DNA-damage response in hematopoietic stem cells: an evolutionary trade-off between blood regeneration and leukemia suppression.

Self-renewing and multipotent hematopoietic stem cells (HSCs) maintain lifelong hematopoiesis. Their enormous regenerative potential coupled with lifetime persistence in the body, in contrast with the Progenitors, demand tight control of HSCs genome stability. Indeed, failure to accurately repair DNA damage in HSCs is associated with bone marrow failure and accelerated leukemogenesis. Recent observations exposed remarkable differences in several DNA-damage response (DDR) aspects between HSCs and Progenitors, especially in their DNA-repair capacities and susceptibility to apoptosis. Human HSCs in comparison with Progenitors exhibit delayed DNA double-strand break rejoining, persistent DDR signaling activation, higher sensitivity to the cytotoxic effects of ionizing radiation and attenuated expression of DNA-repair genes. Importantly, the distinct DDR of HSCs was also documented in mouse models. Nevertheless, physiological significance and the molecular basis of the HSCs-specific DDR features are only partially understood. Taking radiation-induced DDR as a paradigm, this review will focus on the current advances in understanding the role of cell-intrinsic DDR regulators and the cellular microenvironment in balancing stemness with genome stability. Pre-leukemia HSCs and clonal hematopoiesis evolvement will be discussed as an evolutionary compromise between the need for lifelong blood regeneration and DDR. Uniquely for this review, we outline the differences in HSCs-related DDR as highlighted by various experimental systems and attempt to provide their critical analysis.

Quercetin alters the DNA damage response in human hematopoietic stem and progenitor cells via TopoII- and PI3K-dependent mechanisms synergizing in leukemogenic rearrangements.

Quercetin (Que) is an abundant flavonoid in the human diet and high-concentration food supplement with reported pro- and anti-carcinogenic activities. Topoisomerase II (TopoII) inhibition and subsequent DNA damage induction by Que was implicated in the mixed lineage leukemia gene (MLL) rearrangements that can induce infant and adult leukemias. This notion raised concerns regarding possible genotoxicities of Que in hematopoietic stem and progenitor cells (HSPCs). However, molecular targets mediating Que effects on DNA repair relevant to MLL translocations have not been defined. In this study we describe novel and potentially genotoxic Que activities in suppressing non-homologous end joining and homologous recombination pathways downstream of MLL cleavage. Using pharmacological dissection of DNA-PK, ATM and PI3K signalling we defined PI3K inhibition by Que with a concomitant decrease in the abundance of key DNA repair genes to be responsible for DNA repair inhibition. Evidence for the downstream TopoII-independent mutagenic potential of Que was obtained by documenting further increased frequencies of MLL rearrangements in human HSPCs concomitantly treated with Etoposide and Que versus single treatments. Importantly, by engaging a tissue engineered placental barrier, we have established the extent of Que transplacental transfer and hence provided the evidence for Que reaching fetal HSPCs. Thus, Que exhibits genotoxic effects in human HSPCs via different mechanisms when applied continuously and at high concentrations. In light of the demonstrated Que transfer to the fetal compartment our findings are key to understanding the mechanisms underlying infant leukemia and provide molecular markers for the development of safety values.

Sex-specific differences in DNA double-strand break repair of cycling human lymphocytes during aging.

The gender gap in life expectancy and cancer incidence suggests differences in the aging process between the sexes. Genomic instability has been recognized as a key factor in aging, but little is known about sex-specific differences. Therefore, we analyzed DNA double-strand break (DSB) repair in cycling human peripheral blood lymphocytes (PBL) from male and female donors of different age. Reporter-based DSB repair analyses revealed differential regulation of pathway usage in PBL from male and female donors with age: Non-homologous end joining (NHEJ) was inversely regulated in men and women; the activity of pathways requiring end processing and strand annealing steps such as microhomology-mediated end joining (MMEJ) declined with age in women but not in men. Screening candidate proteins identified the NHEJ protein KU70 as well as the end resection regulatory factors ATM and BLM showing reduced expression during aging in women. Consistently, the regulatory factor BLM contributed to the MMEJ proficiency in young but not in old women as demonstrated by knockdown analysis. In conclusion, we show that DSB repair is subject to changes upon aging and age-related changes in DSB repair are distinct in men and women.

Attenuated DNA damage responses and increased apoptosis characterize human hematopoietic stem cells exposed to irradiation.

Failure to precisely repair DNA damage in self-renewing Hematopoietic Stem and early Progenitor Cells (HSPCs) can disrupt normal hematopoiesis and promote leukemogenesis. Although HSPCs are widely considered a target of ionizing radiation (IR)-induced hematopoietic injury, definitive data regarding cell death, DNA repair, and genomic stability in these rare quiescent cells are scarce. We found that irradiated HSPCs, but not lineage-committed progenitors (CPs), undergo rapid ATM-dependent apoptosis, which is suppressed upon interaction with bone-marrow stroma cells. Using DNA repair reporters to quantify mutagenic Non-Homologous End Joining (NHEJ) processes, we found that HSPCs exhibit reduced NHEJ activities in comparison with CPs. HSPC-stroma interactions did not affect the NHEJ capacity of HSPCs, emphasizing its cell autonomous regulation. We noted diminished expression of multiple double strand break (DSB) repair transcripts along with more persistent 53BP1 foci in irradiated HSPCs in comparison with CPs, which can account for low NHEJ activity and its distinct control in HSPCs. Finally, we documented clonal chromosomal aberrations in 10% of IR-surviving HSPCs. Taken together, our results revealed potential mechanisms contributing to the inherent susceptibility of human HSPC to the cytotoxic and mutagenic effects of DNA damage.

SMYD2 lysine methyltransferase regulates leukemia cell growth and regeneration after genotoxic stress.

The molecular determinants governing escape of Acute Myeloid Leukemia (AML) cells from DNA damaging therapy remain poorly defined and account for therapy failures. To isolate genes responsible for leukemia cells regeneration following multiple challenges with irradiation we performed a genome-wide shRNA screen. Some of the isolated hits are known players in the DNA damage response (e.g. p53, CHK2), whereas other, e.g. SMYD2 lysine methyltransferase (KMT), remains uncharacterized in the AML context. Here we report that SMYD2 knockdown confers relative resistance to human AML cells against multiple classes of DNA damaging agents. Induction of the transient quiescence state upon SMYD2 downregulation correlated with the resistance. We revealed that diminished SMYD2 expression resulted in the upregulation of the related methyltransferase SET7/9, suggesting compensatory relationships. Indeed, pharmacological targeting of SET7/9 with (R)-PFI2 inhibitor preferentially inhibited the growth of cells expressing low levels of SMYD2.Finally, decreased expression of SMYD2 in AML patients correlated with the reduced sensitivity to therapy and lower probability to achieve complete remission. We propose that the interplay between SMYD2 and SET7/9 levels shifts leukemia cells from growth to quiescence state that is associated with the higher resistance to DNA damaging agents and rationalize SET7/9 pharmacological targeting in AML.