Kruppel Like Factor 7 Suppresses Hematopoietic Stem and Progenitor Cell Function

Abstract 2356

Kruppel like factor 7 (KLF7) expression is an independent predictor of poor outcome in pediatric acute lymphoblastic leukemia (Flotho, et al; Blood 2007). In addition, KLF7 overexpression is associated with Imatinib-resistant CML (Cammarata, et al; Clinical Leukemia 2007). The kruppel like factor (KLF) family of transcription factors are involved in regulating cellular growth and differentiation in multiple tissue types. KLF7 is important for neurogenesis, and mice lacking KLF7 die perinatally with severe neurologic defects (Laub, et al; Mol Cell Biol 2005). While no specific role for KLF7 in hematopoiesis has been previously reported, loss of the closely related family member KLF6 is associated with defective blood cell production (Matsumoto, et al; Blood 2006), and other KLF family members are involved in multiple aspects of hematopoiesis. Targets of KLF7 include known regulators of hematopoietic stem and progenitor cell (HSPC) function including TRKA, Cebp/a, and CDKN1A (p21). Normal HSPCs appear to have a low level of KLF7 expression based on RNA expression profiling of populations enriched for these cells. Given these findings, we hypothesized that KLF7 may play a role in regulating normal HSPC function, and may contribute to leukemogenesis or resistance to therapy. To test this hypothesis, we first analyzed the effect of the loss of KLF7 on hematopoiesis. Specifically, we generated Klf7^−/− fetal liver chimeras and characterized their hematopoiesis. Long-term multilineage engraftment of Klf7^−/− cells was comparable to control cells. Moreover, HSC self-renewal, as assessed by serial transplantation was not effected by the loss of KLF7. To model the effect of KLF7 overexpression on HSPC function, we generated retroviral and lentiviral vectors that express KLF7. KLF7 expression in wild type bone marrow cells transduced with KLF7 retrovirus was increased approximately 10-fold. Overexpression of KLF7 was associated with a marked suppression of myeloid progenitor cell growth, as assessed using colony-forming cell assays. Relative to the initial transduction efficiency, the number of myeloid colonies produced from KLF7-transduced cells compared to vector-alone transduced cells was reduced 5.7 ± 1.9 fold. We next assessed short- and long-term engraftment of KLF7-transduced cells by bone marrow transplantation. In experiments using bone marrow cells transduced with high efficiency (≥ 60% transduced cells), overexpression of KLF7 resulted in impaired radioprotection. Whereas all (12 of 12) recipients transplanted with control transduced cells survived, only 42% (5 out of 12) of recipients of KLF7 transduced cells survived more than two weeks after transplantation (P < 0.003). When these experiments were performed with a reduced multiplicity of infection to achieve a lower transduction efficiency, all recipient mice survived at least 3 months. Whereas control-transduced cells were readily detected at near input levels (on average, 40% of nucleated blood cells), minimal contribution of KLF7-transduced cells was observed in all lineages except T cells. Interestingly, KLF7-transduced T cells were present at near input levels. In summary, our show that KLF7 is not required for normal HSPC function. However, overexpression of KLF7 leads to a marked suppression of the short- and long-term repopulating activity of HSPC with the exception cells in the T cell lineage. Whether KLF7 expression contributes to T cell leukemogenesis through suppression of other hematopoietic lineages will require further study.