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

Detection of KIT Mutations in Systemic Mastocytosis: How, When, and Why

More than 90% of patients affected by mastocytosis are characterized by a somatic point mutation of KIT, which induces ligand-independent activation of the receptor and downstream signal triggering, ultimately leading to mast cell accumulation and survival. The most frequent mutation is KIT p.D816V, but other rarer mutations can also be found. These mutations often have a very low variant allele frequency (VAF), well below the sensitivity of common next-generation sequencing (NGS) methods used in routine diagnostic panels. Highly sensitive methods are developing for detecting mutations. This review summarizes the current indications on the recommended methods and on how to manage and interpret molecular data for the diagnosis and follow-up of patients with mastocytosis.

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.

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.

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.

Increased TIM-3 and galectin-9 serum levels in patients with advanced systemic mastocytosis

BACKGROUND

Systemic mastocytosis is characterized by expansion of clonal mast cells in various tissues. Several biomarkers with diagnostic and therapeutic potential have recently been characterized in mastocytosis, such as the serum marker tryptase and the immune checkpoint molecule PD-L1.

OBJECTIVE

We aimed to investigate whether serum levels of other checkpoint molecules are altered in systemic mastocytosis and whether these proteins are expressed in mastocytosis infiltrates in the bone marrow.

METHODS

Levels of different checkpoint molecules were analyzed in serum of patients with different categories of systemic mastocytosis and healthy controls and correlated to disease severity. Bone marrow biopsies from patients with systemic mastocytosis were stained to confirm expression.

RESULTS

Serum levels of TIM-3 and galectin-9 were increased in systemic mastocytosis, particularly in advanced subtypes, compared with healthy controls. TIM-3 and galectin-9 levels were also found to correlate with other biomarkers of systemic mastocytosis, such as serum tryptase and KIT D816V variant allele frequency in the peripheral blood. Moreover, we observed expression of TIM-3 and galectin-9 in mastocytosis infiltrates in bone marrow.

CONCLUSIONS

Together, our results demonstrate for the first time that serum levels of TIM-3 and galectin-9 are increased in advanced systemic mastocytosis. Moreover, TIM-3 and galectin-9 are expressed in bone marrow infiltrates in mastocytosis. These findings provide a rationale for exploring TIM-3 and galectin-9 as diagnostic markers and eventually therapeutic targets in systemic mastocytosis, particularly in advanced forms.

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.

Immunohistochemical Staining to Identify Concomitant Systemic Mastocytosis in Acute Myeloid Leukemia with RUNX1::RUNX1T1

Systemic mastocytosis with associated hematological neoplasm (SM-AHN) poses diagnostic challenges because of the coexistence of atypical mast cell proliferation and hematological neoplasms. We assessed the presence of SM-AHN in patients with acute myeloid leukemia (AML) with RUNX1::RUNX1T1 from 2014 to 2020. Bone marrow (BM) samples were evaluated for mast cell aggregates using CD117 and CD25 immunohistochemical (IHC) staining. The KIT D816V variant burden at diagnosis and post induction was assessed using droplet digital PCR. Among 23 patients diagnosed as having AML with RUNX1::RUNX1T1, four (17.4%) were also diagnosed as having SM-AHN. No significant differences in clinical characteristics or overall survival (P=0.565) were observed between patients with or without SM-AHN, except for the presence of KIT variants (P=0.040). After induction therapy, IHC staining revealed the presence of mast cell aggregates in the BM, and the KIT D816V variant burden decreased with decreasing blast count and was similar in BM aspirates, smear slides, and sections. Concomitant SM-AHN was not infrequent in AML patients with RUNX1::RUNX1T1. This study showed the importance of CD117 and CD25 IHC staining after induction chemotherapy for SM-AHN screening, especially in patients with KIT variants.

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.

Mast Cell Diseases in Practice and Research: Issues and Perspectives Raised by Patients and Their Recommendations to the Scientific Community and Beyond

BACKGROUND

Since 2010, patients and physicians have collaborated to understand unmet needs of patients with mast cell diseases, incorporating mastocytosis and mast cell activation disorders, which include mast cell activation syndromes.

OBJECTIVE

This Open Innovation in Science project aims to expand understanding of the needs of patients affected by mast cell diseases, and encourage global communication among patient advocacy groups, physicians, researchers, industry, and government. A major aim is to support the scientific community's efforts to improve diagnosis, management, therapy, and patients' quality of life by addressing unmet needs.

METHODS

In collaboration with mast cell disease specialists, 13 patient advocacy groups from 12 countries and regions developed lists of top patient needs. A core team of leaders from patient advocacy groups collected and analyzed the data and proposed possible actions to address patient needs.

RESULTS

Findings identified similarities and differences among participating countries in unmet needs between patients with mastocytosis and those with mast cell activation syndromes. Issues emphasized struggles relating to the nature and rarity of mast cell diseases, their impact on quality of life, the diagnostic process, access to appropriate care, more effective treatment, and the need for research.

CONCLUSIONS

Solutions vary across countries because situations differ, in particular regarding the existence of and access to centers of excellence and reference centers. Multifaceted mast cell activation syndrome barriers necessitate innovative approaches to improve access to appropriate care. The outcomes of this project should greatly support scientists and clinicians in their efforts to improve diagnosis, management, and treatment of patients with mastocytosis and mast cell activation disorders.

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).

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.

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.

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.

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.

Catching the clinical and biological diversity for an appropriate therapeutic approach in systemic mastocytosis

Systemic mastocytosis (SM) is a rare disease calling for integrated approaches involving onco-hematologic competences for appropriate clinical management and treatment. The wide variability of manifestations and disease course claims for an accurate risk stratification, currently relying on the appraisal of the benefit/risk ratio of treatment modalities within indolent and advanced variants according to WHO classification. More objective parameters are progressively incorporated and integrated into comprehensive models, on which to support the adoption of therapeutic strategies, since the mere clinical distinction between mediator-related signs/symptoms and “true” organ damage can sometimes be complicated. The development of novel targeted drugs is progressively extending the therapeutic alternatives available, which ranges from conventional agents such as interferon and cladribine, to the more modern approach based on KIT inhibition. Ultimately, the choice of the most appropriate therapy should be rationalized on the basis of the clinical picture and molecular data. The focus of the present review is on the areas still open in the current evaluation of SM patients, particularly when considering the need of a treatment.

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.

New developments in the field of mastocytosis and mast cell activation syndromes: a summary of the Annual Meeting of the European Competence Network on Mastocytosis (ECNM) 2019

Mastocytosis are a group of hematologic neoplasms characterized by an accumulation of atypical mast cells in one or several organs/tissues, often accompanied by mast cell activation. Whereas in children the disease manifestations are mostly limited to the skin, in adults the disease is usually systemic (systemic mastocytosis; SM) and involves the bone marrow and/or other internal organs. Several variants of SM have been defined. Whereas most patients have indolent SM, some patients have advanced SM, which underlines the complexity of SM. In 2002, a European consortium of clinicians and scientists initiated a multidisciplinary, multi-national cooperative network, termed the 'European Competence Network on Mastocytosis' (ECNM), with the aim to improve diagnosis and therapy of patients with mastocytosis and other mast cell activation disorders. Since then, members of the ECNM have organized Annual Meetings in several European countries. The present article provides a summary of advances in the field presented during the 17th Annual ECNM meeting held in Salzburg in October 2019.