Unraveling The Leukemic Nature Of hMICL and CD123 Expressing Cells In Acute Myeloid Leukemia

Flow cytometry constitutes a cornerstone in the diagnosis and follow up of acute myeloid leukemia (AML) and it is based on the identification of leukemia-associated immunophenotypes (LAIPs). We have recently demonstrated that the C-type lectin hMICL in combination with CD123 constitute a highly stable and reliable LAIP marker combination at diagnosis and relapse. In addition, we have shown that an hMICL/CD123-based assay is an effective tool for the monitoration of minimal residual disease (MRD) in AML. To what extent hMICL/CD123 marking identifies early leukemic cells is, however, not established. We hypothesized that this could be addressed by studying molecular aberrations in leukemic cell subsets according to their expression of hMICL and CD123. Employing cell sorting and mutational analyses, we here establish the leukemic origin of hMICL and CD123 expressing cell populations.

Analyzing diagnostic AML samples with homozygous FLT3-ITD aberrations allowed for verification of pure malignant clones. Five patients with FLT3-ITD allelic burden of >50% (range 77-93%, median 85%) as measured by DNA fragment analysis by capillary electrophoresis on mononuclear cells (MNC) were identified in our local database of 600 cases. We found that 5/5 patients displayed a normal karyotype and carried NPM1 mutations (NPM1 allelic burden 42-48%, median 46%). In contrast, mutations in FLT3-D835, IDH1-R132, c-KIT-D816V or indel mutations in CEBPA and WT-1 exon 7 were absent. From samples of cryopreserved mononuclear cells (bone marrow (n=4) and peripheral blood (n=1)), CD45low/SSClow blast cell subsets with the following immunophenotypes were sorted by FACS: CD34+/hMICL+/CD123+, CD34+/hMICL+/CD123-, CD34+/hMICL-/CD123+, and CD34+/hMICL-/CD123-. In one case of CD34 negative AML the sorted subsets were CD34-/hMICL+/CD123+, CD34-/hMICL+/CD123-, CD34-/hMICL-/CD123+, and CD34-/hMICL-/CD123-. Sorted cell subsets were analyzed for FLT3-ITD and NPM1 mutations using fragment analysis by capillary electrophoresis.

The results of the fragment analyses are tabulated in the table below. In all cases the hMICL and CD123 expressing subsets of interest closely approximated 100% FLT3-ITD allelic burden. In contrast, hMICL-/CD123- cells approximated only a 50% FLT3-ITD allelic burden.

Of note, an extended search in our AML database, revealed only 9 of 600 patients to have an FLT3-ITD allelic burden >50% (range 52-94%, median 81%) hence indicating a state of either homo- or hemizygosity. Interestingly, with the exception of one case carrying a chromosome 13 duplication, each of these 9 patients also harbored a mutation in the NPM1 gene as the only other known aberration.

In conclusion using AML patients with high FLT3-ITD allelic burdens we have been able to show that blasts expressing hMICL and/or CD123 at diagnosis are indeed malignant thus further substantiating the use of these antigens in AML MRD detection. Additionally, a direct relationship between NPM1 and FLT3-ITD homo-/hemizygosity may be suggested in the evolution of the malignant clone.