Importance of FLT3 Mutations and Other Genetic Abnormalities in Normal Karyotype AML.

Cytogenetic findings at diagnosis are currently considered the most important prognostic factor in AML. By far, the most common cytogenetic finding, found in approximately 40–50% of patients, is a normal karyotype (defined as detection of only normal chromosomes in 20 or more marrow cells). Recent molecular analyses of leukemic blasts in cytogenetically normal AML (CN-AML) have revealed a striking heterogeneity with regard to the presence of acquired gene mutations and changes in gene expression. Multiple submicroscopic genetic alterations with prognostic significance have been discovered, including mutations of the FLT3 gene, both internal tandem duplications (ITDs) and point mutations (TKDs), partial tandem duplication (PTD) of the MLL gene, mutations in the NPM1, CEBPA, and WT1 genes, and high expression of the BAALC, ERG, and MN1 genes. Most of these abnormalities, except for mutations in NPM1 and CEBPA, have been associated with an adverse prognosis. CN-AML patients with FLT3 ITD, NPM1 wild type, or both have worse outcome in the range of 30% long-term survival, unless they also harbor CEBPA mutations. A high expression of the FLT3 ITD allele compared to the FLT3 wild type allele confers an especially poor prognosis. In contrast, CN-AML patients without a FLT3 ITD and with NPM1 mutations have a particularly good outcome in the range of 60% long-term survival, unless they present with high ERG expression. The prognostic significance of FLT3 TKD mutations is controversial, and, recently, the outcome of patients with the MLL PTD has improved when treated with induction therapy, including etoposide and post induction therapy with autologous transplantation. As a result, the 2008 World Health Organization classification of AML recommends evaluation of FLT3, NPM1, and CEBPA mutations in CN-AML. Application of gene expression profiling has also identified signatures that appear to separate CN-AML patients into prognostic subgroups, and, more recently, a microRNA expression signature was found to be associated with outcome in molecular high-risk CN-AML patients (i.e., those with FLT3 ITD, NPM1 wild type, or both). These and similar future findings are likely to have a major impact on the clinical management of CN-AML not only in prognostication, but also in selection of appropriate treatment, since many of the identified genetic alterations already constitute (e.g., FLT3 mutations for tyrosine kinase inhibitors) or will potentially become targets for specific therapeutic interventions.