Disease diversity and FLT3 mutations.

In the last decade, much attention in oncology drug development has focused on exploiting “oncogene addiction,” the premise that, despite multiple genetic lesions, some tumors remain reliant on a single oncogene for maintenance of a malignant phenotype associated with cellular proliferation and survival. Compelling evidence for the concept of oncogene addiction arises from genetically engineered mouse models in which an inducible oncogene expressed in a specific target tissue can give rise to tumorgenesis, but tumor regression is observed when the oncogene is effectively turned off again by genetic or pharmacologic means. Convincing support of oncogene addiction can also be found in the diverse array of human tumors targeted by tyrosine kinase inhibitors (TKIs), in which deep remissions are observed in patients with tumors expressing activated oncogenes, but clinical resistance is clearly associated with reactivation of the target by a mutation that prevents drug binding. This phenomenon was first described in chronic myeloid leukemia (CML) (1), but this paradigm has been extended to multiple human cancers responsive to TKI therapy including epidermal growth factor receptor (EGFR)-mutant lung cancer (2), gastrointestinal stromal tumor (GIST) driven by c-KIT (3), and, recently, acute myeloid leukemia (AML) associated with mutations in Fms-like tyrosine kinase-3 (FLT3) (4). Many oncogenes effectively targeted by current clinical therapeutics encode kinases constitutively activated by mutation through a variety of mechanisms identified in clinical samples, including point mutations and in-frame deletions or duplications as observed in c-KITmutant GIST (5), EGFR-mutant lung cancer (6), and FLT3-mutant AML (7, 8). However, how individual mechanisms activating the same kinase (i.e., point mutation vs. in-frame duplication) may affect disease phenotype and response to therapy remains largely unexplored. In PNAS, Bailey et al. (9) describe a mouse knock-in model of FLT3 activated by a point mutation in the FLT3 kinase activation loop, D835Y, that contrasts phenotypically with their previously described and otherwise genetically identical, knock-in model of FLT3 activated by an in-frame internal tandem duplication (ITD) in the juxtamembrane (JM) domain. This work provides clear evidence that different mutations, although they may result in constitutive activation of the same kinase, may not be equivalent and can result in diverse disease phenotypes. FLT3 is a class III receptor tyrosine kinase that plays an important role in normal hematopoiesis (10) and is mutated in ∼30% of AML. Recent large-scale genomic sequencing efforts have confirmed that FLT3 is the most commonly mutated gene in human AML (11), with ∼20% of mutations consisting of ITD mutations in the JM domain (12) and with an additional subset (∼7–10%) consisting of point mutations in the FLT3 tyrosine kinase domain (TKD), commonly at the activation loop residue D835 (8, 12). FLT3ITD mutations have been clearly associated with poor prognosis (13), whereas the prognostic significance of FLT3 TKD mutations has been less clear (8, 12). Although both FLT3-ITD and FLT3 TKD mutations cause ligand-independent kinase activation, in vitro studies have identified differential autophosphorylation (14) and downstream signaling patterns for FLT3-ITD (15) compared with FLT3 TKD and native FLT3, in particular preferential activation of STAT5 (16) by FLT3-ITD, as well as increased proliferation and clonogenic growth potential in cellular models (16). It has been suggested that this differential signaling is the result, in part, of aberrant trafficking of FLT3-ITD mutant receptors resulting in prolonged retention in the endoplasmic reticulum (ER) and increased exposure to intracellular substrates such as STAT5 (17). In a murine bone marrow (BM) transduction and transplantation model, FLT3 D835Y yields an oligoclonal lymphoid disorder with longer disease latency distinct from the myeloproliferative neoplasm (MPN) observed with FLT3-ITD (18). It is also notable that, although FLT3ITD and D835 mutations have rarely been known to co-occur de novo in individual patients, recent translational studies have established that secondary FLT3 D835 mutations co-occurring on the same allele as FLT3-ITD (FLT3-ITD/D835V/Y/F) are a frequent cause of acquired resistance to FLT3 inhibitors (4). At this point, it is unknown whether these compound FLT3-ITD/D835 mutations function more like FLT3-ITD or FLT3 D835 mutations in regard to signal transduction, proliferative and transforming potential, and effect on disease phenotype. ITD