Abstract: | JAK1 and JAK3 are recurrently mutated in acute lymphoblastic leukemia. These tyrosine kinases associate with heterodimeric cytokine receptors such as IL-7 receptor or IL-9 receptor, in which JAK1 is appended to the specific chain, and JAK3 is appended to the common gamma chain. Here, we studied the role of these receptor complexes in mediating the oncogenic activity of JAK3 mutants. Although JAK3V674A and the majority of other JAK3 mutants needed to bind to a functional cytokine receptor complex to constitutively activate STAT5, JAK3L857P was unexpectedly found to not depend on such receptor complexes for its activity, which was induced without receptor or JAK1 co-expression. Introducing a mutation in the FERM domain that abolished JAK-receptor interaction did not affect JAK3L857P activity, whereas it inhibited the other receptor-dependent mutants. The same cytokine receptor independence as for JAK3L857P was observed for homologous Leu857 mutations of JAK1 and JAK2 and for JAK3L875H. This different cytokine receptor requirement correlated with different functional properties in vivo and with distinct sensitivity to JAK inhibitors. Transduction of murine hematopoietic cells with JAK3V674A led homogenously to lymphoblastic leukemias in BALB/c mice. In contrast, transduction with JAK3L857P induced various types of lymphoid and myeloid leukemias. Moreover, ruxolitinib, which preferentially blocks JAK1 and JAK2, abolished the proliferation of cells transformed by the receptor-dependent JAK3V674A, yet proved much less potent on cells expressing JAK3L857P. These particular cells were, in contrast, more sensitive to JAK3-specific inhibitors. Altogether, our results showed that different JAK3 mutations induce constitutive activation through distinct mechanisms, pointing to specific therapeutic perspectives. |