Rearrangements of the mixed lineage leukaemia (MLL) gene account for approximately 70% of infant and 6% of all adult acute myeloid leukaemias (AMLs), and are associated with particularly poor prognosis. Translocations of the MLL gene with one of over 70 fusion partners produces an in-frame fusion oncoprotein capable of driving inappropriate transcriptional and regulatory programs, often co-occurring with secondary driver mutations in genes such as NRAS and FLT3. Here, we investigate the functional role of NRASG12D and FLT3-ITD mutations in MLL-AF9 driven AML, to determine essentiality and biological function in vitro and in vivo, as well as suggest a novel combination therapy rationale to treat this disease.
We developed inducible syngeneic mouse models of AML driven by coexpression of MLL-AF9 and either NRASG12D or FLT3-ITD. After genetic de-induction of NRASG12D or FLT3-ITD, we assessed the response on cell survival and differentiation in vitro, and disease progression in vivo. We also performed in vitro pre-clinical testing of a novel combination therapy utilising CDK9 inhibition and MEK inhibition (for MLL-AF9/NRASG12D driven AML) and CDK9/FLT3 inhibition (for MLL-AF9/FLT3-ITD driven AML).
Our data indicates MLL-AF9 AMLs are exquisitely sensitive to genetic withdrawal of NRASG12D/FLT3-ITD, leading to rapid induction of apoptosis. In vivo de-induction of these oncogenes led to extended survival of tumor-bearing mice, however this was in general not curative and mice eventually succumbed to disease, suggesting possible acquired resistance. Finally, we also show that in vitro combination therapies of CDK9 and MEK/FLT3 inhibition potently lead to induction of apoptosis in nanomolar concentrations.
Here, we have shown that withdrawal of NRASG12D/FLT3-ITD in MLL-AF9 AMLs leads to rapid apoptosis of leukaemias, and provides significant survival advantage in vivo. This response is recapitulated by a combination therapeutic regimen, combining CDK9 inhibition with either MEK or FLT3 inhibition.