Mast Cell Disorders

Indications for Testing

Individuals who present with symptoms consistent with mast cell activation (eg, flushing, loose stools or diarrhea, anaphylaxis) and/or characteristic skin lesions and in whom secondary causes of mast cell activation have been investigated and excluded should be considered for MCD testing.

Classification

MCDs are categorized as clonal and nonclonal disorders, as described in the following table.

Diagnostic Criteria

Clinical Presentation

Clinical presentation varies greatly depending on the affected organ systems. Some presentations, including anaphylaxis, result from the inappropriate release of mast cell mediators and are frequently associated with a set of characteristic triggers.

In clonal disease (eg, SM), further testing may be prompted by findings that are the result of the abnormal growth and accumulation of neoplastic mast cells in various organs (eg, urticaria pigmentosa, progressive cytopenia, ascites, malabsorption).    

Laboratory Testing

Diagnosis

The European Competence Network on Mastocytosis (ECNM) and the National Comprehensive Cancer Network (NCCN) recommend that initial testing for a suspected MCD include measuring serum tryptase levels and evaluating a peripheral blood sample for the KIT D816V mutation via an ultrasensitive quantitative polymerase chain reaction (qPCR) assay. Depending on results, additional testing, including bone marrow aspiration and core biopsy, may also be indicated. 

Mast Cell Mediators

Serum Tryptase

Serum tryptase is an important initial test in the evaluation of patients with a suspected MCD  and is the most specific marker of mast cell activation available. Mast cells in the tissue produce and constitutively release the alpha form of tryptase, which can be detected by a commercial fluoroimmune enzyme assay. When evaluating patients for nonclonal MCAS (NC-MCAS), a baseline level should be obtained, and tryptase should be measured again shortly (within 4 hours) after a symptomatic episode; if increased, mast cell involvement is suggested.   Alternatively, if a clonal process is suspected (eg, SM), serum tryptase can be tested at any time.

A serum tryptase level >20 ng/mL is a minor diagnostic criterion for SM.  The expert panel for the diagnostic criteria of NC-MCAS agreed that a 20% + 2 ng/mL increase from the baseline level constitutes mast cell activation. 

Tryptase can be consistently elevated in other hematologic malignancies, including myelodysplastic syndromes and acute leukemia; therefore, follow-up of elevated tryptase should include bone marrow aspiration and biopsy.

Other Mast Cell Mediators

Although serum tryptase is the only mast cell mediator included in consensus criteria, other laboratory tests that can be considered include 24-hour urine tests for metabolites of histamine, N-methylhistamine, leukotrine E4, 2,3-Dinor-11beta-prostaglandin F2 alpha, and prostaglandin D2. These markers are most commonly used for the evaluation of MCAS and have been studied in research settings, but lack sensitivity and/or specificity for use in MCD evaluation. 

Bone Marrow Examination

When bone marrow is examined for MCD, good practice recommends examining a blood smear, bone marrow aspirate smear, and a core biopsy. The following ancillary tests are highly useful in this situation :

• Flow cytometry of bone marrow aspirate for mast cells and other abnormalities
• Cytogenetic karyotype on bone marrow aspirate (typically abnormal when an associated hematologic neoplasm is present)
• Fluorescence in situ hybridization (FISH) for eosinophilia (when eosinophilia is present) on blood or aspirate
• KIT D816V testing on aspirate
• Myeloid mutation panel (eg, containing SRSF2, ASXL1, RUNX1) on aspirate
• Immunohistochemistry on bone marrow biopsy:
  • Tryptase: most specific marker for mast cells; can be dim in advanced SM or with certain therapies (eg, in gastrointestinal biopsies with MCD, this is often dim to negative)
  • CD117: uniformly bright marker for mast cells; will also stain early erythroid and myeloid cells
  • CD25: present on abnormal mast cells and associated with the KIT D816V mutation; can be dim with certain therapies and is typically negative in patients with a KIT mutation other than D816V
  • CD34: negative in mast cells, but positive in CD34-positive blasts
  • CD30: brightly expressed in well-differentiated SM (which typically lacks the KIT D816V mutation and lacks CD25 expression) and variably expressed in other types of SM

Tests for other markers are performed based on pathology findings.

KIT Mutational Analysis

All individuals suspected of having SM should undergo KIT mutational analysis.  In adult patients, an activating KIT D816V mutation is found in >80% of all cases.  Individuals with SM and KIT D816V are resistant to certain therapies (eg, imatinib).

KIT testing can be performed on bone marrow or peripheral blood. Traditionally, KIT mutational testing was performed only on bone marrow because bone marrow lesional tissue has a higher concentration of mast cells. However, highly specific allele-specific oligonucleotide (ASO) or digital droplet (dd)-qPCRs can be used to detect the mutation in peripheral blood in nearly all adult patients with typical SM and is the preferred methodology.  If a KIT D186V mutation is detected in peripheral blood, a bone marrow biopsy should be performed to evaluate for SM, including the subtype of disease, and the presence of an associated hematologic neoplasm (found in approximately 30% of all patients with SM). Bone marrow biopsy should also be performed in cases with a high suspicion of SM even when no KIT mutation is detectable in the peripheral blood.  In a small number of cases, patients could have mutations in locations other than KIT D816V, and sequencing of the entire KIT gene by next generation sequencing (NGS) or a similar sensitive methodology might be considered to inform treatment and target therapies. 

Additional Genetic Testing

Tryptase copy number determination is helpful in establishing a diagnosis of hereditary alpha tryptasemia (elevated copy number of the alpha tryptase-encoding gene, TPSAB1). Hereditary alpha tryptasemia is associated with an increase in the basal serum tryptase level and a risk of mast cell activation; this can be present in patients with or without SM.

Monitoring

Yearly monitoring for SM and CM with no complications is appropriate, unless symptoms worsen. Laboratory tests that may be useful for assessment include serum tryptase measurements of mast cell burden, CBC, and chemistry panels. 

Serial measurements of the KIT D816V allele burden by ultrasensitive qPCR techniques appear useful to monitor residual disease in aggressive subtypes during or after cytoreductive therapy or allogeneic stem cell transplantation.  The frequency of measurement of the allele burden should be adapted to the individual situation in each patient. Thus, in patients who have idiopathic SM with low mast cell burden and stable clinical course, the KIT D816V allele burden should be measured at diagnosis, but should not necessarily be repeated unless signs of disease progression occur. In patients with more aggressive forms who are enrolled in clinical trials with cytoreductive therapies, the KIT D816V allele burden should be measured repeatedly before and during therapy.  When symptoms worsen, a bone marrow aspiration and biopsy should be considered.