The JAK3 kinase as a therapeutic target in T-cell acute lymphoblastic leukemia.

The JAK kinases are a family of cytosolic tyrosine kinases that are essential for the signaling of cytokine receptors. All 4 JAK kinases share a common structure with receptor-binding domain at the N-terminus and a regulatory pseudokinase domain and catalytic active kinase domain at the C-terminus. The 2 JAK kinase family members JAK1 and JAK3 are essential components of the heterodimeric interleukin-7 (IL7) receptor, in which the IL7Rα chain is bound by JAK1 and the common gamma chain (γc) is bound by JAK3. Activating mutations were identified in several components of the IL7R signaling complex in T-cell acute lymphoblastic leukemia patients. The most frequently mutated IL7R component is JAK3, which is found in up to 16% of the patients. These mutations were located throughout the entire JAK3 gene. Of all tested JAK3 mutations only the mutations located in the regulatory pseudokinase domain and catalytic active kinase domain were able to transform our Ba/F3 and MOHITO cell models to cytokine independent growth, indicating their oncogenic properties. Subsequently, we showed that mice transplanted with bone marrow progenitor cells expressing JAK3 mutants developed a long-latency transplantable T-ALL like disease.

We were able to show that most JAK3 mutants are depending on the presence of the cytokine receptor complex, including JAK1, for their transforming capacities. Interestingly, 30% of the T-ALL cases harbor secondary JAK3 mutations that either lead to mutation of the second JAK3 allele or to the presence of 2 mutations in the same JAK3 allele. Our data demonstrate that most JAK3 mutants are relatively weak oncogenic alleles that can gain more transforming potential by removal of the competing wild type JAK3 protein or by the acquisition of secondary mutations that lead to stronger downstream signaling.

Finally, using an unbiased comparative and quantitative phosphoproteomics study we identified proteins of the cell cycle, translation, MAPK and PI3K/AKT pathways and proteins implicated in epigenetic regulation to be downstream of mutant JAK3. These findings provided the rationale to test selective inhibitors that target these various pathways in combination with JAK selective inhibitors. Using patient derived xenograft samples we were able to show a synergistic inhibitory effect on JAK3 mutant samples when treated with a JAK selective inhibitor combines with inhibitors targeting the MAPK pathway.