Advances in the understanding and clinical management of mastocytosis and clonal mast cell activation syndromes

Clonal mast cell activation syndromes and indolent systemic mastocytosis without skin involvement are two emerging entities that sometimes might be clinically difficult to distinguish, and they involve a great challenge for the physician from both a diagnostic and a therapeutic point of view. Furthermore, final diagnosis of both entities requires a bone marrow study; it is recommended that this be done in reference centers. In this article, we address the current consensus and guidelines for the suspicion, diagnosis, classification, treatment, and management of these two entities.

Mast cells (MCs) are a key structural and functional component of the immune system and play a key role in inflammatory reactions, and at the same time they are the main effector cells in allergic processes 1-3 . MC disorders might present with a great variety of clinical symptoms or signs, such as skin involvement, which might lead to suspicion of the disease. Nevertheless, among other MC disorders, two entities frequently represent a diagnostic and therapeutic challenge in routine clinical practice in allergy: (i) indolent systemic mastocytosis presenting without skin involvement (ISMs − ) and (ii) clonal mast cell activation syndrome (c-MCAS). Of note, both entities are closely related to anaphylaxis, and their diagnosis requires specific techniques. Here, we review the current consensus and guidelines for the diagnosis, classification, treatment, and management of these two entities.

Mastocytosis and other mast cell disorders: definition and classification Mastocytosis
The term systemic mastocytosis (SM) is used to define a heterogeneous group of rare diseases characterized by the presence of abnormal MCs in various organs and tissues 4 . Two critical biological findings which are linked to the pathogenesis of the disease have been described: (i) activating somatic mutations in the KIT gene (usually the KIT Asp816Val D816V mutation) and the presence of an aberrant immunophenotype associated with the expression of CD25 on (bone marrow [BM]) clonal MCs. The current World Health Organization (WHO) classification of the disease includes up to seven distinct categories that meet the diagnostic criteria for mastocytosis (Table 1). However, the development of new, more sensitive and specific methods, such as multi-parameter flow cytometry and highly sensitive polymerase chain reaction (PCR)based techniques for the detection of aberrant MCs present at very low frequencies 5-8 and the study of the KIT mutation in purified cells 9 or blood 10-12 or both, have led to an unprecedentedly increased rate of detection of phenotypically aberrant and KIT mutated MCs in BM and peripheral blood, pointing out not only the potential need to revise current diagnostic and classification criteria to recognize new entities with very low tumor burden associated with lifethreatening symptoms such as anaphylaxis but also a potential impact on the long-term prognosis of patients with indolent forms of the disease.

Indolent systemic mastocytosis
Based on previous reports in the largest series of patients, indolent systemic mastocytosis (ISM) comprises around 80% of all SM cases 13 . Among them, around 20% of patients lack skin lesions at presentation (ISMs − ) 14 . Despite the great relevance and efficiency of the WHO criteria for the diagnosis of SM, in ISMs − , MCs represent only a very small proportion of all nucleated BM cells (usually fewer than 10 −3 BM MCs, as assessed by flow cytometry) 15 , and BM MC aggregates are frequently (around 30% of cases) not found in such patients with SM 15 , in the absence of significantly increased serum baseline tryptase levels (<20 μg/L). Consequently, the use of highly sensitive and specific methodological approaches to the study of BM MCs becomes critical in order to avoid a misdiagnosis in patients presenting with low tumor burden 16 .

Mast cell activation syndromes
The term MC activation syndrome (MCAS) encompasses a heterogeneous group of diseases which are characterized by systemic symptoms secondary to MC mediator release that (i) might or might not have a known trigger, (ii) might or might not be associated with immunoglobulin E (IgE)-specific antibodies in response to that trigger, (iii) are associated with normal or elevated baseline tryptase levels, and (iv) do not show skin lesions of mastocytosis 17 . In Table 2, the most frequent and relevant clinical symptoms suggesting an underlying MCAS are listed, and Table 3 depicts the diagnostic criteria for MCAS.
The current classification of MCAS is shown in Table 4. Based on the experience of the Spanish Network of Mastocytosis (REMA), the most relevant objective criteria to subclassify MCAS rely on the presence versus absence of clonal MCs as defined by the expression of CD25 (for example, CD25 + versus CD25 − ) or a KIT mutation, particularly KIT D816V, or both. When MCAS diagnostic criteria are fulfilled but there is no evidence of clonality, non-clonal-MCAS should be considered and co-existence of allergy or other underlying diseases should be confirmed or ruled out 18 .

Clinical features of patients with primary mast cell activation syndromes: mastocytosis and clonal mast cell activation syndrome
Symptoms due to the release of MC mediators upon MC activation might be present in every category of MCAS, including mild, severe, or even life-threatening symptoms such as pruritus, flushing, gastrointestinal complaints (abdominal pain or diarrhea), cognitive symptoms, and even anaphylaxis 19,20 .
In indolent systemic mastocytosis with skin lesions (ISMs + ), MC activation symptoms are typically heterogeneous and might vary from recurrent anaphylaxis 21,22 to occasional symptoms triggered by a varying number of different stimuli linked to the MC mediator release episodes (Table 5). In turn, ISMs − are frequently characterized by serious episodes of MC mediator release triggered by different factors-for example, mainly insect sting, drugs, and foods-or they might be idiopathic 23 ; in both situations, such episodes are significantly associated with the presence of anaphylaxis with cardiovascular or vascular collapse symptoms, in the absence of both urticaria and angioedema 23 . Early studies by the REMA have demonstrated that ISMs − patients present unique features that distinguish them from ISMs + cases 23 : (i) a higher prevalence of men versus women, (ii) a lower frequency of symptoms outside of acute episodes, (iii) lower BM MC burden, and (iv) the presence of the KIT mutation usually restricted to the MC lineage 23,24 . Of note, such unique disease features are even more characteristic within ISMs − patients whose symptoms are triggered exclusively by insect stings, whereas ISMs − patients with other triggering factors show clinical characteristics at presentation which are more similar to those of ISMs + cases 24 .

Diagnosis of primary mast cell activation syndromes
The final diagnosis of SM and c-MCAS systematically requires a BM study for the evaluation of all disease characteristics used for the diagnosis of SM, such as BM MC cytology 25 , histology and immunochemistry 26,27 , flow cytometry immunophenotyping using specific gating strategies for the detection of BM MCs present at low frequencies 5-7,28 , and the study of KIT mutation in purified MCs 9,29 , together with a detailed clinical work-up. Usually, these studies are available only in reference centers, and therefore either the patient or the samples should be referred.
The European Competence Network on Mastocytosis recommends using the REMA score ( Figure 1) as a clinically useful tool to predict for the presence of clonal MCs prior to a BM study 23 ; the REMA score is based only on demographic data (gender), the symptoms and signs observed during the acute episodes, and serum baseline tryptase levels. A REMA score of at least 2 predicts with a high sensitivity and specificity for ISMs − (or c-MCAS), whereas a REMA score of less than 2 usually indicates non-clonal disease. Whether "non-clonal disease" means a true absence of any mutations whatsoever or simply the absence of clonality currently detectable in the clinical laboratory remains unclear. The REMA score is a particularly helpful tool since (i) it is based on clinical data and can be used on a routine clinical basis, (ii) it is associated with rather low costs, and (iii) it avoids unnecessary BM studies.

Approach to diagnosis of primary mast cell activation syndromes
Whenever a patient is suspected of having a primary MCAS (either mastocytosis without skin involvement or c-MCAS), the use of the REMA score mentioned above is recommended 23 as an initial screening measure ( Figure 2). Patients with a REMA score of less than 2 will have a low probability of presenting clonal MCs; therefore, in such cases, it is usually not necessary to conduct additional studies other than controlling the symptoms with adequate medication and scheduling periodic follow-up visits. In contrast, patients with a REMA score of at least 2 have a high probability of presenting mastocytosis or c-MCAS. Thus, in the latter patients, it is recommended that appropriate treatment be started and that the patient be evaluated in detail for major complications of the disease, such as hepatomegaly/splenomegaly, osteopenia, and osteoporosis.
To establish the most appropriate timing to perform a BM biopsy to arrive at a firm diagnosis, it might be useful to evaluate the presence of the D816V KIT mutation in peripheral blood (it is typically positive in approximately 80% of primary MCAS cases 12 ) and to monitor baseline serum tryptase levels and wait until they rise above 20 ng/mL, at which point the probability of obtaining a BM sample that is suitable for demonstrating BM involvement increases.
Independently of the time at which the BM study is scheduled, it is recommended that BM MCs be purified prior to molecular studies, which have demonstrated greater sensitivity for the detection of KIT mutations 9,23,30 . As an exception, BM biopsy studies might be performed in suspicious patients with a REMA score of at least 2 and baseline tryptase levels of less than 20 ng/mL who presented with anaphylaxis following a hymenoptera sting and who are candidates for immunotherapy, since patients with mastocytosis or c-MCAS (or both) have a greater risk of having adverse reactions

Treatment of primary mast cell activation syndromes
Patients with c-MCAS or mastocytosis may present symptoms due to the release of MC mediators, associated or not with symptoms related to tissue infiltration by clonal MC (more frequent among aggressive forms of mastocytosis). At present, there is no curative therapy for mastocytosis and the treatment strategies are focused on controlling the frequency and intensity of symptoms secondary to the release of MC mediators; this includes both strict avoidance of triggers (Table 5) and the anti-mediator therapy carefully selected on the basis of the intensity or severity (or both) of the signs and symptoms linked with the activation of MCs. Cytoreductive therapy and targeted therapies with tyrosine kinase inhibitors might be needed in selected cases presenting with elevated MC burden or aggressive and poor prognosis-associated forms of SM or patients who are unresponsive to conventional anti-mediator therapy 32 , but these therapies will not be discussed further in this review.

Avoidance of triggers and counseling
Careful counseling must be given to patients, caretakers, and their physicians to avoid triggers that evoke MC mediator release 32,33 (Table 5). In addition, detailed training to manage MC activationassociated episodes should be provided to patients as well as their physicians.
Anti-mediator therapy for primary mast cell activation disorders Anti-mediator therapy for primary MCAS aims at inhibiting production, interfering with release, blocking the specific receptors, or antagonizing the effects of MC mediators. It is used both to treat and to prevent acute and chronic MC mediator release-associated symptoms 32 . The specific anti-mediator administration schedule (on demand or continuous administration) should be carefully selected for the individual patient on the basis of the intensity or severity (or both) of the signs and symptoms observed during the most severe acute episodes or anaphylaxis as well as the MC mediator-related symptoms recorded between acute episodes 19 .

Histamine receptor blockers.
The biologic effects of histamine released from MCs, through its binding to histamine receptors (mainly H1 and H2 histamine receptors), include (i) increased vascular permeability, (ii) vasodilatation, (iii) contraction of nonvascular smooth muscle, (iv) increase of exocrine gland secretion, and (v) stimulation of the peripheral nervous system; these effects result in symptoms such as pruritus, urticaria, edema, bronchoconstriction, gastric hypersecretion, abdominal cramping, diarrhea, headache, hypotension, and anaphylaxis 34 . The effects of H1 blockers are described in Table 6.
Some H1 antihistamines such as desloratadine and ketotifen have MC-stabilizing properties, and therefore they might also decrease the release of MC mediators 35,36 . Ketotifen has been reported to be effective in the treatment of bone disease as well as cutaneous, gastrointestinal, and neuropsychiatric symptoms in mastocytosis 37-39 . Patients with associated depression might benefit from doxepin because of its effects as an antidepressant and H1 histamine blocker 40 . Rupatadine also has an antagonistic effect against platelet-activating factor, a lipid-derived mediator which is newly synthesized and released by MCs upon their activation, resulting in hypotensive episodes and flushing 41 .
Some patients with mastocytosis require a combination of different H1 blockers to achieve a good control of symptoms 32 . The use of non-sedating H1 blockers is recommended for patients who require daily maintenance therapy; in turn, the use of sedating H1 antihistamines that have a fast-acting effect which makes them suitable drugs to treat acute MC mediator release episodes, frequently in association with corticosteroids or epinephrine or both, and also to prevent MC degranulation during risk situations might be administered at night or on demand 32,42 . Cromolyn sodium. Cromolyn sodium is an MC stabilizer that has been proven to inhibit MC activation and MC release of mediators both in vitro and in vivo despite its limited systemic absorption following ingestion; this suggests an inability of the drug to enter cells and the need for a potential interaction with an as-yet-unidentified cell surface receptor to induce its biologic activity 43 (Table 6). Side effects include headache, sleepiness, irritability, abdominal pain, diarrhea, and constipation, most of which are attenuated by progressive introduction of the drug 44 . In addition, 0.21-4% cromolyn sodium water-soluble creams, as well as aqueous-based skin lotion, may be effective at improving cutaneous symptoms (for example, pruritus and flaring of lesions) 32,34,45,46 .
Antagonists of arachidonic acid metabolites. MCs synthesize de novo mediators and release arachidonic acid metabolites through the action of lipoxygenase and cyclooxygenase enzymes; thus, they produce leukotrienes and prostaglandins (PGs), respectively. Prostaglandin D2 is generated almost entirely by MCs and rapidly converted into active metabolites of prolonged activity rather than the parent compound, such as α11β-PGF2; the two PGs share a biological activity and induce bronchoconstriction, increase of vascular permeability, and vasodilatation, and at the same time they have chemoattractive properties for eosinophils, basophils, and Th2 lymphocytes 47 and are involved in the development of flushing and possibly also hypotensive episodes in patients with c-MCAS and mastocytosis 48 (Table 6).
Aspirin. Non-steroidal anti-inflammatory drugs, especially aspirin, can also inhibit the activation of MCs and their degranulation in some patients with mastocytosis; therefore, this therapy is recommended in patients with known tolerance to these drugs.
Glucocorticoids. The mechanism of action and the effects of glucocorticoids are shown in Table 6. Short cycles of low doses of either prednisone (0.3 mg/kg per day) or oral budesonide (0.1 mg/kg per day) may improve abdominal pain refractory to treatment with cromolyn. Topical corticosteroids may be used for patients who present with skin symptoms, especially among cases with limited skin involvement; however, evidence supporting their topical use is either anecdotal or based on small series of patients 49 .
Anti-immunoglobulin E therapy. Successful anti-IgE therapy has been documented in some conditions such as severe persistent allergic asthma, chronic urticaria 50 , idiopathic anaphylaxis 51 , and mastocytosis (Table 6). Until more information based on clinical trials becomes available, omalizumab in MC diseases should be restricted to selected patients with severe symptoms which have proven unresponsive to intensive anti-mediator therapy 33 .

Hymenoptera venom immunotherapy.
Specific VIT is recommended for mastocytosis and c-MCAS patients with IgE-mediated anaphylaxis to hymenoptera venom; however, it should be managed as a high-risk procedure. VIT has proven effective and safe in these patients 52-56 ; it has a rate of protection from re-stings of 86%, and the frequency of systemic reactions to VIT ranges from 5-25% 14,57 , most of which (75%) were associated with rush inductions (versus 25% using conventional induction) 57 . Whenever adverse reactions to VIT prevent the protective maintenance dose of 100 μg per month from being reached, prophylactic anti-mediator therapy, changes in the extract, and administration of omalizumab therapy may be useful 57,58 . Furthermore, in patients who present anaphylaxis after re-sting, despite the administration of a standard maintenance dose, it is recommended that maintenance doses be increased to 200 μg 57 . An extended maintenance administration (even lifelong) is proposed 14,57 since cases presenting with fatal reactions after discontinuation of VIT have been described 59,60 .
Other therapies Ultraviolet irradiation. In vitro studies demonstrated that longwave ultraviolet radiation (psoralen plus ultraviolet A or ultraviolet A alone) and narrowband ultraviolet B phototherapy irradiation interfere with the release of histamine from skin-activated MCs and induce MC apoptosis 61-63 ; all of them were employed as a second-line therapy to treat cutaneous symptoms (pruritus, whealing, and flare reactions) in patients with typical mastocytosis skin lesions who were not responsive to first-line therapies with antimediator drugs 40,64-66 . There is no information regarding whether these therapies are useful in ISMs − and c-MCAS cases. In addition, fading of hyperpigmented skin lesions (frequently temporary) can be observed in some cases. Furthermore, responses of lifethreatening MC mediator release episodes in children with bullous diffuse cutaneous mastocytosis have been reported 67 .

Treatment of bone mass loss.
This is a frequent finding in both ISMs + and advanced mastocytosis but is less frequently observed among primary MCAS cases, particularly ISMs − and c-MCAS. Fractures are usually developed as a consequence of severe osteoporosis or large osteolytic lesions. Local MC infiltration and disturbances in bone remodeling which are due to the release of MC mediators such as interleukin-6 (IL-6), histamine, heparin, receptor activator of nuclear factor kappa-B ligand, osteoprotegerin, or sclerostin are involved in the pathogenesis of bone manifestations in mastocytosis 68,69 . Calcium and vitamin D supplements, combined with bisphosphonates, are usually the first choice for osteopenia and osteoporosis, respectively 70-72 . Other therapies such as estrogen replacement in postmenopausal women and denosumab or interferon-alpha 73,74 in patients with severe osteoporosis at risk of pathologic bone fractures unresponsive to conventional treatments might also be considered. Regarding peptidergic/peptidomimetic drugs such as teriparatide, caution is urged since these drugs have been associated with driving MC activation via MRGPRX2 75 .
Selective serotonin reuptake inhibitors. MCs contain numerous mediators, including neurotransmitters (for example, serotonin), cytokines, and chemokines, that play a role in stress response, behavior, and emotion regulation 76, 77 . Furthermore, the elevation of the circulating levels of tumor necrosis factor-alpha and IL-6 with, for example, endotoxin leads to depressive symptoms, and it has been previously described that some selective serotonin reuptake inhibitors (SSRIs) such as citalopram can reduce endotoxin-induced symptoms 78 . Based on this information, together with their good tolerability profile, SSRIs have emerged as an option to improve symptoms of depression in patients with mastocytosis 79 .

Other conditions
With regard to pregnancy, two different series of women with mastocytosis have been reported in the literature 80,81 . The larger series 81 included 45 pregnancies and deliveries in women with nonaggressive categories of the disease. Based on their results, anti-mediator therapy (dexchlorpheniramine, loratadine, cetirizine, ranitidine, oral corticosteroids, and adrenaline), if required during pregnancy, as well as systematic administration of prophylactic anti-mediator therapy at the beginning of labor based on drugs with a good well-known safety profile, is recommended 81 .

Competing interests
The authors declare that they have no competing interests. Editorial Note on the Review Process are commissioned from members of the prestigious and are edited as a F1000 Faculty Reviews F1000 Faculty service to readers. In order to make these reviews as comprehensive and accessible as possible, the referees provide input before publication and only the final, revised version is published. The referees who approved the final version are listed with their names and affiliations but without their reports on earlier versions (any comments will already have been addressed in the published version).

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