Existence of a differentiation blockage at the stage of a megakaryocyte precursor in the thrombocytopenia and absent radii (TAR) syndrome

The thrombocytopenia and absent radii (TAR) syndrome is a rare disease asso ciating bilateral radial agenesis and congenital thrombocytopenia, Here, we investigated in vitro megakaryocyte (MK) differentiation and expression of c-mpl in 6 patients. Using blood or marrow CD34(+) cells, the colony-formi ng unit (CFU)-MK number was markedly reduced. CD34(+) cells were also cultu red in liquid medium in the presence of a combination of 3 cytokines (stem cell factor, interleukin-3, and interleukin-6) or megakaryocyte growth and development factor (PEG-rHuMGDF) with or without SCF, In the presence of PE G-rHuMGDF, the majority of mature megakaryocytes (CD41 high, CD42 high) und erwent apoptosis, This phenomenon was also observed in cultures stimulated by three cytokines, However, this last combination of cytokines allowed a m ore complete terminal MK differentiation. Surprisingly, a homogeneous popul ation of CD34(-)CD41(+)CD42(-) cells accumulated during the cultures. This population was unable to differentiate along the myeloid pathways. This res ult suggests that a fraction of MK cells is unable to differentiate in the TAR syndrome, We subsequently investigated whether this could be related to an abnormality in c-mpl, No mutation or rearrangement in the c-mpl gene wa s found by Southern blots or by sequencing of the c-mpl coding region and i ts promoter in any of the patients. Using Western blot analysis, a decrease d level of MpI was found in patient platelets. A decreased level of c-mpl m essenger RNA in TAR platelets was also detected with a lower c-mpl-P to c-m pl-K ratio in comparison to adult platelets. Altogether, these results demo nstrate that the thrombocytopenia of the TAR syndrome is associated with a dysmegakaryocytopoiesis characterized by cells blocked at an early stage of differentiation. (C) 2000 by The American Society of Hematology.