Thrombopoietin in Patients With Congenital Thrombocytopenia and Absent

Radii: Elevated Serum Leveis, Normal Receptor Expression, But Defective

Reactivity to Thrombopoietin

By Matthias Ballmaier, Harald Schulze, Gabriele Strauß, Klara Cherkaoui, Nicole Wittner, Stefanle Lynen,

Susanne Wolters, Jakob Bogenberger, and Karl WeIte

The pathophysiology of thrombocytopenia in the syndrome of thrombocytopenia with absent radil (TAR) is not yet un derstood. We examined thrombopoietin (TPO) serum leveis and the in vitro reactivity of platelets to TPO in five patients affected with TAR syndrome. We found elevated TPO serum leveis in all patients tested, excluding a TPO production de fect as cause for thrombocytopenia in TAR syndrome. In addition, we found similar expression of the TPO receptor c-MpI on the surface of platelets from TAR patients (5 of 5) and a similar molecular weight of the receptor as compared with healthy controls (4 of 4). Platelet response to adenosine diphosphate or thrombin receptor agonist peptide SFLLRN

(TRAP) was normal in TAR patients. However, in contrast to results with healthy controls we could show absence öf in vitro reactivity of platelets from TAR patients to recombi nant TPO as measured by testing TPO synergism to adenine diphosphate and TRAP in platelet activation. TPO induced tyrosine phosphorylation of platelet proteins was com pletely absent (3 of 4) or markedly decreased (1 of 4). Our results indicate that defective megakaryocytopoiesisl thrombocytopoiesis in TAR syndrome is not caused by a defect in TPO production but a lack of response to TPO in the signal transduction pathway of c-Mpl.

© 1997 by The American Society of Hematology.

THROMBOCYTOPENIA WITH absent radii (TAR) is a rare congenital defect wit« hypomegakaryocytic

thrombocytopenia and bilateral radial aplasia. Although first described in 1929,1 it was defined as a syndrome by Hall et al in l969.² TAR seems to be inherited in an autosomal recessive manner.³ The pathogenesis of TAR is poorly un derstood. Bone marrow aspirates from TAR patients show either decreased or absent megakaryocytopoiesis, indicating that the thrombocytopenia is caused by a defective platelet production.

There are inconsistent reports about “thrombopoietin like” activity and megakaryocyte colony-stimulating activ ity (Mega-CSA) in the serum of TAR patients.^4~7

Uhe presence of colony forming units for megakaryocytes (CFU-Mega) in the bone marrow of TAR patients is controversial, too. Whereas some investigators did not find any growth of megakaryocytic colonies from the bone marrow of TAR patients,^4,6 de Alarcon et a1⁷ showed normal CFU Mega counts with an abnormal colony morphology. Cells in these colonies were sma1ler, and the number of cells per colony was much higher than in normal CFU-Mega-derived colonies.

Platelet function seems to be normal in TAR patients,³ but thei-e are also some reports about abnormal platelet mor phology and functioh, ^8,9

In 1994 several groups purified and cloned a new hemato poietic factor: thrombopoietin (TPO).¹⁰-¹⁴ Since then, this

cytokine has been ~hown to be the major reuulator of mega karyocytopoiesis and thrombocytopoiesis. Investigations of the in vitro and in vivo effects of TPO showed that it acts as a Mega-CSF as weil as a megakaryocyte maturation factor and pösitive regulator of platelet produetion in vivo.¹⁵ The receptor for TPO, the product of the proto-oncogene c-mpl, ¹'⁶ is expressed in the megakaryocytic lineage from progenitor cells to platelets.¹⁷ c-mpl, a cellular homologue of the v-mpl oncogene,'⁸ was found to be a member of the cytokine receptor superfamily ^16,19 that share common signal transduc tion pathways.²⁰ Binding of the ligand leads to tyrosine phosphorylation of some cellular proteins in cell lines and platelets.^21~23

Our aim was to elucidate a possible relationship between the TAR-related thrombocytopenia and the TPO-c-Mpl sys tem. Here we present the first report about TPO serum levels in TAR patients and about the in vitro reactivity of platelets from TAR patients to recombinant human (rh) TPO. Our results indicate that defective megakaryocytopoiesislthrom bocytopoiesis in TAR syndrome is not caused by a defect in TPO production but by a lack of response to TPO in the signal transduction of c-Mpl.

MATERIALS AND METHODS

Patients. We examined samples from five unrelated chudren with TAR syndrome. The patients' characteristics are described in detail in Table 1. lt is important to note that four of the patients required platelet transfusions.

Patient 1 was a 3-year-old girl with bilateral absent radii and hypoplastic, curved ulna and with platelet counts usually about 50,000 /µL. She needed three platelet transfusions during the first year of life. Aside from this, a clinically irrelevant ventride septum defect (VSD) was stated.

Patient 2 was a 3-year-old gin with i~Iateral absent radii and short ulna with platelet counts usually approximately 20,000 /µL. During infections she required platelet transfusions because of bleedings from nose and rnouth.

Patient 3 was a 6-week-old boy at first examination. He showed bilateral absent radii, a short ulna right, an ulna aplasia leif, and a bilateral hip Üranslocation. Because of a severe thrombocytopenia he needed platelet transfusions twice a month for the first 3 months of life. Platelet counts were between 25,000 at the time of the first examination and 72,000 at the age of 10 months.

Patient 4 was a 6-month-old baby gin. Bilateral absent radii and

Froni tile Department of Pediatn~: Hematology and Oncology, Medical Sch 001 Haim oy'e r, Ge rinan v and A ~ngen Inc, Thotisand öaks. CA. The first ~vo autllors contributed equally' to this wo rk.

SL£hlflitted April 1, 1996; accepted March 6,1997.

St'pported by Grant No. We 942/5-1 from Deutsche Forschungs gemeinschaft (DFÜ).

Address reprint requests to Karl Weite, MD, PhD, Abt Pädia trische Häinatologie und Onkologie, Medizüiische Hochschule Hannover, D-30623 Hannover, Germany.

The publication costs ofthis artide were defrayed in part by page charge payrnent. This artide must therefore be hereby marked ,advertisement" in accordance wüh 18 US. C. section 1734 soleiy to i'idicate this filcL

© 1997 by The American Sociey of Hematoiogy.

0006-4971/97/9002-001 7$3. 00/0

612 Blood, Vol 90, No 2 (JuIy 15), 1997: pp 612-619