Page 30 - 2019_05-HaematologicaMondo-web

A. Tuval and L.I. Shlush et al.
be leukemic; this is a small insertion/duplication in the ter- minal exon of NPM1 that causes the mutant protein to be aberrantly localized in the cytoplasm (hence, designated NPM1c). Detection of this mutation accompanies a dra- matic change in the phenotype of the clone with a rapid proliferation and acquisition of additional mutations. When individuals were found to harbor an NPM1c mutat- ed clone, they were subsequently diagnosed with NPM1c AML within up to 3 months.22,35 This mutation was most clearly shown to be a marker for leukemic blasts when risk for relapse was found to be correlated with its pres- ence in blood samples of AML patients in remission.36
Targeting cells harboring transforming, leukemic, muta- tions seems plausible not only for monitoring purposes, but also for therapy.37 In fact, genomic editing of this mutation in cell lines as well as in primary human AML samples using the CRISPR/Cas9 system disrupted the mutant allele and led to nuclear re-localization of the pro- tein. This reverted the leukemic phenotype, resulting in differentiation and a reduced proliferation rate. A nuclear export inhibitor had a similar effect on NPM1c, both at the molecular level as well as at the cellular level, when tested on a cell line and on primary AML samples. It also resulted in prolonged survival of NPM1c-mutated leukemic mice.38
Late events
Late events are mutations that appear later on during leukemic evolution, represent clonal selection and hetero- geneity, and are not shared by all leukemic blasts. Examples for such variants are activating mutations in tyrosine kinase receptors (such as FLT3-ITD, KIT, RAS),
emphasizing their role in increasing clonal proliferation capacity.39 Other examples are WT1,39 transcription factor CEBPA bi-allelic mutations,19 and del(7q) in TP53-mutated AML.23 Interestingly, there are some exceptions to these “rules”: some mutations that were detected in healthy individuals, for example, IDH2, were also described as late events in AML.34 In addition, the FLT3 D835 mutation, usually considered as a late event, was found to be pre- leukemic.9
Some late events co-occur with certain leukemic events, for example, the FLT3-ITD and NPM1 mutations.4 This strong link between leukemic mutations and late events, rather than between pre-leukemic mutations and late events, was recently demonstrated by Höllein et al.40 They described patients with NPM1-mutant AMLs that devel- oped a NPM1 wild-type AML after therapy. Both types of leukemia evolved on a similar pre-leukemic background. Interestingly, FLT3-ITD was significantly more frequent among NPM1-mutant AML.40 Late events, such as FLT3- ITD or KIT mutations, can be found in unique AML sub- types, such as APL and CBF-AML, respectively, as described below.
Targeting cells according to late events can prove bene- ficial and was employed using tyrosine kinase inhibitors.41 Nevertheless targeting a few subclones can result in a pos- itive selection of other subclones with a different genomic landscape.42 Monitoring residual clones using late events as clonal markers should be done with great care, since this approach might miss subclones lacking these markers.43,44
Unique acute myeloid leukemia subtypes
A few AML subtypes, such as CBF-AML and APL, are
Figure 2. Pre-leukemic clones have inherent chemoresistance. Pre-leukemic clones (blue) undergo positive selection by chemotherapy administered for a non-relat- ed cancer [other than acute myeloid leukemia (AML)]. They expand and evolve into t-AML (green). Pre-leukemic hematopoietic stem and progenitor cells (HSPCs) have inherent chemoresistance, thus they also survive following AML induction chemotherapy and reconstitute clonal hematopoiesis. Most relapses occur within the first 2 years and originate from residual leukemic clones that can be identified at diagnosis and that were not eradicated by AML therapy. Rare events of second AML (red) stem from (mostly, the same) pre-leukemic clones that evolved again into AML following a more prolonged latency.
876
haematologica | 2019; 104(5)