The international consensus classification of mastocytosis and related entities

Systemic mastocytosis: current status and challenges in 2024

ABSTRACT

Systemic mastocytosis (SM) is a rare disease and has had significant discoveries in its biology, prognostication, and management in the past 2 decades. The latest update of the World Health Organization classification and the new International Consensus Classification are current standards in the diagnosis and prognostication of SM. In clinical practice, SM can be divided into 2 main categories: nonadvanced SM (nonAdvSM) and advanced SM (AdvSM). The integration of clinical signs and symptoms as well as bone marrow morphologic, immunophenotypic, and molecular results is required to diagnose SM variants. In the modern era, data with KIT inhibitors (ie, avapritinib) suggest prolongation of survival in AdvSM. Although this is encouraging progress, and we now have effective drugs for managing both patients with indolent SM and AdvSM, there are remaining challenges in SM. For example, optimal initial treatment in certain patient subsets, such as SM with an associated hematologic neoplasm (SM-AHN), remains under debate (eg, treatments targeting AHN or SM, monotherapy, or combinations). Prospective studies evaluating drugs with different mechanisms of action are needed for such patients. This review provides an updated overview of SM, including the latest methods for diagnosis, patient classification based on their prognosis, and management according to the most significant clinical trials, covering both patients with nonadvSM and AdvSM.

High-sensitivity KIT D816V variation analysis by droplet digital polymerase chain reaction: The reference laboratory perspective

OBJECTIVE

Systemic mastocytosis is a hematologic malignancy characterized by clonal expansion of neoplastic mast cells. Detection of this variation is critical for screening and diagnosis, with recent guidelines emphasizing the need for high-sensitivity assays that identify variants at a variant allele frequency below 0.05%. Our reference laboratory offers droplet digital polymerase chain reaction (ddPCR) for detection of KIT D816V at a limit of detection of 0.03% variant allele frequency-substantially higher sensitivity than next-generation sequencing (NGS).

METHODS

Because high-sensitivity KIT D816V testing is still not widely available, we present our 3-year experience with KIT D816V ddPCR in a clinical setting. From January 2021 to March 2024, KIT D816V variation was detected in 14.9% (1232/8272) of samples.

RESULTS

Peripheral blood and bone marrow positivity rates were 11.1% and 34.9%, respectively. Among 181 samples tested by both ddPCR and NGS, ddPCR identified 37.6% as positive, while NGS identified only 6.0% as positive. Next-generation sequencing showed 16% sensitivity and 100% specificity for KIT D816V detection compared with ddPCR as the gold standard, which detected the variant in 84% more samples because of its lower limit of detection. A 20-ng/mL serum tryptase threshold to screen for detecting KIT D816V by ddPCR had 73.7% sensitivity and 91.2% specificity, but lowering the serum tryptase threshold to 11.5 ng/mL increased sensitivity to 97.5%, with 70.7% specificity.

CONCLUSIONS

Overall, ddPCR for detection of KIT D816V dramatically increases sensitivity over NGS tests used for myeloid malignancies, including systemic mastocytosis. Our findings also provide support for the use of a lower serum tryptase threshold (>11.4 ng/mL instead of >20ng/mL) to initiate workup for a mast cell neoplasm.

Mast Cells at the Crossroads of Hypersensitivity Reactions and Neurogenic Inflammation

Although mast cells have long been known, they are not yet fully understood. They are traditionally recognized for their role in allergic reactions through the IgE/FcεRI axis, but different groups of surface receptors have since been characterized, which appear to be involved in the manifestation of peculiar clinical features. In particular, MRGPRX2 has emerged as a crucial receptor involved in degranulating human skin mast cells. Because of mast cells' close proximity to peripheral nerve endings, it may play a key role in neuroimmune interactions. This paper provides an overview of mast cell contributions to hypersensitivity and so-called "pseudoallergic" reactions, as well as an update on neuroinflammatory implications in the main models of airway and skin allergic diseases. In particular, the main cellular characteristics and the most relevant surface receptors involved in MC pathophysiology have been reappraised in light of recent advancements in MC research. Molecular and clinical aspects related to MC degranulation induced by IgE or MRGPRX2 have been analyzed and compared, along with their possible repercussions and limitations on future therapeutic perspectives.

Harmonization of Diagnostic Criteria in Mastocytosis for Use in Clinical Practice: WHO vs ICC vs AIM/ECNM

Mastocytosis is a clonal myeloid disorder defined by an increase and accumulation of mast cells (MCs) in one or multiple organ systems. The complex pathology of mastocytosis results in variable clinical presentations, courses, and outcomes. The World Health Organization (WHO) divides the disease into cutaneous mastocytosis (CM), several forms of systemic mastocytosis (SM), and MC sarcoma. In most patients with SM, a somatic KIT mutation, usually D816V, is identified. Patients diagnosed with CM or nonadvanced SM, including indolent SM, have a near-normal life expectancy, whereas those with advanced SM, including aggressive SM and MC leukemia, have limited life expectancy. Since 2001, a multidisciplinary consensus group consisting of experts from the European Competence Network on Mastocytosis and the American Initiative in Mast Cell Diseases has supported the field by developing diagnostic criteria for mastocytosis. These criteria served as the basis for the WHO classification of mastocytosis over 2 decades. More recently, an International Consensus Classification group proposed slightly modified diagnostic criteria and a slightly revised classification. In this article, these changes are discussed. Furthermore, we propose harmonization among the proposals of the American Initiative in Mast Cell Diseases/European Competence Network on Mastocytosis consensus group, WHO, and the International Consensus Classification Group. Such harmonization will facilitate comparisons of retrospective study results and the conduct of prospective trials.

MASTering systemic mastocytosis: Lessons learned from a large patient cohort

Background

Systemic mastocytosis (SM), a rare condition affecting about 32,000 individuals in the United States, is often misdiagnosed or underdiagnosed owing to its nonspecific symptoms and the need for invasive biopsies.

Objective

Our aim was to identify, classify, and characterize the natural history of patients with SM.

Methods

In a retrospective cohort study, administrative data from a large managed care organization was used to identify patients with confirmed SM, based on World Health Organization criteria. Demographic data, delay to diagnosis, disease progression, and health care resource utilization were examined.

Results

Of 116 patients with confirmed SM, 77% had indolent SM, 2% had smoldering SM, 12% had SM with associated hematologic neoplasm, 9% had aggressive SM, and none had mast cell leukemia. In all, 5 patients were misclassified as having a less advanced SM subtype initially and 3 were completely undiagnosed (missed diagnosis). The average delay to diagnosis of SM was 58.3 plus or minus 73.1 months. In all, 18% of patients progressed from a nonadvanced form of SM (indolent or smoldering SM) to an advanced form of SM (aggressive SM, SM with associated hematologic neoplasm, or mast cell leukemia) over an average of 88.3 plus or minus 82.7 months. Patients with SM had increased health care utilization, including increases in their numbers of hospital admissions, emergency room visits, urgent care visits, and specialty provider visits, after diagnosis versus before.

Conclusions

Rare diseases such as SM would benefit from increased understanding and awareness to improve diagnostic accuracy. Prospective studies are needed to better characterize this patient population and determine the type of follow-up needed to recognize advanced forms of SM so that appropriate treatment can be implemented.

Case Report: Multidisciplinary management of a patient with indolent systemic mastocytosis and refractory symptoms

Systemic mastocytosis (SM) is a rare hematologic condition characterized by the proliferation and accumulation in tissue of clonal mast cells in multiple organ systems. The release of mast cell mediators in the indolent disease type and the predominant mast cell infiltration of tissues in advanced disease contribute to the heterogeneous clinical presentation. The disease driver in >90% of adult cases is an activating KIT mutation, with D816V being the most frequent. Here we describe a case of a young adult male presenting with osteoporosis with associated symptoms of reflux and a history of bee sting anaphylaxis. A multidisciplinary approach to the diagnosis and management was required to minimize morbidities and prevent complications. Current best supportive care was inadequate to control the patient's disease, and a selective KIT D816V inhibitor (avapritinib) was initiated. Conventional, and advanced therapies, including those in the treatment pipeline for SM are discussed.

Primary Mast Cell Sarcoma of the Maxillary Sinus and Gingiva Mimicking Malignant Neuroendocrine Tumor: A Case Report

Mast cell sarcoma (MCS) is an extremely rare and aggressive malignancy primarily affecting bones, with limited literature associating it with neuroendocrine marker expression. This report presents a rare case of MCS arising in the maxillary sinus and gingiva. A 74-year-old man presented with a progressively enlarging ulcer on the right-sided upper gingiva. Magnetic resonance imaging revealed a 3.4 cm tumor on the floor of the right maxillary sinus. The patient underwent an inferior maxillectomy and right-sided neck dissection. Microscopically, the tumor consisted of monotonous round cells with oval nuclei, vesicular chromatin, inconspicuous nucleoli, and brisk mitoses. A panel of immunohistochemical stains was initially applied to exclude common sinonasal undifferentiated neoplasms, such as sinonasal undifferentiated carcinoma, melanoma, rhabdomyosarcoma, Ewing sarcoma, and lymphoma. The tumor cells showed patchy staining for INSM1 and synaptophysin, but were negative for AE1/AE3, CAM5.2, p40, chromogranin, S100, HMB45, NKX2.2, desmin, CD45 (LCA), CD3, and CD20, with intact INI1 and BRG1 expression. No specific diagnosis could be rendered based on the staining results, leading to consideration of other rare malignancies. Additional staining revealed positivity for CD117, mast cell tryptase, CD13, CD33, CD43, and CD68, confirming the MCS diagnosis. Molecular testing for KIT mutation was negative. Subsequent bone marrow biopsy demonstrated infiltration of atypical mast cells, which led to a diagnosis of mast cell leukemia. Despite high-dose chemotherapy, the patient died three months after the initial diagnosis. The undifferentiated epithelioid morphology and unusual aberrant neuroendocrine marker expression posed significant diagnostic challenges. The major differential diagnoses were discussed in this report.

Systemic Mastocytosis: State of the Art

PURPOSE OF REVIEW

Since identification of Systemic mastocytosis (SM) as a distinct disease entity by the World Health Organisation (WHO), there has been a wealth of new research in therapeutic targeting of the pathogenic C-KIT D816V mutation.

RECENT FINDINGS

Avapritinib, the first licensed drug in SM capable of disease modification alongside the increasingly potent, oral and highly selective KIT tyrosine kinase inhibitors (TKIs) Bezuclastinib and now Elenestinib have enabled the prospect of long-term remissions. Studies have shown improved survival and symptomatic control in patients with SM. Of great triumph, this has been achieved in an outpatient setting with apparent tolerable and minimal toxicity. The importance of molecular profiling is being demonstrated in administering combination therapies for SM with an associated haematological neoplasm (AHN), allowing more personalised and streamlined treatment regimes. This review focuses on current management strategies of SM, focusing on state-of-the-art directed therapies, the evidence behind their use with presentation of two clinical cases to highlight key messages.

Treatment refractory mast cell leukemia with dominant gastrointestinal manifestation and concomitant skin symptoms: A case report

BACKGROUND

Mast cell leukemia (MCL), a subtype of systemic mastocytosis (SM), is an extremely rare clinical entity characterized by a very poor prognosis. Chemotherapy, tyrosine kinase inhibitors, and allogeneic hematopoietic cell transplantation are the only treatment options, but they cannot provide the desired outcomes in most cases of MCL. However, other types of SM can be successfully treated. The disease has no specific manifestation, but gastroenterological symptoms are present in most cases.

CASE SUMMARY

The authors, hereby, report a case of a 46-year-old female patient diagnosed with MCL-the rarest subtype of SM. The patient presented to the gastroenterology clinic with multiple, various, and unspecific gastroenterological symptoms. Concomitance of skin lesions significantly contributed to a relatively prompt diagnosis. The serum tryptase level was extremely high and bone the marrow aspirate showed an infiltration of atypical mast cells. The disease was rapidly progressive and primary refractory to chemotherapy and the patient succumbed to the illness about a month after the initiation of treatment.

CONCLUSION

Despite its "hematological nature", MCL, in most cases presents dominantly with unspecific gastroenterological symptoms. Thus, a high disease awareness among physicians other than hematologists is necessary to improve treatment outcomes. Serum tryptase level, due to its non-invasive nature and easy access, may serve as an initial step to estimate the probability of mastocytosis.

Biopsy samples from patients with irritable bowel syndrome, but not from those with mastocytosis or unspecific gastrointestinal complaints reveal unique nerve activation in all gut regions independent of mast cell density, histamine content or specific gastrointestinal symptoms

Introduction

We previously showed enteric nerve activation after application of colonic mucosal biopsy supernatants from patients with irritable bowel syndrome (IBS). The question remains whether this is a region-specific or a generalized sensitization. We tested the nerve-activating properties of supernatants from large and small intestinal regions of IBS patients with diarrhea (IBS-D) in comparison to those from mastocytosis patients with diarrhea (MC-D) or non-IBS/non-MC patients with GI-complaints. MC-D patients were included to test samples from patients with an established, severe mast cell disorder, because mast cells are suggested to play a role in IBS.

Methods

Voltage-sensitive dye imaging was used to record the effects of mucosal biopsy supernatants from IBS-D, MC-D, and non-IBS/non-MC on guinea pig submucous neurons. Mast cell density and histamine concentrations were measured in all samples.

Results

The median neuroindex (spike frequency × % responding neurons in Hz × %) was significantly (all p < 0.001) increased for IBS-D (duodenum and colon, proximal and distal each, 49.3; 50.5; 63.7; 71.9, respectively) compared to non-IBS/non-MC (duodenum and colon, proximal and distal each, 8.7; 4.9; 6.9; 5.4, respectively) or MC-D supernatants (duodenum and colon, proximal and distal each, 9.4; 11.9; 0.0; 7.9, respectively). Nerve activation by MC-D and non-IBS/non-MC supernatants was comparable (p>0.05). Mast cell density or histamine concentrations were not different between IBS-D, MC-D, and non-IBS/non-MC samples.

Discussion

Nerve activation by biopsy supernatants is an IBS hallmark that occurs throughout the gut, unrelated to mast cell density or histamine concentration. At least as important is our finding that GI complaints per se were not associated with biopsy supernatant-induced nerve activation, which further stresses the relevance of altered nerve behavior in IBS.

Hematological Neoplasms with Eosinophilia

Eosinophils in peripheral blood account for 0.3-5% of leukocytes, which is equivalent to 0.05-0.5 × 109/L. A count above 0.5 × 109/L is considered to indicate eosinophilia, while a count equal to or above 1.5 × 109/L is defined as hypereosinophilia. In bone marrow aspirate, eosinophilia is considered when eosinophils make up more than 6% of the total nuclear cells. In daily clinical practice, the most common causes of reactive eosinophilia are non-hematologic, whether they are non-neoplastic (allergic diseases, drugs, infections, or immunological diseases) or neoplastic (solid tumors). Eosinophilia that is associated with a hematological malignancy may be reactive or secondary to the production of eosinophilopoietic cytokines, and this is mainly seen in lymphoid neoplasms (Hodgkin lymphoma, mature T-cell neoplasms, lymphocytic variant of hypereosinophilic syndrome, and B-acute lymphoblastic leukemia/lymphoma). Eosinophilia that is associated with a hematological malignancy may also be neoplastic or primary, derived from the malignant clone, usually in myeloid neoplasms or with its origin in stem cells (myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions, acute myeloid leukemia with core binding factor translocations, mastocytosis, myeloproliferative neoplasms, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic neoplasms). There are no concrete data in standardized cytological and cytometric procedures that could predict whether eosinophilia is reactive or clonal. The verification is usually indirect, based on the categorization of the accompanying hematologic malignancy. This review focuses on the broad differential diagnosis of hematological malignancies with eosinophilia.

Approach to Idiopathic Anaphylaxis in Adolescents

Anaphylaxis is a potentially-life threatening condition. Adolescents are particularly vulnerable due to increased risk-taking behaviors, poor disease management, and minimized perception of risk. Although most anaphylaxis can be attributed to food, drug, or venom allergy via a detailed history and confirmatory studies, in nearly 1 in 5 cases, the cause may not be obvious. Clinical differentials including rare allergens, cofactors, mast-cell disorders, and mimic disorders can increase the likelihood of discovering of the cause of anaphylaxis.

Guide to the Diagnosis of Myeloid Neoplasms: A Bone Marrow Pathology Group Approach

OBJECTIVES

The practicing pathologist is challenged by the ever-increasing diagnostic complexity of myeloid neoplasms. This guide is intended to provide a general roadmap from initial case detection, often triggered by complete blood count results with subsequent blood smear review, to final diagnosis.

METHODS

The integration of hematologic, morphologic, immunophenotypic, and genetic features into routine practice is standard of care. The requirement for molecular genetic testing has increased along with the complexity of test types, the utility of different testing modalities in identifying key gene mutations, and the sensitivity and turnaround time for various assays.

RESULTS

Classification systems for myeloid neoplasms have evolved to achieve the goal of providing a pathology diagnosis that enhances patient care, outcome prediction, and treatment options for individual patients and is formulated, endorsed, and adopted by hematologists/oncologists.

CONCLUSIONS

This guide provides diagnostic strategies for all myeloid neoplasm subtypes. Special considerations are provided for each category of testing and neoplasm category, along with classification information, genetic testing requirements, interpretation information, and case reporting recommendations based on the experience of 11 Bone Marrow Pathology Group members.

Bone and Cytokine Markers Associated With Bone Disease in Systemic Mastocytosis

BACKGROUND

Mastocytosis encompasses a heterogeneous group of diseases characterized by tissue accumulation of clonal mast cells, which frequently includes bone involvement. Several cytokines have been shown to play a role in the pathogenesis of bone mass loss in systemic mastocytosis (SM), but their role in SM-associated osteosclerosis remains unknown.

OBJECTIVE

To investigate the potential association between cytokine and bone remodeling markers with bone disease in SM, aiming at identifying biomarker profiles associated with bone loss and/or osteosclerosis.

METHODS

A total of 120 adult patients with SM, divided into 3 age and sex-matched groups according to their bone status were studied: (1) healthy bone (n = 46), (2) significant bone loss (n = 47), and (3) diffuse bone sclerosis (n = 27). Plasma levels of cytokines and serum baseline tryptase and bone turnover marker levels were measured at diagnosis.

RESULTS

Bone loss was associated with significantly higher levels of serum baseline tryptase (P = .01), IFN-γ (P = .05), IL-1β (P = .05), and IL-6 (P = .05) versus those found in patients with healthy bone. In contrast, patients with diffuse bone sclerosis showed significantly higher levels of serum baseline tryptase (P < .001), C-terminal telopeptide (P < .001), amino-terminal propeptide of type I procollagen (P < .001), osteocalcin (P < .001), bone alkaline phosphatase (P < .001), osteopontin (P < .01), and the C-C Motif Chemokine Ligand 5/RANTES chemokine (P = .01), together with lower IFN-γ (P = .03) and RANK-ligand (P = .04) plasma levels versus healthy bone cases.

CONCLUSIONS

SM with bone mass loss is associated with a proinflammatory cytokine profile in plasma, whereas diffuse bone sclerosis shows increased serum/plasma levels of biomarkers related to bone formation and turnover, in association with an immunosuppressive cytokine secretion profile.

Mast cell ontogeny: From fetal development to life-long health and disease

Mast cells (MCs) are evolutionarily ancient innate immune cells with important roles in protective immunity against bacteria, parasites, and venomous animals. They can be found in most organs of the body, where they also contribute to normal tissue functioning, for example by engaging in crosstalk with nerves. Despite this, they are most widely known for their detrimental roles in allergy, anaphylaxis, and atopic disease. Just like macrophages, mast cells were conventionally thought to originate from the bone marrow. However, they are already present in fetal tissues before the onset of bone marrow hematopoiesis, questioning this dogma. In recent years, our view of myeloid cell ontogeny has been revised. We now know that the first mast cells originate from progenitors made in the extra-embryonic yolk sac, and later get supplemented with mast cells produced from subsequent waves of hematopoiesis. In most connective tissues, sizeable populations of fetal-derived mast cells persist into adulthood, where they self-maintain largely independently from the bone marrow. These developmental origins are highly reminiscent of macrophages, which are known to have critical functions in development. Mast cells too may thus support healthy development. Their fetal origins and longevity also make mast cells susceptible to genetic and environmental perturbations, which may render them pathological. Here, we review our current understanding of mast cell biology from a developmental perspective. We first summarize how mast cell populations are established from distinct hematopoietic progenitor waves, and how they are subsequently maintained throughout life. We then discuss what functions mast cells may normally have at early life stages, and how they may be co-opted to cause, worsen, or increase susceptibility to disease.

Mast Cell Leukemia: An Update with a Practical Review

Mast cell leukemia (MCL) is the leukemic form of SM with at least 20% mostly immature mast cells on bone marrow aspirate. MCL may develop de novo, in the absence of a prior SM, or it may represent a progression from a previous SM. MCL may be sub-divided into the more frequent, aggressive acute form with signs of organ damage (C-findings) and the chronic form lacking C-findings and presenting a more stable course, although over time, progression to acute MCL is common. The 2022 WHO subtype of MCL with an associated hematological neoplasm was renamed MCL with an associated myeloid neoplasm in the 2022 International Consensus Classification (ICC). The relevance of the distinction between the leukemic and aleukemic forms based on the percentage of circulating mast cells is a matter of debate. The current knowledge on MCL is restricted mainly to single reports or case series with a limited number of larger studies. Our aim is to provide a comprehensive overview of this rare disease in terms of clinical manifestations, morphology, phenotype, molecular characteristics, differential diagnosis, outcome and treatment. A general overview on mastocytosis is also included.

Systemic Lymphadenopathic Mastocytosis with Eosinophilia

Systemic mastocytosis is a neoplastic proliferation of mast cells that most frequently involves cutaneous sites. Mastocytosis involves various extracutaneous sites, but the lymph node is rare. We present an interesting image of systemic mastocytosis in the lymph node with marked eosinophilia. It is a rare subtype of systemic mastocytosis requiring high suspicion levels for the correct diagnosis.