Patients with well-differentiated forms of cancer may derive substantial clinical benefit from therapies that modify the aberrant behavior of the malignant clone, without damaging normal tissues, even if the tumor is not eradicated. In this case, the value of therapy lies in reducing the symptoms caused by the neoplasm for a meaningful time period. A classic example of this paradigm is the use of anti-proliferative agents, such as hydroxyurea, in the treatment of chronic myelogenous leukemia (CML), a myeloproliferative neoplasm driven by the aberrant fusion kinase Bcr-Abl (15
). Even as highly effective small-molecule-targeted inhibitors of the Bcr-Abl kinase have markedly prolonged the survival of patients with CML, the causative mutation often remains detectable at low levels, and clinical experience supports the idea that a highly beneficial treatment does not always eliminate all mutant stem cells (16
). In our studies, Mx1-Cre, KrasG12D
mice treated with PD0325901 demonstrated a rapid and sustained improvement in hematopoietic abnormalities, even though the KrasG12D
oncogene persisted at high levels in the bone marrow. This is similar to patients with myeloproliferative neoplasms that undergo clinical, but not molecular, remissions. An implication of this idea is that a reduction in oncogene ‘allele burden,’ which is frequently used as a surrogate of disease response in clinical trials, may not always be required for a therapeutic agent to demonstrate clinical utility.
Our data indicate that the dramatic hematological responses observed in Mx1-Cre
mice treated with the MEK inhibitor PD0325901 are not due to a purely anti-proliferative effect, as observed with conventional chemotherapy. The effects of MEK inhibition in Mx1-Cre
mice also contrast with those of Abl kinase inhibitors in patients with CML, where treatment provides a selective advantage to normal myeloid progenitor populations (17
). Instead, the resolution of both leukocytosis and anemia indicates that PD0325901 rebalances the output of the hematopoietic system, despite continued KrasG12D
expression. This “rebalancing” might indicate that MEK activity regulates the expression of genes controlling lineage choice during hematopoietic differentiation. Such a model would be consistent with studies showing that cytokine stimulation instructs lineage choice in multipotent progenitors (18
), and that Ras activation promotes monocytic over granulocytic differentiation (19
). Further experiments to identify genes that respond to MEK activity in multipotent cells, and to test their influence on cell fate, are required to address this hypothesis.
Curative therapy for myeloproliferative neoplasia will need to eliminate neoplastic stem cells. Whether or not Kras
mutant alleles could be eliminated by MEK inhibition was not fully addressed in these experiments. Because recombination in the bone marrow of Mx1-Cre
mice is usually incomplete, Mx1-Cre
mice can be expected to harbor a small pool of stem cells in which the conditional KrasLSL-G12D
allele remains in the germline, non-expressed configuration. We have demonstrated such genetic chimerism in the bone marrow of young Mx1-Cre, KrasG12D
). However, we did not determine if this persisted until the initiation of PD0325901 treatment. Interestingly, a similar question applies to patients with myeloproliferative neoplasia; some patients might lose their normal hematopoietic elements over the course of their disease. Although we did not observe emergence of cells lacking oncogenic Kras
in these studies, it is possible that longer or more intense MEK inhibition might be more effective. Alternatively, inhibition of other signals evoked by mutant Kras
may be required for elimination of mutant stem cells. The retention of KrasG12D
stem cells in the bone marrow likely contributed to the development of T-ALL in some mice after prolonged drug treatment, because KrasG12D
stem cells can create a reservoir of lymphoid progenitors that are susceptible to undergoing leukemic transformation (20
). The observation that these acute leukemias arise during ongoing treatment suggests they are less dependent than myeloproliferative neoplasia on hyperactive Raf/MEK/ERK signaling.
In this study, we have provided direct evidence in vivo
that aberrant hematopoiesis caused by hyperactive Ras signaling is mediated by the Raf/MEK/ERK pathway. These results are consistent with a previous investigation, which showed that constitutive activation of MEK could block erythroid differentiation in vitro
). However, the requirement for MEK in myeloproliferative neoplasia has been called into question by our previous biochemical studies, which revealed that endogenous levels of KrasG12D
do not strongly activate Raf/MEK/ERK signaling (3
). This implied that aberrant hematopoiesis might be mediated by other effector pathways regulated by Ras (22
). Therefore, the importance of Raf/MEK/ERK signaling in mediating the myeloproliferative phenotype provides insight into the mechanism by which excessive Ras activity causes hematologic malignancies.
Our preclinical data imply that MEK inhibitors might be useful for treating patients with JMML and CMML, by alleviating the burdens of anemia, splenomegaly, and complications from myeloid cell overproduction such as tissue infiltration. In particular, treating children with JMML prior to hematopoietic stem cell transplantation might improve their clinical status by reducing the morbidity caused by infiltration of organs with leukemic cells (1
). Many adults with CMML suffer substantial morbidity owing to chronic anemia (23
), and a treatment that both reduces myeloproliferation and enhances erythropoiesis, as seen with MEK inhibitors, could prove clinically beneficial.