes haematologists, haematopathologists, dermatologist, immunologist and molecular diagnosticians. Over 500 patients have been reviewed in our quarterly SM-MDT. We present a large single centre experience of a rare disorder.
Method: We have reviewed prospective and retrospectively collected clinical data for 125 adult patients from 2009. The data captures clinical, laboratory, diagnostic results and management. Local patients with a tryptase level of >20ug/L are offered haematology review and bone marrow investigation with KIT D816V mutation analysis using digital PCR. Patients seen for a second opinion have their diagnostic material reviewed. Since 2018, next generation sequencing (NGS) based myeloid gene panels are carried out on patients with advanced SM (AdvSM).
Results: Classification of mastocytosis patients:
In 125 patients no gender bias was seen: 56 (44.8%) were male and 69 (55.2%) female. Median age of patients with indolent SM (ISM) was 49 years (range 22-78); and for AdvSM: (aggressive SM, SM with an associated haematological neoplasm and mast cell leukaemia) was 64years (range 5-77). The median serum tryptase level was 48.9ug/L for ISM patients (range 4-682) and 120ug/L for AdvSM patients (range 24-854).
112/125 patients had a bone marrow biopsy. 13 patients chose not to have a biopsy. 88% (110/125) of patients had a mast cell disorder. 4% (5/125) had no marrow involvement. 84% (105/125) met WHO criteria for a diagnosis of SM - Figure 1.These were sub-classified: 71/105 (67%) ISM; 2/105 (2%) smouldering SM (SSM); 3/105 (3%) aggressive SM (ASM); 2/105 (2%) mast cell leukaemia (MCL) and 27/105 (26%) SM with an associated hematologic neoplasm (SM-AHN). The most frequent associated haematological neoplasms were MPN and CMML. Lymphomas (n=3) and plasma cell dyscrasias (n=1) were seen patients. In 10/27 (37%) cases of SM-AHN the SM was the initial diagnosis with subsequent development of an AHN. In 5/27(19%) cases the AHN component preceded the SM and in 12/27 (44%) cases the 2 components were co-diagnosed.
Mutation status : 24/32 patients with AdvSM were noted to be positive for an activating KIT mutation (D816V, n=23; D816L, n=1). 15/32 AdvSM patients had NGS myeloid gene panel analysis for the SAR panel (SRSF2, ASXL1 and RUNX1) which confers poor prognosis. 4 of the15 patients harboured at least 2 out of 3 SAR mutations. JAK2 V617F mutation was detected in 7 of 8 classical MPN patients and in 1 of 4 patients with MPNu/MDS overlap. 4 patients had a TET2 mutation. 1 patient with mast cell leukaemia had a GATA2 mutation in addition to all 3 in the SAR panel.
Cutaneous involvement: 70% (88/125) of patients had skin involvement: 62% (78/125) urticaria pigments, 3% (4/125) telangiectasia macularis eruptiva perstans and 5% (6/125) having non-specific involvement. Skin involvement was more common in ISM (78%) than AdvSM (54% in SM-AHN, none in the 2 patients with MCL).
Management : Treatments in ISM are usually for symptom control. Antihistamines, anti-inflammatory agents, anti-leukotriene agents, mast cell stabilising agents, bisphosphonates, proton-pump inhibitors, PUVA and steroids are used. Various cytoreductive therapies have been used in AdvSM (hydroxycarbamide, pegylated interferon-α, cladrabine, cytarabine, azacitadine alone/combination with midostaurin). Patients with SM-AHN have tailored management depending on the disease component that requires treatment. Access to targeted therapies for AdvSM patients in a trial or compassionate use programme has been successful: 7/32 patients with advanced SM have been enrolled into trials (midostaurin 1; avapritinib 6).
Conclusions: Our data reflects the heterogeneous nature of the SM classification spectrum clinically and diagnostically. An experienced MDT provides holistic high standard of care within a comprehensive centre for good patient outcomes (Figure 2). Development of a portfolio of novel therapeutic options in multi-centred, international collaborative trials is beneficial to patients and pivotal for research in this rare disorder.

Disclosures
Cross: Novartis: Consultancy, Research Funding; Incyte: Consultancy. McLornan:Jazz Pharmaceuticals: Honoraria, Speakers Bureau; Novartis: Honoraria. Radia:Blueprint Medicines: Consultancy; Novartis: Consultancy, Speakers Bureau.

OffLabel Disclosure:
There are very few medications available in mastocytosis and various off label medications have been used including pegylated interferon alpha.