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CDC25A is a member of the CDC25 family of phosphatases that plays a major role in cell cycle progression. Here, we present evidence that expression of CDC25a in T-ALL is regulated at the transcriptional level by oncogenic Casein Kinase II (CK2) via direct phosphorylation of Ikaros, a transcription factor and tumor suppressor protein. Global chromatin immunoprecipitation coupled with next-generation sequencing (ChIP-seq) studies in both primary human acute lymphoblastic leukemia cells and cell lines, demonstrated that Ikaros binds to the promoter of the CDC25a gene. Ikaros functions as a tumor suppressor protein and deletion of which is associated with development of T-ALL. Ikaros binding to CDC25a promoter was confirmed by quantitative chromatin immunoprecipitation (qChIP) in primary T-ALL cells. Ikaros knock-down with shRNA results in increased transcription of CDC25a in T-ALL. In mice, T-ALL cells that were derived from Ikaros-knockout mice express high levels of CDC25a. Transduction of these cells with Ikaros-containing retrovirus results in sharp reduction of CDC25a expression. Overexpression of CK2 via retroviral transduction resulted in increased transcription of the CDC25a gene, as measured by qRT-PCR, as well as increased overall expression of CDC25a, as measured by Western blot. Increased expression of CK2 was associated with a loss of Ikaros binding to the CDC25a gene promoter. Molecular inhibition of CK2 using shRNA, as well as pharmacological inhibition with a specific CK2 inhibitor resulted in reduced expression of CDC25a in primary human T-ALL. CK2 inhibition was also associated with strong reduction in AKT phosphorylation, emphasizing that CK2 inhibition downregulates CDC25a and other cell cycle progression genes. Inhibition of CK2 was associated with increased Ikaros binding at the promoter of CDC25a. Ikaros knock-down restored high expression of CDC25a in T-ALL cells that were treated with CK2 inhibitors. These data showed that CK2 and Ikaros are major transcriptional regulators of CDC25a transcription in T-ALL and that CK2 inhibition represses CDC25a transcription via Ikaros-mediated repression. In conclusion, these results indicate that expression of the CDC25a oncogene in T-ALL is regulated by the CK2 which modulates Ikaros activity. Presented data revealed a novel mechanism of therapeutic action of CK2 inhibitors - repression of CDC25a expression via Ikaros. Results provide a rationale for the use of novel CK2 inhibitors in T-ALL. Citation Format: Soumya C. Iyer, Shriya Kane, Chandrika Gowda, Chunhua Song, Yali Ding, Jon Payne, Pavan Kumar Dhanyam Raju, Bihua Tan, Mary McGrath, Yevgeniya Bamme, Mario Solimon, Nathalia Moreno Cury, Dhimant Desai, Arati Sharma, Kimberly J. Payne, Sinisa Dovat. Regulation of CDC25a expression by the ikaros and casein kinase II (CK2) in T-cell acute lymphoblastic leukemia (T-ALL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 871.