MEIS1 Regulates Early Erythroid and Megakaryocytic Lineage Decision

Abstract 2365

The transcription factor MEIS1 is essential for definitive hematopoiesis in the mouse embryo and during zebrafish development. MEIS1 deficient mice die at mid gestation, mainly due to a poorly developed HSC compartment and a reduction in myeloerythroid colony forming cells. We have previously shown by comparative transcriptional profiling of all human adult blood cell lineages that MEIS1 is specifically expressed in megakaryocytes (MK). It is however not yet known if and where MEIS1 participates in the human hematopoietic transcription factor hierarchy.

We established that primary hematopoietic stem and progenitor (CD34⁺) cells in human bone marrow had high expression of MEIS1, which was comparable to CD34+ cells in cord blood (CB) and even higher (p<0.01) in mobilized peripheral blood (MPB). Compared to other bone marrow progenitor subsets, MEIS1 expression was highest in hematopoietic stem cells (HSC:CD34⁺/CD38⁻) and significantly reduced (p<0.01) in downstream granulocyte monocyte progenitors (GMP:CD34⁺/CD38⁺/CD110⁻/CD45RA⁺) and megakaryocyte-erythrocyte progenitors (MEP:CD34⁺/CD38⁺/CD110⁺/CD45RA⁻). We then analyzed the hematopoietic potential of CD34⁺ cells derived from CB or MPB, in which MEIS1 expression was silenced with short hairpins or overexpressed using lentiviral vectors. Cells were sorted for GFP expression at 48 hours after transduction and evaluated for colony forming potential. MEIS1 knockdown in human CD34+ cells resulted in a near absence of BFU-E, a 70% reduction in CFU-GM and a 25% reduction in CFU-MEG (p<0.01). When transduced CD34⁺ cells were cultured in suspension with EPO, SCF and IL-3 promoting differentiation towards erythroblasts or with TPO and IL-1β towards megakaryocytes, knockdown of MEIS1 significantly inhibited the proliferation of both erythroblasts and megakaryocytes (p<0.01). In addition, the expression of CD235a on erythroblasts was reduced by 60% (p<0.01) upon MEIS1 knockdown, whereas no changes were detected in the expression of CD41 on MKs. On the other hand, overexpression of MEIS1 altered lineage differentiation of human CD34⁺ cells without influencing the total outgrowth of colonies, inducing a 3-fold increase of BFU-E and a 2.5-fold increase in CFU-MEG at the expense of a 3-fold reduction in CFU-GM. Furthermore, since established erythroblast cultures were not affected by knockdown of MEIS1, we hypothesized that MEIS1 might regulate the erythroid/megakaryocytic lineage decision in the common myeloid progenitor (CMP). To test this hypothesis, we sorted CMPs (CD34⁺/CD38⁺/CD110⁻/CD45RA⁻), transduced the cells with MEIS1 and evaluated their colony forming potential. Indeed, overexpression of MEIS1 in CMP induced an increase in CFU-MEG and BFU-E and a striking absence of CFU-GM. When MEIS1 was overexpressed in GMP no differences were observed compared to the empty vector.

In summary, MEIS1 expression in primary human progenitor cells is essential for the formation of myeloid cells, erythroblasts and megakaryocytes. However, a surplus of MEIS1 inhibits myeloid differentiation and enhances erythroid and megakaryocytic development. We therefore conclude that MEIS1 is a novel regulator for differentiation of human CMPs and that gene dosage of MEIS1 determines its lineage fate decisions.