Evi-1 Regulates Myelopoiesis and Hematopoietic Stem Cell Development in Zebrafish and Human Pluripotent Stem Cells

Abstract 1281

The Evi-1 locus was originally identified as a common site of retroviral integration in murine myeloid tumors. Over the last years, Evi-1 evolved as one of the most potent oncogenes associated with human and murine myeloid leukemia. More recent studies in knockout mice suggest also involvement of Evi-1 in the regulation of developmental hematopoiesis, but the role of Evi-1 in this context is poorly understood. Here, we analyzed zebrafish embryo and human pluripotent stem cells (PSC) to understand how Evi-1 modulates early hematopoietic development.

We examined the hematopoietic development in zebrafish embryo by in situ hybridization (ISH) for hematopoietic markers. The zebrafish homologue evi-1 was shown to be expressed in co-localization with scl in the posterior blood islands, indicating a role during early blood development. We also performed loss-of-function studies were by injecting morpholino oligonucleotides (MO) in zebrafish zygotes to inhibit evi-1 pre-mRNA splicing. Inhibition of evi-1 was confirmed in MO-injected versus control embryos. N=100 zebrafish embryos were analyzed per experiment in each group. To control for off-target effects, two separate MO were designed and injected. MO mediated evi-1 knockdown severely reduced numbers of circulating blood cells and induced hemorrhages. ISH performed in evi-1 morphants versus control fish revealed strongly impaired formation of myeloid embryonic cells (measured by pu.1 expression), while no changes were observed in primitive erythroid progenitor cells (monitored by gata1 expression) or overall in blood and endothelial precursors in the posterior lateral plate mesoderm (as monitored by scl expression). Moreover, analyses at 36 hours and 5 days post fertilization showed strong reduction of runx1+/cmyb+ cells and rag1+ lymphoid cells, indicating a role of evi-1 in developing hematopoietic stem cells (HSC).

Previous reports in adult murine hematopoietic cells suggest that Evi-1 affects hematopoietic stem cell proliferation through regulation of Gata2. To test whether Gata2 is a putative downstream regulator of Evi-1 in our system, we performed a rescue experiment and co-injected gata2 mRNA in evi-1 MO treated fish. Indeed, ectopic gata2 rescued the impaired myeloid phenotype, as shown by re-occurrence of mpo, l-plastin as well as pu.1 expressing cells.

To assess whether these molecular interactions are conserved during human developmental hematopoiesis, we surveyed in vitro differentiating human pluripotent stem cells (PSC) genetically modified to suppress EVI-1. EVI-1 expression was detected during differentiation of human PSC in embryoid bodies, especially around day 9 when hematopoietic progenitors start to emerge in this system. Treatment with EVI-1 shRNA strongly reduced the generation of myeloid colonies from human PSC in vitro as well as the numbers of emerging CD34+ and CD45+ cells. Molecularly, EVI-1 suppression inhibited the expression of PU.1 and GATA2 during the course of development, while leaving SCL and GATA1 expression unaltered.

Taken together, our data suggest that, in both fish and human, Evi-1 regulates embryonic myelopoiesis through interactions with Gata2 and independently of Gata1 and embryonic erythropoiesis. Moreover, Evi-1 appears crucial for HSC development. Currently ongoing experiments in our laboratory focus on the further elucidation of the molecular mechanisms underlying the Evi-1 effects during developmental hematopoiesis.