Thrombocytopenia with absent radii (TAR) syndrome: from hemopoietic progenitor to mesenchymal stromal cell disease?

Thrombocytopenia with absent radii (TAR) syndrome is a rare autosomal recessive disease characterized by hypomegakaryocytic thrombocytopenia and bilateral radial aplasia. Its expression includes skeletal, hematologic, and cardiac system abnormalities. According to some authors, the association of disparate skeletal and hematologic abnormalities is related to simultaneous development of the heart, radii, and megakaryocytes at 6 to 8 weeks' gestation. Thrombocytopenia that generally presents at birth or during the neonatal period can also occur subsequently. Data as to the physiopathology of TAR syndrome are scanty because of the low frequency of the disease and frequent unavailability of samples for bone marrow. The few studies on colony formation suggest that thrombocytopenia could be due to a decreased response to thrombopoietin that affects both proliferation and differentiation. The genetic basis of this syndrome remains unclear because c-mpl gene mutations are not a likely cause of thrombocytopenia and they are also frequent in the normal population. This is also the case for the mutations to the multifunctional growth factor transforming growth factor (TGF)-β2 gene as described in our laboratory. Finally, the deletion on chromosome 1q21.1 described by Klopocki and colleagues is not considered sufficient to determine the TAR syndrome phenotype. We have reported that bone marrow adherent stromal cells from patients with TAR syndrome do not express CD105 antigen (expressed in normal mesenchymal cells), part of the receptor complex for TGF-β1 and TGF-β3. Thus, the hypothesis that the clinical phenotype of TAR could derive from damage to a common osteo/chondrogenic and hemopoietic progenitor warrants further study.