Serial Analysis of Hematopoietic Progenitors in Mpl^−/− Mice.

Thrombopoietin (TPO) is the pivotal regulator of megakaryocytopoiesis but also an important factor for early multipotent hematopoietic progenitors. Loss-of-function mutations in the human MPL gene coding for the TPO receptor are the cause of congenital amegakaryocytic thrombocytopenia (CAMT), a rare inherited bone marrow failure syndrome presenting as isolated thrombocytopenia at birth and progressing to aplastic pancytopenia during the first years of life. In contrast, Mpl^−/− mice although exhibiting thrombocytopenia and severely reduced numbers of early hematopoietic progenitors do not develop any signs of pancytopenia or generalized bone marrow failure during lifetime. To identify parallels and differences in the impact of TPO on hematopoietic development between mice and men we serially analyzed subpopulations of hematopoietic cells from early progenitors to mature peripheral blood cells from Mpl^−/− mice (kindly provided by Warren Alexander, WEHI, Melbourne, Australia) over a period of 24 months. We determined frequency, clonogenicity and proliferative potential of hematopoietic progenitors from different hematopoietic tissues. Compared to wild type mice we found a strong reduction of multipotent stem cells (HSC), of common myeloid progenitors (CMP), of megakaryocyte/erythrocyte lineage restricted progenitors (MEP) but also of common lymphoid progenitors (CLP) in Mpl^−/− mice. We did not find any significant changes in the frequency of these cells over a period of 24 months. For the CMP and MEP subpopulations of myeloid progenitors we found not only a decreased frequency but also a reduced clonogenicity and proliferative capacity. Interestingly, these functional restrictions significantly increased significantly during aging, a finding that was similar to our results obtained with myeloid progenitors from CAMT patients. Impaired proliferation of myeloid progenitors rather than a reduced number could be the reason for development of pancytopenia in CAMT patients.