Regulation Of The Cdc42 Signaling Pathway By Ikzf1 In T-Cell Acute Lymphoblastic Leukemia

The IKZF1 gene encodes Ikaros—a DNA-binding, Kruppel-like zinc finger protein that functions as a tumor suppressor in T-cell acute lymphoblastic leukemia (T-ALL). Ikaros regulates transcription of a large number of genes that are involved in control of cellular proliferation. However, the molecular mechanisms through which Ikaros regulates proliferation of T-ALL cells are still unknown. The use of quantitative chromatin immunoprecipitation (qChIP) showed that Ikaros binds to the promoter regions of CDC42 in vivo in primary T-ALL patient samples as well as in T-ALL cell lines. Ikaros overexpression in B-ALL cells via retroviral transduction results in decreased transcription of CDC42, as evidenced by qRT-PCR and Western blot. The luciferase reporter assay showed that Ikaros represses CDC42 transcription by directly binding to its promoter. The shRNA-mediated knock-down of Ikaros in T-ALL resulted in increased expression of CDC42. These results suggest that Ikaros functions as a transcriptional repressor of CDC42 in T-ALL. Next, we studied the upstream signaling pathways that regulate Ikaros-mediated control of CDC42 transcription in T-ALL. Because Ikaros activity in leukemia is regulated via direct phosphorylation by Casein Kinase II (CK2), we tested the effect of CK2 inhibition on Ikaros-mediated regulation of CDC42 expression. Treatment of T-ALL cells with a specific CK2 inhibitor, CX-4945, resulted in reduced expression of CDC42 in T-ALL cells. This was associated with an increase in DNA-binding of Ikaros to the promoter of the CDC42 gene. Ikaros knock-down restored high expression of CDC42 in T-ALL cells that were treated with CK2 inhibitors. These data suggest that transcriptional repression of the CDC42 gene by Ikaros is impaired in T-ALL due to increased CK2 activity and reduced Ikaros DNA-binding affinity toward the CDC42 gene promoter. Inhibition of CK2 restores Ikaros function and results in transcriptional repression of the CDC42 gene. In conclusion, our results suggest that Ikaros regulates CDC42 signaling pathway via transcriptional repression of the small GTPase, CDC42, in T-ALL. Ikaros-mediated repression of CDC42 in T-ALL is negatively regulated by CK2. These results suggest that regulation of CDC42 expression is one of the mechanisms through which Ikaros regulates cellular proliferation in T-ALL. Citation Format: Mario Soliman, Chunhua Song, Jonathon L. Payne, Zheng Ge, Chandrika Gowda, Yali Ding, Kimberly J. Payne, Sinisa Dovat. Regulation of the CDC42 signaling pathway by IKZF1 in T-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1512.