ONCOGENIC COOPERATION BETWEEN TAL1 EXPRESSION AND PI3K/AKT PATHWAY
ACTIVATION IN T-CELL ACUTE LYMPHOBLASTIC LEUKEMIA
Naomi Thielemans, Sofie Demeyer, Nicole Mentens, Sarah Provost, Olga Gielen and Jan Cools
VIB-KU Leuven Center for Cancer Biology, Leuven, Belgium
Background
T-cell acute lymphoblastic leukemia (T-ALL) is caused by the accumulation of multiple genetic
defects, including mutually exclusive overexpression of transcription factors (such as TAL1),
mutations in the NOTCH pathway, JAK-STAT pathway or PI3K pathway. Analysis of sequencing
data revealed a significant correlation between TAL1 expression and mutations causing
PI3K/AKT pathway activation. These data suggest that there could be a synergy between TAL1
expression and PI3K/AKT signaling during leukemia development.
Methods
CD2-Cre mice donor cells were transduced with inducible retroviral vectors containing TAL1,
mutant AKT(E17K) or both, which would only be activated by Cre recombinase in CD2 positive
cells. Transduced cells were injected in the tail vein of wild type recipient mice. In vitro we
usedimmature T cells (pro-T cells) derived from Cre-ER mice that were cultured ex vivo in the
presence of Dll4, Scf and Il7. These cells were transduced with above mentioned inducible
vectors. Expression was induced by 4OH-Tamoxifen. ChIP-seq and RNA-seq analysis was
performed on the different models.
Results
Primary transplantation of TAL1 transduced cells did not cause any disease. Even more,
constitutive transplantation of TAL1 showed decreased survival of transplanted bone marrow
cells with lower infiltration compared to empty vector suggesting proliferative disadvantage
of TAL1 expressing cells. In contrast, induced expression of AKT(E17K) alone or TAL1 + AKT(E17K)
caused T cell lymphoblastic lymphomas with similar latency. The lymphoma cells were of late
cortical stage in both conditions. The TAL1 + AKT(E17K) lymphoma cells were transplantable and
caused leukemia with increased white blood cell counts, while the AKT(E17K) only lymphoma
cells were not transplantable to secondary recipients. In agreement with our findings in vivo,
induced expression of TAL1 alone inhibited the growth of pro-T cells ex vivo, which could be
rescued by co-expression of an AKT(E17K) activating mutant.
To get insight in the mechanism of TAL1 induced growth disadvantage and rescue mechanism
of PI3K pathway hyperactivation we performed gene expression profiling on sorted TAL1+
thymus cells and on pro-T-cells after different times of TAL1 expression induction. RNA-seq
and CHIP-seq analyses of TAL1+ pro-T cells or sorted thymus cells showed a direct
upregulation of transcription factor E2F2 and GSEA showed strong upregulation of E2F target
genes. E2F2 is known to have a tumour suppressor role in T cell malignancies. E2F2 has been
shown to play a role in DNA stability and DNA repair. Indeed, both in vivo and in vitro model
showed upregulation of DNA repair in TAL1+ cells compared to wild type. Therefore,
combining PI3K inhibition and PARP inhibition was tested in T-ALL cellines and showed
synergistic effects.
Conclusion
Our models identify vulnerabilities associated with TAL1 expression in T-ALL and indicates that
TAL1+ T-ALL patients might be good candidates for therapy with PI3K inhibitors in
combination with PARP inhibitors.
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