Abstract B32: A novel preclinical model to identify therapies for CRLF2 B-ALL and reduce childhood cancer health disparities.

Hispanic children with Acute Lymphoblastic Leukemia (ALL) have a 39% higher death rate than white children. A major contributor to this disparity is the lack of therapies that specifically target CRLF2 B cell ALL. This high-risk leukemia occurs five times more frequently among children of Hispanic/Latino ethnicity than others and represents the most significant biological component of childhood cancer health disparities identified to date. CRLF2 B-ALL is caused by genetic alterations that result in B cells that overexpress the cytokine receptor component, CRLF2, leading to malignant transformation and high-risk leukemia with poor prognosis. CRLF2 pairs with the IL-7Rα, to form a receptor complex that is activated by the cytokine, TSLP. Activation of the receptor complex stimulates downstream JAK-STAT5 phosphorylation that has been shown to induce proliferation and survival of B-ALL cells. Activating JAK mutations are found in many cases of CRLF2 B-ALL and have led to speculation that stimulation of the receptor by TSLP is not a factor in this disease. In preliminary studies to address this question we evaluated the effect of TSLP in CRLF2 B-ALL cells with JAK defects. Our data show that TSLP increases STAT5 phosphorylation in these cell lines and also in primary CRLF2 B-ALL cells. These data suggest that TSLP can activate JAK-STAT5 signaling to induce downstream survival and proliferation in CRLF2-B-ALL cells, including those with JAK defects. Bone marrow (BM) provides the tumor microenvironment that can harbor chemoresistant B-ALL cells responsible for relapse in B-ALL. We used RT-PCR and ELISA assays to show that TSLP is expressed in BM cells from pediatric patients and thus could provide an in vivo source of TSLP to stimulate CRLF2 B-ALL cells. Human-mouse xenograft models produced by transplanting human leukemia cells into immune deficient mice mimic the in vivo environment and are the model of choice for identifying therapies that target the mechanisms of chemoresistance that are characteristic of high-risk leukemia. However, mouse TSLP is different from most other cytokines produced in the xenograft in that it is species-specific and does not activate the human TSLP receptor complex that contains CRLF2. Thus, traditional xenograft models do not provide the TSLP-CRLF2 interactions that our data implicate as a contributing factor in CRLF2 B-ALL. To overcome this obstacle we have engineered immune deficient mice to express human TSLP (hTSLP+ mice) as well as control mice that lack the TSLP cytokine (hTSLP– mice). ELISA assays show hTSLP levels in the hTSLP+ mice that approximate the normal range in human plasma. We used this hTSLP+/- xenograft model system to study the in vivo effects of TSLP on mice transplanted with a CRLF2 B-ALL cell line harboring a JAK defect (MUTZ5) and with primary pre-B ALL cells from a Hispanic patient. Mice were euthanized at 5 weeks and BM disease was evaluated. In recipients of MUTZ5 B-ALL cells the percentage of viable leukemia cells in hTSLP+ mice was twice that observed in hTSLP- mice. Similarly, in recipients of primary B-ALL, the percentage of viable leukemia cells was higher in hTSLP+ than hTSLP- mice. These data provide evidence that the TSLP produced in this model is active and that TSLP-CRLF2 interactions contribute in vivo to CRLF2-B-ALL. This model will be particularly important for identifying therapies that can effectively target CRLF2 B-ALL and reduce cancer health disparities in Hispanic childhood B-ALL. Citation Format: Francis L. Olivia, Shannalee R. Martinez, Terrence Bennett, Ineavely Baez, Terry-Ann Milford, Christopher L. Morris, Ross O. Fisher, Xiao-Bing Zhang, Rui-Jun Su, Sinisa Dovat, Kimberly J. Payne. A novel preclinical model to identify therapies for CRLF2 B-ALL and reduce childhood cancer health disparities. [abstract]. In: Proceedings of the Fifth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2012 Oct 27-30; San Diego, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2012;21(10 Suppl):Abstract nr B32.