Primary human cells offer a potentially ideal target for the study of oncogene function. Using human cell–based models, we demonstrated dual functions of RUNX1 in myeloid leukemogenesis. RUNX1 overexpression inhibits the growth of CB cells by inducing myeloid differentiation, whereas a certain level of RUNX1 activity is required for the growth of AML cells. Human AML cells are more sensitive to RUNX1 inhibition probably because their RUNX activity is already reduced. We also used a mouse AML model and showed that the combined loss of RUNX1/CBFB inhibits the development of murine MLL fusion leukemia, while single loss of RUNX1 accelerates it (11
). Based on these results, we propose a model to explain the dosage-dependent function of RUNX in myeloid leukemogenesis (Figure ). Partial loss of RUNX activity blocks myeloid maturation and supports AML development, while a further reduction of RUNX activity results in cell cycle arrest and cell death.
Proposed model illustrating dosage-dependent functions of RUNX in myeloid leukemogenesis.
In the past few decades, the scientific community has primarily focused on “oncogenes” that are overexpressed in tumor cells as ideal therapeutic targets. However, such oncogenes are usually also important for the maintenance of normal stem cells, and therefore it remains unclear whether inhibiting such oncogenes would favor normal cells over tumor cells. Instead, genes that are underexpressed in cancer cells but are essential for their survival will be promising targets to selectively eradicate cancer cells while preserving normal stem cells. Importantly, many so-called “tumor suppressors” appear to have such features. In addition to RUNX1, two other well-known tumor suppressors, PML and FOXOs, were shown to have survival roles in leukemia stem cells (38
). Interestingly, recent genome-wide sequence studies in myeloid neoplasms found inactivating mutations in genes that were shown to have oncogenic properties, including GATA2, EZH2, and NOTCH pathway genes (40
). Although these findings reveal a potential tumor suppressor role for these oncogenes, it is highly likely that these genes still retain some growth-promoting function. Indeed, several recent reports showed an important role for EZH2 in the development of MLL fusion leukemia (45
). Dosage-dependent functions of these genes in myeloid leukemogenesis need to be clarified in future studies.
Mouse models have proven to be invaluable tools for the understanding of human cancer. Nevertheless, significant differences exist between mouse models and clinical diseases. The current study highlights the potential influence on tumor development of a compensatory mechanism in mice lacking individual genes. Our data, together with a recent report showing that RUNX2 is upregulated by MLL-AF9 in mouse bone marrow cells (48
), strongly indicate the functional redundancy between RUNX1 and RUNX2 in MLL fusion leukemia. The upregulated RUNX2 probably provides sufficient RUNX activity for Runx1
-deficient murine MLL-ENL cells to develop leukemia. It is not clear why we did not see the compensatory effects in human cell–based models. Given that RUNX1 is not mutated in patients with MLL fusion leukemia, human MLL fusion AML cells may require stronger RUNX activity than the corresponding mouse cells for optimal growth. Alternatively, adverse effects of shRNAs used in human cells and/or different experimental assays could affect the results. While this manuscript was under review, Wilkinson et al. showed that RUNX1 is a transcriptional target of MLL-AF4 and supports the growth of MLL-AF4 cells (49
). Although they argued that RUNX1 is specifically important for the growth of MLL-AF4 cells, our study clearly demonstrates that other types of MLL fusion leukemia also require a certain level of RUNX activity. Compounds targeting RUNX/CBFB function will show substantial efficacy in MLL fusion leukemia, and these compounds can be expected to exhibit a reasonable therapeutic window relative to normal blood stem cells.
We identified BCL2 as an important mediator for the survival effect of RUNX1 in human MLL fusion leukemia. However, BCL2 did not rescue the cell cycle arrest induced by RUNX1 depletion. Furthermore, Runx1-deficient murine MLL-AF9 cells still developed leukemia despite reduced BCL2 expression. Therefore, other factors must also contribute to the RUNX1-mediated leukemia cell growth. The mechanisms for RUNX1-mediated cell cycle progression remain elusive, but CDKN1A appears to be at least involved in this process, given its consistent upregulation in RUNX1-inhibited leukemia cells. Further characterization of RUNX/CBFB-mediated gene regulation may reveal the way to specifically block the survival function of RUNX without preventing its tumor suppressor role in promoting myeloid differentiation.
In summary, we found an unexpected survival- and growth-promoting role for RUNX1 in the development and maintenance of AML with leukemogenic fusion proteins. Recent efforts to target this transcription factor complex in CBF leukemia (31
) should be extended to AML associated with MLL rearrangements and potentially to other myeloid neoplasms with RUNX1 dysfunction.