Loss of the Cell Cycle Regulator p26^INCA1 Induces Exhaustion of Leukemic Stem Cells

Abstract 96

Acute myeloid leukemia is a clonal disease characterized by a malignant proliferation and accumulation of myeloid progenitor cells. Current therapeutic strategies are often not able to eradicate the leukemic cells. Malignancy is associated with deregulation of cell cycle check- points and the deregulation of checkpoints is associated with altered stem cell properties. A better understanding of malignant stem cells and their cell cycle regulation might help to develop new therapies. Recently, we identified p26^INCA1 as a novel cell cycle regulator. GST pulldown assays revealed binding of INCA1 predominantly to CDK2- specific Cyclins and we demonstrated an inhibitory effect of INCA1 on CDK2/ CyclinA complexes in kinase activity assays. The loss of INCA1 and its inhibitory effect on the cell cycle regulation led to an increased cell cycling and consequently to an enlarged stem cell pool in vivo. Upon cytotoxic stress, the loss of Inca1 enhanced cell cycling and bone marrow exhaustion. To analyze a potential role of INCA1 in leukemogenesis we retrovirally transduced wildtype and Inca1^−/− bone marrow cells with AML1-ETO9a (A1E9a) and transplanted these cells into wildtype recipients. Most of the wildtype cell- transplanted recipients died due to AML. In contrast, only one of the Inca1^−/− cell- transplanted mice developed AML. Engraftment was higher upon transplantion of Inca1^−/− cells but engraftment was not sustained. To consider the repopulation capacity of the leukemic cells we performed transplantation of primary leukemic cells into secondary recipients. A1E9a induced leukemia in Inca1 wildtype cells was transplantable and lethal. However Inca1^−/− leukemic cells were severely impaired in leukemia development in secondary recipients. Colony forming units and replating capacity were significantly reduced in A1E9a Inca1^−/− bone marrow cells although these cells showed increased CDK2 activity. Exhaustion of leukemic cells in the absence of Inca1 was confirmed by cloning efficiency assays. Further analyses were performed with c-myc induced leukemias. Interestingly, Inca1 deletion precluded the development of leukemias in secondary recipients. Taken together, these findings identify an important role for p26^INCA1 in the maintenance of leukemia and potentially the self-renewal capacity of leukemic stem cells.