Novel Activating STAT5B Mutations As Drivers Of T-ALL

T-cell acute lymphoblastic leukemia (T-ALL) is caused by the cooperation of multiple oncogenic lesions. Recent evidence supports that IL-7 and its receptor IL-7R contribute to T-ALL development (Zenatti et al, 2011). The two main pathways induced by IL-7R are JAK/STAT5 and PI3K/Akt/mTOR. Activating mutations to IL7R, JAK1, JAK2 or JAK3 are estimated to occur in 20-30% of all T-ALL patients (Cools 2013). STAT5 plays an important role in many hematologic malignancies but constitutive STAT5 activation often is a secondary event. Mutations in STAT5B (N642H) were recently described in LGL-leukemia in patients with an unusually aggressive and fatal form of the disease (Rajala et al, 2013). In other cancers, including ALL, patients with mutations in STAT5B have not been described. Here we report novel activating STAT5B mutations as drivers of T-ALL.

We performed exome sequencing of bone marrow (BM) samples from an 18-year-old female with relapsed T-ALL. Targeted next-generation amplicon sequencing and Sanger sequencing was used to analyze the region encoding the STAT5B SRC homology 2 (SH2) domain including the N642, T648 and I704 codons in a cohort of 17 adult and pediatric T-ALL patients treated at HUCH 2008-2013. For functional studies STAT5B expression vectors with the N642H, T648S or I704L mutation and an expression vector with both N642H and T648S mutations were used to transiently transfect HEK293 cells. To investigate the effect on transcriptional activity we co-transfected the mutant constructs with a STAT5 luciferase reporter plasmid and used Western blot analysis to study the phosphorylation status of the generated constructs. For drug sensitivity of STAT5B mutated cells we performed ex vivo drug testing on primary blasts from the index patient using a comprehensive set of 202 oncology drugs (approved and in clinical development). Each drug was tested over a 10,000-fold concentration range.

Sequencing of the index patient revealed 3 different somatic missense mutations in STAT5B (T648S, N642H, I704L) and mutations in KRAS, WT1 and SUZ12. No mutations affecting the JAK genes or IL7R were detected. All STAT5B mutations were located in the SH2 domain, which mediates dimerization and activation by trans-phosphotyrosine binding. The same three STAT5B mutations were also found in the diagnostic sample and most likely represent founding events in leukemogenesis. The N642H and T648S mutations occurred on the same allele with tumor mutation frequencies of approximately 40% while the I704L mutation occurred on a different allele with a similar tumor mutation frequency. To investigate the prevalence of STAT5B mutations in T-ALL we sequenced 17 BM samples from T-ALL patients. In this cohort we could not detect any other patients carrying mutations in the STAT5B SH2 domain.

Western blot analysis made with mutant constructs showed that the N642H and I704L mutations induced constitutive phosphorylation of STAT5B. Compared to wild type STAT5B the N642H and I704L mutants induced 47- and 6-fold increases in transcriptional activity, respectively, while T648S mutation had no effect in the assays. The construct with both the N642H and T648S mutations showed the highest amount of constitutive phosphorylation and induced a 56-fold increase in transcriptional activity compared to wild type STAT5B. Using ex vivo drug testing the STAT5B mutated blasts were resistant (EC50≥1 uM) to inhibitors of PI3K (e.g. idelalisib, XL147), dual inhibitors of PI3K/mTOR (PF-04691502, dactolisib) and mTOR inhibitors (temsirolimus, everolimus). Furthermore the blasts showed no response to AKT1 inhibitors (MK-2220) or JAK inhibitors (ruxolitinb, tofacitinib). In contrast, the cells were most sensitive to the BCL-2/BCL-XL inhibitor navitoclax (EC50 83 nM).

STAT5B mutations are uncommon in T-ALL but their occurrence underlines the significance of the IL7R-JAK-STAT5 pathway in the pathogenesis of T-ALL. While STAT5B mutant blasts were not sensitive to inhibitors targeting JAK kinases, the cells were unusually sensitive to inhibitors of target molecules of STAT5B, including anti-apoptotic BCL-2 proteins. These results suggest that BCL-2/BCL-XL inhibitors such as navitoclax are novel candidate therapies for T-ALL patients.

Mustjoki:Novartis: Consultancy, Speakers Bureau; BMS: Consultancy, Speakers Bureau. Porkka:BMS: Consultancy, Research Funding, Speakers Bureau; Novartis: Consultancy, Research Funding, Speakers Bureau.