The KIT D816V allele burden predicts survival in patients with mastocytosis and correlates with the WHO type of the disease

Management of Advanced Systemic Mastocytosis: Clinical Challenges

Advanced systemic mastocytosis (AdvSM) is a rare hematologic malignancy with organ damage and compromised life expectancy arising from organ accumulation of neoplastic mast cells. Identification of the gain-of-function KITD816V in the majority of cases has accelerated pharmaceutical development culminating with the development of selective KIT inhibitors such as avapritinib. While the advent of these therapies has improved the quality and quantity of life in patients with AdvSM, current challenges remain in the management of this disease. In this review, we summarize the present and future therapeutics landscape of AdvSM, highlighting the development of novel KIT inhibitors including elenestinib and bezuclastinib. We also explore the continued role of additional treatment modalities including allogeneic stem cell transplantation before discussing unresolved clinical challenges in the management of AdvSM.

SOHO State of the Art Update and Next Questions: Current and Emerging Therapies for Systemic Mastocytosis

Systemic mastocytosis (SM) is a heterogeneous myeloid neoplasm, characterized by clonal proliferation of mast cells (MCs) in ≥ 1 extracutaneous organs, including the bone marrow (BM) and gastrointestinal tract. Aberrant MC proliferation is driven by mutation KIT D816V in ≈90-95% of SM patients. Indolent SM (ISM) is the most common SM subtype with various symptoms that can be severe. Advanced SM (AdvSM) has markedly poor prognosis. The advent of KIT inhibitors, targeting mutant KIT and neoplastic MCs, led to a paradigm shift in SM management and markedly improved outcomes. Midostaurin inaugurated the era of KIT inhibitors and was approved for AdvSM in 2017. Avapritinib is the first highly potent and selective inhibitor of KIT D816V that was approved to treat AdvSM and symptomatic ISM (platelets ≥ 50 × 109/L), in the US, in 2021 and 2023, respectively. Pooled analysis of the EXPLORER and PATHFINDER studies, assessing avapritinib in AdvSM, demonstrated rapid and profound reductions (≥ 50%) in markers of MC burden, high response rates (71-75%), and prolonged survival. In the PIONEER study, avapritinib significantly and rapidly improved symptoms/quality of life, and reduced markers of MC burden in ISM patients. The investigational agents bezuclastinib and elenestinib are highly potent and selective inhibitors of KIT D816V with minimal blood-brain barrier penetration. Bezuclastinib reduced markers of MC burden by ≥ 50% in ≈50% of AdvSM patients and ≈90-100% of nonAdvSM patients and reduced symptoms (≥ 50%) in the APEX and SUMMIT studies, respectively. Elenestinib demonstrated dose-dependent efficacy in reducing MC burden markers and improved symptoms in ISM patients in the HARBOR study.

Unraveling the Rare Entity of KIT D816V-Negative Systemic Mastocytosis

Systemic mastocytosis (SM) is a rare type of myeloproliferative neoplasm characterized by abnormal proliferation and infiltration of different tissue by clonal mast cells. The uncontrolled proliferation and activation of mast cells trigger the release of vasoactive and inflammatory mediators, resulting in a cascade of systemic symptoms. Around 95% of SM arise from a gain-of-function mutation at the KIT gene, specifically at codon 816, which highlights its essential role in SM and makes it an attractive target for therapy. Although KIT-negative SM is exceptionally rare, the increased number of cases documented in the literature makes it an intriguing dimension of this disorder. The reported clinical manifestations of KIT-negative SM are widely variable, but many are similar to KIT-positive SM. KIT-targeted therapeutic options have been a game-changer in KIT-positive SM, however their role in KIT-negative SM remains controversial. This report aimed to further understand KIT-negative SM by presenting two cases of KIT-negative SM, one of which was responsive to KIT-targeted therapy, and analyzing reported cases in the existing literature.

Diagnosis of systemic mastocytosis with cryptic deletion of TET2 and DNMT3A resulting from unbalanced translocation

Systemic mastocytosis (SM) is a rare haematological neoplasm associated with the gain of function mutation KIT D816V in 90% of adult patients. Classically, cytogenetic aberrations are not common except in cases of SM associated with another haematological neoplasm. We highlight here an unusual clinical presentation of SM and demonstrate the utility of advanced cytogenetic analysis (optical genome mapping, OGM) in detecting a novel cytogenetic abnormality resulting in an unusual mechanism of DNMT3A and TET2 loss of function.

Tumor necrosis factor α promotes clonal dominance of KIT D816V+ cells in mastocytosis: role of survivin and impact on prognosis

ABSTRACT

Systemic mastocytosis (SM) is defined by the expansion and accumulation of neoplastic mast cells (MCs) in the bone marrow (BM) and extracutaneous organs. Most patients harbor a somatic KIT D816V mutation, which leads to growth factor-independent KIT activation and accumulation of MC. Tumor necrosis factor α (TNF) is a proapoptotic and inflammatory cytokine that has been implicated in the clonal selection of neoplastic cells. We found that KIT D816V increases the expression and secretion of TNF. TNF expression in neoplastic MCs is reduced by KIT-targeting drugs. Similarly, knockdown of KIT or targeting the downstream signaling cascade of MAPK and NF-κB signaling reduced TNF expression levels. TNF reduces colony formation in human BM cells, whereas KIT D816V+ cells are less susceptible to the cytokine, potentially contributing to clonal selection. In line, knockout of TNF in neoplastic MC prolonged survival and reduced myelosuppression in a murine xenotransplantation model. Mechanistic studies revealed that the relative resistance of KIT D816V+ cells to TNF is mediated by the apoptosis-regulator BIRC5 (survivin). Expression of BIRC5 in neoplastic MC was confirmed by immunohistochemistry of samples from patients with SM. TNF serum levels are significantly elevated in patients with SM and high TNF levels were identified as a biomarker associated with inferior survival. We here characterized TNF as a KIT D816V-dependent cytokine that promotes clonal dominance. We propose TNF and apoptosis-associated proteins as potential therapeutic targets in SM.

Clinical impact of the TPSAB1 genotype in mast cell diseases: A REMA study in a cohort of 959 individuals

BACKGROUND

A close association between hereditary alpha-tryptasemia (HAT) and mast cell (MC) disorders has been previously reported. However, the relationship between HAT and the diagnostic subtypes and clinical features of MC disorders still remains to be established.

OBJECTIVE

To determine the prevalence of HAT in healthy donors (HD) vs patients with different diagnostic subtypes of MC activation syndromes (MCAS) and mastocytosis, and its relationship with the clinical behavior of the disease.

METHODS

A total of 959 subjects were studied including 346 healthy donors (HD), 464 mastocytosis, and 149 non-clonal MCAS patients. Molecular studies to assess the TPSAB1 genotype were performed, and data on serum baseline tryptase (sBT) and basal MC-mediator release episodes and triggers of anaphylaxis were collected.

RESULTS

HAT was detected in 15/346 (4%) HD versus 43/149 (29%) non-clonal MCAS and 84/464 (18%) mastocytosis cases. Among mastocytosis, HAT was more frequently found in patients with MC-restricted KITD816V (21% vs. 10% among multilineage KITD816V patients; p = .008). Overall, median sBT was higher in cases presenting with HAT (28.9 vs. 24.5 ng/mL; p = .008), while no significant differences in sBT were observed among HAT+ mastocytosis patients depending on the presence of 1 vs. ≥2 extra copies of the α-tryptase gene (44.1 vs. 35.2 ng/mL, p > .05). In turn, anaphylaxis was more frequently observed in HAT+ versus HAT- mastocytosis patients (76% vs. 65%; p = .018), while HAT+ and HAT- patients who did not refer anaphylaxis as the presenting symptom (n = 308) showed a similar prevalence of subsequent anaphylaxis (35% vs. 36%, respectively).

CONCLUSION

The frequency of HAT in MC disorders varies according to the diagnostic subtype of the disease. HAT does not imply a higher risk (and severity) of anaphylaxis in mastocytosis patients in whom anaphylaxis is not part of the presenting symptoms of the disease.

KIT Mutations and Other Genetic Defects in Mastocytosis: Implications for Disease Pathology and Targeted Therapies

A KIT activating mutation (usually KIT D816V) is detected in neoplastic cells in greater than 90% of indolent patients with systemic mastocytosis (SM). In more advanced variants of SM, additional genetic defects can be found in several myeloid malignancy-related genes, which can be detected by applying next-generation sequencing. Currently, the techniques recommended to detect the KIT D816V mutation and quantify the mutational burden in peripheral blood, bone marrow, or other organs/tissues are allele specific-quantitative PCR or droplet digital PCR. These techniques are useful for diagnosis, prognostication, follow-up and monitoring of therapeutic efficacy of cytoreductive agents in patients with SM.

Recent advances in diagnosis and therapy in systemic mastocytosis

Mastocytosis is a heterogeneous neoplasm characterized by accumulation of neoplastic mast cells in various organs. There are three main types: cutaneous mastocytosis (CM), systemic mastocytosis (SM), and mast cell sarcoma. CM mainly affects children and is confined to the skin, whereas SM affects adults and is characterized by extracutaneous involvement, with or without cutaneous involvement. Most cases of SM have an indolent clinical course; however, some types of SM have aggressive behavior and a poor prognosis. Recent advances in the understanding of the molecular changes in SM have changed the diagnosis and treatment of aggressive and advanced SM subtypes. The International Consensus Classification and World Health Organization refined the diagnostic criteria and classification of SM as a result of accumulation of clinical experience and advances in molecular diagnostics. Somatic mutations in the KIT gene, most frequently KIT D816V, are detected in 90% of patients with SM. Expression of CD30 and any KIT mutation were introduced as minor diagnostic criteria after the introduction of highly sensitive screening methods. SM has a wide spectrum of clinical features, and only a few drugs are effective at treating advanced SM. Currently, the mainstay of SM treatment is limited to the management of chronic symptoms related to release of mast cell mediators. Small-molecule kinase inhibitors targeting the KIT-downstream and KIT-independent pathways were recently approved for treating advanced SM. I describe recent advances in diagnosis of SM, and review the currently available and emerging therapeutic options for SM management.

Case report: Schnitzler-like syndrome without monoclonal gammopathy

Schnitzler syndrome is a rare autoinflammatory disorder characterized by urticarial rash, joint pain, recurrent fever, leucocytosis, elevated C-reactive protein (CRP) and serum amyloid A (SAA), and monoclonal IgM or IgG gammopathy. According to the Strasbourg criteria, both urticarial rash and gammopathy are mandatorily required for the diagnosis of Schnitzler’s syndrome. However, incomplete variants lacking either skin symptoms or monoclonal gammopathy have also been described. Here, we report a case in which the diagnosis of Schnitzler-like syndrome was made despite the absence of gammopathy, based on neutrophilic dermal inflammation, episodic and excessive increase in inflammatory parameters, and prompt response to anakinra, a soluble IL1 receptor antagonist (sIL-1RA). In addition, we detected neutrophil epitheliotropism, which is highly suggestive of autoinflammatory disease. Using whole-exome sequencing, we were unable to find a causative pathogenic mutation but did find several mutations possibly related to the inflammatory processes in this patient. This and other cases highlight that the existing Strasbourg criteria are too strict to capture Schnitzler-like syndromes that may respond well and rapidly to IL1 inhibition. Recurrent episodes of disease with normalization of inflammatory symptoms in the interval, rapid response to anakinra, and neutrophilic epitheliotropism in a lesional skin biopsy may help confirm the diagnosis of Schnitzler-like syndrome.

Acute mast cell leukemia without KIT D816V mutation and lack of CD2 and CD25-a case report of rare entity

Systemic mastocytosis (SM) is a rare hematological neoplasm caused by the excessive proliferation of pathological mast cells that accumulate in the bone marrow (BM) and other extracutaneous organs leading to multi-organ damage and failure. Mast cell leukemia (MCL) is a rare form of systemic mastocytosis, accounting for < 1% of all cases of mastocytosis. MCL usually behaves aggressively with poor responses to current treatment options. Here, we report a diagnostic challenge with the leukemic subtype of MCL with a primary suspicion of pancreatic cancer. A cytomorphological, immunophenotypic, and histopathological examination of the bone marrow was performed. The diagnosis was based on the presence of ≥ 20% atypical and immature mast cells in the bone marrow and ≥ 10% mast cells among the peripheral white blood cells. The neoplastic cell population was identified as mast cell lineage by the expression of CD117 and tryptase. Only 3% of neoplastic cells displayed surface markers characteristic for clonal mast cells: CD25 and CD2. The D816V KIT mutation was not found. Neoplastic mast cells expressed CD30, a marker that is currently considered as a new minor criterion for SM. In the presented case, the primary suspicion of pancreatic cancer with osteosclerotic, lung, and pleural metastases was misleading, and a differential diagnosis based on hematological findings was performed. The patient's severe symptoms were likely the result of organ damage from mast cell infiltration. Despite the use of intensive acute myeloid leukemia (AML)-like polychemotherapy, the patient died during the course of post-induction myelosuppression due to bleeding complications.

New Insights into the Pathogenesis of Mastocytosis: Emerging Concepts in Diagnosis and Therapy

Mastocytosis is a heterogeneous group of neoplasms defined by a numerical increase and accumulation of clonal mast cells (MCs) in various organ systems. The disease may present as cutaneous mastocytosis or systemic mastocytosis (SM). On the basis of histopathological and molecular features, clinical variables, and organ involvement, SM is divided into indolent SM, smoldering SM, SM with an associated hematologic neoplasm, aggressive SM, and MC leukemia. Each variant is defined by unique diagnostic criteria and a unique spectrum of clinical presentations. A key driver of MC expansion and disease evolution is the oncogenic machinery triggered by mutant forms of KIT. The genetic background, additional somatic mutations, and comorbidities also contribute to the course and prognosis. Patients with SM may also suffer from mediator-related symptoms or even an MC activation syndrome. This article provides an update of concepts on the genetics, etiology, and pathology of mastocytosis, with emphasis on diagnostic criteria and new treatment concepts.

The international consensus classification of mastocytosis and related entities

Mastocytosis is a neoplasm characterized by a clonal proliferation of mast cells, which accumulate in one or multiple organs, associated with an extremely heterogeneous clinical presentation. The disease can be limited to the skin (cutaneous mastocytosis) that is mostly seen in childhood and usually behaves in a benign fashion. Adult patients most often present with systemic disease with or without skin lesions. This includes indolent forms such as indolent systemic mastocytosis and its subvariant bone marrow mastocytosis, and smoldering systemic mastocytosis as well as aggressive forms including aggressive systemic mastocytosis, systemic mastocytosis with an associated myeloid neoplasm (previously called systemic mastocytosis with an associated hematologic neoplasm), and mast cell leukemia. In addition, mast cell sarcoma is a rare aggressive form of mastocytosis that can present in the skin as well as at extracutaneous sites. This review article focuses on the updates in mastocytosis of the 2022 international consensus classification (ICC).

Diagnosis and treatment of systemic mastocytosis in Brazil: Recommendations of a multidisciplinary expert panel

INTRODUCTION

Systemic Mastocytosis comprises a group of neoplastic diseases characterized by clonal expansion and infiltration of mast cells into several organs. The diagnosis and treatment of this disease may be challenging for non-specialists.

OBJECTIVE

Make suggestions or recommendations in Systemic Mastocytosis based in a panel of Brazilian specialists.

METHOD AND RESULTS

An online expert panel with 18 multidisciplinary specialists was convened to propose recommendations on the diagnosis and treatment of Systemic Mastocytosis in Brazil. Recommendations were based on discussions of topics and multiple-choice questions and were graded using the Oxford Centre for Evidence-Based Medicine 2011 Levels of Evidence Chart.

CONCLUSION

Twenty-two recommendations or suggestions were proposed based on a literature review and graded according to the findings.

Clinical Outcomes of Adults with Systemic Mastocytosis: A 15-Year Multidisciplinary Experience

Systemic mastocytosis (SM) is a rare, clonal, clinically heterogeneous disorder of the mast cells (MCs), and mainly affects adults. The present study aims to describe the clinical and laboratory features as well as the outcomes of SM. A 15-year retrospective study was conducted on 195 consecutive SM patients (aged ≥ 18 years) diagnosed in 2006−2020 at the Multidisciplinary Mastocytosis Center at Karolinska University Hospital. Patients with indolent SM (ISM) represented the most common SM variant (88.2%). Furthermore, the frequencies of aggressive SM and SM with associated non-mast-cell hematological neoplasm were 4.1% and 7.7%, respectively. The prevalence of SM in the adult population of the Stockholm region was estimated to be 10.6/100,000 inhabitants, and the mean incidence of SM cases in the Stockholm region was 0.77/100,000 people per year. In this series, tryptase levels were below 20 ng/mL in 51 patients (26%). Osteoporosis was present in 21.9% of all cases. Interestingly, there was no progression from ISM to advanced SM variants in our study. Furthermore, overall survival was significantly better in ISM patients compared to advanced SM patients (p < 0.0001). Our data suggest that the early recognition and correct diagnosis of SM has prognostic significance.

Histopathology and Molecular Genetics in Systemic Mastocytosis: Implications for Clinical Management

The diagnosis of systemic mastocytosis (SM) is based on various clinical, dermatological, serological, and hematological findings but essentially relies on histological evidence of an abnormal increase in tissue-localized mast cells (MCs). The extra-cutaneous organ most frequently affected is the bone marrow (BM), and therefore, histological examination of trephine biopsy specimens of the iliac crest is mandatory on suspicion of SM. At microscopic examination, neoplastic MCs show aberrant morphology, usually with prominent spindling. Immunohistochemistry is a useful tool in the diagnosis of SM because mast cell (MC) infiltrates may be slight and scarce, in a mixed background of lymphohistiocytic cells, eosinophils, and plasma cells. Moreover, neoplastic MCs exhibit an aberrant phenotype. Recent evidence, largely derived from molecular genetics, has enhanced the diagnostic capability of SM, also providing the basis for adequate prognostic and therapeutic evaluation. The cases herein reported illustrate the variable clinical manifestations and disease course of SM, focusing on diagnostic and therapeutic challenges. In accordance with the World Health Organization (WHO) classification and the International Consensus Classification (ICC) systems, our findings emphasize the importance of an integrated diagnostic approach for SM, with proper application of diverse assessment methodologies in order to improve SM classification and treatment effectiveness.

Proposed European Competence Network on Mastocytosis-American Initiative in Mast Cell Diseases (ECNM-AIM) Response Criteria in Advanced Systemic Mastocytosis

Advanced systemic mastocytosis (AdvSM) is characterized by the presence of KIT D816V and other somatic mutations (eg, in SRSF2, ASXL1, and RUNX1) in 95% and 60% to 70% of patients, respectively. The biological and clinical consequences of AdvSM include multilineage involvement (eg, associated hematologic neoplasm) in 60% to 80% of patients, variable infiltration and damage (C-findings) of predominantly bone marrow and visceral organs through affected mast cell (MC) and non-MC lineages, and elevated levels of serum tryptase. Recently, the treatment landscape has substantially changed with the introduction of the multikinase/KIT inhibitor midostaurin and the selective KIT D816V inhibitor avapritinib. In this review, we discuss the evolution of AdvSM response criteria that have been developed to better capture clinical benefit (eg, improved responses and progression-free and overall survival). We propose refined response criteria from European Competence Network on Mastocytosis and American Initiative in Mast Cell Diseases investigators that use a tiered approach to segregate the effects of histopathologic (eg, bone marrow MC burden, tryptase), molecular (eg, KIT D816V variant allele frequency), clinical (eg, C-findings), and symptom response on long-term outcomes. These response criteria require evaluation in future prospective clinical trials of selective KIT inhibitors and other novel agents.

Refined Treatment Response Criteria for Indolent Systemic Mastocytosis Proposed by the ECNM-AIM Consortium

Indolent systemic mastocytosis (ISM) has a favorable prognosis and normal life expectancy. However, many patients suffer from mast cell (MC) mediator-related symptoms, which significantly affect quality of life (QoL). Cutaneous, gastrointestinal, and neurological complaints, musculoskeletal pain, and the presence of skin lesions, anaphylaxis, and osteoporosis are the main symptoms and signs in ISM and must be assessed in all patients before and during treatment. Validated mastocytosis-specific patient-reported outcome measures (PROMs) should be used for this purpose. Serum tryptase and KIT D816V allele burden are recommended as secondary outcome parameters, noting that they do not reflect the severity of signs, symptoms, and related QoL impairment, but indirectly express MC burden. Changes from baseline of 90%, 60%, and 30% indicate complete response >90%, major response 60% to 90%, partial response 30% to 60%, and no response <30% to treatment. To conclude, we recommend the use of PROMs as primary outcome parameters to define treatment response in patients with ISM in clinical trials and in everyday clinical practice.

Systemic Mastocytosis and Other Entities Involving Mast Cells: A Practical Review and Update

Evidence in the recent literature suggests that the presentation spectrum of mast cell neoplasms is broad. In this article, we elaborate on recent data pertaining to minor diagnostic criteria of systemic mastocytosis (SM), including sensitive testing methods for detection of activating mutations in the KIT gene or its variants, and adjusted serum tryptase levels in cases with hereditary α-tryptasemia. We also summarize entities that require differential diagnosis, such as the recently reclassified SM subtype named bone marrow mastocytosis, mast cell leukemia (an SM subtype that can be acute or chronic); the rare morphological variant of all SM subtypes known as well-differentiated systemic mastocytosis; the extremely rare myelomastocytic leukemia and its differentiating features from mast cell leukemia; and mast cell activation syndrome. In addition, we provide a concise clinical update of the latest adjusted risk stratification model incorporating genomic data to define prognosis in SM and new treatments that were approved for advanced SM (midostaurin, avapritinib).

Drugs and Vaccines Hypersensitivity in Children with Mastocytosis

Mastocytosis, a heterogeneous mastcell disease, include three different entities: cutaneous mastocytosis, systemic mastocytosis (SM) and mast-cell sarcoma. Tryptase levels can differentiate cutaneous mastocytosis from SM. In mastocytosis, quick onset drug hypersensitivity reactions (DHRs) that are facilitated by mastcell mediators, are investigated in adults. Due to the limited number of children with mastcell disease and increased serum tryptase levels, the role of drugs in this age group is less studied. In this review, we critically assessed relevant papers related with immediate DHRs in children with mastocytosis and discuss practical issues of the management. In childhood mastocytosis, anaphylaxis is frequently idiopathic, and elevated level of basal tryptase, and high burden of disease may increase the risk. Among drugs, antibiotics, NSAIDs and opioids can potentially induce anaphylaxis, anyway avoidance should be recommended only in case of previous reactions. Moreover, vaccinations are not contraindicated in patients with mastocytosis. The risk of severe systemic reactions after drugs intake seems to be extremely low and in general lower in children than in adults. Anyway, studies on this topic especially focusing on children, are missing to state final recommendations.

Comprehensive Analysis of Acquired Genetic Variants and Their Prognostic Impact in Systemic Mastocytosis

Systemic mastocytosis (SM) is a rare clonal haematopoietic stem cell disease in which activating KIT mutations (most commonly KIT D816V) are present in virtually every (>90%) adult patient at similar frequencies among non-advanced and advanced forms of SM. The KIT D816V mutation is considered the most common pathogenic driver of SM. Acquisition of this mutation early during haematopoiesis may cause multilineage involvement of haematopoiesis by KIT D816V, which has been associated with higher tumour burden and additional mutations in other genes, leading to an increased rate of transformation to advanced SM. Thus, among other mutations, alterations in around 30 genes that are also frequently mutated in other myeloid neoplasms have been reported in SM cases. From these genes, 12 (i.e., ASXL1, CBL, DNMT3A, EZH2, JAK2, KRAS, NRAS, SF3B1, RUNX1, SF3B1, SRSF2, TET2) have been recurrently reported to be mutated in SM. Because of all the above, assessment of multilineage involvement of haematopoiesis by the KIT D816V mutation, in the setting of multi-mutated haematopoiesis as revealed by a limited panel of genes (i.e., ASXL1, CBL, DNMT3A, EZH2, NRAS, RUNX1 and SRSF2) and associated with a poorer patient outcome, has become of great help to identify SM patients at higher risk of disease progression and/or poor survival who could benefit from closer follow-up and eventually also early cytoreductive treatment.

Personalized Management Strategies in Mast Cell Disorders: ECNM-AIM User's Guide for Daily Clinical Practice

Mastocytosis is a myeloid neoplasm defined by expansion and focal accumulation of clonal mast cells (MCs) in one or more organs. The disease exhibits a complex pathology and may be complicated by MC activation, bone abnormalities, neurological problems, gastrointestinal symptoms, and/or hematologic progression. The World Health Organization divides mastocytosis into cutaneous forms, systemic mastocytosis (SM) and MC sarcoma. In most patients with SM, somatic mutations in KIT are detected. Patients with indolent SM have a normal to near-normal life expectancy, whereas patients with advanced SM, including aggressive SM and MC leukemia, have a poor prognosis. In those with advanced SM, multiple somatic mutations and an associated hematologic neoplasm may be detected. Mediator-related symptoms can occur in any type of mastocytosis. Symptoms may be mild, severe, or even life-threatening. In patients with severe acute symptoms, an MC activation syndrome may be diagnosed. In these patients, relevant comorbidities include IgE-dependent and IgE-independent allergies. Management of patients with SM is an emerging challenge in daily practice and requires in-depth knowledge and a multidisciplinary and personalized approach with selection of appropriate procedures and interventions. In this article, we review the current knowledge on SM and MC activation syndrome, with emphasis on multidisciplinary aspects in diagnosis and patient-specific management. In addition, we provide a user's guide for application of markers, algorithms, prognostic scores, and treatments for use in daily practice.

Standards of Genetic Testing in the Diagnosis and Prognostication of Systemic Mastocytosis in 2022: Recommendations of the EU-US Cooperative Group

Mastocytosis comprises rare heterogeneous diseases characterized by an increased accumulation of abnormal mast cells in various organs/tissues. The pathogenesis of mastocytosis is strongly linked to the presence of KIT-activating mutations. In systemic mastocytosis (SM), the most frequent mutation encountered is KIT p.D816V, whose presence constitutes one of the minor diagnostic criteria. Different techniques are used to search and quantify the KIT p.D816V mutant; however, allele-specific quantitative PCR and droplet digital PCR are today the most sensitive. The analysis of the KIT p.D816V allele burden has undeniable interest for diagnostic, prognostic, and therapeutic monitoring. The analysis of non-mast cell hematological compartments in SM is similarly important because KIT p.D816V multilineage involvement is associated with a worse prognosis. In addition, in advanced forms of SM, mutations in genes other than KIT are frequently identified and affect negatively disease outcome and response to therapy. Thus, combined quantitative and sensitive analysis of KIT mutations and next-generation sequencing of other recurrently involved myeloid genes make it possible to better characterize the extent of the affected cellular compartments and additional molecular aberrations, providing a more detailed overview of the complex mutational landscape of SM, in relation with the clinical heterogeneity of the disease. In this article, we report the latest recommendations of the EU-US Cooperative Group presented in September 2020 in Vienna during an international working conference, on the techniques we consider standard to detect and quantify the KIT p.D816V mutant in SM and additional myeloid mutations found in SM subtypes.

Refined diagnostic criteria for bone marrow mastocytosis: a proposal of the European competence network on mastocytosis

In the current classification of the World Health Organization (WHO), bone marrow mastocytosis (BMM) is a provisional variant of indolent systemic mastocytosis (ISM) defined by bone marrow involvement and absence of skin lesions. However, no additional diagnostic criteria for BMM have been proposed. Within the registry dataset of the European Competence Network on Mastocytosis, we compared characteristics and outcomes of 390 patients with BMM and 1175 patients with typical ISM. BMM patients were significantly older, predominantly male, had lower tryptase and lower burden of neoplastic mast cells, and displayed a higher frequency of allergic reactions, mainly triggered by Hymenoptera, than patients with typical ISM. The estimated 10-year progression-free survival of BMM and typical ISM was 95.9% and 92.6%, respectively. In BMM patients defined by WHO-based criteria, the presence of one B-Finding and tryptase level ≥125 ng/mL were identified as risk factors for progression in multivariate analyses. BMM patients without any of these risk factors were found to have better progression-free survival (p < 0.05) and better overall survival (p < 0.05) than other ISM patients. These data support the proposal to define BMM as a separate SM variant characterized by SM criteria, absence of skin lesions, absence of B-Findings, and tryptase levels <125 ng/mL.

Updated Diagnostic Criteria and Classification of Mast Cell Disorders: A Consensus Proposal

Mastocytosis is a hematologic neoplasm characterized by expansion and focal accumulation of neoplastic mast cells (MC) in diverse organs, including the skin, bone marrow (BM), spleen, liver, and gastrointestinal tract. The World Health Organization classification divides the disease into prognostically distinct variants of cutaneous mastocytosis (CM) and systemic mastocytosis (SM). Although this classification remains valid, recent developments in the field and the advent of new diagnostic and prognostic parameters created a need to update and refine definitions and diagnostic criteria in MC neoplasms. In addition, MC activation syndromes (MCAS) and genetic features predisposing to SM and MCAS have been identified. To discuss these developments and refinements in the classification, we organized a Working Conference comprised of experts from Europe and the United States in August 2020. This article reports on outcomes from this conference. Of particular note, we propose adjustments in the classification of CM and SM, refinements in diagnostic criteria of SM variants, including smoldering SM and BM mastocytosis (BMM), and updated criteria for MCAS and other conditions involving MC. CD30 expression in MC now qualifies as a minor SM criterion, and BMM is now defined by SM criteria, absence of skin lesions and absence of B- and C-findings. A basal serum tryptase level exceeding 20 ng/mL remains a minor SM criterion, with recognition that hereditary alpha-tryptasemia and various myeloid neoplasms may also cause elevations in tryptase. Our updated proposal will support diagnostic evaluations and prognostication in daily practice and the conduct of clinical trials in MC disorders.

Systemic Mastocytosis: Multidisciplinary Approach

Systemic mastocytosis (SM) is a heterogeneous group of diseases that affect almost exclusively adults and are defined by the proliferation and accumulation of clonal mast cells (MC) in various tissues. Disease subtypes range from indolent to rare aggressive forms. Although SM is classified as a rare disease, it is believed to be likely underdiagnosed. Major signs and symptoms mainly depend on MC activation and less frequent organ infiltration, typical of more aggressive variants. Diagnosis may be challenging, and symptoms can be aspecific and involve several organs. Therefore, it is advisable to refer patients to specialized centers, having sufficient knowledge of the disease, sensitive diagnostic procedures, offering a personalized and multidisciplinary diagnostic approach, including at least hematological, allergological, dermatological, and rheumatological evaluations. A precise and timely diagnosis is required for: a) adequate counseling of patients and their physicians; b) beginning of symptomatic treatment (anti-mediator therapy); c) prevention of severe manifestations of the disease (i.e., recurrent anaphylaxis, osteoporosis, and bone fractures); d) cytoreductive treatment of advanced SM variants. This review summarizes the disease's main manifestations and describes the ideal diagnostic approach for adult patients with suspected SM, giving physicians the main notions for correct patient diagnosis and management. This review also highlights the importance of a multidisciplinary approach in this very complex disease.

Avapritinib for Systemic Mastocytosis

INTRODUCTION

Systemic mastocytosis (SM) is a rare myeloid neoplasm driven in ≈95% of cases by activating KIT mutations, usually D816V. SM can be indolent (ISM), smoldering (SSM) and advanced (AdvSM), the latter characterized by organ damage resulting from infiltrating neoplastic mast cells. The vast majority of cases are indolent, with near-normal life expectancy, although symptoms can be severe. AdvSM, comprising aggressive SM, SM with an associated hematologic neoplasm and mast cell leukemia, however, carries a poor prognosis. Avapritinib is a highly potent and selective inhibitor of mutant KIT.

AREAS COVERED

We provide an overview of SM, including the current therapeutic landscape, and discuss avapritinib in detail: its chemistry and discovery, pharmacodynamic and pharmacokinetic data, current approval status and safety and efficacy profiles in both advanced and non-advanced SM.

EXPERT OPINION

With a response rate of 75% amongst evaluable patients with AdvSM and marked reductions observed in measures of mast cell and disease burden, avapritinib stands out as a highly effective targeted therapy for this mutant KIT-driven disease. Cognitive impairment may occur, and intracranial hemorrhage has been reported, particularly in association with severe thrombocytopenia. Early results in patients with ISM/SSM are encouraging. Avapritinib is now approved in the US for AdvSM.

Systemic Mastocytosis: Molecular Landscape and Implications for Treatment

Over the past decade, we have witnessed significant advances in the molecular characterization of systemic mastocytosis (SM). This has provided important information for a better understanding of the pathogenesis of the disease but has also practically impacted the way we diagnose and manage it. Advances in molecular testing have run in parallel with advances in therapeutic targeting of constitutive active KIT, the major driver of the disease. Therefore, assessing the molecular landscape in each SM patient is essential for diagnosis, prognosis, treatment, and therapeutic efficacy monitoring. This is facilitated by the routine availability of novel technologies like digital PCR and NGS. This review aims to summarize the pathogenesis of the disease, discuss the value of molecular diagnostic testing and how it should be performed, and provide an overview of present and future therapeutic concepts based on fine molecular characterization of SM patients.

New Insights into the Pathogenesis of Systemic Mastocytosis

Mastocytosis is a type of myeloid neoplasm characterized by the clonal, neoplastic proliferation of morphologically and immunophenotypically abnormal mast cells that infiltrate one or more organ systems. Systemic mastocytosis (SM) is a more aggressive variant of mastocytosis with extracutaneous involvement, which might be associated with multi-organ dysfunction or failure and shortened survival. Over 80% of patients with SM carry the KIT D816V mutation. However, the KIT D816V mutation serves as a weak oncogene and appears to be a late event in the pathogenesis of mastocytosis. The management of SM is highly individualized and was largely palliative for patients without a targeted form of therapy in past decades. Targeted therapy with midostaurin, a multiple kinase inhibitor that inhibits KIT, has demonstrated efficacy in patients with advanced SM. This led to the recent approval of midostaurin by the United States Food and Drug Administration and European Medicines Agency. However, the overall survival of patients treated with midostaurin remains unsatisfactory. The identification of genetic and epigenetic alterations and understanding their interactions and the molecular mechanisms involved in mastocytosis is necessary to develop rationally targeted therapeutic strategies. This review briefly summarizes recent developments in the understanding of SM pathogenesis and potential treatment strategies for patients with SM.

Hereditary α tryptasemia is a valid genetic biomarker for severe mediator-related symptoms in mastocytosis

Mastocytosis is a hematopoietic neoplasm characterized by expansion of KIT D816V-mutated clonal mast cells in various organs and severe or even life-threatening anaphylactic reactions. Recently, hereditary α-tryptasemia (HαT) has been described as a common genetic trait with increased copy numbers of the α-tryptase encoding gene, TPSAB1, and associated with an increased basal serum tryptase level and a risk of mast cell activation. The purpose of our study was to elucidate the clinical relevance of HαT in patients with mastocytosis. TPSAB1 germline copy number variants were assessed by digital polymerase chain reaction in 180 mastocytosis patients, 180 sex-matched control subjects, 720 patients with other myeloid neoplasms, and 61 additional mastocytosis patients of an independent validation cohort. α-Tryptase encoding TPSAB1 copy number gains, compatible with HαT, were identified in 17.2% of mastocytosis patients and 4.4% of the control population (P < .001). Patients with HαT exhibited higher tryptase levels than patients without HαT (median tryptase in HαT+ cases: 49.6 ng/mL vs HαT- cases: 34.5 ng/mL, P = .004) independent of the mast cell burden. Hymenoptera venom hypersensitivity reactions and severe cardiovascular mediator-related symptoms/anaphylaxis were by far more frequently observed in mastocytosis patients with HαT than in those without HαT. Results were confirmed in an independent validation cohort. The high prevalence of HαT in mastocytosis hints at a potential pathogenic role of germline α-tryptase encoding TPSAB1 copy number gains in disease evolution. Together, our data suggest that HαT is a novel emerging robust biomarker in mastocytosis that is useful for determining the individual patient´s risk of developing severe anaphylaxis.

Adverse Prognostic Impact of the KIT D816V Transcriptional Activity in Advanced Systemic Mastocytosis

In systemic mastocytosis (SM), qualitative and serial quantitative assessment of the KIT D816V mutation is of diagnostic and prognostic relevance. We investigated peripheral blood and bone marrow samples of 161 patients (indolent SM (ISM), n = 40; advanced SM, AdvSM, n = 121) at referral and during follow-up for the KIT D816V variant allele frequency (VAF) at the DNA-level and the KIT D816V expressed allele burden (EAB) at the RNA-level. A round robin test with four participating laboratories revealed an excellent correlation (r > 0.99, R² > 0.98) between three different DNA-assays. VAF and EAB strongly correlated in ISM (r = 0.91, coefficient of determination, R² = 0.84) but only to a lesser extent in AdvSM (r = 0.71; R² = 0.5). However, as compared to an EAB/VAF ratio ≤2 (cohort A, 77/121 patients, 64%) receiver operating characteristic (ROC) analysis identified an EAB/VAF ratio of >2 (cohort B, 44/121 patients, 36%) as predictive for an advanced phenotype and a significantly inferior median survival (3.3 vs. 11.7 years; p = 0.005). In terms of overall survival, Cox-regression analysis was only significant for the EAB/VAF ratio >2 (p = 0.006) but not for VAF or EAB individually. This study demonstrates for the first time that the transcriptional activity of KIT D816V may play an important role in the pathophysiology of SM.

Genetic Regulation of Tryptase Production and Clinical Impact: Hereditary Alpha Tryptasemia, Mastocytosis and Beyond

Tryptase is a serine protease that is predominantly produced by tissue mast cells (MCs) and stored in secretory granules together with other pre-formed mediators. MC activation, degranulation and mediator release contribute to various immunological processes, but also to several specific diseases, such as IgE-dependent allergies and clonal MC disorders. Biologically active tryptase tetramers primarily derive from the two genes TPSB2 (encoding β-tryptase) and TPSAB1 (encoding either α- or β-tryptase). Based on the most common gene copy numbers, three genotypes, 0α:4β, 1α:3β and 2α:2β, were defined as “canonical”. About 4–6% of the general population carry germline TPSAB1-α copy number gains (2α:3β, 3α:2β or more α-extra-copies), resulting in elevated basal serum tryptase levels. This condition has recently been termed hereditary alpha tryptasemia (HαT). Although many carriers of HαT appear to be asymptomatic, a number of more or less specific symptoms have been associated with HαT. Recent studies have revealed a significantly higher HαT prevalence in patients with systemic mastocytosis (SM) and an association with concomitant severe Hymenoptera venom-induced anaphylaxis. Moreover, HαT seems to be more common in idiopathic anaphylaxis and MC activation syndromes (MCAS). Therefore, TPSAB1 genotyping should be included in the diagnostic algorithm in patients with symptomatic SM, severe anaphylaxis or MCAS.

Genome-wide association study identifies novel susceptibility loci for KIT D816V positive mastocytosis

Mastocytosis is a rare myeloid neoplasm characterized by uncontrolled expansion of mast cells, driven in >80% of affected individuals by acquisition of the KIT D816V mutation. To explore the hypothesis that inherited variation predisposes to mastocytosis, we performed a two-stage genome-wide association study, analyzing 1,035 individuals with KIT D816V positive disease and 17,960 healthy control individuals from five European populations. After quality control, we tested 592,007 SNPs at stage 1 and 75 SNPs at stage 2 for association by using logistic regression and performed a fixed effects meta-analysis to combine evidence across the two stages. From the meta-analysis, we identified three intergenic SNPs associated with mastocytosis that achieved genome-wide significance without heterogeneity between cohorts: rs4616402 (p_meta = 1.37 × 10^-15, OR = 1.52), rs4662380 (p_meta = 2.11 × 10^-12, OR = 1.46), and rs13077541 (p_meta = 2.10 × 10^-9, OR = 1.33). Expression quantitative trait analyses demonstrated that rs4616402 is associated with the expression of CEBPA (p_eQTL = 2.3 × 10^-14), a gene encoding a transcription factor known to play a critical role in myelopoiesis. The role of the other two SNPs is less clear: rs4662380 is associated with expression of the long non-coding RNA gene TEX41 (p_eQTL = 2.55 × 10^-11), whereas rs13077541 is associated with the expression of TBL1XR1, which encodes transducin (β)-like 1 X-linked receptor 1 (p_eQTL = 5.70 × 10^-8). In individuals with available data and non-advanced disease, rs4616402 was associated with age at presentation (p = 0.009; beta = 4.41; n = 422). Additional focused analysis identified suggestive associations between mastocytosis and genetic variation at TERT, TPSAB1/TPSB2, and IL13. These findings demonstrate that multiple germline variants predispose to KIT D816V positive mastocytosis and provide novel avenues for functional investigation.

Mastocytosis

OBJECTIVES

The 2019 Workshop of the Society for Hematopathology/European Association for Haematopathology received and reviewed cases covering the spectrum of mastocytosis and related diseases, including morphologic mimics, focusing on recent updates and relevant findings for pathologists.

METHODS

The workshop panel reviewed 99 cases of cutaneous and systemic mastocytosis (SM) and SM and associated hematologic neoplasms (SM-AHN).

RESULTS

Despite a common theme of KIT mutation (particularly D816V), mastocytosis is a heterogeneous neoplasm with a wide variety of presentations. This spectrum, including rare subtypes and extramedullary organ involvement, is discussed and illustrated by representative cases.

CONCLUSIONS

In the age of targeted treatment aimed at KIT, the accurate diagnosis and classification of mastocytosis has major implications for therapy and further interventions. Understanding the clinical, pathologic, and genetic findings of mastocytosis is crucial for selecting the proper tests to perform and subsequent arrival at a correct diagnosis in this rare disease.

Scoring the Risk of Having Systemic Mastocytosis in Adult Patients with Mastocytosis in the Skin

BACKGROUND

Mastocytosis in adults often presents with skin lesions. A bone marrow biopsy is necessary to confirm or exclude the presence of systemic mastocytosis (SM) in these cases. When a bone marrow biopsy is not performed, the provisional diagnosis is mastocytosis in the skin (MIS). No generally accepted scoring system has been established to estimate the risk of SM in these patients.

OBJECTIVE

To develop a risk score to predict SM in adults with MIS.

METHODS

We examined 1145 patients with MIS from the European Competence Network on Mastocytosis Registry who underwent a bone marrow biopsy. A total of 944 patients had SM and 201 patients had cutaneous mastocytosis; 63.7% were female, and 36.3% were male. Median age was 44 ± 13.3 years. The median serum tryptase level amounted to 29.3 ± 81.9 ng/mL. We established a multivariate regression model using the whole population of patients as a training and validation set (bootstrapping). A risk score was developed and validated with receiver-operating curves.

RESULTS

In the multivariate model, the tryptase level (P < .001), constitutional/cardiovascular symptoms (P = .014), and bone symptoms/osteoporosis (P < .001) were independent predictors of SM (P < .001; sensitivity, 90.7%; specificity, 69.1%). A 6-point risk score was established (risk, 10.7%-98.0%) and validated.

CONCLUSIONS

Using a large data set of the European Competence Network on Mastocytosis Registry, we created a risk score to predict the presence of SM in patients with MIS. Although the score will need further validation in independent cohorts, our score seems to discriminate safely between patients with SM and with pure cutaneous mastocytosis.

Genotypic and phenotypic characteristics of Chinese neonates with cutaneous mastocytosis: a case report and literature review

Mastocytosis is an accumulation of clonal mast cells within tissues and it is most commonly caused by an activating mutation in the KIT gene. In this study, we report a neonatal case who presented with diffuse cutaneous mastocytosis (CM) at birth. In China, nine other cases of neonatal-onset CM have been reported in the literature since 2006. In those cases, diffuse CM and urticaria pigmentosa were the main symptoms, and mutations in exon 17 at codon 816 in KIT were identified.

Clinical features and survival of patients with indolent systemic mastocytosis defined by the updated WHO classification

BACKGROUND

In indolent systemic mastocytosis (ISM), several risk factors of disease progression have been identified. Previous studies, performed with limited patient numbers, have also shown that the clinical course in ISM is stable and comparable to that of cutaneous mastocytosis (CM). The aim of this project was to compare the prognosis of patients with ISM with that of patients with CM.

METHODS

We employed a dataset of 1993 patients from the registry of the European Competence Network on Mastocytosis (ECNM) to compare outcomes of ISM and CM.

RESULTS

We found that overall survival (OS) is worse in ISM compared to CM. Moreover, in patients with typical ISM, bone marrow mastocytosis (BMM), and smoldering SM (SSM), 4.1% of disease progressions have been observed (4.9% of progressions in typical ISM group, 1.7% in BMM, and 9.4% in SSM). Progressions to advanced SM were observed in 2.9% of these patients. In contrast, six patients with CM (1.7%) converted to ISM and no definitive progression to advanced SM was found. No significant differences in OS and event-free survival (EFS) were found when comparing ISM, BMM, and SSM. Higher risk of both progression and death was significantly associated with male gender, worse performance status, and organomegaly.

CONCLUSION

Our data confirm the clinical impact of the WHO classification that separates ISM from CM and from other SM variants.

Recent Advances in the Molecular Biology of Systemic Mastocytosis: Implications for Diagnosis, Prognosis, and Therapy

In recent years, molecular characterization and management of patients with systemic mastocytosis (SM) have greatly benefited from the application of advanced technologies. Highly sensitive and accurate assays for KIT D816V mutation detection and quantification have allowed the switch to non-invasive peripheral blood testing for patient screening; allele burden has prognostic implications and may be used to monitor therapeutic efficacy. Progress in genetic profiling of KIT, together with the use of next-generation sequencing panels for the characterization of associated gene mutations, have allowed the stratification of patients into three subgroups differing in terms of pathogenesis and prognosis: i) patients with mast cell-restricted KIT D816V; ii) patients with multilineage KIT D816V-involvement; iii) patients with "multi-mutated disease". Thanks to these findings, new prognostic scoring systems combining clinical and molecular data have been developed. Finally, non-genetic SETD2 histone methyltransferase loss of function has recently been identified in advanced SM. Assessment of SETD2 protein levels and activity might provide prognostic information and has opened new research avenues exploring alternative targeted therapeutic strategies. This review discusses how progress in recent years has rapidly complemented previous knowledge improving the molecular characterization of SM, and how this has the potential to impact on patient diagnosis and management.

New developments in diagnosis, prognostication, and treatment of advanced systemic mastocytosis

Systemic mastocytosis (SM) has greatly benefited from the broad application of precision medicine techniques to hematolymphoid neoplasms. Sensitive detection of the recurrent KIT D816V mutation and use of next-generation sequencing (NGS) panels to profile the genetic landscape of SM variants have been critical adjuncts to the diagnosis and subclassification of SM, and development of clinical-molecular prognostic scoring systems. Multilineage KIT involvement and multimutated clones are characteristic of advanced SM (advSM), especially SM with an associated hematologic neoplasm (AHN). A major challenge is how to integrate conventional markers of mast cell disease burden (percentage of bone marrow mast cell infiltration and serum tryptase levels) with molecular data (serial monitoring of both KIT D816V variant allele frequency and NGS panels) to lend more diagnostic and prognostic clarity to the heterogeneous clinical presentations and natural histories of advSM. The approval of the multikinase/KIT inhibitor midostaurin has validated the paradigm of KIT inhibition in advSM, and the efficacy and safety of second-generation agents, such as the switch-control inhibitor ripretinib (DCC-2618) and the D816V-selective inhibitor avapritinib (BLU-285) are being further defined in ongoing clinical trials. Looking forward, perhaps the most fruitful marriage of the advances in molecular genetics and treatment will be the design of adaptive basket trials that combine histopathology and genetic profiling to individualize treatment approaches for patients with diverse AHNs and relapsed/refractory SM.

Molecular quantification of tissue disease burden is a new biomarker and independent predictor of survival in mastocytosis

A high allele burden of the KIT D816V mutation in peripheral blood or bone marrow aspirates indicates multi-lineage hematopoietic involvement and has been associated with an aggressive clinical course of systemic mastocytosis. Since mast cells are substantially underrepresented in these liquid specimens, their mutation burden likely underestimates the tumor burden of the disease. We used a novel previously validated digital polymerase chain reaction (PCR) method for KIT D816V analysis to systematically analyze the mutation burden in formalin-fixed, paraffin-embedded bone marrow tissue sections of 116 mastocytosis patients (91 with indolent and 25 with advanced systemic mastocytosis), and to evaluate for the first time the clinical value of the tissue mutation burden as a novel biomarker. The KIT D816V mutation burden in the tissue was significantly higher and correlated better with bone marrow mast cell infiltration (r=0.68 vs. 0.48) and serum tryptase levels (r=0.68 vs. 0.58) compared to that in liquid specimens. Furthermore, the KIT D816V tissue mutation burden was: (i) significantly higher in advanced than in indolent systemic mastocytosis (P=0.001); (ii) predicted survival of patients in multivariate analyses independently; and (iii) was significantly reduced after response to cytoreductive therapy. Finally, digital PCR was more sensitive in detecting KIT D816V in bone marrow sections of indolent systemic mastocytosis patients than melting curve analysis after peptide nucleic acid-mediated PCR clamping (97% vs. 89%; P<0.05). In summary, digital PCR-based measurement of KIT D816V mutation burden in the tissue represents a novel biomarker with independent prognostic significance that can also be employed for monitoring disease progression and treatment response in systemic mastocytosis.

The Spectrum of Aggressive Mastocytosis: A Workshop Report and Literature Review

Most cases of mastocytosis are indolent, usually cutaneous mastocytosis or indolent systemic mastocytosis (SM). Aggressive mast cell (MC) diseases are very rare and often fatal. They can develop de novo or due to progression of indolent forms and can present in different ways; either as MC sarcoma or as advanced SM which includes aggressive SM, MC leukemia, and SM with an associated hematological neoplasm. This review will describe these different aggressive forms of mastocytosis, illustrated by cases submitted to the workshop of the 18th Meeting of the European Association for Haematopathology, Basel 2016, organized by the European Bone Marrow Working Group. In addition, the diagnostic criteria for identifying myelomastocytic leukemia, an aggressive myeloid neoplasm with partial MC differentiation that falls short of the criteria for SM, and disease progression in patients with established mastocytosis are discussed.

International prognostic scoring system for mastocytosis (IPSM): a retrospective cohort study

BACKGROUND

The WHO classification separates mastocytosis into distinct variants, but prognostication remains a clinical challenge. The aim of this study was to improve prognostication for patients with mastocytosis.

METHODS

We analysed data of the registry of the European Competence Network on Mastocytosis including 1639 patients (age 17-90 years) diagnosed with mastocytosis according to WHO criteria between Jan 12, 1978, and March 16, 2017. Univariate and multivariate analyses with Cox regression were applied to identify prognostic variables predicting survival outcomes and to establish a prognostic score. We validated this International Prognostic Scoring System in Mastocytosis (IPSM) with data of 462 patients (age 17-79 years) from the Spanish network Red Española de Mastocitosis diagnosed between Jan 22, 1998, and Nov 2, 2017.

FINDINGS

The prognostic value of the WHO classification was confirmed in our study (p<0·0001). For patients with non-advanced mastocytosis (n=1380), we identified age 60 years or older (HR 10·75, 95% CI 5·68-20·32) and a concentration of alkaline phosphatase 100 U/L or higher (2·91, 1·60-5·30) as additional independent prognostic variables for overall survival. The resulting scoring system divided patients with non-advanced mastocytosis into three groups: low (no risk factors), intermediate 1 (one risk factor), and intermediate 2 (two risk factors). Overall survival and progression-free survival differed significantly among these groups (p<0·0001). In patients with advanced mastocytosis (n=259), age 60 years or older (HR 2·14, 95% CI 1·42-3·22), a concentration of tryptase 125 ng/mL or higher (1·81, 1·20-2·75), a leukocyte count of 16 × 10⁹ per L or higher (1·88, 1·27-2·79), haemoglobin of 11 g/dL or lower (1·71, 1·13-2·57), a platelet count of 100 × 10⁹ per L or lower (1·63, 1·13-2·34), and skin involvement (0·46, 0·30-0·69) were prognostic variables. Based on these variables, a separate score for advanced mastocytosis with four risk categories was established, with significantly different outcomes for overall survival and progression-free survival (p<0·0001). The prognostic value of both scores was confirmed in 413 patients with non-advanced disease and 49 with advanced mastocytosis from the validation cohort.

INTERPRETATION

The IPSM scores for patients with non-advanced and advanced mastocytosis can be used to predict survival outcomes and guide treatment decisions. However, the predictive value of the IPSM needs to be confirmed in forthcoming trials.

FUNDING

Austrian Science Fund, Deutsche Forschungsgemeinschaft, Koeln Fortune Program, Charles and Ann Johnson Foundation, Instituto de Salud Carlos III, Fondos FEDER, Research-Foundation Flanders/Fonds Wetenschappelijk Onderzoek, Clinical Research-Fund of the University Hospitals Leuven, and Research-Foundation Flanders/Fonds Wetenschappelijk Onderzoek.

An evaluation of masitinib for treating systemic mastocytosis

Introduction: Systemic Mastocytosis (SM) is a complex family of rare diseases, against which pharmacological therapies are still very few. It is a c-kit driven disease, whose disregulation leads to uncontrolled activation and proliferation of mast cells (MCs) with consequent release of effector molecules which are responsible for its clinical manifestations. Areas covered: Masitinib is a relatively new potential drug against SM and its chemical structure strictly derives from imatinib, the first tyrosine kinase inhibitor which entered the pharmaceutical market about 15 years ago. In this review, the authors present masitinib in all its properties, from chemistry to pharmacology and toxicity to its potential clinical application in SM, focusing the discussion on the few clinical trials in which it has been involved, with a particular attention on the still open challenge to determine how to measure the response to therapy. Expert opinion: In spite of their similarity in chemistry and biological activity against submolecular targets, masitinib is much more selective towards c-kit receptors than other tyrosine kinases, such as Bcl-Abl. Furthermore, its ability to inhibit degranulation, cytokine production and MCs migration from bone marrow gives it a great chance to become an important therapeutic option for selected SM patients.

Frequency and prognostic impact of KIT and other genetic variants in indolent systemic mastocytosis

Indolent systemic mastocytosis (ISM) patients have a normal life expectancy, except in the 5% to 10% of cases that progress to more advanced SM (advSM), which has a significantly poorer outcome. Mutations in genes other than KIT frequently found in myeloid neoplasms have been associated with a poorer outcome among advSM, whereas limited information exists about their frequency and prognostic impact in ISM. We investigated the frequency and prognostic impact of variants in 18 genes, found to be altered in advSM, in 322 ISM patients (median follow-up, 5.7 years) divided into discovery (n = 200) and validation (n = 122) cohorts. Overall, 71 genetic variants were detected in 55 of 322 (17%) patients. Mutated ISM cases, particularly those carrying ASXL1, RUNX1, and/or DNMT3A (A/R/D) pathogenic variant allele frequencies (VAFs) ≥ 30%, exhibited significantly shortened (P &lt; .001) progression-free survival (PFS) and overall survival (OS). Multivariate analysis showed that serum β2-microglobulin (sβ2M) levels &gt; 2.5 µg/mL (hazard ratio [HR], 9.8; P = .001), together with a KIT D816V VAF ≥ 1% in bone marrow (BM) (HR, 10.1; P = .02) and pathogenic variants of A/R/D VAFs ≥ 30% (HR, 4.2; P = .02), were the best combination of independent predictors for PFS. In turn, A/R/D gene pathogenic VAF ≥ 30% was the only independent predictor for OS (HR, 51.8; P &lt; .001). Based on these variables, 2 scoring systems were constructed for risk stratification of ISM at diagnosis with significantly different 10-year PFS (100%, 91%, 0% for scores of 0, 1, ≥2, respectively) and OS (100% and 50% for scores of 0 and 1) rates.

Mast Cells, Stress, Fear and Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a developmental condition characterized by impaired communication and obsessive behavior that affects 1 in 59 children. ASD is expected to affect 1 in about 40 children by 2020, but there is still no distinct pathogenesis or effective treatments. Prenatal stress has been associated with higher risk of developing ASD in the offspring. Moreover, children with ASD cannot handle anxiety and respond disproportionately even to otherwise benign triggers. Stress and environmental stimuli trigger the unique immune cells, mast cells, which could then trigger microglia leading to abnormal synaptic pruning and dysfunctional neuronal connectivity. This process could alter the "fear threshold" in the amygdala and lead to an exaggerated "fight-or-flight" reaction. The combination of corticotropin-releasing hormone (CRH), secreted under stress, together with environmental stimuli could be major contributors to the pathogenesis of ASD. Recognizing these associations and preventing stimulation of mast cells and/or microglia could greatly benefit ASD patients.

Multidisciplinary Challenges in Mastocytosis and How to Address with Personalized Medicine Approaches

Mastocytosis is a hematopoietic neoplasm defined by abnormal expansion and focal accumulation of clonal tissue mast cells in various organ-systems. The disease exhibits a complex pathology and an equally complex clinical behavior. The classification of the World Health Organization (WHO) divides mastocytosis into cutaneous forms, systemic variants, and localized mast cell tumors. In >80% of patients with systemic mastocytosis (SM), a somatic point mutation in KIT at codon 816 is found. Whereas patients with indolent forms of the disease have a normal or near-normal life expectancy, patients with advanced mast cell neoplasms, including aggressive SM and mast cell leukemia, have a poor prognosis with short survival times. In a majority of these patients, multiple somatic mutations and/or an associated hematologic neoplasm, such as a myeloid leukemia, may be detected. Independent of the category of mastocytosis and the serum tryptase level, patients may suffer from mediator-related symptoms and/or osteopathy. Depending on the presence of co-morbidities, the symptomatology in such patients may be mild, severe or even life-threatening. Most relevant co-morbidities in such patients are IgE-dependent allergies, psychiatric, psychological or mental problems, and vitamin D deficiency. The diagnosis and management of mastocytosis is an emerging challenge in clinical practice and requires vast knowledge, a multidisciplinary approach, and personalized medicine procedures. In this article, the current knowledge about mastocytosis is reviewed with special emphasis on the multidisciplinary aspects of the disease and related challenges in daily practice.

The upregulation of Pim kinases is essential in coordinating the survival, proliferation, and migration of KIT D816V-mutated neoplastic mast cells

About ˜80% of mast cell neoplasm patients harbor the c-Kit activating mutation D816 V, which is associated with c-Kit inhibitor resistance and poor prognosis. However, the molecular basis for these effects is not fully known. To address this issue, in this study we screened molecules whose expression is altered by KIT D816 V mutation and found that Pim kinases were overexpressed in D816V-mutant neoplastic mast cells. This was accompanied by upregulation of signal transducer and activator of transcription (STAT) and mammalian target of rapamycin (mTOR) and downregulation of Akt and extracellular signal-regulated kinase (ERK1/2). Activated Pim kinases promoted the survival of D816 V cells by maintaining mTOR and p70S6K activation even under nutrient starvation. Conversely, cell proliferation was suppressed by inhibiting Pim kinases. The mRNA level of C-X-C chemokine receptor type 4 (CXCR4) was about 2-fold higher in D816 V cells; this was associated with a 2-fold increase in migratory capacity, which was modulated by Pim kinases. We also confirmed that upregulation of Pim kinases is a feature specific to cells with the D816 V mutation and is not observed in cells with the c-Kit activating N822 K mutation. These data suggest Pim kinases as a promising therapeutic target for the treatment of mast cell neoplasms with KIT D816 V mutation.