CDKN1C Modulates the Stress-Reponse of Hematopoietic Stem Cells Rendering Hematopoiesis Resistant to Chemotherapeutics

Abstract 3162

The cyclin-dependent kinase inhibitor (CDKI) CDKN1C (p57) is a tumor suppressor gene with strong differential expression in both human and murine hematopoietic stem cells (HSC). Whereas the expression of other CDKIs is normal in most hematopoietic malignancies, p57 expression is silenced in 30 to 55% of acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), and B-Cell lymphoma patients suggesting a role as suppressor of hematopoietic cell transformation. p57 has a unique role in embryogenesis functioning in tissue-specific developmental programs to coordinate proliferation, differentiation and apoptosis. We have identified p57 as an early transcriptional target of the tumor suppressor TGFβ which is required for TGFβ-induced cytostasis of human CD34+ hematopoietic progenitor/stem cells; functions that provide a mechanistic basis for its expression being silenced in many aggressive human myeloid and lymphoid malignancies. To understand the role of p57 in HSC function, we used an engineered mouse strain deficient in p57. Using limiting-dilution HSC transplantation, we found that the fetal livers of p57-null mice have ∼4-fold fewer HSCs than their wild-type littermate controls. When we transplanted wt recipient mice with a mixture of p57-wt and p57-null fetal liver mononuclear cells (FLMC), we found that the p57-null hematopoietic cells were underrepresented in the blood of the recipient animals at steady-state. Both the limiting-dilution and competitive repopulation experiments indicate that the absence of p57 compromises HSC development. Strikingly, despite this quantitative deficit, we found that p57-null HSCs are qualitatively superior to p57-wt HSCs. Using serial transplantation, an assay of in vivo HSC self-renewal, we found that p57-null HSCs can be transplanted for two generations beyond which p57-wt HSCs are exhausted and unable to reconstitute hematopoiesis in the recipient animals. This result demonstrates under the tonic strain of serial transplantation, p57 normally serves to restrain HSC self-renewal. Importantly, mice with p57-null hematopoiesis are less sensitive to myelotoxic stress induced by the chemotherapeutics, 5-Fluorouracil (5FU) and cytarabine and have a shallower and shorter nadir following such treatment. Mice with p57-null hematopoiesis also recovered more briskly to the haemolytic agent phenylhydrazine suggesting that p57 serves a critical function restraining the stress-response of hematopoiesis. We next examined whether p57 might be involved in regulating the maintenance of LKSCD34- Flk2- long-term HSCs, LKSCD34+ Flk2- short-term HSCs, or LKSCD34+ Flk2+ MPPs (multipotent progenitors), LKSca-CD34+ FcRlow CMPs (common myeloid progenitors), LKSca- CD34+ FcR+ GMPs (granulocyte-monocyte progenitors) and LKSca- CD34- FcR- MEPs (megakaryocyte erythroid progenitors). Unlike in steady-state where the amount of cells in each subset in p57-null and wt bone marrow is comparable, we found significant variations in the p57-null bone marrow reconstitution after 5FU treatment. Furthermore when we administered 5FU to mice transplanted with mixtures of p57-null and p57-wt HSCs, we found that p57-null hematopoiesis contributed disproportionately to hematopoietic recovery and that this enhanced competition was durable. Our findings reveal a novel role of p57 to restrain HSC self-renewal during periods of hematopoietic stress. Deletion of p57 decelerates hematopoietic cells exhaustion due to serial transplantation and improves long-term engraftment, largely because of increased self-renewing divisions of HSCs in vivo. These results suggest that cells deficient in p57 have an advantage over p57-expressing cells only after hematopoietic stress suggesting that silenced p57 expression can serve to promote malignant hematopoiesis at the expense of normal hematopoiesis and contribute to chemotherapy resistance.