MARS: Mutation-Adjusted Risk Score for Advanced Systemic Mastocytosis

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.

Precision Medicine in Systemic Mastocytosis

Mastocytosis is a rare hematological neoplasm characterized by the proliferation of abnormal clonal mast cells (MCs) in different cutaneous and extracutaneous organs. Its diagnosis is based on well-defined major and minor criteria, including the pathognomonic dense infiltrate of MCs detected in bone marrow (BM), elevated serum tryptase level, abnormal MCs CD25 expression, and the identification of KIT D816V mutation. The World Health Organization (WHO) classification subdivides mastocytosis into a cutaneous form (CM) and five systemic variants (SM), namely indolent/smoldering (ISM/SSM) and advanced SM (AdvSM) including aggressive SM (ASM), SM associated to hematological neoplasms (SM-AHN), and mast cell leukemia (MCL). More than 80% of patients with SM carry a somatic point mutation of KIT at codon 816, which may be targeted by kinase inhibitors. The presence of additional somatic mutations detected by next generation sequencing analysis may impact prognosis and drive treatment strategy, which ranges from symptomatic drugs in indolent forms to kinase-inhibitors active on KIT. Allogeneic stem cell transplant (SCT) may be considered in selected SM cases. Here, we review the clinical, diagnostic, and therapeutic issues of SM, with special emphasis on the translational implications of SM genetics for a precision medicine approach in clinical practice.

Low risk of contrast media-induced hypersensitivity reactions in all subtypes of systemic mastocytosis

BACKGROUND

Patients with systemic mastocytosis (SM) are at increased risk of hypersensitivity reactions (HRs). Although Hymenoptera venoms are the predominant triggers, cases of contrast media-induced HR (CMIHR) have also been reported and prophylactic premedication is often performed. However, data from larger series are limited and differences between indolent and advanced SM have not yet been investigated.

OBJECTIVE

To determine the incidence and severity of CMIHR in all subtypes of SM.

METHODS

We analyzed 162 adult patients with SM (indolent systemic mastocytosis [ISM], n = 65; advanced systemic mastocytosis [advSM], n = 97). First, the cumulative incidence of CMIHR was retrospectively assessed in the patient's history. Second, at our institution, patients underwent 332 contrast media (CM)-enhanced imaging including 80 computed tomography (CT) scans with iodine-based contrast agent and 252 magnetic resonance imaging (MRI) with a gadolinium-based contrast agent, and tolerance was assessed.

RESULTS

Previous CMIHRs to CT (vomiting, n = 1, erythema, n = 1, cardiovascular shock, n = 1), and MRI (dyspnea, n = 1, cardiovascular shock, n = 1) had been reported by 4 out of 162 (2.5%) patients (ISM, n = 3; advSM, n = 1). In contrast, during or after 332 CM-enhanced CT or MRI examinations at our institution, no CMIHRs were reported. Premedication was solely given to 3 patients before CT scans, including 1 with previous CMIHR, who tolerated the imaging well.

CONCLUSION

We conclude that: (1) there is a substantial discrepancy between the perception and prevalence of HRs to CM in SM; (2) reactions are scarce in ISM and even rarer in advSM; and (3) in SM patients without previous history of CM hypersensitivity, prophylactic premedication before CM-enhanced CT or MRI is dispensable.

Decoding the intricacies of the mast cell compartment

Historically, understanding of the human mast cell (MC) compartment has lagged behind the appreciation of other cell lineages. MCs exist in vascularised tissues but do not under normal circumstances circulate in blood, and there has been no pharmacological agent identified that totally and selectively inhibits human MC function. There are no substantiated accounts of an apparently healthy individual who is severely lacking in MCs. Thus, some of the approaches employed to understand the function of a specific immune cell are not available to the MC biologist. The disease categories that have provided the greatest insight into MC biology have been monoclonal and IgE-mediated MC disorders. This has led to the categorisation of MC diseases as intrinsic or extrinsic to the MC compartment and to the recognition of the role of mediators in MC activation disorders. Mastocytosis as a clonal disorder not only impacts the MC compartment through changes intrinsic to the MC, but also by the effects of episodes of significant release of MC mediators. The availability of newer therapeutic approaches developed to treat monoclonal MC disorders offer insights into how to more selectively approach management of MC centric diseases.

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.

Clinical and histopathological features of myeloid neoplasms with concurrent Janus kinase 2 (JAK2) V617F and KIT proto-oncogene, receptor tyrosine kinase (KIT) D816V mutations

We report on 45 patients with myeloid neoplasms and concurrent Janus kinase 2 (JAK2) V617F and KIT proto-oncogene, receptor tyrosine kinase (KIT) D816V (JAK2^{pos .} /KIT^{pos .} ) mutations, which are individually identified in >60% of patients with classical myeloproliferative neoplasms (MPN) and >90% of patients with systemic mastocytosis (SM) respectively. In SM, the concurrent presence of a clonal non-mast cell neoplasm [SM with associated haematological neoplasm (SM-AHN)] usually constitutes a distinct subtype associated with poor survival. All 45 patients presented with a heterogeneous combination of clinical/morphological features typical of the individual disorders (e.g. leuco-/erythro-/thrombocytosis and elevated lactate dehydrogenase for MPN; elevated serum tryptase and alkaline phosphatase for SM). Overlapping features identified in 70% of patients included splenomegaly, cytopenia(s), bone marrow fibrosis and additional somatic mutations. Molecular dissection revealed discordant development of variant allele frequency for both mutations and absence of concurrently positive single-cell derived colonies, indicating disease evolution in two independent clones rather than monoclonal disease in >60% of patients examined. Overall survival of JAK2^{pos .} /KIT^{pos .} patients without additional somatic high-risk mutations [HRM, e.g. in serine and arginine-rich splicing factor 2 (SRSF2), additional sex combs like-1 (ASXL1) or Runt-related transcription factor 1 (RUNX1)] at 5 years was 77%, indicating that the mutual impact of JAK2 V617F and KIT D816V on prognosis is fundamentally different from the adverse impact of additional HRM in the individual disorders.

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.

Systemic mastocytosis in adults: 2021 Update on diagnosis, risk stratification and management

OVERVIEW

Systemic mastocytosis (SM) results from a clonal proliferation of abnormal mast cells (MC) in extra-cutaneous organs.

DIAGNOSIS

The major criterion is presence of multifocal clusters of spindled MC in the bone marrow. Minor diagnostic criteria include elevated serum tryptase level, abnormal MC CD25 expression, and presence of KITD816V mutation.

RISK STRATIFICATION

Establishing SM subtype as per the World Health Organization classification system is an important first step. Broadly, patients either have indolent/smoldering SM (ISM/SSM) or advanced SM, the latter includes aggressive SM (ASM), SM with associated hematological neoplasm (SM-AHN), and mast cell leukemia (MCL). Identification of poor-risk mutations (ie, ASXL1, RUNX1, SRSF2, NRAS) further refines the risk stratification. Recently, clinical and hybrid clinical-molecular risk models have been developed to more accurately assign prognosis in SM patients.

MANAGEMENT

Treatment goals for ISM patients are primarily directed towards anaphylaxis prevention/symptom control/osteoporosis treatment. Patients with advanced SM frequently need MC cytoreductive therapy to ameliorate disease-related organ dysfunction. High response rates have been seen with small-molecule inhibitors that target mutant-KIT, including midostaurin (Food and Drug Administration approved) or avapritinib (investigational). Other options for MC cytoreduction include cladribine or interferon-α, although head-to-head comparisons are lacking. Treatment of SM-AHN primarily targets the AHN component, particularly if an aggressive disease such as acute myeloid leukemia is present. Allogeneic stem cell transplant can be considered in such patients, or in those with relapsed/refractory advanced SM. Imatinib has a limited therapeutic role in SM; effective cytoreduction is limited to those with imatinib-sensitive KIT mutations.

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.

Myeloid somatic mutation panel testing in myeloproliferative neoplasms

Myeloproliferative neoplasms are characterised by somatic mutations in pathways that regulate cell proliferation, epigenetic modifications, RNA splicing or DNA repair. Assessment of the mutational profile assists diagnosis and classification, but also aids assessment of prognosis, and may guide the use of emerging targeted therapies. The most practical way to provide information on numerous genetic variants is by using massively parallel sequencing, commonly in the form of disease specific next generation sequencing (NGS) panels. This review summarises the diagnostic and prognostic value of somatic mutation testing in Philadelphia-negative myeloproliferative neoplasms: polycythaemia vera, essential thrombocythaemia, primary myelofibrosis, chronic neutrophilic leukaemia, systemic mastocytosis, and chronic eosinophilic leukaemia. NGS panel testing is increasing in routine practice and promises to improve the accuracy and efficiency of pathological diagnosis and prognosis.

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.

Proposed global prognostic score for systemic mastocytosis: a retrospective prognostic modelling study

BACKGROUND

Several risk stratification models have been proposed in recent years for systemic mastocytosis but have not been directly compared. Here we designed and validated a risk stratification model for progression-free survival (PFS) and overall survival (OS) in systemic mastocytosis on the basis of all currently available prognostic factors, and compared its predictive capacity for patient outcome with that of other risk scores.

METHODS

We did a retrospective prognostic modelling study based on patients diagnosed with systemic mastocytosis between March 1, 1983, and Oct 11, 2019. In a discovery cohort of 422 patients from centres of the Spanish Network on Mastocytosis (REMA), we evaluated previously identified, independent prognostic features for prognostic effect on PFS and OS by multivariable analysis, and designed a global prognostic score for mastocytosis (GPSM) aimed at predicting PFS (GPSM-PFS) and OS (GPSM-OS) by including only those variables that showed independent prognostic value (p<0·05). The GPSM scores were validated in an independent cohort of 853 patients from centres in Europe and the USA, and compared with pre-existing risk models in the total patient series (n=1275), with use of Harrells' concordance index (C-index) as a readout of the ability of each model to risk-stratify patients according to survival outcomes.

FINDINGS

Our GPSM-PFS and GPSM-OS models were based on unique combinations of independent prognostic factors for PFS (platelet count ≤100 × 10⁹ cells per L, serum β2-microglobulin ≥2·5 μg/mL, and serum baseline tryptase ≥125 μg/L) and OS (haemoglobin ≤110 g/L, serum alkaline phosphatase ≥140 IU/L, and at least one mutation in SRSF2, ASXL1, RUNX1, or DNMT3A). The models showed clear discrimination between low-risk and high-risk patients in terms of worse PFS and OS prognoses in the discovery and validation cohorts, and further discrimination of intermediate-risk patients. The GPSM-PFS score was an accurate predictor of PFS in systemic mastocytosis (C-index 0·90 [95% CI 0·87-0·93], vs values ranging from 0·85 to 0·88 for pre-existing models), particularly in non-advanced systemic mastocytosis (C-index 0·85 [0·76-0·92], within the range for pre-existing models of 0·80 to 0·93). Additionally, the GPSM-OS score was able to accurately predict OS in the entire cohort (C-index 0·92 [0·89-0·94], vs 0·67 to 0·90 for pre-existing models), and showed some capacity to predict OS in advanced systemic mastocytosis (C-index 0·72 [0·66-0·78], vs 0·64 to 0·73 for pre-existing models).

INTERPRETATION

All evaluated risk classifications predicted survival outcomes in systemic mastocytosis. The REMA-PFS and GPSM-PFS models for PFS, and the International Prognostic Scoring System for advanced systemic mastocytosis and GPSM-OS model for OS emerged as the most accurate models, indicating that robust prognostication might be prospectively achieved on the basis of biomarkers that are accessible in diagnostic laboratories worldwide.

FUNDING

Carlos III Health Institute, European Regional Development Fund, Spanish Association of Mastocytosis and Related Diseases, Rare Diseases Strategy of the Spanish National Health System, Junta of Castile and León, Charles and Ann Johnson Foundation, Stanford Cancer Institute Innovation Fund, Austrian Science Fund.

Clinical Impact of Inherited and Acquired Genetic Variants in Mastocytosis

Mastocytosis is a rare and complex disease characterized by expansion of clonal mast cells (MC) in skin and/or various internal organ systems. Involvement of internal organs leads to the diagnosis of systemic mastocytosis (SM). The WHO classification divides SM into indolent SM, smoldering SM and advanced SM variants, including SM with an associated hematologic neoplasm, aggressive SM, and MC leukemia. Historically, genetic analysis of individuals with pure cutaneous mastocytosis (CM) and SM have focused primarily on cohort studies of inherited single nucleotide variants and acquired pathogenic variants. The most prevalent pathogenic variant (mutation) in patients with SM is KIT p.D816V, which is detectable in most adult patients. Other somatic mutations have also been identified-especially in advanced SM-in TET2, SRSF2, ASXL1, RUNX1, CBL and JAK2, and shown to impact clinical and cellular phenotypes. Although only small patient cohorts have been analyzed, disease associations have also been identified in several germline variants within genes encoding certain cytokines or their receptors (IL13, IL6, IL6R, IL31, IL4R) and toll-like receptors. More recently, an increased prevalence of hereditary alpha-tryptasemia (HαT) caused by increased TPSAB1 copy number encoding alpha-tryptase has been described in patients with SM. Whereas HαT is found in 3-6% of general Western populations, it is identified in up to 17% of patients with SM. In the current manuscript we review the prevalence, functional role and clinical impact of various germline and somatic genetic variants in patients with mastocytosis.

Cytogenetic and molecular aberrations and worse outcome for male patients in systemic mastocytosis

In systemic mastocytosis (SM), the clinical features and survival vary greatly. Patient-related factors determining the outcome in SM are largely unknown. Methods: We examined the impact of sex on the clinical features, progression-free survival (PFS), and overall survival (OS) in 3403 patients with mastocytosis collected in the registry of the European Competence Network on Mastocytosis (ECNM). The impact of cytogenetic and molecular genetic aberrations on sex differences was analyzed in a subset of patients. Results: Of all patients enrolled, 55.3% were females. However, a male predominance was found in a subset of advanced SM (AdvSM) patients, namely SM with an associated hematologic neoplasm (SM-AHN, 70%; p < 0.001). Correspondingly, organomegaly (male: 23% vs. female: 13%, p = 0.007) was more, whereas skin involvement (male: 71% vs. female: 86%, p = 0.001) was less frequent in males. In all patients together, OS (p < 0.0001) was significantly inferior in males, and also within the WHO sub-categories indolent SM, aggressive SM (ASM) and SM-AHN. PFS was significantly (p = 0.0002) worse in males when all patients were grouped together; due to low numbers of events, this significance persisted only in the subcategory smoldering SM. Finally, prognostically relevant cytogenetic abnormalities (10% vs. 5%, p = 0.006) or molecular aberrations (SRSF2/ASXL1/RUNX1 profile; 63% vs. 40%, p = 0.003) were more frequently present in males. Conclusions: Male sex has a major impact on clinical features, disease progression, and survival in mastocytosis. Male patients have an inferior survival, which seems related to the fact that they more frequently develop a multi-mutated AdvSM associated with a high-risk molecular background.

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.

Trichinella spiralis-induced mastocytosis and erythropoiesis are simultaneously supported by a bipotent mast cell/erythrocyte precursor cell

Anti-helminth responses require robust type 2 cytokine production that simultaneously promotes worm expulsion and initiates the resolution of helminth-induced wounds and hemorrhaging. However, how infection-induced changes in hematopoiesis contribute to these seemingly distinct processes remains unknown. Recent studies have suggested the existence of a hematopoietic progenitor with dual mast cell-erythrocyte potential. Nonetheless, whether and how these progenitors contribute to host protection during an active infection remains to be defined. Here, we employed single cell RNA-sequencing and identified that the metabolic enzyme, carbonic anhydrase (Car) 1 marks a predefined bone marrow-resident hematopoietic progenitor cell (HPC) population. Next, we generated a Car1-reporter mouse model and found that Car1-GFP positive progenitors represent bipotent mast cell/erythrocyte precursors. Finally, we show that Car1-expressing HPCs simultaneously support mast cell and erythrocyte responses during Trichinella spiralis infection. Collectively, these data suggest that mast cell/erythrocyte precursors are mobilized to promote type 2 cytokine responses and alleviate helminth-induced blood loss, developmentally linking these processes. Collectively, these studies reveal unappreciated hematopoietic events initiated by the host to combat helminth parasites and provide insight into the evolutionary pressure that may have shaped the developmental relationship between mast cells and erythrocytes.

Helminth parasites infect approximately 2 billion people and represent a significant public health concern. Helminths undertake complex developmental life cycles through multiple organs and as a result cause substantial tissue damage. To combat this, mammals have evolved mechanisms to initiate balanced immune responses that promote inflammation needed to seclude parasites in granulomas, reduce parasitic burdens and mitigate the consequences of helminth-induced wounds. Despite their clinical importance, the mechanisms that regulate these events remain poorly defined. Here we have uncovered a unique progenitor cell that supports both proinflammatory mast cell responses and red blood cell development, thereby simultaneously initiating both of these host-protective responses. Collectively, these studies reveal unappreciated events initiated by the host to combat pathogens that infect billions of individuals worldwide.

Importance of Adequate Diagnostic Workup for Correct Diagnosis of Advanced Systemic Mastocytosis

BACKGROUND

Little is known about the epidemiology of advanced systemic mastocytosis (advSM).

OBJECTIVES

To investigate epidemiologic features and diagnostic pitfalls of advSM in Germany.

METHODS

Therefore, 140 patients from a single German reference center of the European Competence Network on Mastocytosis between 2003 and 2018 were analyzed.

RESULTS

The patients' median age was 68 years (range, 26-86 years), and male versus female ratio was 2:1. An elevated serum tryptase, a KIT D816 mutation, and additional somatic mutations, for example, in SRSF2, ASXL1, or RUNX1, were identified in 95%, 91%, and 74% of patients, respectively. Median overall survival was 3.5 years (range, 0.03-14.3 years; male vs female 2.6 vs 4.2 years; P = .02). Two categories of misdiagnoses were identified in 51 of 140 (36%) patients: First, systemic mastocytosis (SM) was overlooked in 28 of 140 (20%) patients primarily diagnosed with various subtypes of myeloid neoplasms. Second, 23 of 140 (16%) patients were diagnosed with supposed progression from indolent SM to advSM; however, combination of an elevated KIT D816V variant allele frequency in peripheral blood (n = 22), monocytosis (n = 9), eosinophilia (n = 6), and/or mutations in SRSF2, ASXL1, or RUNX1 (n = 10) suggest that distinct signs of potential advSM were overlooked in virtually all patients. Based on locally diagnosed patients in an area of 2.5 million inhabitants, but obviously without considering more, yet unrecognized cases, the incidence and prevalence of advSM is at least 0.8 and 5.2, respectively, per 1 million inhabitants.

CONCLUSIONS

Adequate analyses of tryptase levels, bone marrow morphology, and genetics in patients with myeloid neoplasms or SM would help to prevent the significant underdiagnosis of advSM.

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.

An increased bone mineral density is an adverse prognostic factor in patients with systemic mastocytosis

PURPOSE

Systemic mastocytosis (SM) is characterized by the expansion of clonal mast cells that infiltrate various organ systems. The extent of organ infiltration and subsequent organ damage distinguishes between indolent SM (ISM) defined by a nearly normal life expectancy and advanced SM (AdvSM) defined by poor prognosis. In ISM, measurement of the bone mineral density (BMD) frequently reveals osteoporosis. In contrast, the clinical implication of an increased BMD and osteosclerosis remains unclear.

METHODS

BMD was evaluated in 61 patients with mastocytosis (ISM, n = 29, 48%; AdvSM, n = 32, 52%). We correlated the prevalence of osteoporosis, increased BMD and osteosclerosis with clinical parameters, disease variant and prognosis.

RESULTS

Osteoporosis was detected in 11/29 (38%) patients with ISM but only in 2/32 (6%) patients with AdvSM (p = 0.004). An increased BMD was detected in 1/29 (3%) patients with ISM and 24/32 (75%) patients with AdvSM (p < 0.001) while osteosclerosis was only detected in AdvSM patients (16/32, 50%). AdvSM patients with increased BMD had higher levels of bone marrow mast cell infiltration, higher serum tryptase and alkaline phosphatase levels compared to ISM as well as higher number of high-molecular risk mutations (p < 0.05). In addition, we found that the prognosis of AdvSM patients with increased BMD is inferior compared to those without increased BMD (median overall survival 3.6 years versus not reached, p = 0.031).

CONCLUSIONS

Osteoporosis is a common feature in ISM but not in AdvSM. An increased BMD is frequently present in AdvSM but not in ISM and is associated with more advanced disease and inferior outcome.

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.