Transcriptional Regulation of Cell Cycle Progression in T‐Cell Leukemia

Abstract

The IKZF1 gene encodes the Ikaros protein that acts as a tumor suppressor in acute lymphoblastic leukemia (ALL). The role of Ikaros in T‐cell acute lymphoblastic leukemia (T‐ALL) has been documented by the presence of Ikaros deletions in a significant subset of human T‐ALL. Ikaros functions as a transcriptional regulator of a large set of its target genes and a tumor suppressor in B‐cell acute lymphoblastic leukemia. However, the molecular mechanisms of the tumor suppressor activity of Ikaros in T‐ALL is unclear. The use of quantitative chromatin immunoprecipitation (qChIP) showed that Ikaros binds to the promoter regions of the CDK2 and CDK6 cell cycle genes in primary T‐ALL cells in vivo. Ikaros gain‐of‐function experiments performed using retroviral transduction of T‐ALL cells showed that Ikaros overexpression in T‐ALL results in the reduced expression of CDK2 and CDK6, as evidenced by quantitative RT‐PCR (qRT‐PCR) and western blot. The knock‐down of Ikaros with shRNA in T‐ALL resulted in increased transcription of CDK2 and CDK6 as measured using qRT‐PCR. These data suggest that Ikaros can regulate cell cycle progression in T‐ALL by direct transcriptional repression of the CDK2 and CDK6 genes. Next, we studied the mechanisms that regulate Ikaros' ability to repress CDK2 and CDK6 in T‐ALL. Ikaros' function as a transcriptional repressor is regulated by its direct phosphorylation by Casein Kinase II (CK2). CK2 is an oncogenic kinase that is overexpressed in hematopoietic malignancies. We tested the effect of CK2 inhibition on Ikaros' ability to regulate transcription of CDK2 and CDK6 in human T‐ALL. Molecular inhibition with shRNA, as well as pharmacological inhibition of CK2 with a specific inhibitor, CX‐4945, resulted in reduced transcription of CDK2 and CDK6. This was associated with increased DNA‐binding of Ikaros to the promoters of CDK2 and CDK6 as indicated by qChIP. These data suggest that CK2 impairs Ikaros' ability to transcriptionally repress CDK2 and CDK6 and to regulate cell cycle progression in T‐ALL. Inhibition of CK2 restores Ikaros' function as a transcriptional repressor of CDK2 and CDK6 and as a regulator of cell cycle progression in T‐ALL. In conclusion, our results demonstrate that the Ikaros tumor suppressor can control cell cycle progression in T‐ALL via transcriptional regulation of CDK2 and CDK6 expression. Overexpression of CK2 impairs Ikaros' ability to repress CDK2 and CDK6 expression, which contributes to deregulation of cell cycle control in T‐ALL. Results reveal the therapeutic mechanism of targeted inhibition of CK2 as treatment for T‐ALL.