Acute lymphoblastic leukemia is the most common cancer of children. Individual cases of leukemia may have multiple genetic lesions, and identifying those that drive leukemogenesis will be important in the development of targeted therapy. Approximately 10% of pediatric T-cell acute lymphoblastic leukemia (T-ALL) cases have a mutation in IL-7Rα. These mutations are thought to be oncogenic, but little is known about the effects of the mutation on T-cell development. In addition, the mutation does not seem to induce leukemia in the absence of other genetic lesions, suggesting that collaborative mutations are required for leukemogenesis. Based on patient data, potential collaborators include TLX3 expression, HOXA gene cluster overexpression, and NRAS mutation.

Given the current state of knowledge regarding mutant IL-7Rα, this project was developed with two specific aims. The first was to investigate the effects of mutant IL-7Rα gain-of-function (IL-7Rα-GOF) on T-cell development in vitro and in vivo. The second was to determine whether candidate collaborative genetic lesions would drive T-ALL formation when combined with mutated IL-7Rα. To address these aims, immature murine thymocytes were cultured on an OP9-DL4 stromal cell system, transduced with retroviral vectors, and injected into sub-lethally irradiated Rag1-/- mice. Resultant diseases were analyzed using a variety of techniques including flow cytometry, histology, immunohistochemistry, ligation-mediated PCR, TCRβ clonality assessment, RNA-sequencing, serial passage, and limiting dilution assay.

Studies showed that IL-7Rα-GOF mutation caused an increase of CD8+ cells in vitro. When thymocytes transduced with IL-7Rα-GOF mutation were injected into mice, animals developed a multi-systemic inflammatory disease. This inflammation was not due to imbalance in populations of Treg and Th17 cells, as had been hypothesized.

Assessing collaborations with TLX3 expression, HOXA overexpression, and NRAS mutation showed that combination of these genetic lesions with IL-7Rα-GOF mutation caused different neoplastic diseases. The combination of IL-7Rα-GOF mutation and TLX3 expression caused low-penetrance, late-onset T-cell lymphoma. Thymocytes overexpressing the HOXA gene cluster and transduced with IL-7Rα-GOF mutation caused a rapid-onset myeloid leukemia. Combination of IL-7Rα-GOF mutation with mutant NRAS yielded rapid-onset, full-penetrance T-cell lymphoblastic leukemia, suggesting that this combination of mutations was sufficient to induce T-ALL. These experimental results may help to lay the foundation for the development of targeted therapy for pediatric T-ALL.