PS1122 RECURRENT XPO1 MUTATIONS IDENTIFIED AS PREDICTIVE MARKER FOR ADVANCED CLL PROGRESSION
Authors: J.S. Walker, Z.A. Hing, B. Harrington, C. Oakes, H.G. Ozer, A. Lehman, J. Skinner, C. Cempre, V. Coppola, J.C. Byrd, R. Lapalombella
Journal: HemaSphere (2019)

Background: Exportin 1 (XPO1/CRM1) is responsible for nuclear‐to‐cytosol export of RNAs and protein cargos, including various tumor suppressors ( e.g . p53, IκB) and growth regulators ( e.g . RB, p21). Our previous work has shown that XPO1 is upregulated in chronic lymphocytic leukemia (CLL), leading to mislocalization of critical cargos, which may contribute to activation of pathways involved in leukemogenesis and disease progression. Whole exome sequencing has revealed recurrent mutations to a highly conserved amino acid (E571G/E571K) near the XPO1 cargo binding groove in ∼5% of CLL cases and in ∼25% of CLL patients that develop a clinical complication of their disease with a very abysmal prognosis known as Richter's transformation. Despite inherent association with advanced hematologic malignancies, conflicting reports describe the merit of XPO1 mutations as a predictive marker for poor clinical outcome in CLL. Aims: We aimed to assess the significance of recurrent XPO1 E571 mutations on CLL progression and overall outcome by concurrent analysis of a human CLL clinical cohort and a large scale evaluation of a novel XPO1 transgenic mouse model. Methods: Retrospective analysis of an independent sample set of CLL cases (N = 466) was used to correlate XPO1 mutation status (N = 18, &gt;30% clonal population CLL cells) with clinical data. These included time to first treatment, overall survival, IgHV mutation status, ZAP‐70 expression (%), and status of epigenetic programming. Differential gene expression analysis in newly diagnosed untreated CLL patient lymphocytes was constructed from RNA‐sequencing, with downstream pathway analysis performed by Ingenuity Pathway Analysis software. A novel XPO1 transgenic mouse model (Eμ‐XPO1) was developed with human XPO1 (WT/G/K) gene under the heavy chain promoter/enhancer of the B cell expression, and was then crossed with the Eμ‐TCL1 mouse to assess contribution of XPO1 mutations in concordance with murine CLL. Disease onset and progression was determined via flow cytometry (% CD19/CD5 double positive cells) analysis of peripheral blood. Results: In the CLL cohort we analyzed, presence of XPO1 E571 mutations (18/466, 4%) confers disease characteristics consistent with poor clinical prognosis; including association with low epigenetic programming (18/18), unmutated IgHV region (17/17), elevated ZAP‐70 expression (p = 0.012), and reduction in treatment free progression (p = 0.002) and overall survival (p = 0.068) compared with XPO1 un‐mutated CLL. Similarly, overexpression of E571WT, K or G as the sole genetic abnormality (Eμ‐XPO1) resulted in the development of lymphoid neoplasms at advanced ages (&gt;20 mo) compared to age‐matched B6 mice. Furthermore, overexpression of E571G (N = 174, p = 0.001) and E571K (N = 188, p = 0.093), but not E571WT (N = 151, p = 0.713) XPO1 showed a trend of accelerated disease onset in the Eμ‐XPO1xTCL1 model consistent with a role of mutant XPO1 in disease progression. Lastly, RNA‐seq in XPO1 E571 or wild‐type CLL cells revealed unique clustering of differentially expressed genes specific to lymphocyte activation, signaling, and regulation, suggesting amplification of pro‐survival and anti‐apoptotic intracellular pathways. Summary/Conclusion: We report evidence for XPO1 E571 mutations as a predictive marker for poor CLL clinical prognosis warranting further investigation for selective targeting of mutated XPO1 as a personalized cancer therapy in CLL.