Abstract 142: Loss of AID exacerbates the malignant progression of CLL

Abstract

Abstract Activation-induced cytidine deaminase (AID) is an enzyme that has been implicated as both a positive and a negative factor in the progression of B cell chronic lymphocytic leukemia (CLL), but the role that it plays in the development and progression of this disease is still unclear. Higher AID expression in patients has been correlated with a worse prognosis, but patients with mutated immunoglobulin (IgHV) genes, which requires AID activity, have a better prognosis. To elucidate the role of AID in leukemic progression, we generated an AID-deficient mouse model of CLL, AID-/-/Eμ-TCL1, and found that these mice die significantly earlier than their AID-proficient counterparts. AID-/-/Eμ-TCL1 mice have higher percentages of germinal center B cells and increased infiltration of CLL cells into the lungs, liver, and peritoneal cavity. Both AID-deficient B cells and CLL cells downregulated the tumor suppressive Smad1/S1PR2 signaling pathway, which normally restrains expansion of the germinal center, potentially leading to altered proliferation and homing of leukemia cells. Furthermore, AID-/-/Eμ-TCL1 CLL cells exhibited higher levels of B cell receptor (BCR) signaling, supporting their growth and survival. Upregulated BCR signaling led us to investigate the IRE1α/XBP1s pathway in these cells, which is necessary to support proper BCR signaling and is upregulated in CLL cells. IRE1α is an endoplasmic reticulum (ER) stress sensor that cleaves XBP1 mRNA to produce the functional transcription factor XBP1s. XBP1s promotes synthesis of lipids and chaperones necessary for BCR signaling, and inhibition of this pathway compromises BCR signaling and CLL survival. Compared to Eμ-TCL1 controls, AID-/-/Eμ-TCL1 CLL cells indeed respond to various ER stressors with a higher ER stress response, particularly through the IRE1α/XBP1s pathway, resulting in cells that are more resistant to the stressed conditions within the tumor microenvironment. Mechanistically, this upregulation in the ER stress response cannot be solely attributed to an accelerated leukemic progression, as B cells from AID-/- mice also express increased levels of XBP1s and are more responsive to treatment with pharmacological ER stressors. These studies are translatable to human CLL, as shRNA knockdown of AID in the CLL cell line WaC3 resulted in an increased ER stress response, and CLL cells from patients with IgHV-unmutated disease express higher levels of XBP1 mRNA than those from patients with mutated CLL. Additionally, treatment of both mouse and human CLL cells with a combination of the S1PR2 agonist CYM-5520 and the IRE1 inhibitor B-I09 led to improved induction of apoptosis, revealing a novel way to treat CLL. Overall, these results demonstrate multiple pathways through which the loss of AID leads to worsened CLL and may explain why IgHV-unmutated CLL has such a high proliferative rate that requires therapeutic intervention. Citation Format: Avery C. Lee, Sai R. Pingali, Javier A. Pinilla-Ibarz, Chih-Hang A. Tang, Chih-Chi A. Hu. Loss of AID exacerbates the malignant progression of CLL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 142.