Structural and functional alterations of FLT3 in acute myeloid leukemia

Abstract

Hematopoiesis is highly regulated through cytokine-induced stimulation of multiple signal transduction pathways in order to mediate appropriate differentiation and proliferation of specific progenitor populations. Ligand-induced stimulation of the FMS-like tyrosine kinase 3 (FLT3) leads to activation of multiple downstream effector pathways resulting in differentiation and proliferation of specific progenitor cell populations. Genomic alterations of the FLT3 gene, including FLT3 internal tandem duplication (FLT3/ITD) and FLT3 activation loop mutation (FLT3/ALM) lead to autonomous receptor activation, dysregulation of FLT3 signal transduction pathways, contribute to myeloid pathogenesis, and have been linked to response to therapy and clinical outcome. Exploring the mechanisms by which these FLT3 alterations lead to dysregulated proliferation should provide a better understanding of the molecular pathogenesis of acute myeloid leukemia (AML) and may provide insights into potential therapeutic interventions. FLT3 inhibitors are under evaluation for their efficacy in AML patients with FLT3 mutations.

Figure 1. FLT3 signal transduction pathway

FLT3 receptor monomer is composed of an extracellular domain (ECD), a transmemberane domain (TMD), a Juxtamembrane domain (JMD) and a tyrosine kinase domain (TKD) interrupted by a short kinase insert. Binding to FLT3 ligand (FL) leads to receptor dimerization and activation of the intracellular kinase. Tyrosine kinase activation leads to phosphorylation of multiple sites in the intracellular kinase moiety. The activated receptor recruits a number of proteins in the cytoplasm including SHC and GRB2 to form a complex of protein-protein interactions, leading to activation of a number of intracellular mediators including AKT, MAPK and STAT. Activated mediators interact with HSP90 which protects them from inactivation and chaperones the active mediators to the nuclear interphase, where they are released into the nucleus and act to mediate vital cellular functions including cell growth, differentiation, apoptosis, DNA repair and proliferation.

Figure 2. Loss of heterozygosity (LOH) as a due to aUPD

Acquired uniparental disomy, resulting in homozygous state can involve the entire chromosome or be confined to a segment of a chromosome (Segmental aUPD). Segmental aUPD is mediated through homologous recombination mediated somatic crossing over between two homologous non-sister chromatids. In baseline, non mutated state, genes are represented by two different alleles (A, heterozygous state). Molecular alteration is acquired in one allele, leading to a heterozygous mutation (B). Evolution of UPD leads to conversion of the wild type allele to mutant, and evolution of homozygous mutation. When interstitial, the segmental UPD results from two symmetrical breaks flanking a segment of the chromosome, where during repair process, the region flanked by the breaks in the wild type allele is used as a template and as a result, a segment of chromosome undergoes conversion from heterozygous to homozygous state (acquired UPD). Terminal segmental UPD results from a single symmetrical break in each of two homologous non-sister chromatids and the resultant repair product demonstrates UPD (LOH) of the region telomeric to the region of double strand break.