Transforming Growth-Interacting Factor (TGIF) Modulates Hematopoietic Stem Cell Functions

Abstract 1279

We have shown that quantitative expression of the stem cell expressed Transforming-growth-interfering factor (TGIF) is a predictor of patient survival in acute myelogenous leukemia (AML). By Kaplan-Meier analysis, patients whose leukemic cells expressed decreased levels of TGIF RNA had a mean survival of 12 months, while patients whose leukemic cells expressed TGIF at a higher level had a mean survival of 60 months (p=0.00001).

TGIF is a homeobox transcriptional repressor and although it has been implicated in holoprosencephaly (HPE), whether it has a direct role in hematopoiesis is not known. It is known, however, that TGIF is expressed in hematopoietic stem cells (HSCs) and that it can down-regulate both TGF-b and retinoic acid (RA) signaling, and there is incontrovertible evidence that both of these pathways play an important role in hematopoiesis.

To understand the biological basis for our clinical finding, we hypothesized a role for TGIF in hematopoiesis and in particular HSC function. We then proceeded to test this hypothesis in a Tgif knockout mouse model. Flow cytometric analysis of non-lineage depleted bone marrow (BM) cells showed that Tgif-null mice had statistically increased numbers of long-term HSCs (LT-HSCs) as defined by CD34-Flk2-Lin-ckit+Sca1+ status or by CD150+CD48-Lin-cKit+Sca+ status and decreased numbers of multipotent progenitors (MPP) as defined by CD34+FlK+Lin-ckit+Sca1+ status. The short-term (ST) HSC (defined by CD34+Flk2-lin-ckit+Sca1+ or by CD150-CD48-lin-ckit+Sca+) populations were not statistically different between the null and the WT-type mice. Tgif-null BM cells produced statistically significant higher total colony number in methylcellulose colony forming unit (CFU) compared to the WT-mice though the percentages of specific colonies types remained the same. We then compared the Tgif-null BM cells with WT-BM cells in competitive repopulation assays (CRA). These experiments showed that Tgif-null BM cells were more competitive and showed higher engraftment compared to the WT-BM cells.

In conclusion, our data suggest that Tgif has an important role in hematopoietic stem cell function and may play a role in determining the balance between quiescence and self-renewal. One hypothesis to explain these data is that HSCs with no or low Tgif are more quiescent and if so this provides one biological explanation of how might TGIF affect AML prognosis; increased quiescence of HSCs or the Leukemic stem cells derived from these HSCs may make them more resistant to myelotoxic injury following chemotherapy, increasing the likelihood of relapse and/or poor long-term survival. These hypotheses continue to be an active area or research in our laboratory.