title: Two patterns of thrombopoietin signaling suggest no coupling between platelet production and thrombopoietin reactivity in thrombocytopenia-absent radii syndrome.
abstract: 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.
  
  
  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.
authors.affiliation: Freie Universität Berlin, Dept. Biochemistry, Germany.
authors.forename: Janine
authors.initials: J
authors.lastname: Fiedler
authors.forename: Gabriele
authors.initials: G
authors.lastname: Strauss
authors.forename: Martin
authors.initials: M
authors.lastname: Wannack
authors.forename: Silke
authors.initials: S
authors.lastname: Schwiebert
authors.forename: Kerstin
authors.initials: K
authors.lastname: Seidel
authors.forename: Katja
authors.initials: K
authors.lastname: Henning
authors.forename: Eva
authors.initials: E
authors.lastname: Klopocki
authors.forename: Markus
authors.initials: M
authors.lastname: Schmugge
authors.forename: Gerhard
authors.initials: G
authors.lastname: Gaedicke
authors.forename: Harald
authors.initials: H
authors.lastname: Schulze
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001792:Blood Platelets / Q000378:metabolism*; D002460:Cell Line; D002648:Child; D002675:Child, Preschool; D002872:Chromosome Deletion; D000080984:Congenital Bone Marrow Failure Syndromes; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008533:Megakaryocytes / Q000378:metabolism; D010976:Platelet Count; D011884:Radius / Q000002:abnormalities / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism; D015398:Signal Transduction; D013921:Thrombocytopenia / Q000235:genetics / Q000378:metabolism*; D013926:Thrombopoietin / Q000378:metabolism*; D038062:Upper Extremity Deformities, Congenital / Q000235:genetics / Q000378:metabolism*; D055815:Young Adult
chemical_list: D011956:Receptors, Cell Surface; D013926:Thrombopoietin