Children with Down syndrome (DS) have an approximately 20-fold higher incidence of leukemia than the general population. The majority of leukemia cases associated with DS are acute megakaryoblastic leukemia (AMKL). Although GATA1 mutations have been found in almost all cases of transient myeloproliferative disorder (TMD) “a preleukemia” that may be present in as many as 10% of newborn infants with DS and AMKL accompanying DS (DS-AMKL), GATA1 mutation alone may not be sufficient for development of leukemia. Following identification of acquired activating JAK3 mutations in DS-AMKL, JAK3 mutations have been reported also in TMD patients. However, the frequency and functional consequence of JAK3 mutations in TMD remain unknown. To further understand how JAK3 mutations are involved in the development and/or progression of leukemia in DS, we screened TMD patients and two DS-AMKL cell lines for JAK3 mutation, and examined the functional consequences of these JAK3 mutations. In one out of the two DS-AMKL cell lines, MGS, we identified novel JAK3 mutations (JAK3Q501H mutation in the SH2 domain and JAK3R657Q mutation in the psuedokinase domain in the same allele). JAK3Q501H and JAK3R657Q each constitutively phosphorylates STAT5 and transformes Ba/F3 cells to factor-independent growth, whereas the double mutant (JAK3Q501H and JAK3R657Q) has more potent transforming activity than each mutant. Biochemichal analysis in Ba/F3 cells revealed that the degrees of phosphorylation of STAT5 in the cells transduced with each JAK3 mutant were correlated with its transforming activity. Although we previously identified a JAK3I87T mutation in one of two TMD patients, no JAK3 mutations were detected in another 9 TMD patients. Together with the previous results, we found JAK3 mutations in each of 11 TMD and 11 DS-AMKL patients. Although the number of the patients analyzed was small, these results indicate that there are no significant differences in the frequency of JAK3 mutations between TMD and DS-AMKL. In this study, we showed for the first time that the TMD patient-derived JAK3 mutation was also an activating one. JAK3I87T transformed Ba/F3 cells to factor-independent growth. Treatment with JAK3 inhibitors (WHI-P131 and WHI-P154) resulted in a significant decrease in the growth and viability of Ba/F3 cells expressing each activating JAK3 mutant. These results suggest that the JAK3 activating mutation is an early event during the development of AMKL in DS. Furthermore these results provide a proof-of-principle that JAK3 inhibitor should have therapeutic effects on the AMKL and TMD patients carrying the activating JAK3 mutations.

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Disclosure: No relevant conflicts of interest to declare.