Transcription Factor Hes1 Functions As a Tumor Suppressor in MLL-Associated Acute Leukemia

The transcription factor Hairy enhancer of split1 (Hes1) is well characterised as a downstream target of Notch signaling. Hes1 is a basic helix-loop-helix-type protein, and represses target gene expression. Notch signaling has been proposed to play bivalent roles in tumorigenesis, depending on the cell types. Notch/Hes1 pathway promotes leukemogenesis in T-cell acute lymphoblastic leukemia, and Hes1 also plays an oncogenic role in a mouse model of blastic transformation of chronic myelogenous leukemia. Recent studies, however, demonstrated that Notch signaling serves as a tumor suppressor for chronic myelomonocytic leukemia. Meanwhile, the role of Notch signaling in acute myeloid leukemia (AML) remains to be elucidated. To explore whether Hes1 could possibly play oncongenic or tumor suppressive roles in AML, we focused on a mouse leukemia model established by retroviral transduction with MLL-AF9.

Common myeloid progenitors (CMPs), purified from Hes1^−/− or Hes1^+/+mouse fetal liver, were retrovirally transduced with MLL-AF9 cDNA. MLL-AF9-transduced cells were subjected to a colony-forming assay, with multiple rounds of replating. These cells were transplanted into lethally irradiated syngenic mice. CMPs from wild-type mouse bone marrow (BM-CMPs) were transduced with MLL-AF9 and dominant negative Hes1 (dnHes1), which lacks WRPW domain, and these cells were transplanted into irradiated mice. Secondary transplantation was performed by infusing leukemic bone marrow cells obtained from the primary recipients. Expression levels of downstream targets were evaluated by cDNA array and quantitative RT-PCR.

MLL-AF9-transduced Hes1^−/− CMPs formed higher number of colonies at third replating compared to MLL-AF9-transduced Hes1^+/+ CMPs, while colony numbers were not different in first and second plating. When infused to irradiated syngenic mice, MLL-AF9/Hes1^−/− cells developed leukemia at shorter latencies than MLL-AF9/Hes1^+/+ cells (MLL-AF9/Hes1^−/−, 7–10 weeks, n=18 vs MLL-AF9/Hes1^+/+, 10–14 weeks, n=18; p<0.001). Both MLL-AF9/Hes1^−/−- and MLL-AF9/Hes1^+/+- derived leukemia cells prepared from bone marrow of the first recipients were transplantable and caused leukemia in the secondary recipients with shorter and similar latencies (MLL-AF9/Hes1^−/−, 4 weeks, n=10 vs MLL-AF9/Hes1^+/+, 4 weeks, n=5). Mice receiving BM-CMPs transduced with MLL-AF9 and dnHes1 also developed leukemia at shorter latencies than mice receiving BM-CMPs transduced with MLL-AF9 and mock (dnHes1/MLL-AF9 8–10 weeks, n=10 vs mock/MLL-AF9 14–20 weeks, n=10, p<0.01). Leukemia cells induced by both MLL-AF9 and dnHes1 were transplantable and caused leukemia in the secondary recipients with latencies shorter than those induced by MLL-AF9 alone (dnHes1/MLL-AF9 3–4 weeks, n=10 vs mock/MLL-AF9 4–5 weeks, n=5, p<0.05). MLL-AF9/Hes1^−/− and MLL-AF9/Hes1^+/+ leukemia cells were then compared for mRNA expression by cDNA microarray. Among the genes with different expression levels between MLL-AF9/Hes1^−/− and MLL-AF9/Hes1^+/+leukemia cells, Meis1 was expressed at significantly higher levels in MLL-AF9/Hes1−/− leukemia cells.

These results imply that Hes1 serves as a tumor suppressor in MLL-AF9-induced AML. In most of the MLL-associated leukemias, dysregulation of HOX genes is thought to be a mainstay of leukemogenesis. Meis1 is a key cooperative factor for the function of each HOX protein, and thus, presumed to play an important role in development of MLL-associated leukemias. Repression of Meis1 might be among mechanisms how Hes1 functions as a tumor suppressor in MLL-associated leukemia.

No relevant conflicts of interest to declare.