Leukemia Risk Gene ARID5B is a Crucial Regulator of B-Cell Development
Authors: Goodings-Harris Charnise , Zhang Hui , Zhao Xuijie , Xu Heng , Abdel-Wahab Omar I , McKinney-Freeman Shannon , Yang Jun J. 
Published: October 2019

Abstract
Acute lymphoblastic leukemia (ALL) is the most common cancer in children. Genome wide association studies by us and others have identified germline genetic variants at the ARID5B locus that strongly influence susceptibility to ALL (Nat Genet 2009, J Clin Oncol 2012). Despite compelling results from these genetic association studies, the molecular mechanisms by which ARID5B contributes to ALL pathogenesis remains largely unknown. ARID5B is a member of the ARID transcription family characterized by a conservative AT rich binding domain, but the physiological functions of ARID5B are poorly understood. Therefore we sought to develop mouse models to comprehensively characterize the roles of ARID5B in normal and malignant hematopoiesis. To this end, we first established a Vav-1 specific Arid5b overexpression (AOE) mouse model. Mice were designed with a tetO element knocked in before the start codon of Arid5b to create an Arid5b tetO mouse. Arid5b tetO mice were then crossed with Vav-1-tTA mice and, overexpression of Arid5b confirmed in the hematopoietic system of progenies with the desired genotype.
At 6-8 weeks old complete blood counts (CBC) analysis revealed that AOE mice had a large decrease in white blood cells (WBCs) and a slight reduction in red blood cells (RBCs) and hemoglobin (Hb). We also noted a significant reduction in the bone marrow cellularity of our Arid5b overexpressing mice. To further characterize our AOE mouse model we used flow cytometry to quantify the proportions of mature as well as various stem and progenitor cell populations during hematopoiesis. We found that AOE mice showed an increase in the MegE biased MPP2 (LSK flt3−/CD150+/CD48+) population and a decrease in the lymphoid biased MPP4 (LSK (Flt3+/CD150−/CD48+/−) population. Overexpression of Arid5b resulted in a significant reduction in the proportion of all bone marrow B cell populations (Hardy Fraction A-F) (Figure 1A) and B220+ cells in the spleen. In vitro methylcellulose colony forming assays further revealed a loss of functional pre B lymphoid progenitor in AOE bone marrow. To determine if the hematopoietic defects seen in younger AOE mice was due to a delay in hematopoiesis we aged AOE mice. We found that as we aged AOE mice there was still a reduction in bone marrow cellularity and all bone marrow B cell populations. Surprisingly, as mice were aged we found that AOE mice were prone to sudden death and displayed a dramatic reduction in overall survival when compared to wildtype littermates with a mean survival age of 8 months (Figure 1B). At the time of death, we found that AOE mice presented with severely enlarged spleens. To predict the death of AOE mice we performed ultrasounds to track spleen volume. Based on ultrasounds performed on AOE mice and wildtype littermates, AOE mice spleens become larger than those of their littermates as early as 6 months old, however spleen volume did not predict sudden death. In aged AOE mice, while the spleens were grossly enlarged and contained larger amounts of red pulp and increased Ter119+ cells, there was still a reduction in the proportion of B cells and no increase in the proportions of other white blood cells (Figure 1D). Older AOE mice exhibited anemia, hypergammaglobulinemia, and splenomegaly. By 30 weeks, defects in B cell proportions were seen in the lymph nodes of AOE mice via immunohistochemistry (Figure 1C). Compared to wildtype littermates, AOE mice displayed reduced white blood cells, red blood cells, and platelets (Figure 1D). The anemia was associated with higher reticulocyte numbers and increased serum erythropoietin concentration. The life span of erythrocytes from AOE mice was less than that of wildtype littermates. Together, these results indicated that Arid5b plays an important role in B lymphopoiesis and erythropoiesis.

Disclosures
No relevant conflicts of interest to declare.