Glivec/STI571 Treatment Stimulates Megakaryopoiesis and Normalizes PDGF Receptor beta Kinase Expression in Thrombocytopenic Patients with Myeloid Metaplasia with Myelofibrosis.

Myeloid Metaplasia with Myelofibrosis (MMM) is a Philadelphia negative chronic myeloproliferative disorder associated with myeloid metaplasia in the spleen and liver, bone marrow fibrosis and neoangiogenesis. The myeloproliferation is characterized by an increased number of circulating CD34⁺ hematopoietic progenitors (up to 200 folds) with a prominent amplification of dystrophic megakaryocytes (Mk). Alteration of tyrosine kinase signals is suspected to play a key role in the myeloproliferation and in the increased sensitivity of hematopoietic progenitors to growth factors. Glivec is a small-molecule tyrosine kinase inhibitor of the ABL fusion gene, platelet derived growth factor receptor beta (PDGFRB) and SCF receptor (c-kit). In this study, we investigated the effect of Glivec/STI571 on the megakaryopoiesis and the kinase expression of 11 MMM patients enrolled in a European EUMNET clinical trial. We showed that 50 percents of the treated patients exhibited a tendency to a decrease in the CD34⁺ cell number from 261/μL to 56/μL with a normalization of the CD34⁺ cell count (1–10/μL) in 20 percents of these patients (2/10). We also observed a reduction of a CD34⁺CD41⁺ Mk subpopulation co-expressing the CD9 tetraspanin that is suggested to be a marker of MMM. Normalization of the CD9 gene expression level was further confirmed by quantitative RT-PCR in MMM mononuclear cells. In contrast, 55 percents of MMM patients (5/9 patients) showed an increase in their platelet count from 191 x 10⁹/L to 282 x 10⁹/L during treatment with Glivec. Interestingly, an increase in the platelet count was also observed in three of the five thrombopenic patients who exhibited an increase greater than 50 x 10⁹/L (p= 0.02) being associated with an augmentation in the number of clonogenic megakaryocytic progenitors (CFU-Mk). The effect of Glivec treatment on megakaryopoiesis was further illustrated by a morphometric analysis on bone marrow histological sections showing a normalization of the size and shape of the dystrophic MMM Mk. We also analyzed by quantitative RT-PCR the effect of Glivec treatment on receptor kinase gene expression in circulating mononuclear cells from MMM patients and demonstrated a normalization of the receptor beta of the PDGF, a cytokine involved in Mk differentiation. Actually, the PDGFB expression that was down-regulated in MMM PBMC before Glivec as compared to normal PBMC (−1.14 +/− 0.76 and − 0.14 +/− 0.42, respectively; p= 0.02) was significantly increased during therapy, reaching a normal level in 60% of tested patients. In conclusion, our results show that Glivec treatment reduced the CD34⁺ cell number and stimulated the megakaryopoiesis of MMM thrombopenic patients, probably by restoring Mk progenitor differentiation.