Induction of AML Preleukemic Fusion Genes in HSPCs and DNA Damage Response in Preleukemic Fusion Gene Positive Samples

Preleukemic fusion genes (PFGs) occurring after DNA damage in hematopoietic stem progenitor cells (HSPCs) in utero often represent the initial event in the development of childhood leukemia. While the incidence of PFGs characteristic for acute lymphoblastic leukemia (ALL) was relatively well examined by several research groups and estimated to be 1–5% in umbilical cord blood (UCB) of healthy newborns, PFGs that are relevant to acute myeloid leukemia (AML) were poorly investigated. Therefore, this study is focused on the estimation of the incidence of the most frequent AML PFGs in newborns. For the first time, this study considered the inducibility of AML PFGs in different subsets of UCB HSPCs by low-dose γ-rays and also compared endogenous DNA damage, apoptosis, and reactive oxygen species (ROS) level between UCB samples containing or lacking AML PFGs. We found that: (i) the incidence of AML PFGs in UCB was 3.19% for RUNX1-RUNX1T1, 3.19% for PML-RARα, and 1.17% for KMT2A-MLLT3, (ii) 50 cGy of γ-rays did not induce RUNX1-RUNX1T1, PML-RARα, or KMT2A-MLLT3 PFGs in different subsets of sorted and expanded HSPCs, and (iii) the AML PFG⁺ samples accumulated the same level of endogenous DNA damage, as measured by the γH2AX/53BP1 focus formation, and also the same ROS level, and apoptosis as compared to PFG⁻ controls. Our study provides critical insights into the prevalence of AML PFGs in UCB of newborns, without the evidence of a specific HSPC population more susceptible for PFG formation after irradiation to low-dose γ-rays or increased amount of ROS, apoptosis and DNA damage.

Preleukemic fusion genes typical for acute myeloid leukemia

Acute myeloid leukemia (AML) is a highly heterogeneous subtype of leukemia, accounting for 25 % of childhood leukemias. By the presence of genetic mutations in hematopoietic/ progenitor stem cells, the bone marrow produces a large number of abnormal undifferentiated leukocytes (blasts), which significantly impairs the proper differentiation of cells. AML is induced by two interventions. Chromosomal translocation during hematopoiesis of intrauterine development is the first intervention. This creates preleukemic fusion genes (PFG), which can later be transformed by a second intervention (point genetic mutation - deletion, insertion ) into a functional malignant clone. Characteristic AML fusion genes include AML1-ETO, PML-RARA or MLL-AF9, which in turn produce hybrid proteins with altered function. Several studies suggest that these PFGs are considered an important prognostic tool in disease assessment. While the incidence of PFG characteristic of acute lymphoblastic leukemia (ALL) has been relatively well studied by several research groups and has been estimated at 1 to 5% in the umbilical cord blood of healthy neonates, PFG relevant to AML are still not sufficiently clarified.

Incidence of preleukemic fusion genes in healthy subjects

The diagnostics of leukemia relies upon multi-parametric approach involving a number of different pathology disciplines such as flow cytometry, histopathology, cytogenetics and molecular genetics [fluorescent in situ hybridization (FISH) and polymerase chain reaction (PCR)]. Childhood leukemia is often determined by the presence of specific chromosomal translocation that entails the generation of preleukemic fusion genes (PFG). In the last two decades, several studies have reported observations that PFG are present in healthy population and not necessarily result in leukemia. The first such study by Limpens and colleagues on t(14/18)/ BCL2-JH [1] and next in line [2, 3] led to many questions regarding the significance of these chromosomal translocations in leukemogenesis. However, the data on the incidence of PFG are contradictive. This review aims to highlight the molecular genetic approaches used by various studies with regard to differences in diagnostics and incidence of PFG in healthy subjects. The focus is on the incidence and prevalence of the most common PFG such as TEL-AML1, MLL-AF4, BCR-ABL (p190), AML1-ETO, PML-RARA, and CBFB-MYH11 detected in umbilical cord blood, in neonatal blood spots (Guthrie cards (GC)), bone marrow, peripheral blood and tissues of amortized fetuses. We conclude that the incidence of PFG is significantly higher than incidence of leukemia and more sophisticated analysis of PFG in leukemogenic cell populations is warranted to relate the occurrence of PFG with leukemia. The emerging notion is that only those PFG may contribute to development of leukemia which arise in stem cells at specific time windows during development. Thus, screening of PFG in subpopulations of stem cells may be a challenge for assessment of predisposition to leukemia and for validation of cell transplant to minimize donor cell-derived leukemia.

Backtracked analysis of preleukemic fusion genes and DNA repair foci in umbilical cord blood of children with acute leukemia

The first event in origination of many childhood leukemias is a specific preleukemic fusion gene (PFG) that arises, often in utero, in hematopoietic stem/progenitor cells (HSPC) from misrepaired DNA double strand break (DSB). An immanently elevated level of DSB and impaired apoptosis may contribute to origination and persistence of PFG and donor cell-derived leukemia in recipients of allogeneic transplantation of umbilical cord blood (UCB). We investigated DSB, apoptosis and PFG in the backtracked UCB cells of leukemic patients. RNA from UCB of three patients with acute lymphoblastic leukemia, patient with acute megakaryoblastic leukemia and Down syndrome, and four healthy children was screened for common PFG by RT-qPCR. Presence of PFG was validated by sequencing. Endogenous γH2AX and 53BP1 DNA repair foci, cell populations, and apoptosis were analyzed in UCB CD34+/- cells with imaging and standard flow cytometry. We found MLL₂-AF4 and BCR-ABL (p190) fusion genes in UCB of two out from four pediatric patients, apparently not detected at diagnosis, while UCB cells of TEL-AML1+ ALL patient were tested negative for this PFG and no PFG were detected in UCB cells of healthy children. No significant difference in DNA damage and apoptosis between UCB CD34+/- cells from healthy children and leukemic patients was observed, while Down syndrome trisomy increased DNA damage and resulted in distribution of cell populations resembling transient abnormal myelopoiesis. Our findings indicate increased genetic instability in UCB HSPC of leukemic patients and may be potentially used for diagnostics and exclusion of possibly affected UCB from transplantation.