Response and resistance to MEK inhibition in leukaemias initiated by hyperactive Ras

Abstract

In a mouse model for myeloproliferative disorder (MPD) driven by loss of NF-1 (which activates the Ras–MEK–MAPK pathway), additional mutations were created by insertional mutagenesis that led to progression of the disease to acute myeloid leukaemias (AMLs). Interestingly, MEK inhibitors are not effective in mice with MPD, but lead to tumour regression in AMLs. However, these mice eventually develop resistance to MEK inhibitors. This was linked to insertional mutagenesis at genes encoding RasGRP1 and p38 in subclones of the AMLs that are present prior to and selected for during treatment with MEK inhibitors. In human cancers with deregulated Ras signalling, including tumours that have inactivated the Nf1 tumour suppressor, the cascade comprising Raf, mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated kinase (ERK) is a therapeutic target. Here, in mice, the effects of inhibitors of MEK in a model of myeloproliferative disorder initiated by inactivating Nf1 and in a model of acute myeloid leukaemia are compared. The cascade comprising Raf, mitogen-activated protein kinase kinase (MEK) and extracellular signal-regulated kinase (ERK) is a therapeutic target in human cancers with deregulated Ras signalling, which includes tumours that have inactivated the Nf1 tumour suppressor1,2. Nf1 encodes neurofibromin, a GTPase-activating protein that terminates Ras signalling by stimulating hydrolysis of Ras–GTP. We compared the effects of inhibitors of MEK in a myeloproliferative disorder (MPD) initiated by inactivating Nf1 in mouse bone marrow and in acute myeloid leukaemias (AMLs) in which cooperating mutations were induced by retroviral insertional mutagenesis. Here we show that MEK inhibitors are ineffective in MPD, but induce objective regression of many Nf1-deficient AMLs. Drug resistance developed because of outgrowth of AML clones that were present before treatment. We cloned clone-specific retroviral integrations to identify candidate resistance genes including Rasgrp1, Rasgrp4 and Mapk14, which encodes p38α. Functional analysis implicated increased RasGRP1 levels and reduced p38 kinase activity in resistance to MEK inhibitors. This approach represents a robust strategy for identifying genes and pathways that modulate how primary cancer cells respond to targeted therapeutics and for probing mechanisms of de novo and acquired resistance.