The Balance of Gfi-1 and Gfi-1b Maintains the Undifferentiated, Multipotent State of Hematopoietic Stem Cells and Regulates Lineage-Commitment

Abstract 1210

Gfi-1 and Gfi-1b are homologous transcriptional repressors that are expressed in hematopoietic stem cells (HSCs). Gfi-1 is crucial for the terminal maturation of neutrophils, and Gfi-1b is critical for erythropoiesis and thrombopoiesis. HSCs give rise to all mature blood lineages through a tightly regulated multistep differentiation process, but the mechanism of their early lineage specification remains largely elusive. Here, we have dissected the role of the Gfi-family factors in HSC maintenance and early lineage-commitment. To this end, we generated conditional targeted alleles for Gfi-1 and Gfi-1b that allowed for time controlled induced disruption of their genes. Acute disruption of Gfi-1 resulted in a rapid, severe decrease of HSCs numbers in the bone marrow and ablated their function in competitive repopulation assays. Surprisingly, and sharply contradicting recent claims to the opposite, acute disruption of Gfi-1b also led to decreased numbers of long-term repopulating HSCs in the bone marrow and decreased fitness in competitive transplantation. After induced, combined disruption of both factors, no HSC and progenitor cells were maintained in the bone marrow for more than 2 weeks. To elucidate the molecular mechanisms of the Gfi-family mediated HSC maintenance we performed global gene expression profiling of Gfi-1^−/− and Gfi-1b^−/− HSCs. Unexpectedly, both factors regulate highly distinct gene sets involved in differentiation of alternative lineages. Thus, surprisingly, their action in HSCs is not redundant but synergistic. Consistent with this, disruption of individual Gfi-family factors renders HSCs prone to differentiation to specific alternative lineages, while combined disruption is entirely incompatible with HSCs maintenance, in large part due to unchecked differentiation. Together, our data reveal that balanced expression of Gfi-1 and Gfi-1b is required for maintaining the undifferentiated, multipotent state of HSCs, while altering the balance is sufficient for inducing commitment to specific lineages.