Forced expression of the Ikaros 6 isoform in human placental blood CD34(+)cells impairs their ability to differentiate toward the B-lymphoid lineage

Studies in mice suggest that the Ikaros (Ik) gene encodes several isoforms and is a critical regulator of hematolymphoid differentiation. Little is kn own on the role of Ikaros in human stem cell differentiation. Herein, the b iological consequences of the forced expression of Ikaros 6 (Ik6) in human placental blood CD34(+) progenitors are evaluated. Ik6 is one of the isofor ms produced from the Ikaros premessenger RNA by alternative splicing and is thought to behave as a dominant negative isoform of the gene product becau se it lacks the DNA binding domain present in transcriptionally active isof orms. The results demonstrate that human cord blood CD34(+) cells that expr ess high levels of Ik6 as a result of retrovirally mediated gene transfer h ave a reduced capacity to produce lymphoid B cells in 2 independent assays: (1) in vitro reinitiation of human hematopoiesis during coculture with the MS-5 murine stromal cell line and (2) xenotransplantation in nonobese diab etic-severe combined immunodeficient mice. These results suggest that Ikaro s plays an important role in stem cell commitment in humans and that the ba lance between the different isoforms is a key element of this regulatory sy stem; they support the hypothesis that posttranscriptional events can parti cipate in the control of human hematopoietic differentiation. (C) 2001 by T he American Society of Hematology.