Stage specific over-expression of the dominant negative Ikaros 6 reveals distinct role of Ikaros throughout human B-cell differentiation.

Ikaros is a transcription factor that acts both as an activator and as an inhibitor of gene expression in several hematopoietic lineages. Ikaros functions in hematopoiesis have mostly been studied in mice, and are notably crucial for lymphopoiesis. Deregulation of Ikaros expression was evidenced in several leukemia subtypes, including pre-B-ALL. Here, we studied the role of Ikaros in human B lymphoid differentiation through xeno-transplantation of genetically modified cord blood (CB) human hematopoietic progenitor cells (HPC) in NOD/SCID mice. We used lentiviral vectors to force expression of Ikaros 6 (Ik6), a known dominant negative (DN) protein that interferes with normal Ikaros and structurally related proteins in HPC and their progeny. Two types of vectors were used: a vector containing the EF1alpha promoter which produces strong gene expression in all hematopoietic lineages, and a recently validated B-specific vector containing an enhanced CD19 derived promoter that strongly favors expression in the B-cell lineage. Ik6 transduction of CB CD34(+) cells with these vectors produced distinct consequences in the B-cell differentiation profiles of xenografted human cells. While the ubiquitous vector favored a specific block at the early pro-B/pre-B stage of differentiation, with an increase in Lambda Like transcript expression in the bone marrow (BM), B-specific Ik6 expression provoked a global decrease in the CD19(+) cell population in both BM and spleen, associated with a decrease in IgM+ immature B-cells in the spleen. We conclude that Ikaros proteins are active throughout human B-cell differentiation, before and after CD19 appearance.

The role of Ikaros in human erythroid differentiation.

Ikaros--a factor that positively or negatively controls gene transcription--is active in murine adult erythroid cells, and involved in fetal to adult globin switching. Mice with Ikaros mutations have defects in erythropoiesis and anemia. In this paper, we have studied the role of Ikaros in human erythroid development for the first time. Using a gene-transfer strategy, we expressed Ikaros 6 (Ik6)--a known dominant--negative protein that interferes with normal Ikaros activity-in cord blood or apheresis CD34(+) cells that were induced to differentiate along the erythroid pathway. Lentivirally induced Ik6-forced expression resulted in increased cell death, decreased cell proliferation, and decreased expression of erythroid-specific genes, including GATA1 and fetal and adult globins. In contrast, we observed the maintenance of a residual myeloid population that can be detected in this culture system, with a relative increase of myeloid gene expression, including PU1. In secondary cultures, expression of Ik6 favored reversion of sorted and phenotypically defined erythroid cells into myeloid cells, and prevented reversion of myeloid cells into erythroid cells. We conclude that Ikaros is involved in human adult or fetal erythroid differentiation as well as in the commitment between erythroid and myeloid cells.

[Ikaros and erythropoiesis]. / Ikaros, facteur de transcription impliqué, aussi, dans l'érythropoïèse.

Mostly studied in murine models, Ikaros--a factor that positively or negatively controls gene transcription--was first described as essential to lymphopoiesis until its involvement in numerous hematopoietic lineages was documented. Indeed, Ikaros is also active in murine erythropoiesis and more precisely during fetal to adult globin switching. Recently, these observations were confirmed by our team in a human context. We here review some of the most important characteristics of Ikaros, and detail more precisely how defects of Ikaros activity either by gene inactivation or mutation in mice, or by forced expression of dominant negative isoforms in human precursor cells, modify the erythroid differentiation. An increased cell death, together with decreased cell proliferation, decreased expression of erythroid-specific genes including GATA1, and a delay in fetal to adult globin switching were observed. At the same time, myeloid differentiation was slightly favoured thus suggesting that Ikaros could be involved in the control of the myeloid/erythroid commitment.