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IMiDs block megakaryocytic maturation
Figure 6. Downregulation of GATA1 by IMiDs induces renewal and expansion of hematopoietic progenitors with a concomitant block of megakaryocytic matura- tion. Treatment with IMiDs induces self-renewal of hematopoietic progenitors and upregulates megakaryocytic colonies (CFU-Meg) by inhibiting apoptosis and increasing proliferation of early megakaryocytic progenitors. IMiDs down-regulate the transcription factor GATA1 and thereby affect transcription factors and cell cycle regulators controlled by GATA1. The subsequent decrease of ZFPM1 and NFE2 leads to expansion of megakaryocytic progenitors with concomitant inhibition of maturation. A decrease of cyclin D1 and an increase of p16 results in the block of megakaryocytic maturation (Adapted from Figure 7 of reference 25, with per- mission of Dr. John D. Crispino).
bition of megakaryocytic maturation. Most importantly, OE of GATA1 widely abrogated the effects of IMiDs on lineage commitment (Figure 5D). The numbers of BFU-E colonies increased significantly (P<0.001), whereas the numbers of CFU-G/GM colonies in the IMiDs group decreased significantly (P<0.001) when compared to EV- transfected cells. OE of GATA1 abrogated the IMiDs- induced shift of lineage commitment toward myelopoiesis at the expense of erythropoiesis/megakary- opoiesis.
Discussion
Despite the fact that IMiDs are not directly cytotoxic, their use is associated with severe thrombocytopenia (grade 3/4) in up to 25% of patients with MM.10,30 The mechanism for induction of thrombocytopenia is unknown. Here, we determined that IMiDs dramatically expand CD34+ hematopoietic progenitors in liquid cul- tures and significantly induce megakaryopoiesis and megakaryocytic colony formation. Interestingly, IMiDs generated mainly immature CD41a+ /CD42b– megakary- ocytes. This was further confirmed by transmission elec- tron microscopy, revealing structural abnormalities of Mks
that also suggested a maturational block in megakary- opoiesis.31,32 More strikingly, we were able to maintain a small pool of CD34+ cells for as long as 4 months in liquid culture. Besides CD34+ expression, the long-term cultured cells maintained concomitant expression of myeloid (CD33, CD11b) and megakaryocytic (CD41, CD61) mark- ers, reflecting a phenotype that is similar to acute megakaryoblastic leukemia (AMKL) with GATA1 muta- tions33,34. GATA1 is a transcription factor critical for devel- opment of erythroid and megakaryocytic cells. GATA1 mutations in humans act as a dominant leukemogenic oncogene in megakaryocytic progenitors and cause inher- ited thrombocytopenia.31 In our studies, the IMiDs- induced proliferation of hematopoietic progenitors and inhibition of maturation of Mks were associated with a loss of GATA1, ZFPM1 (FOG-1) and NFE2 expression. ZFPM1 acts as a cofactor for GATA1 and provides a para- digm for the regulation of cell type-specific gene expres- sion by GATA1.22 GATA1 also drives the expression of another important transcription factor, NFE2 that exists as a heterodimer comprised of p45 and p18 subunits. p45Nfe2-/- mice present with an increase in Mks, a marked defect in maturation and profound thrombocytopenia.35,36 This indicates that terminal maturation of Mks depends heavily on the GATA1/NFE2 axis. In addition to promot-
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