Residency · Residency · Allergy Immunology

Mast Cell Disorders - Mastocytosis and MCAS

Overview

Mast cell disorders encompass a spectrum of conditions unified by abnormal mast cell biology, including mastocytosis (clonal mast cell proliferation with organ infiltration), mast cell activation syndrome (MCAS, in which mast cells activate inappropriately without clonal proliferation), and hereditary alpha-tryptasemia (HaT, a genetic condition involving extra copies of the TPSAB1 gene encoding alpha-tryptase). The shared clinical feature across these entities is episodic mast cell activation manifesting as flushing, pruritus, hypotension, and gastrointestinal symptoms. Tryptase, a serine protease stored preformed in mast cell granules, serves as the key biomarker that links these conditions and guides diagnostic evaluation.

Mastocytosis

Classification (WHO 2022)

The World Health Organization 2022 classification divides mastocytosis into cutaneous and systemic forms with distinct clinical implications and prognostic trajectories.

Cutaneous mastocytosis (CM) is confined to the skin and occurs primarily in the pediatric population. The most common variant is urticaria pigmentosa, also termed maculopapular cutaneous mastocytosis, which presents as red-brown macules and papules that demonstrate the Darier sign -- the development of a wheal and flare response upon stroking the lesion due to local mast cell degranulation. Diffuse cutaneous mastocytosis involves the entire skin surface with a characteristic peau d'orange (orange-peel) texture and represents the most severe cutaneous form. Solitary mastocytoma presents as a single or small number of lesions in young children. An important clinical point is that the majority of childhood cutaneous mastocytosis resolves spontaneously by adolescence.

Systemic mastocytosis (SM) is defined by mast cell infiltration of extracutaneous organs, with the bone marrow being the most commonly affected site. Indolent systemic mastocytosis (ISM) is the most common adult form, comprising approximately 65% of SM cases, and carries a near-normal life expectancy. Smoldering systemic mastocytosis (SSM) is characterized by a higher mast cell burden, meeting two or more B-findings, but without evidence of organ damage. Systemic mastocytosis with an associated hematologic neoplasm (SM-AHN) describes the coexistence of SM with myelodysplastic syndrome, myeloproliferative neoplasm, chronic myelomonocytic leukemia, or other hematologic malignancies. Aggressive systemic mastocytosis (ASM) is defined by the presence of organ damage, termed C-findings, and carries a poor prognosis. Mast cell leukemia (MCL), defined by 20% or more mast cells in the bone marrow aspirate, is exceedingly rare and associated with the worst prognosis. Mast cell sarcoma, a localized destructive mast cell tumor, is an extremely rare entity.

WHO Diagnostic Criteria for Systemic Mastocytosis

SM Subtypes and Prognosis

The diagnosis of systemic mastocytosis requires meeting either the major criterion plus one minor criterion, or three minor criteria. The major criterion is the identification of multifocal dense aggregates containing 15 or more mast cells in bone marrow biopsy or in biopsy of another extracutaneous organ, identified by tryptase immunohistochemistry. The four minor criteria are: first, greater than 25% of mast cells in the biopsy displaying spindle-shaped or atypical morphology; second, detection of the KIT D816V mutation in bone marrow, peripheral blood, or other extracutaneous tissue; third, aberrant expression of CD25 and/or CD2 on mast cells by flow cytometry, which are markers not normally expressed by mast cells; and fourth, a persistently elevated baseline serum tryptase exceeding 20 ng/mL.

B-Findings and C-Findings

B-findings reflect a high mast cell burden without end-organ damage and include bone marrow biopsy showing greater than 30% mast cell infiltration, serum tryptase exceeding 200 ng/mL, hepatomegaly, splenomegaly, or lymphadenopathy without organ dysfunction, and dysplasia or myeloproliferation in non-mast cell lineages. The presence of two or more B-findings in the absence of C-findings defines smoldering SM.

C-findings represent mast cell-mediated organ damage and define aggressive SM. These include cytopenia with an absolute neutrophil count below 1,000, hemoglobin below 10 g/dL, or platelet count below 100,000; hepatomegaly with liver dysfunction including ascites, elevated liver enzymes, or portal hypertension; splenomegaly with hypersplenism; malabsorption with weight loss exceeding 10% over six months; pathologic fractures resulting from osteolysis; and large osteolytic lesions on skeletal survey.

KIT D816V Mutation

The KIT D816V point mutation is present in over 90% of adult systemic mastocytosis and represents a gain-of-function mutation in the KIT receptor tyrosine kinase. This mutation results in constitutive, ligand-independent activation of the KIT signaling pathway, driving autonomous mast cell proliferation and survival. Detection requires highly sensitive molecular techniques including allele-specific PCR, digital droplet PCR, or next-generation sequencing, as the mutant allele burden is often very low in peripheral blood samples. A critically important therapeutic implication of the D816V mutation is that it confers resistance to imatinib, a first-generation tyrosine kinase inhibitor that effectively targets wild-type KIT. This resistance drove the development of newer agents, specifically midostaurin and avapritinib, which retain activity against the D816V-mutated KIT protein.

<image>A comprehensive diagnostic pathway for systemic mastocytosis. Starting point: "Suspected mastocytosis: unexplained anaphylaxis, flushing, urticaria pigmentosa, elevated baseline tryptase." Step 1: Measure baseline serum tryptase (fasting, no recent mast cell activation). If >20 ng/mL (or >11.4 ng/mL with clinical suspicion): proceed to bone marrow biopsy and aspirate. Bone marrow evaluation panel: (1) H&E and tryptase immunohistochemistry looking for mast cell aggregates (>=15 cells), (2) Mast cell immunophenotyping by flow cytometry (CD25, CD2 expression on KIT+ mast cells), (3) KIT D816V mutation analysis (sensitive PCR assay), (4) Cytogenetics and additional molecular studies. WHO diagnostic criteria shown as a checklist with major + 1 minor or 3 minor criteria. Classification outcome boxes: ISM, SSM (B-findings), SM-AHN, ASM (C-findings), MCL. Side panel: hereditary alpha-tryptasemia screening (TPSAB1 copy number) if tryptase elevated but no clonal mast cell disease found.</image>

Treatment of Mastocytosis

Symptom Management (All SM Subtypes)

Symptomatic management of mast cell mediator-related symptoms is appropriate for all subtypes of systemic mastocytosis. H1 antihistamines, such as cetirizine or fexofenadine, are administered at standing doses and may require two to four times the standard dose for adequate symptom control. H2 antihistamines, particularly famotidine, serve the dual purpose of gastric acid reduction and amelioration of gastrointestinal symptoms. Cromolyn sodium, administered orally at 200 mg four times daily, acts as a mast cell stabilizer with poor systemic bioavailability but local efficacy within the gastrointestinal tract, making it particularly useful for abdominal symptoms. Leukotriene receptor antagonists such as montelukast can help control flushing and abdominal cramping by blocking the effects of cysteinyl leukotrienes released during mast cell degranulation.

All patients with systemic mastocytosis should carry epinephrine auto-injectors due to their significantly increased risk of anaphylaxis. For patients with venom allergy and concurrent SM, lifelong venom immunotherapy is recommended without discontinuation, as the risk of fatal anaphylaxis to re-stings remains elevated indefinitely. Aspirin may reduce flushing in selected patients through inhibition of prostaglandin D2, but it should only be administered after confirming NSAID tolerance, as some patients with mast cell disease experience mast cell-mediated reactions to nonsteroidal anti-inflammatory drugs. Omalizumab has been reported in case series to reduce the frequency of anaphylactic episodes in SM, though the mechanism remains unclear given that mast cell activation in SM is not primarily IgE-driven.

Cytoreductive Agents for Advanced SM

Cytoreductive Therapy (Advanced SM)

Midostaurin (Rydapt), a multikinase inhibitor with activity against KIT D816V, was the first FDA-approved therapy for advanced systemic mastocytosis, encompassing ASM, SM-AHN, and MCL. Administered at 100 mg twice daily orally, midostaurin achieves an overall response rate of approximately 60%, including improvements in symptoms, reductions in serum tryptase, and reversal of organ damage. The principal side effects are gastrointestinal, including nausea, vomiting, and diarrhea, which can be mitigated with antiemetics and administration with food.

Avapritinib (Ayvakit) represents a major therapeutic advance as a highly selective and potent inhibitor of KIT D816V. For advanced SM (ASM, SM-AHN, MCL), avapritinib is administered at 200 mg daily with an overall response rate of approximately 75%, making it the most effective cytoreductive agent currently available for these indications. Notably, avapritinib has also been approved for indolent SM at a dose of 25 mg daily, based on the PIONEER trial. This landmark study demonstrated significant reductions in the Total Symptom Score, serum tryptase levels, and skin lesion burden in patients with ISM, establishing avapritinib as the first approved targeted therapy for indolent disease. Important side effects include periorbital edema, cognitive effects (affecting memory and attention), and, rarely, intracranial hemorrhage, necessitating careful monitoring. Cladribine, a purine analog, provides cytoreductive activity for advanced SM refractory to kinase inhibitors. Interferon-alpha, while historically used, has been largely supplanted by kinase inhibitors due to its substantial side effect burden. HSCT may be considered for aggressive or refractory SM-AHN or MCL.

Anaphylaxis in Mastocytosis

Patients with systemic mastocytosis face a significantly elevated risk of severe and potentially fatal anaphylaxis. Hymenoptera venom stings are a particularly important trigger, and venom-induced anaphylaxis may, in fact, be the presenting manifestation that leads to the diagnosis of underlying SM. This observation mandates that any patient presenting with severe or recurrent unexplained anaphylaxis undergo measurement of baseline serum tryptase and consideration of a comprehensive SM workup. An elevated baseline tryptase level exceeding 11.4 ng/mL independently increases the severity of anaphylactic reactions, even in patients without mastocytosis.

The perioperative period represents a time of heightened risk for mast cell-mediated reactions. Premedication with H1 and H2 antihistamines, and in some protocols corticosteroids, is recommended before surgical procedures and other interventions that may trigger mast cell degranulation. Venom immunotherapy in patients with SM should be continued lifelong without discontinuation, as the protection rate of 90 to 95% achieved during active therapy may decline upon cessation, and the consequences of unprotected re-sting anaphylaxis in this population can be fatal.

Hereditary Alpha-Tryptasemia (HaT)

Overview

Hereditary alpha-tryptasemia is a recently characterized genetic condition resulting from extra germline copies of the TPSAB1 gene, which encodes alpha-tryptase. This condition is surprisingly prevalent, affecting approximately 5 to 6% of the general population, making it a common genetic trait rather than a rare disorder. HaT follows autosomal dominant inheritance, and each additional copy of TPSAB1 raises the baseline serum tryptase level by approximately 8 ng/mL.

The clinical phenotype of HaT extends beyond simply elevated tryptase levels. Associated features include increased severity of anaphylactic reactions, autonomic dysfunction, gastrointestinal dysmotility, connective tissue symptoms including joint hypermobility, and episodic flushing. Critically, HaT is not a clonal mast cell disease, and the bone marrow is histologically normal. This distinction is clinically important because many patients referred for evaluation of elevated baseline tryptase who do not meet criteria for systemic mastocytosis will be found to have HaT upon genetic testing. Diagnosis is established by TPSAB1 copy number assessment using droplet digital PCR, with commercial testing now available. Management is symptomatic, focusing on antihistamines and epinephrine availability for anaphylaxis; no cytoreductive therapy is indicated.

Mast Cell Activation Syndrome (MCAS)

Diagnostic Criteria (2019 Consensus)

The 2019 consensus diagnostic criteria for MCAS require that all three of the following conditions be met. First, the patient must experience episodic symptoms consistent with mast cell mediator release affecting two or more organ systems, which may include cutaneous manifestations (flushing, pruritus, urticaria, angioedema), gastrointestinal symptoms (cramping, diarrhea, nausea), cardiovascular signs (hypotension, tachycardia, syncope or presyncope), respiratory symptoms (wheezing, throat tightness), and neurologic complaints (headache, cognitive difficulty).

Second, there must be objective laboratory evidence of mast cell mediator elevation temporally related to the symptomatic episode. The most specific marker is an increase in serum tryptase of at least 20% plus 2 ng/mL above the individual's established baseline, measured during or within four hours of a symptomatic episode. Alternative mediator markers include elevations in 24-hour urine N-methylhistamine, 11-beta-prostaglandin F2-alpha, or leukotriene E4.

Third, the patient must demonstrate a clinical response to mast cell-directed therapy, including H1 and H2 antihistamines, mast cell stabilizers, or anti-leukotriene agents. The requirement for all three criteria helps ensure diagnostic specificity and prevents the overdiagnosis of MCAS, which has become an increasingly recognized concern.

Primary (mastocytosis) and secondary causes of mast cell activation (including IgE-mediated allergic reactions, drug reactions, and physical urticaria) must be excluded before a diagnosis of idiopathic MCAS is assigned.

Classification

MCAS is classified into three subtypes based on the presence or absence of clonal markers and identifiable triggers. Primary MCAS is characterized by the presence of KIT D816V positivity or aberrant CD25 expression on mast cells, but the patient does not meet the full diagnostic criteria for systemic mastocytosis. Secondary MCAS occurs when an identifiable trigger for mast cell activation is present, such as IgE-mediated allergy, drug reactions, or physical stimuli. Idiopathic MCAS is a diagnosis of exclusion, assigned when no clonal markers are found and no identifiable trigger can be identified.

Approach to Evaluation

The evaluation of suspected MCAS begins with measurement of baseline serum tryptase when the patient is asymptomatic. Event tryptase should be obtained within one to four hours of a symptomatic episode to document the temporal elevation. If the baseline tryptase is elevated, a bone marrow biopsy should be performed to evaluate for systemic mastocytosis. If the baseline tryptase is normal, TPSAB1 copy number assessment should be considered to screen for hereditary alpha-tryptasemia. Collection of 24-hour urine for mast cell mediators during a symptomatic period can provide additional objective evidence of mast cell activation. A thorough evaluation for allergic triggers, physical urticaria, and other secondary causes of mast cell activation should be performed systematically.

<image>A Venn diagram illustrating the overlap and distinctions between mastocytosis, MCAS, and hereditary alpha-tryptasemia. Three overlapping circles: (1) Mastocytosis (clonal): characterized by KIT D816V, CD25+ mast cells, BM aggregates, tryptase >20 ng/mL. (2) MCAS (non-clonal): episodic mast cell mediator release, meets consensus criteria (symptoms + mediator elevation + treatment response), no BM aggregates. (3) Hereditary alpha-tryptasemia: extra TPSAB1 copies, elevated baseline tryptase, NOT clonal, associated with dysautonomia and anaphylaxis severity. Overlap zones: primary MCAS (clonal markers but not full SM criteria) between mastocytosis and MCAS; HaT + SM can coexist (additive tryptase elevation). Center overlap: rare patients may have all three. Below: diagnostic approach flowchart starting with "Elevated baseline tryptase or recurrent mast cell activation symptoms" leading to tryptase measurement, TPSAB1 copy number, and bone marrow biopsy decision points.</image>

Key Clinical Pearls

• KIT D816V is present in >90% of adult systemic mastocytosis and confers resistance to imatinib; midostaurin and avapritinib target this mutation
• Avapritinib (PIONEER trial) is the first FDA-approved targeted therapy for indolent SM; significantly reduces symptoms, tryptase, and skin lesions
• Baseline tryptase >20 ng/mL is a minor criterion for SM; but HaT (5-6% prevalence) is a much more common cause of mildly elevated tryptase
• ALL patients with systemic mastocytosis should carry epinephrine auto-injectors regardless of anaphylaxis history
• Venom immunotherapy in SM should be lifelong (no discontinuation) due to persistent high risk of re-sting anaphylaxis
• MCAS diagnosis requires ALL three criteria: episodic multi-system symptoms + objective mediator elevation + response to mast cell therapy
• HaT is common (5-6% of general population) and should be considered in anyone with elevated baseline tryptase before pursuing invasive workup
• Bone marrow biopsy with KIT D816V testing, tryptase immunohistochemistry, and mast cell flow cytometry is the gold standard for diagnosing SM

References

1. Valent P, et al. Updated diagnostic criteria and classification of mast cell disorders: a consensus proposal. Hemasphere. 2021;5(11):e646.
2. Gotlib J, et al. Avapritinib for advanced systemic mastocytosis. N Engl J Med. 2021;384(7):618-630.
3. DeAngelo DJ, et al. Avapritinib in indolent systemic mastocytosis (PIONEER). N Engl J Med. 2021;385(17):1540-1552.
4. Lyons JJ, et al. Elevated basal serum tryptase identifies a multisystem disorder associated with increased TPSAB1 copy number. Nat Genet. 2016;48(12):1564-1569.
5. Akin C. Mast cell activation syndromes. J Allergy Clin Immunol. 2017;140(2):349-355.

Read this lecture as Markdown