Diagnosis of platelet function disorders: A standardized, rational, and modular flow cytometric approach.

A high proportion of patients with mucocutaneous bleeding diathesis and suspected inherited or acquired platelet disorder remain without diagnosis even after comprehensive laboratory testing. Since flow cytometry allows investigation of resting and activated platelets on the single cell level by requiring only minimal amounts of blood, this method has become an important assay within the diagnostic algorithm, especially in pediatrics. We therefore developed a standardized and modular flow cytometric approach that contributes to clarify impaired platelet function in a rational step-by-step manner. Due to simultaneous analysis of four fluorophores in a basic panel design, we are able to readily detect the most common and clinically significant platelet disorders: Glanzmann thrombasthenia or Glanzmann-like diseases (fibrinogen receptor GPIIb-IIIa), Bernard-Soulier syndrome (von Willebrand-factor receptor complex GPIb-IX-V) and less well characterized ß1-integrins that serve as the collagen, laminin or fibronectin receptor (CD29-CD49b, e and f, respectively). Platelet reactivity was investigated in response to the agonists adenosine diphosphate (ADP) and thrombin receptor activator peptide 6 (TRAP6) in suboptimal and optimal concentrations by quantifying surface expression of activation markers CD62P and CD63 as well as binding of PAC-1 antibody to the high affinity conformation of the fibrinogen receptor. For advanced diagnostic questions, several further modules were implemented: (i) calcium mobilization for evaluation of early signal transduction, (ii) a kinetically resolved mepacrine assay for estimation of delta-granule content and release, and (iii) a module to determine platelet reactivity upon additional agonists like the thromboxane A2-analogue U46619 or collagen. Blood withdrawn from a healthy control cohort allowed generating preliminary standard values for all parameters. The modules were validated by analysis of patients with known or suspected platelet defects (leukocyte-adhesion deficiency type III, Wiskott-Aldrich syndrome, acute myeloid leukemia, sickle cell disease and chronic immune thrombocytopenia).

Two patterns of thrombopoietin signaling suggest no coupling between platelet production and thrombopoietin reactivity in thrombocytopenia-absent radii syndrome.

BACKGROUND: Thrombocytopenia with absent radii syndrome is defined by bilateral radius aplasia and thrombocytopenia. Due to impaired thrombopoietin signaling there are only few bone marrow megakaryocytes and these are immature; the resulting platelet production defect improves somewhat over time. A microdeletion on chromosome 1q21 is present in all patients but is not sufficient to form thrombocytopenia with absent radii syndrome. We aimed to refine the signaling defect in this syndrome. DESIGN AND METHODS: We report an extended study of 23 pediatric and adult patients suffering from thrombocytopenia with absent radii syndrome in order to scrutinize thrombopoietin signal transduction by immunoblotting and gel electrophoretic shift assays. In addition, platelet immunotyping and reactivity were analyzed by flow cytometry. Results were correlated with clinical data including age and platelet counts. RESULTS: Two distinct signaling patterns were identified. Juvenile patients showed abrogated thrombopoietin signaling (pattern #1), which is restored in adults (pattern #2). Phosphorylated Jak2 was indicative of activation of STAT1, 3 and 5, Tyk2, ERK, and Akt, showing its pivotal role in distinct thrombopoietin-dependent pathways. Jak2 cDNA was not mutated and the thrombopoietin receptor was present on platelets. All platelets of patients expressed normal levels of CD41/61, CD49b, and CD49f receptors, while CD42a/b and CD29 were slightly reduced and the fibronectin receptor CD49e markedly reduced. Lysosomal granule release in response to thrombin receptor activating peptide was diminished. CONCLUSIONS: We show a combined defect of platelet production and function in thrombocytopenia with absent radii syndrome. The rise in platelets that most patients have during the first years of life preceded the restored thrombopoietin signaling detected at a much later age, implying that these events are uncoupled and that an unknown factor mediates the improvement of platelet production.