The Function And Mechanism Of DNA Repair Protein HMCES In Hematopoietic Stem Cells

Hematopoietic stem cells(HSCs)comprise a rare population of adult stem cells that maintain lifelong blood cell production through their self-renewal and multi-lineage capacity.HSCs have unique property to limit the accumulation of DNA damage.Reactive oxygen species(ROS)is one of the major sources of DNA damage,which causes oxidative DNA damage including apurinic/apyrimidinic(AP)sites.In the steady state,most HSCs reside in the quiescent phase so as to minimize the generation of ROS.However,hematopoietic stress can drive quiescent HSCs to proliferate,resulting in a rapid increase of ROS and AP sites.Moreover,a significant amount of single-stranded DNA(ss DNA)is generated during DNA replication upon HSC activation.Unrepaired AP sites in the replication-associated ss DNA,where no complementary strand is available,can precipitate DNA double-strand breaks(DSBs)upon their removal by the AP endonuclease;alternatively,they may be processed by error-prone polymerases leading to mutations.Therefore,we reasoned that timely repair or shielding of the burst of AP sites in HSCs when entry into cell cycle is critical for protecting their genome integrity and regenerative capacity.Here,we showed that HMCES,an evolutionarily conserved protein,plays a pivotal role in this process using Hmces knock-out mice.HMCES can covalently crosslink to ss DNA AP sites generating a DNA-protein crosslink(DPC)intermediate and effectively prevent DSBs and mutations.Although HMCES was not required for normal hematopoiesis in the steady state,Hmces-deficient HSCs were drastically reduced after transplantation or 5-fluorouracil(5-FU)treatment.Moreover,transcriptome analysis revealed that Hmces-deficient HSCs displayed broad attenuation of DNA damage response and repair pathways.Consistently,Hmces depletion resulted in accumulation of DNA lesions upon activation of HSCs out of quiescence using 5-FU or polyinosinic:polycytidylic acid(p I:p C),which may be due to the elevated AP sites and downregulated DNA damage response and repair pathways.Besides increased sensitivity to activation-induced intrinsic DNA lesions,we also showed that Hmces deficiency led to hypersensitivity of LT-HSCs to exogenous DNA insults and DNAdamaging metabolites,such as ionizing radiation(IR)and acetaldehyde.Taken together,these data suggested that HMCES safeguards genome integrity and long-term selfrenewal of HSCs during stress responses.Moreover,HMCES also protects leukemia cells from ROS-induced oxidative DNA damage and is essential for leukemia cell survival.Importantly,elevated expression of HMCES occurs frequently in acute lymphocytic leukemia(ALL)and is associated with poor prognosis.Given that HMCES is not required for normal hematopoiesis in the steady state,it may serve as a potential selective target against ALL while sparing normal hematopoiesis.Collectively,our data not only defined essential functions of HMCES in protecting the genome integrity and long-term self-renewal of HSCs during stress responses,but also revealed its clinical relevance in ALL and provided a promising new therapeutic target for leukemia.