Diagnostic Value of Bone Marrow Tryptase in Systemic Mastocytosis

Mastocytosis are a group of disorders characterized by clonal accumulation of mast cells in various organs. Diagnosis of mastocytosis is challenging and patients remain undiagnosed for years. Using the diagnostic criteria defined by the World Health Organization (WHO) mastocytosis can be divided between Cutaneous Mastocytosis (CM) and Systemic Mastocytosis (SM).

Study suggested that measurement of tryptase in plasma from a Bone Marrow Aspirate (from BMA) could be useful for the diagnosis of SM. Results from a preliminary study at Toulouse University Hospital suggested that a Bone Marrow Tryptase Level (MT) of 50 µg/L or more may be a more sensitive diagnostic criterion (sensitivity of 94.7%) than the histological result of BMB (the major diagnostic criterion for SM with a sensitivity of 68 to 80% in expert centers) for diagnosis of SM in patients with mastocytosis in skin.

Because this preliminary study was done on a limited number of patients and in a single center, the diagnostic value of MT for the diagnosis of SM, has to be confirmed on a larger and more representative population.

In addition, the investigators propose to evaluate:

• The diagnostic accuracy of MT for diagnosis of SM in patients without mastocytosis in skin;
• The diagnostic accuracy of absolute and corrected MT for diagnosis of SM with and without mastocytosis in skin;
• The diagnostic accuracy of MT/ST ratio for diagnosis of SM with and without mastocytosis in skin;
• The diagnostic accuracy of flow cytometry performed on cells collected by BMA and maintained in a preservative solution for diagnosis of SM with and without mastocytosis in skin;
• The diagnostic accuracy of quantification of the fraction of KIT mutation-positive cells in peripheral blood for diagnosis of SM with and without mastocytosis in skin;
• Correlation the results of quantification of the fraction of KIT mutation-positive cells in peripheral blood with results of MT level, results of MT/ST ratio and results of absolute and corrected MT;
• Correlation the results of the CF performed both on the EDTA/TransFix® tubes and on sodium heparin tubes and validate the transport conditions (20-25°C within 24 to 96H) for the CF; these analyzes will be performed on the first 50 samples of BMA.

The samples for these diagnosis tests will be made on the day of inclusion. For each enrolled patient, one 5 mL sample of peripheral blood (EDTA) and 1 mL samples of BM aspirate (0,5 mL in a EDTA/TransFix® tube et 0,5 mL in a sodium heparinate tube) will be collected the same day, and shipped in less than 24 to 96H (+20-25°C) to a central laboratory.

For each patient, genomic DNA from total leucocytes will be purified from the peripheral blood EDTA samples, and used later for the quantification of the kit mutations by real-time qPCR. The plasma tryptase level will be measured from the peripheral blood EDTA sample after an aliquot has been taken for DNA extraction. The bone marrow aspirate sample will be used first for immunophenotyping of BM mastocytes by flow cytometry (FC), and then, after centrifugation, for measurement of BM tryptase level. The degree of dilution of the BM aspirate by peripheral blood will be estimated by comparing the percentage of CD16bright vs. CD16low granulocytes in BM samples, and used to adjust the BM tryptase. Tryptase measurements will be made from EDTA or Na, heparinate plasma, using a standardized fluoro enzyme immunoassay on an automated analyzer. Immunophenotyping of BM mastocytes will be performed on BM aspiration stabilized by TransFix® (a Cellular Antigen Stabilisation Reagent) (and for firsts 50 samples on BM aspiration in Na, heparinate) using a pre-defined mixture of anti-CD45/CD117/CD34/CD2/CD25/CD16 antibodies, coupled to appropriate fluorochromes. Genomic DNA from BM leucocytes will be extracted from 300-500µL of peripheral blood (both anti coagulated with EDTA) using a Promega DNA automated extractor, and subsequently stored at +4°C. Quantitative PCR will be performed using a pre-validated qPCR assay (Life Technologies, Foster City, CA) with a 7900HT Fast Real-Time PCR System (Life Technologies).

The end of study visit is performed the same day that the patient is seen to be made aware of the results of the clinical and laboratory investigations made. This is also the day when the planned medical management of mastocytosis is discussed.

Inclusion criteria

• Suspicion of systemic mastocytosis, for whom bone marrow sampling (bone marrow biopsy and/or bone marrow aspiration) is carried out as part of normal workup of the disease in the center of reference and centers of competence in mastocytosis.
• Patient whose written informed consent has been obtained.

Exclusion criteria

• Patients with a contra-indication to marrow sampling (anticoagulant and/or anti-clotting treatments,
• Thrombocytopenia < 50 000/mm2) for whom it is impossible to formally conclude the final type of mast cell disease: SM, CM, mast cell activation syndrome.