Abstract 5426: Regulation of mTOR signaling pathway by Ikaros and CK2 in high risk leukemia
Authors: Gowda Chandrika S. , Song Chunhua , McGrath Mary , Annageldiyev Charyguly , payne Kimberly , Dovat Sinisa 
Published: August 2018

Acute Lymphoblastic Leukemia (ALL) is the most common cancer in children. Alterations in IKZF1 have proven to be an indicator of inferior outcome in patients with high-risk ALL. Casein Kinase II (CK2) is a pro-oncogenic protein which is overexpressed in various cancers including leukemia. CK2 phosphorylates Ikaros and impairs its tumor suppressor functions. Targeted inhibition of CK2 restores Ikaros tumor suppressor function in high-risk B-ALL even in cases with single allele Ikaros deletion. Global chromatin immunoprecipitation (ChIP) coupled with the next-generation sequencing (ChIP-seq) studies in primary human B-ALL cells and in cell lines, demonstrated Ikaros occupancy of the promoter of genes involved in mTOR pathway. mTOR (mechanistic target of rapamycin) is the key component of the PI3K/AKT/mTOR pathway - a major signaling pathway that stimulates cellular proliferation in B-ALL. Several specific inhibitor of mTOR (rapamycin, Everolimus, sirolimus) are currently being used for the treatment of relapsed/refractory leukemia.

Objective of this study is to understand the mechanisms of transcriptional regulation of mTOR in B- ALL. We hypothesize that Ikaros negatively regulates the mTOR pathway by repressing transcription of mTOR and PI3K genes.

Ikaros binding to promoters of mTOR and genes that promote the PI3K pathway was confirmed using quantitative ChIP. Functional experiments such as overexpression of Ikaros in B-ALL cells results in transcriptional repression of mTOR and PI3K genes whereas Ikaros silencing using shRNA resulted in increased transcription. These results suggest that Ikaros negatively regulates the mTOR pathway by repressing transcription of mTOR and PI3K genes. Molecular inhibition of CK2 with shRNA targeting the CK2 catalytic subunit, as well as pharmacological targeting of CK2 with CX4945 resulted in transcriptional repression of mTOR and PI3K genes. CK2 inhibition was associated with increased Ikaros DNA-binding to the promoters of mTOR and PI3K genes. However, the ability of CX4945 to repress mTOR and PI3K genes is lost or severely reduced, in cells with shRNA silencing of Ikaros, as compared to cells with intact Ikaros. Moreover, similar results were noted following treatment with CX4945 in leukemia cells obtained from high risk B-ALL patients with deletion of one IKZF1 allele. Ikaros binds poorly to promoters of mTOR and PI3K genes in these cells. Treatment with CX4945 restores Ikaros DNA-binding to the promoters of both mTOR and PI3K genes, which is associated with strong repression of mTOR and PI3K genes.

In conclusion, these results suggest that CK2 inhibition with CX4945 represses the mTOR pathway by enhancing Ikaros-mediated transcriptional repression of mTOR and PI3K pathway genes. Results provide the rationale for the use of CK2 inhibitors in combination with mTOR inhibitors in high risk leukemia with dysregulation of mTOR/PI3K pathway.

Citation Format: Chandrika S. Gowda, Chunhua Song, Mary McGrath, Charyguly Annageldiyev, Kimberly payne, Sinisa Dovat. Regulation of mTOR signaling pathway by Ikaros and CK2 in high risk 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 5426.