Mast Cell Disease (Mastocytosis)

WHO 2022 classification, KIT D816V–driven pathogenesis, diagnosis of systemic mastocytosis, and targeted therapy with midostaurin and avapritinib

Key Points

• Mastocytosis is clonal mast cell disease driven in >95% of cases by the KIT D816V point mutation, which constitutively activates the KIT receptor tyrosine kinase independent of stem cell factor (SCF) binding.
• Systemic mastocytosis (SM) is diagnosed by bone marrow biopsy — the major criterion is multifocal dense clusters of ≥15 mast cells; at least 1 major + 1 minor criterion, or 3 minor criteria, is required for diagnosis.
• Serum tryptase >20 ng/mL is both a diagnostic minor criterion and the primary biomarker for mast cell burden; it should be measured at diagnosis, with each treatment change, and during surveillance.
• SM subtypes range from indolent (ISM — normal life expectancy) to aggressive (ASM), SM with associated hematologic neoplasm (SM-AHN), and mast cell leukemia (MCL) — accurate subclassification is essential as treatment intensity and prognosis differ dramatically.
• Avapritinib (Ayvakit), a potent selective KIT D816V inhibitor, is FDA-approved for advanced SM (ASM/SM-AHN/MCL) and for symptomatic indolent SM — it achieves responses including complete remission in advanced SM that were previously unattainable.
• All SM patients, regardless of subtype, require mediator symptom management (H1 and H2 antihistamines, cromolyn, epinephrine autoinjector for anaphylaxis) and evaluation for osteoporosis.

Classification, Pathogenesis, and Epidemiology

**WHO 2022 Classification of Mastocytosis:** *Cutaneous mastocytosis (CM):* Mast cell infiltration limited to the skin; no systemic organ involvement. Subtypes: - **Urticaria pigmentosa (UP) / Maculopapular cutaneous mastocytosis (MPCM)**: most common; tan-brown macules and papules that urticateWith stroking (Darier sign); predominant form in adults - **Diffuse cutaneous mastocytosis (DCM)**: rare; predominantly in young children; diffuse skin thickening - **Mastocytoma of skin**: solitary lesion; predominantly in children Pediatric CM: most cases regress spontaneously by adolescence; KIT…

Clinical Features and Mediator-Related Symptoms

Mastocytosis causes symptoms through two distinct mechanisms: **mediator release** (dominant in ISM) and **organ infiltration/dysfunction** (dominant in ASM/MCL). Both can coexist. **Mediator-release symptoms (can occur in all SM subtypes):** Activated mast cells release histamine, prostaglandin D2 (PGD2), leukotrienes, tryptase, heparin, chymase, and cytokines: *Skin:* - Flushing (episodic, often triggered; can be severe) - Urticaria and pruritus (especially with physical stimuli — Darier sign) - Dermatographism *Cardiovascular:* - Anaphylaxis — the most dangerous manifestation; can be…

Diagnostic Criteria and Workup

**SM diagnostic criteria (WHO 2022):** Diagnosis requires: **1 major + 1 minor** OR **3 minor criteria** *Major criterion:* - **Multifocal dense clusters of mast cells** (≥15 mast cells per aggregate) in bone marrow sections or other extracutaneous organ; confirmed by tryptase IHC or Giemsa stain *Minor criteria (4 total):* 1. **Morphology**: >25% of mast cells in BM smear or tissue sections are spindle-shaped or atypical (abnormal morphology) 2. **Immunophenotype**: mast cells in BM express CD25 and/or CD2 (aberrant co-expression not seen in normal mast cells); CD25 is the most sensitive…

Treatment — Mediator Control and Targeted Therapy

Treatment of SM is organized into two parallel tracks: **mediator symptom management** (relevant for all patients) and **cytoreductive/targeted therapy** (for advanced disease or symptomatic ISM). **Track 1 — Mediator symptom management (all SM subtypes):** *H1 antihistamines:* - Non-sedating (fexofenadine, cetirizine, loratadine): daily scheduled dosing; 2nd-generation preferred for daytime use - Sedating (hydroxyzine, diphenhydramine): useful at bedtime or for acute reactions - Dose to tolerance — SM patients often require 2–4× the standard allergy dose *H2 antihistamines:* - Famotidine or…