Abstract 2527: Regulation of mitotic progression in T-cell acute lymphoblastic leukemia by the Ikaros tumor suppressor

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that represents a therapeutic challenge. Next-generation sequencing revealed that a subset of high-risk T-ALL which is associated with poor prognosis harbor inactivating mutations or deletion of one allele of the IKZF1 tumor suppressor. The IKZF1 gene encodes the Ikaros protein that functions as a regulator of transcription and a tumor suppressor. However, the molecular mechanism of Ikaros' tumor suppressor function in T-ALL is unclear. The use of quantitative chromatin immunoprecipitation (qChIP) determined that Ikaros binds to the promoter regions of Anaphase Promoting Complex Subunit 1 (ANAPC1) and Anaphase Promoting Complex Subunit 7 (ANAPC7) cell cycle genes in T-ALL primary cells in vivo. ANAPC1 and ANAPC7 genes encode proteins whose function is essential for progression through mitosis, as well as through the G1 phase of the cell cycle. Ikaros overexpression in T-ALL via retroviral transduction, results in reduced expression of ANAPC1and ANAPC7, as evidenced by quantitative RT-PCR (qRT-PCR) and Western blot. The luciferase reporter assay further confirmed Ikaros' function as a transcriptional repressor of ANAPC1 and ANAPC7. The knock-down of Ikaros with shRNA in T-ALL resulted in increased transcription of ANAPC1 and ANAPC7, as evidenced by qRT-PCR. These data suggest that Ikaros can regulate mitotic progression in T-ALL by repressing transcription of ANAPC1 and ANAPC7 genes. Next, we studied the mechanisms that regulate Ikaros' ability to repress ANAPC1 and ANAPC7 in T-ALL. Ikaros' function as a transcriptional repressor is regulated by Casein Kinase II (CK2). CK2 is overexpressed in hematopoietic malignancies and increased expression of CK2 results in T-ALL in murine models. We tested the effect of CK2 inhibition on Ikaros' ability to regulate transcription of ANAPC1 and ANAPC7 in human T-ALL. Molecular inhibition of CK2 with shRNA against the CK2 catalytic subunit resulted in reduced transcription of ANAPC1 and ANAPC7, as evidenced by qRT-PCR. This was associated with increased DNA-binding of Ikaros to the promoters of ANAPC1 and ANAPC7, as evidenced by qChIP. These data suggest that CK2 impairs Ikaros' ability to transcriptionally repress ANAPC1 and ANAPC7 and to regulate mitotic progression in T-ALL. Inhibition of CK2 enhances transcriptional repression of ANAPC1 and ANAPC7 by Ikaros, resulting in the cell cycle arrest of T-ALL. In conclusion, our results show