Thrombopoietin-Increased DNA-PK-Dependent DNA Repair Limits Hematopoietic Stem and Progenitor Cell Mutagenesis in Response to DNA Damage

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

BACKGROUND

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has yet to be elucidated.

METHODS

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 h post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

RESULTS

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

CONCLUSIONS

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

Background

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating the regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is a key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has not yet been elucidated.

Methods

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 hours post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

Results

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

Conclusions

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Alternative treatment modality for severe aplastic anemia in a resource-limited setting: a single-institution prospective cohort study from Upper Egypt

This study compared the efficacy and safety of CsA monotherapy with eltrombopag (E-PAG) + CsA combined treatment in children with severe aplastic anemia (SAA). The study including 30 children had SAA. Ten were a retrospective cohort treated with CsA monotherapy. The other 20 were prospective cohort received E-PAG + CsA. All patients were evaluated for partial (PR) and complete (CR) hematological response at 3, 6, and 12 months. overall response (OR), overall survival rates (OS), and treatment safety. OR for the E-PAG patients was 40% after 3 months of therapy. At 6 months, this had increased to 75% with significantly higher CR rate (40%) than in the CsA group (p = 0.0001). After a year of treatment, the CR for the E-PAG + CsA regimen had increased to 50% and the OR to 85%, compared to 20% in the CsA group (p = 0.0001). The OS at 12 months was 100% in the E-PAG+ CsA group compared to 80% in the CsA cohort. At 24 months, the OS in the E-PAG + CsA group was 90%. In conclusion, E-PAG+ CsA was found to be a safe and effective alternative treatment for children with SAA particularly in countries with limited resources.

Multimodal cartography of human lymphopoiesis reveals B and T/NK/ILC lineages are subjected to differential regulation

The developmental cartography of human lymphopoiesis remains incompletely understood. Here, we establish a multimodal map demonstrating that lymphoid specification follows independent direct or stepwise hierarchic routes converging toward the emergence of newly characterized CD117lo multi-lymphoid progenitors (MLPs) that undergo a proliferation arrest before entering the CD127- (NK/ILC/T) or CD127+ (B) lymphoid pathways. While the differentiation of CD127- early lymphoid progenitors is mainly driven by Flt3 signaling, emergence of their CD127+ counterparts is regulated cell-intrinsically and depends exclusively on the divisional history of their upstream precursors, including hematopoietic stem cells. Further, transcriptional mapping of differentiation trajectories reveals that whereas myeloid granulomonocytic lineages follow continuous differentiation pathways, lymphoid trajectories are intrinsically discontinuous and characterized by sequential waves of cell proliferation allowing pre-commitment amplification of lymphoid progenitor pools. Besides identifying new lymphoid specification pathways and regulatory checkpoints, our results demonstrate that NK/ILC/T and B lineages are under fundamentally distinct modes of regulation. (149 words).

PEGylated thrombopoietin mimetic, JNJ‑26366821 a novel prophylactic radiation countermeasure for acute radiation injury

Thrombopoietin (TPO) is the primary regulator of platelet generation and a stimulator of multilineage hematopoietic recovery following exposure to total body irradiation (TBI). JNJ‑26366821, a novel PEGylated TPO mimetic peptide, stimulates platelet production without developing neutralizing antibodies or causing any adverse effects. Administration of a single dose of JNJ‑26366821 demonstrated its efficacy as a prophylactic countermeasure in various mouse strains (males CD2F1, C3H/HeN, and male and female C57BL/6J) exposed to Co-60 gamma TBI. A dose dependent survival efficacy of JNJ‑26366821 (- 24 h) was identified in male CD2F1 mice exposed to a supralethal dose of radiation. A single dose of JNJ‑26366821 administered 24, 12, or 2 h pre-radiation resulted in 100% survival from a lethal dose of TBI with a dose reduction factor of 1.36. There was significantly accelerated recovery from radiation-induced peripheral blood neutropenia and thrombocytopenia in animals pre-treated with JNJ‑26366821. The drug also increased bone marrow cellularity and megakaryocytes, accelerated multi-lineage hematopoietic recovery, and alleviated radiation-induced soluble markers of bone marrow aplasia and endothelial damage. These results indicate that JNJ‑26366821 is a promising prophylactic radiation countermeasure for hematopoietic acute radiation syndrome with a broad window for medical management in a radiological or nuclear event.

Molecular and cellular basis of the dose-rate-dependent adverse effects of radiation exposure in animal models. Part II: Hematopoietic system, lung and liver

While epidemiological data have greatly contributed to the estimation of the dose and dose-rate effectiveness factor (DDREF) for human populations, studies using animal models have made significant contributions to provide quantitative data with mechanistic insights. The current article aims at compiling the animal studies, specific to rodents, with reference to the dose-rate effects of cancer development. This review focuses specifically on the results that explain the biological mechanisms underlying dose-rate effects and their potential involvement in radiation-induced carcinogenic processes. Since the adverse outcome pathway (AOP) concept together with the key events holds promise for improving the estimation of radiation risk at low doses and low dose-rates, the review intends to scrutinize dose-rate dependency of the key events in animal models and to consider novel key events involved in the dose-rate effects, which enables identification of important underlying mechanisms for linking animal experimental and human epidemiological studies in a unified manner.

rhTPO Ameliorates Radiation-Induced Long-Term Hematopoietic Stem Cell Injury in Mice

Exposure to medium and high doses of ionizing radiation (IR) can induce long-term bone marrow (BM) suppression. We previously showed that recombinant human thrombopoietin (rhTPO) significantly promotes recovery from hematopoietic-acute radiation syndrome, but its effect on long-term BM suppression remains unknown. C57BL/6 mice were exposed to 6.5 Gy γ-rays of total body irradiation (TBI) at a dose-rate of 63.01 cGy per minute, and the mice were treated with rhTPO (100 μg; intramuscular injection) or vehicle at 2 h after TBI. All mice were killed one or two months after TBI for analysis of peripheral blood cell counts, long-term hematopoietic stem cell (HSC) frequency, and BM-derived clonogenic activity. The HSC self-renewal capacity was analyzed by BM transplantation. The levels of reactive oxygen species (ROS) production and ratios of γH2AX+ and p16, p53, and p21 mRNA in HSCs were measured by flow cytometry and real-time polymerase chain reaction, respectively. Treatment with rhTPO reduced long-term myelosuppression by improving long-term hematopoietic reconstitution (p < 0.05) after transplantation and resting state maintenance of HSCs (p < 0.05). Moreover, rhTPO treatment was associated with a sustained reduction in long-term ROS production, reduction of long-term DNA damage, diminished p53/p21 mRNA expression, and prevention of senescence after TBI. This study suggests rhTPO is an effective agent for treating IR-induced long-term BM injury because it regulates hematopoietic remodeling and HSC cycle disorder through the ROS/p53/p21/p16 pathway long term after IR.

RPRM deletion preserves hematopoietic regeneration by promoting EGFR-dependent DNA repair and hematopoietic stem cell proliferation post ionizing radiation

Reprimo (RPRM), a target gene of p53, is a known tumor suppressor. DNA damage induces RPRM, which triggers p53-dependent G2 arrest by inhibiting cyclin B1/Cdc2 complex activation and promotes DNA damage-induced apoptosis. RPRM negatively regulates ataxia-telangiectasia mutated by promoting its nuclear-cytoplasmic translocation and degradation, thus inhibiting DNA damage. Therefore, RPRM plays a crucial role in DNA damage response. Moreover, the loss of RPRM confers radioresistance in mice, which enables longer survival and less severe intestinal injury after radiation exposure. However, the role of RPRM in radiation-induced hematopoietic system injury remains unknown. Herein, utilizing a RPRM-knockout mouse model, we found that RPRM deletion did not affect steady-state hematopoiesis in mice. However, RPRM knockout significantly alleviated radiation-induced hematopoietic system injury and preserved mouse hematopoietic regeneration in hematopoietic stem cells (HSCs) against radiation-induced DNA damage. Further mechanistic studies showed that RPRM loss significantly increased EGFR expression and phosphorylation in HSCs to activate STAT3 and DNA-PKcs, thus promoting HSC DNA repair and proliferation. These findings reveal the critical role of RPRM in radiation-induced hematopoietic system injury, confirming our hypothesis that RPRM may serve as a novel target for radiation protection.

NF-κB signaling controls H3K9me3 levels at intronic LINE-1 and hematopoietic stem cell genes in cis

Ionizing radiations (IR) alter hematopoietic stem cell (HSC) function on the long term, but the mechanisms underlying these effects are still poorly understood. We recently showed that IR induces the derepression of L1Md, the mouse young subfamilies of LINE-1/L1 retroelements. L1 contributes to gene regulatory networks. However, how L1Md are derepressed and impact HSC gene expression are not known. Here, we show that IR triggers genome-wide H3K9me3 decrease that occurs mainly at L1Md. Loss of H3K9me3 at intronic L1Md harboring NF-κB binding sites motifs but not at promoters is associated with the repression of HSC-specific genes. This is correlated with reduced NFKB1 repressor expression. TNF-α treatment rescued all these effects and prevented IR-induced HSC loss of function in vivo. This TNF-α/NF-κB/H3K9me3/L1Md axis might be important to maintain HSCs while allowing expression of immune genes during myeloid regeneration or damage-induced bone marrow ablation.

Molecular Pathogenesis of Myeloproliferative Neoplasms: From Molecular Landscape to Therapeutic Implications

Despite distinct clinical entities, the myeloproliferative neoplasms (MPN) share morphological similarities, propensity to thrombotic events and leukemic evolution, and a complex molecular pathogenesis. Well-known driver mutations, JAK2, MPL and CALR, determining constitutive activation of JAK-STAT signaling pathway are the hallmark of MPN pathogenesis. Recent data in MPN patients identified the presence of co-occurrence somatic mutations associated with epigenetic regulation, messenger RNA splicing, transcriptional mechanism, signal transduction, and DNA repair mechanism. The integration of genetic information within clinical setting is already improving patient management in terms of disease monitoring and prognostic information on disease progression. Even the current therapeutic approaches are limited in disease-modifying activity, the expanding insight into the genetic basis of MPN poses novel candidates for targeted therapeutic approaches. This review aims to explore the molecular landscape of MPN, providing a comprehensive overview of the role of drive mutations and additional mutations, their impact on pathogenesis as well as their prognostic value, and how they may have future implications in therapeutic management.

Hematopoietic stem cells temporally transition to thrombopoietin dependence in the fetal liver

Tissue stem cells temporally change intrinsic mechanisms to meet physiological demands. However, little is known whether and how stem cells rely on distinct extrinsic maintenance mechanisms over time. Here, we found that hematopoietic stem cells (HSCs) temporally transition to depend on thrombopoietin (TPO), a key extrinsic factor, from E16.5 onward in the developing liver. Deletion of Tpo reduced mTOR activity, induced differentiation gene expression, and preferentially depleted metabolically active HSCs. Ectopic activation of the JAK2 or MAPK pathway did not rescue HSCs in Tpo^-/- mice. Enforced activation of the mTOR pathway by conditionally deleting Tsc1 significantly rescued HSCs and their gene expression in Tpo^-/- mice. Lin28b intrinsically promoted mTOR activation in HSCs, and its expression diminished over time. Conditional deletion of Lin28b further reduced mTOR activity and strongly exacerbated HSC depletion in Tpo^-/- mice. Therefore, HSCs temporally transition from intrinsic LIN28B-dependent to extrinsic TPO-dependent maintenance in the developing liver.

Slc20a1b is essential for hematopoietic stem/progenitor cell expansion in zebrafish

Hematopoietic stem and progenitor cells (HSPCs) are able to self-renew and can give rise to all blood lineages throughout their lifetime, yet the mechanisms regulating HSPC development have yet to be discovered. In this study, we characterized a hematopoiesis defective zebrafish mutant line named smu07, which was obtained from our previous forward genetic screening, and found the HSPC expansion deficiency in the mutant. Positional cloning identified that slc20a1b, which encodes a sodium phosphate cotransporter, contributed to the smu07 blood phenotype. Further analysis demonstrated that mutation of slc20a1b affects HSPC expansion through cell cycle arrest at G2/M phases in a cell-autonomous manner. Our study shows that slc20a1b is a vital regulator for HSPC proliferation in zebrafish early hematopoiesis and provides valuable insights into HSPC development.

The Y14-p53 regulatory circuit in megakaryocyte differentiation and thrombocytopenia

Thrombocytopenia-absent radius (TAR) syndrome is caused by RBM8A insufficiency. We generated megakaryocyte-specific Rbm8a knockout (Rbm8aKO^MK) mice that exhibited marked thrombocytopenia, internal hemorrhage, and splenomegaly, providing evidence that genetic deficiency of Rbm8a causes a disorder of platelet production. Rbm8aKO^MK mice accumulated low-ploidy immature megakaryocytes in the bone marrow and exhibited defective platelet activation and aggregation. Accordingly, depletion of Y14 (RBM8A) in human erythroleukemia (HEL) cells compromised phorbol-ester-induced polyploidization. Notably, Y14/RBM8A deficiency induced both p53 and p21 in megakaryocytes and HEL cells. Treatment with a p53 inhibitor restored ex vivo differentiation of Rbm8aKO^MK megakaryocytes and unexpectedly activated Y14 expression in HEL cells. Trp53 knockout partially restored megakaryocyte differentiation by reversing cell-cycle arrest and increased platelet counts of Rbm8aKO^MK, indicating that excess p53 in part accounts for thrombocytopenia in TAR syndrome. This study provides evidence for the role of the Y14-p53 circuit in platelet production and a potential therapeutic strategy.

Involvement of classic and alternative non-homologous end joining pathways in hematologic malignancies: targeting strategies for treatment

Chromosomal translocations are the main etiological factor of hematologic malignancies. These translocations are generally the consequence of aberrant DNA double-strand break (DSB) repair. DSBs arise either exogenously or endogenously in cells and are repaired by major pathways, including non-homologous end-joining (NHEJ), homologous recombination (HR), and other minor pathways such as alternative end-joining (A-EJ). Therefore, defective NHEJ, HR, or A-EJ pathways force hematopoietic cells toward tumorigenesis. As some components of these repair pathways are overactivated in various tumor entities, targeting these pathways in cancer cells can sensitize them, especially resistant clones, to radiation or chemotherapy agents. However, targeted therapy-based studies are currently underway in this area, and furtherly there are some biological pitfalls, clinical issues, and limitations related to these targeted therapies, which need to be considered. This review aimed to investigate the alteration of DNA repair elements of C-NHEJ and A-EJ in hematologic malignancies and evaluate the potential targeted therapies against these pathways.

Nf1 and Sh2b3 mutations cooperate in vivo in a mouse model of juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is initiated in early childhood by somatic mutations that activate Ras signaling. Although some patients have only a single identifiable oncogenic mutation, others have 1 or more additional alterations. Such secondary mutations, as a group, are associated with an increased risk of relapse after hematopoietic stem cell transplantation or transformation to acute myeloid leukemia. These clinical observations suggest a cooperative effect between initiating and secondary mutations. However, the roles of specific genes in the prognosis or clinical presentation of JMML have not been described. In this study, we investigate the impact of secondary SH2B3 mutations in JMML. We find that patients with SH2B3 mutations have adverse outcomes, as well as higher white blood cell counts and hemoglobin F levels in the peripheral blood. We further demonstrate this interaction in genetically engineered mice. Deletion of Sh2b3 cooperates with conditional Nf1 deletion in a dose-dependent fashion. These studies illustrate that haploinsufficiency for Sh2b3 contributes to the severity of myeloproliferative disease and provide an experimental system for testing treatments for a high-risk cohort of JMML patients.

Thrombopoietin from hepatocytes promotes hematopoietic stem cell regeneration after myeloablation

The bone marrow niche plays critical roles in hematopoietic recovery and hematopoietic stem cell (HSC) regeneration after myeloablative stress. However, it is not clear whether systemic factors beyond the local niche are required for these essential processes in vivo. Thrombopoietin (THPO) is a key cytokine promoting hematopoietic rebound after myeloablation and its transcripts are expressed by multiple cellular sources. The upregulation of bone marrow-derived THPO has been proposed to be crucial for hematopoietic recovery and HSC regeneration after stress. Nonetheless, the cellular source of THPO in myeloablative stress has never been investigated genetically. We assessed the functional sources of THPO following two common myeloablative perturbations: 5-fluorouracil (5-FU) administration and irradiation. Using a Thpo translational reporter, we found that the liver but not the bone marrow is the major source of THPO protein after myeloablation. Mice with conditional Thpo deletion from osteoblasts and/or bone marrow stromal cells showed normal recovery of HSCs and hematopoiesis after myeloablation. In contrast, mice with conditional Thpo deletion from hepatocytes showed significant defects in HSC regeneration and hematopoietic rebound after myeloablation. Thus, systemic THPO from the liver is necessary for HSC regeneration and hematopoietic recovery in myeloablative stress conditions.

Thrombopoietin receptor agonists for marrow failure: A concise clinical review

Bone marrow failure is characterized by a disruption of hematopoietic stem cell (HSC) homeostasis and function, which causes decreased blood counts. Germline and somatic mutations within HSCs and immune dysregulation contribute to the pathogenesis of marrow failure. Allogeneic HSC transplant is a potentially curative therapy for marrow failure, although not all patients are candidates for this procedure. Immune suppressive therapy (IST) is an effective treatment for patients with aplastic anemia (AA) and select patients with myelodysplastic syndromes, but some patients fail to respond or relapse after IST. Over the past decade, the oral thrombopoietin receptor agonist eltrombopag has become a therapeutic option for AA in combination with frontline IST, and as a single agent for relapsed and refractory patients after IST. In this review, we highlight current knowledge of thrombopoietin receptor agonist mechanisms of action, and clinical indications and toxicities in patients with marrow failure, including the risk of clonal evolution.

Eltrombopag for patients with moderate aplastic anemia or uni-lineage cytopenias

There is no standard or widely effective treatment of patients with moderate aplastic anemia (MAA) or hypo-productive uni-lineage cytopenias (UC). Eltrombopag (EPAG), a small molecule thrombopoietin mimetic, has previously been shown to result in durable multi-lineage hematologic responses with low toxicity in patients with refractory severe aplastic anemia (SAA). Its safety and efficacy in MAA are unknown. This prospective phase 2 study enrolled previously untreated and treated MAA and UC patients with clinically relevant cytopenias. EPAG was administered at doses escalating from 50 to 300 mg/d. Hematologic responses were assessed at 16 to 20 weeks. Responding patients were continued on EPAG until reaching defined robust or stable blood counts. EPAG was reinstituted for relapse. Thirty-four patients were enrolled between 2012 and 2017, including 31 with MAA and 3 with UC. Seventeen patients responded in at least 1 eligible lineage by the primary end point. A striking improvement in anemia was observed in a patient with Diamond-Blackfan anemia. EPAG was well tolerated, and it was discontinued for robust or stable blood counts in 12 of 17 patients after a median of 8 months. A majority required re-initiation of EPAG for declining counts, and all regained response. Two of 34 patients developed non-chromosome 7 bone marrow cytogenetic abnormalities while taking EPAG, without dysplasia or increased blasts. Somatic mutation allele frequencies in cancer genes did not increase overall on EPAG. EPAG is a well-tolerated oral treatment of cytopenias in patients with MAA/UC. This trial was registered at www.clinicaltrials.gov as #NCT01328587.

Prolonged maintenance of hematopoietic stem cells that escape from thrombopoietin deprivation

Hematopoietic stem cells (HSC) rarely divide, rest in quiescence, and proliferate only upon stress hematopoiesis. The cytokine thrombopoietin (Thpo) has been perplexingly described to induce quiescence and promote self-renewal divisions in HSCs. To clarify the contradictory effect of Thpo, we conducted a detailed analysis on conventional (Thpo-/-) and liver-specific (Thpofl/fl;AlbCre+/-) Thpo-deletion models. Thpo-/- HSCs exhibited profound loss of quiescence, impaired cell cycle progression, and increased apoptosis. Thpo-/- HSCs also exhibited diminished mitochondrial mass and impaired mitochondrial bioenergetics. Abnormal HSC phenotypes in Thpo-/- mice were reversible after HSC transplantation into wild-type recipients. Moreover, Thpo-/- HSCs acquired quiescence with extended administration of a Thpo receptor agonist, romiplostim, and were prone to subsequent stem cell exhaustion during competitive bone marrow transplantation. Thpofl/fl;AlbCre+/- HSCs exhibited similar stem cell phenotypes but to a lesser degree compared with Thpo-/- HSCs. HSCs that survive Thpo deficiency acquire quiescence in a dose-dependent manner through the modification of their metabolic state.

MPN: The Molecular Drivers of Disease Initiation, Progression and Transformation and their Effect on Treatment

Myeloproliferative neoplasms (MPNs) constitute a group of disorders identified by an overproduction of cells derived from myeloid lineage. The majority of MPNs have an identifiable driver mutation responsible for cytokine-independent proliferative signalling. The acquisition of coexisting mutations in chromatin modifiers, spliceosome complex components, DNA methylation modifiers, tumour suppressors and transcriptional regulators have been identified as major pathways for disease progression and leukemic transformation. They also confer different sensitivities to therapeutic options. This review will explore the molecular basis of MPN pathogenesis and specifically examine the impact of coexisting mutations on disease biology and therapeutic options.

Non-Hodgkin's lymphoma in an elderly patient with renal dysfunction: a case report

OBJECTIVE

This study was performed to examine the treatment regimen used for an elderly patient with diffuse large B-cell lymphoma (DLBCL) complicated with renal dysfunction.

CASE REPORT

An 85-year-old man presented with nasal and sinus disorders in May 2018. He was also found to have renal insufficiency caused by long-term consumption of compound aminopyrine phenacetin tablets. Physical examination revealed irritation of the nasal mucous membrane on the right side and dark red nasal passages with a smooth surface. The right side of the neck contained several small peanut-sized lymph nodes. A biopsy of the right nasal neoplasm revealed germinal center type DLBCL. The mini-rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone regimen (mini-R-CHOP) was administered as the main chemotherapy regimen. Additionally, the use of thrombopoietin prevented further deterioration in renal function. This individualized treatment program helped the patient to achieve complete remission. The creatinine level decreased and was well maintained.

CONCLUSION

The mini-R-CHOP and rituximab cross-use regimen was found to be safe in an elderly patient with chronic renal insufficiency. Thrombopoietin exerted a protective effect on renal function.

Mitigative efficacy of the clinical dosage administration of granulocyte colony-stimulating factor and romiplostim in mice with severe acute radiation syndrome

BACKGROUND

It has been reported that the high-dosage administration of domestically approved pharmaceutical drugs, especially granulocyte colony-stimulating factor (G-CSF) and romiplostim (RP), is a rapid and appropriate medical treatment for preventing severe acute radiation syndrome (ARS) of victims exposed to lethal total-body irradiation (TBI). However, it remains unclear whether or not the clinical dosage administration of these drugs can ameliorate TBI-induced ARS and related high mortality in order to find various drug treatment options and less toxic optimum protocol depending on the situation surrounding the radiological accidents.

METHODS

We assessed the clinical dosage administration in combination with G-CSF and RP as intraperitoneal injection in C57BL/6 J mice exposed to more than 7-Gy lethal dose of X-ray TBI for the survival study evaluated by the log-rank test. Bone marrow and splenic cells were collected on the 21st day, when 1 week has passed from last administration, to detect the level of cell apoptosis, intracellular reactive oxygen species (ROS), and nuclear factor erythroid 2-related factor 2 (Nrf2)-related anti-oxidative gene expressions, and enzyme-linked immune sorbent assay using sera was performed for cell senescence and inflammation status analyzed with one-way ANOVA and Tukey-Kramer or Bonferroni/Dunn multiple comparison tests.

RESULTS

The combined once-daily administration of 10 μg/kg G-CSF for 4 times and 10 μg/kg RP once a week for 3 times improve the 30-day survival rate of lethal TBI mice compared with untreated TBI mice, accompanied by a gradual increase in the body weight and hematopoietic cell numbers. The radio-mitigative effect is probably attributed to the scavenging of ROS and the reduction in cell apoptosis. These changes were associated with the upregulation of Nrf2 and its downstream anti-oxidative targets in TBI mice. Furthermore, this combination modulated TBI-induced cell senescence an d inflammation markers.

CONCLUSIONS

This study suggested that the clinical dosage administration in combination with G-CSF and RP may also have radio-mitigative effects on mice exposed to lethal TBI and may be a potent therapeutic agent for mitigating radiation-induced severe ARS.

Single-Dose Administration of Recombinant Human Thrombopoietin Mitigates Total Body Irradiation-Induced Hematopoietic System Injury in Mice and Nonhuman Primates

PURPOSE

Recombinant human thrombopoietin (rhTPO) has been evaluated as a therapeutic intervention for radiation-induced myelosuppression. However, the immunogenicity induced by a repeated-dosing strategy raises concerns about the therapeutic use of rhTPO. In this study, single-dose administration of rhTPO was evaluated for efficacy in the hematopoietic response and survival effect on mice and nonhuman primates exposed to total body irradiation (TBI).

METHODS AND MATERIALS

Survival of lethally (9.0 Gy) irradiated C57BL/6J male mice was observed for 30 days after irradiation. Hematologic evaluations were performed on C57BL/6J male mice given a sublethal dose of radiation (6.5 Gy). Furthermore, in sublethally irradiated mice, we performed bone marrow (BM) histologic evaluation and evaluated BM-derived clonogenic activity. Next, the proportion and number of hematopoietic stem cells (HSCs) were analyzed. Competitive repopulation experiments were conducted to assess the multilineage engraftment of irradiated HSCs after BM transplantation. Flow cytometry was used to evaluate DNA damage, cell apoptosis, and cell cycle stage in HSCs after irradiation. Finally, we evaluated the efficacy of a single dose of rhTPO administered after 7 Gy TBI in male and female rhesus monkeys.

RESULTS

A single administration of rhTPO 2 hours after irradiation significantly mitigated TBI-induced death in mice. rhTPO promoted multilineage hematopoietic recovery, increasing peripheral blood cell counts, BM cellularity, and BM colony-forming ability. rhTPO administration led to an accelerated recovery of BM HSC frequency and multilineage engraftment after transplantation. rhTPO treatment reduced radiation-induced DNA damage and apoptosis and promoted HSC proliferation after TBI. Notably, a single administration of rhTPO significantly promoted multilineage hematopoietic recovery and improved survival in nonhuman primates after TBI.

CONCLUSIONS

These findings indicate that early intervention with a single administration of rhTPO may represent a promising and effective radiomitigative strategy for victims of radiation disasters.

Epidermal growth factor receptor-dependent DNA repair promotes murine and human hematopoietic regeneration

Chemotherapy and irradiation cause DNA damage to hematopoietic stem cells (HSCs), leading to HSC depletion and dysfunction and the risk of malignant transformation over time. Extrinsic regulation of HSC DNA repair is not well understood, and therapies to augment HSC DNA repair following myelosuppression remain undeveloped. We report that epidermal growth factor receptor (EGFR) regulates DNA repair in HSCs following irradiation via activation of the DNA-dependent protein kinase-catalytic subunit (DNA-PKcs) and nonhomologous end joining (NHEJ). We show that hematopoietic regeneration in vivo following total body irradiation is dependent upon EGFR-mediated repair of DNA damage via activation of DNA-PKcs. Conditional deletion of EGFR in hematopoietic stem and progenitor cells (HSPCs) significantly decreased DNA-PKcs activity following irradiation, causing increased HSC DNA damage and depressed HSC recovery over time. Systemic administration of epidermal growth factor (EGF) promoted HSC DNA repair and rapid hematologic recovery in chemotherapy-treated mice and had no effect on acute myeloid leukemia growth in vivo. Further, EGF treatment drove the recovery of human HSCs capable of multilineage in vivo repopulation following radiation injury. Whole-genome sequencing analysis revealed no increase in coding region mutations in HSPCs from EGF-treated mice, but increased intergenic copy number variant mutations were detected. These studies demonstrate that EGF promotes HSC DNA repair and hematopoietic regeneration in vivo via augmentation of NHEJ. EGF has therapeutic potential to promote human hematopoietic regeneration, and further studies are warranted to assess long-term hematopoietic effects.

Proteomic changes by radio-mitigative thrombopoietin receptor agonist romiplostim in the blood of mice exposed to lethal total-body irradiation

PURPOSE

The thrombopoietin receptor agonist romiplostim (RP) is a therapeutic agent for immune thrombocytopenia that can achieve complete survival in mice exposed to a lethal dose of ionizing radiation. The estimated mechanism of the radio-protective/mitigative effects of RP has been proposed; however, the detailed mechanism of action remains unclear. This study aimed to elucidate the mechanism of the radio-protective/mitigative effects of RP, the fluctuation of protein in the blood was analyzed by proteomics.

MATERIALS AND METHODS

Eight-week-old female C57BL/6J mice were randomly divided into 5 groups; control at day 0, total-body irradiation (TBI) groups at day 10 and day 18, and TBI plus RP groups at day 10 and day18, consisting of 3 mice per group, and subjected to TBI with 7 Gy of ¹³⁷Cs γ-rays at a dose rate of 0.74 Gy/min. RP was administered intraperitoneally to mice at a dose of 50 µg/kg once daily for 3 days starting 2 hours after TBI. On day 10 and day 18 after TBI, serum collected from each mouse was analyzed by liquid chromatography tandem mass spectrometry.

RESULTS

Nine proteins were identified by proteomics methods from 269 analyzed proteins detected in mice exposed to a lethal dose of TBI: keratin, type II cytoskeletal 1 (KRT1), fructose-1, 6-bisphosphatase (FBP1), cytosolic 10-formyltetrahydrofolate dehydrogenase (ALDH1L1), peptidyl-prolyl cis-trans isomerase A (PPIA), glycine N-methyltransferase (GNMT), glutathione S-transferase Mu 1 (GSTM1), regucalcin (RGN), fructose-bisphosphate aldolase B (ALDOB) and betain-homocysteine S-methyltransferase 1 (BHMT). On the 10th day after TBI, KRT1 was significantly increased (p < 0.05) by 4.26-fold compared to the control group in the TBI group and significantly inhibited in the TBI plus RP group (p < 0.05). Similarly, the expression levels of other 8 proteins detected at 18th day after TBI were significantly increased by 4.29 to 27.44-fold in the TBI group, but significantly decreased in the TBI plus RP group compared to the TBI group, respectively.

CONCLUSION

Nine proteins were identified by proteomics methods from 269 analyzed proteins detected in mice exposed to a lethal dose of TBI. These proteins are also expected to be indicators of the damage induced by high-dose radiation.

Dysregulated haematopoietic stem cell behaviour in myeloid leukaemogenesis

Haematopoiesis is governed by haematopoietic stem cells (HSCs) that produce all lineages of blood and immune cells. The maintenance of blood homeostasis requires a dynamic response of HSCs to stress, and dysregulation of these adaptive-response mechanisms underlies the development of myeloid leukaemia. Leukaemogenesis often occurs in a stepwise manner, with genetic and epigenetic changes accumulating in pre-leukaemic HSCs prior to the emergence of leukaemic stem cells (LSCs) and the development of acute myeloid leukaemia. Clinical data have revealed the existence of age-related clonal haematopoiesis, or the asymptomatic clonal expansion of mutated blood cells in the elderly, and this phenomenon is connected to susceptibility to leukaemic transformation. Here we describe how selection for specific mutations that increase HSC competitive fitness, in conjunction with additional endogenous and environmental changes, drives leukaemic transformation. We review the ways in which LSCs take advantage of normal HSC properties to promote survival and expansion, thus underlying disease recurrence and resistance to conventional therapies, and we detail our current understanding of leukaemic 'stemness' regulation. Overall, we link the cellular and molecular mechanisms regulating HSC behaviour with the functional dysregulation of these mechanisms in myeloid leukaemia and discuss opportunities for targeting LSC-specific mechanisms for the prevention or cure of malignant diseases.

Molecular Profiling for Predictors of Radiosensitivity in Patients with Breast or Head-and-Neck Cancer

Nearly half of all cancers are treated with radiotherapy alone or in combination with other treatments, where damage to normal tissues is a limiting factor for the treatment. Radiotherapy-induced adverse health effects, mostly of importance for cancer patients with long-term survival, may appear during or long time after finishing radiotherapy and depend on the patient’s radiosensitivity. Currently, there is no assay available that can reliably predict the individual’s response to radiotherapy. We profiled two study sets from breast (n = 29) and head-and-neck cancer patients (n = 74) that included radiosensitive patients and matched radioresistant controls.. We studied 55 single nucleotide polymorphisms (SNPs) in 33 genes by DNA genotyping and 130 circulating proteins by affinity-based plasma proteomics. In both study sets, we discovered several plasma proteins with the predictive power to find radiosensitive patients (adjusted p < 0.05) and validated the two most predictive proteins (THPO and STIM1) by sandwich immunoassays. By integrating genotypic and proteomic data into an analysis model, it was found that the proteins CHIT1, PDGFB, PNKD, RP2, SERPINC1, SLC4A, STIM1, and THPO, as well as the VEGFA gene variant rs69947, predicted radiosensitivity of our breast cancer (AUC = 0.76) and head-and-neck cancer (AUC = 0.89) patients. In conclusion, circulating proteins and a SNP variant of VEGFA suggest that processes such as vascular growth capacity, immune response, DNA repair and oxidative stress/hypoxia may be involved in an individual’s risk of experiencing radiation-induced toxicity.

Human hematopoietic stem/progenitor cells display reactive oxygen species-dependent long-term hematopoietic defects after exposure to low doses of ionizing radiations

Hematopoietic stem cells are responsible for life-long blood cell production and are highly sensitive to exogenous stresses. The effects of low doses of ionizing radiations on radiosensitive tissues such as the hematopoietic tissue are still unknown despite their increasing use in medical imaging. Here, we study the consequences of low doses of ionizing radiations on differentiation and self-renewal capacities of human primary hematopoietic stem/progenitor cells (HSPC). We found that a single 20 mGy dose impairs the hematopoietic reconstitution potential of human HSPC but not their differentiation properties. In contrast to high irradiation doses, low doses of irradiation do not induce DNA double strand breaks in HSPC but, similar to high doses, induce a rapid and transient increase of reactive oxygen species (ROS) that promotes activation of the p38MAPK pathway. HSPC treatment with ROS scavengers or p38MAPK inhibitor prior exposure to 20 mGy irradiation abolishes the 20 mGy-induced defects indicating that ROS and p38MAPK pathways are transducers of low doses of radiation effects. Taken together, these results show that a 20 mGy dose of ionizing radiation reduces the reconstitution potential of HSPC suggesting an effect on the self-renewal potential of human hematopoietic stem cells and pinpointing ROS or the p38MAPK as therapeutic targets. Inhibition of ROS or the p38MAPK pathway protects human primary HSPC from low-dose irradiation toxicity.

Mechanisms and therapeutic prospects of thrombopoietin receptor agonists

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

Diverse functions of the thrombopoietin receptor agonist romiplostim rescue individuals exposed to lethal radiation

In cases of radiological accidents, especially victims exposed to high-dose ionizing radiation, the administration of appropriate approved pharmaceutical drugs is the most rapid medical treatment. However, currently, there are no suitable candidates. The thrombopoietin receptor (TPOR) agonist romiplostim (RP) is a therapeutic agent for immune thrombocytopenia and has potential to respond to such victims. Here, we show that RP administration in mice exposed to lethal-dose radiation leads not only to the promotion of haematopoiesis in multiple organs, including the lungs but also a reduction in damage to organs and cells. RP also causes a rapid increase in the number of mesenchymal stem cells in the spleen. In addition, RP suppresses the expression of several miRNAs involved in radiation-induced leukemogenesis, suggesting the presence of targets other than TPOR. Among the currently approved pharmaceutical drugs, RP is the most suitable candidate for victims exposed to high-dose ionizing radiation.

Eltrombopag promotes DNA repair in human hematopoietic stem and progenitor cells

A causal link between hematopoietic stem/progenitor cell (HSPC) dysfunction and DNA damage accrual has been proposed. Clinically relevant strategies to maintain genome integrity in these cells are needed. Here we report that eltrombopag, a small molecule agonist of the thrombopoietin (TPO) receptor used in the clinic, promotes DNA double-strand break (DSB) repair in human HSPCs. We found that eltrombopag specifically activates the classic nonhomologous end-joining (C-NHEJ) DNA repair mechanism, a pathway known to support genome integrity. Eltrombopag-mediated DNA repair results in enhanced genome stability, survival, and function of primary human HSPCs, as demonstrated in karyotyping analyses, colony-forming unit assays and after transplantation in immunodeficient NSG mice. Eltrombopag may offer a new therapeutic modality to protect human HSPCs against genome insults.

Molecular and epigenetic regulatory mechanisms of normal stem cell radiosensitivity

Ionizing radiation (IR) therapy is a major cancer treatment modality and an indispensable auxiliary treatment for primary and metastatic cancers, but invariably results in debilitating organ dysfunctions. IR-induced depletion of neural stem/progenitor cells in the subgranular zone of the dentate gyrus in the hippocampus where neurogenesis occurs is considered largely responsible for deficiencies such as learning, memory, and spatial information processing in patients subjected to cranial irradiation. Similarly, IR therapy-induced intestinal injuries such as diarrhea and malabsorption are common side effects in patients with gastrointestinal tumors and are believed to be caused by intestinal stem cell drop out. Hematopoietic stem cell transplantation is currently used to reinstate blood production in leukemia patients and pre-clinical treatments show promising results in other organs such as the skin and kidney, but ethical issues and logistic problems make this route difficult to follow. An alternative way to restore the injured tissue is to preserve the stem cell pool located in that specific tissue/organ niche, but stem cell response to ionizing radiation is inadequately understood at the molecular mechanistic level. Although embryonic and fetal hypersensity to IR has been very well known for many decades, research on embryonic stem cell models in culture concerning molecular mechanisms have been largely inconclusive and often in contradiction of the in vivo observations. This review will summarize the latest discoveries on stem cell radiosensitivity, highlighting the possible molecular and epigenetic mechanism(s) involved in DNA damage response and programmed cell death after ionizing radiation therapy specific to normal stem cells. Finally, we will analyze the possible contribution of stem cell-specific chromatin's epigenetic constitution in promoting normal stem cell radiosensitivity.

Lnk/Sh2b3 deficiency restores hematopoietic stem cell function and genome integrity in Fancd2 deficient Fanconi anemia

Fanconi anemia (FA) is a bone marrow failure (BMF) syndrome that arises from mutations in a network of FA genes essential for DNA interstrand crosslink (ICL) repair and replication stress tolerance. While allogeneic stem cell transplantation can replace defective HSCs, interventions to mitigate HSC defects in FA do not exist. Remarkably, we reveal here that Lnk (Sh2b3) deficiency restores HSC function in Fancd2^−/− mice. Lnk deficiency does not impact ICL repair, but instead stabilizes stalled replication forks in a manner, in part, dependent upon alleviating blocks to cytokine−mediated JAK2 signaling. Lnk deficiency restores proliferation and survival of Fancd2^−/− HSCs, while reducing replication stress and genomic instability. Furthermore, deletion of LNK in human FA-like HSCs promotes clonogenic growth. These findings highlight a new role for cytokine/JAK signaling in promoting replication fork stability, illuminate replication stress as a major underlying origin of BMF in FA, and have strong therapeutic implications.

Loss of Fancd2 leads to replication stress intolerance and Fanconi Anemia, where haematopoietic stem cell (HSC) function is compromised. Here, the authors show that Lnk/Sh2b3 loss restores HSC proliferation and survival in Fancd2 knockout mice and ameliorates replication stress in a cytokine/JAK2 signaling dependent manner.

The thrombopoietin mimetic romiplostim leads to the complete rescue of mice exposed to lethal ionizing radiation

For the primary treatment of emergency exposure to high-dose radiation, such as in the event of a radiation accident, the top priority is the reconstitution and restoration of haematopoiesis. In most radiation accidents, drug therapy is chosen as the most suitable treatment; the chosen drug should already be approved domestically, stably supplied and regularly stockpiled. In the present study, a single administration of romiplostim (RP), an approved thrombopoietin receptor agonist, produced a 100% survival rate in C57BL/6 J mice exposed to a lethal dose (7 Gy) of ¹³⁷Cs γ-rays, and all irradiated mice survived for more than 30 days with both 3- and 5-day consecutive administrations. By day 30, the peripheral blood cells, bone marrow cells and haematopoietic progenitor cells of the RP-administered irradiated mice had all recovered to a level that was not significantly different from that in non-irradiated mice. In contrast to myelosuppression, which did not fully recover until day 30, the expression of several bone marrow cell surface antigens recovered sooner, and DNA repair concurrently increased in haematopoietic cells, speeding the resolution of double strand breaks and reducing the rates of apoptosis. These findings suggest that RP may be a clinic-ready countermeasure to treat victims of radiation accidents.

Thrombopoietin protects hematopoietic stem cells from retrotransposon-mediated damage by promoting an antiviral response

Propagation of retrotransposons induces genomic instability. Their roles in HSCs remain poorly studied. Barbieri et al. show that retrotransposon expression and mobilization are involved in long-lasting HSC impairment upon irradiation. These effects are counteracted by the self-renewal cytokine THPO through induction of interferon-like response.

Maintenance of genomic integrity is crucial for the preservation of hematopoietic stem cell (HSC) potential. Retrotransposons, spreading in the genome through an RNA intermediate, have been associated with loss of self-renewal, aging, and DNA damage. However, their role in HSCs has not been addressed. Here, we show that mouse HSCs express various retroelements (REs), including long interspersed element-1 (L1) recent family members that further increase upon irradiation. Using mice expressing an engineered human L1 retrotransposition reporter cassette and reverse transcription inhibitors, we demonstrate that L1 retransposition occurs in vivo and is involved in irradiation-induced persistent γH2AX foci and HSC loss of function. Thus, RE represents an important intrinsic HSC threat. Furthermore, we show that RE activity is restrained by thrombopoietin, a critical HSC maintenance factor, through its ability to promote a potent interferon-like, antiviral gene response in HSCs. This uncovers a novel mechanism allowing HSCs to minimize irradiation-induced injury and reinforces the links between DNA damage, REs, and antiviral immunity.

The roles of JAK2 in DNA damage and repair in the myeloproliferative neoplasms: Opportunities for targeted therapy

The JAK2V617F-positive myeloproliferative neoplasms (MPN) serve as an excellent model for the study of genomic instability accumulation during cancer progression. Recent studies highlight the implication of JAK2 activating mutations in the development of DNA damage via reactive oxygen species (ROS) production, replication stress induction and the accumulation of genomic instability via the increased degradation of p53 and acquisition of a "mutagenic" phenotype. The accumulation of genomic instability and acquisition of mutations in critical DNA damage repair (DDR) mediators appears to be implicated in the progression of JAK2V617F-positive MPN. On the other hand, JAK2 signaling normally induces DDR through activation of repair mediators such as Chk1, RAD51 and RECQL5. These opposing effects on DNA integrity in the setting of JAK2V617F have significant clinical implications and have led to the introduction of novel combinational therapies for these diseases. The inhibition of MDM2 with Nutlin-3 improves the efficacy of IFN-α via decreased p53 degradation, the combination of hydroxyurea with Ruxolitinib, and their combination with PARP inhibitors have significant anti-tumor effects. A better understanding of the implication of JAK2 in the development and repair of DNA damage can improve our understanding of the biology of these neoplasms, meliorate the risk stratification of our patients and enrich our therapeutic armamentarium.

Ruxolitinib-induced defects in DNA repair cause sensitivity to PARP inhibitors in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) often carry JAK2(V617F), MPL(W515L), or CALR(del52) mutations. Current treatment options for MPNs include cytoreduction by hydroxyurea and JAK1/2 inhibition by ruxolitinib, both of which are not curative. We show here that cell lines expressing JAK2(V617F), MPL(W515L), or CALR(del52) accumulated reactive oxygen species-induced DNA double-strand breaks (DSBs) and were modestly sensitive to poly-ADP-ribose polymerase (PARP) inhibitors olaparib and BMN673. At the same time, primary MPN cell samples from individual patients displayed a high degree of variability in sensitivity to these drugs. Ruxolitinib inhibited 2 major DSB repair mechanisms, BRCA-mediated homologous recombination and DNA-dependent protein kinase-mediated nonhomologous end-joining, and, when combined with olaparib, caused abundant accumulation of toxic DSBs resulting in enhanced elimination of MPN primary cells, including the disease-initiating cells from the majority of patients. Moreover, the combination of BMN673, ruxolitinib, and hydroxyurea was highly effective in vivo against JAK2(V617F)+ murine MPN-like disease and also against JAK2(V617F)+, CALR(del52)+, and MPL(W515L)+ primary MPN xenografts. In conclusion, we postulate that ruxolitinib-induced deficiencies in DSB repair pathways sensitized MPN cells to synthetic lethality triggered by PARP inhibitors.

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.

Interorgan Communication Pathways in Physiology: Focus on Drosophila

Studies in mammals and Drosophila have demonstrated the existence and significance of secreted factors involved in communication between distal organs. In this review, primarily focusing on Drosophila, we examine the known interorgan communication factors and their functions, physiological inducers, and integration in regulating physiology. Moreover, we describe how organ-sensing screens in Drosophila can systematically identify novel conserved interorgan communication factors. Finally, we discuss how interorgan communication enabled and evolved as a result of specialization of organs. Together, we anticipate that future studies will establish a model for metazoan interorgan communication network (ICN) and how it is deregulated in disease.

Adult human megakaryocyte-erythroid progenitors are in the CD34+CD38mid fraction

Bipotent megakaryocyte/erythroid progenitors (MEPs) give rise to progeny limited to the megakaryocyte (Mk) and erythroid (E) lineages. We developed a novel dual-detection functional in vitro colony-forming unit (CFU) assay for single cells that differentiates down both the Mk and E lineages (CFU-Mk/E), which allowed development and validation of a novel purification strategy for the identification and quantitation of primary functional human MEPs from granulocyte colony-stimulating factor-mobilized peripheral blood and bone marrow. Applying this assay to fluorescence-activated cell sorter-sorted cell populations, we found that the Lin(-)CD34(+)CD38(mid)CD45RA(-)FLT3(-)MPL(+)CD36(-)CD41(-) population is much more highly enriched for bipotent MEPs than any previously reported subpopulations. We also developed purification strategies for primary human lineage-committed Mk and E progenitors identified as CFU-Mk and burst forming unit-E. Comparative expression analyses in MEP, MkP, and ErP populations revealed differential expression of MYB We tested whether alterations in MYB concentration affect the Mk-E fate decision at the single cell level in MEPs and found that short hairpin RNA-mediated MYB knockdown promoted commitment of MEPs to the Mk lineage, further defining its role in MEP lineage fate. There are numerous applications for these novel enrichment strategies, including facilitating mechanistic studies of MEP lineage commitment, improving approaches for in vitro expansion of Mk and E cells, and developing improved therapies for benign and malignant hematologic disease.

DNA Repair and Cytokines: TGF-β, IL-6, and Thrombopoietin as Different Biomarkers of Radioresistance

Double strand breaks (DSBs) induced by radiotherapy are highly cytotoxic lesions, leading to chromosomal aberrations and cell death. Ataxia-telangiectasia-mutated (ATM)-dependent DNA-damage response, non-homologous end joining, and homologous recombination pathways coordinately contribute to repairing DSBs in higher eukaryotes. It is known that the expression of DSB repair genes is increased in tumors, which is one of the main reasons for radioresistance. The inhibition of DSB repair pathways may be useful to increase tumor cell radiosensitivity and may target stem cell-like cancer cells, known to be the most radioresistant tumor components. Commonly overexpressed in neoplastic cells, cytokines confer radioresistance by promoting proliferation, survival, invasion, and angiogenesis. Unfortunately, tumor irradiation increases the expression of various cytokines displaying these effects, including transforming growth factor-beta and interleukin-6. Recently, the capabilities of these cytokines to support DNA repair pathways and the ATM-dependent DNA response have been demonstrated. Thrombopoietin, essential for megakaryopoiesis and very important for hematopoietic stem cell (HSC) homeostasis, has also been found to promote DNA repair in a highly selective manner. These findings reveal a novel mechanism underlying cytokine-related radioresistance, which may be clinically relevant. Therapies targeting specific cytokines may be used to improve radiosensitivity. Specific inhibitors may be chosen in consideration of different tumor microenvironments. Thrombopoietin may be useful in fending off irradiation-induced loss of HSCs.

Balancing self-renewal against genome preservation in stem cells: How do they manage to have the cake and eat it too?

Stem cells are endowed with the awesome power of self-renewal and multi-lineage differentiation that allows them to be major contributors to tissue homeostasis. Owing to their longevity and self-renewal capacity, they are also faced with a higher risk of genomic damage compared to differentiated cells. Damage on the genome, if not prevented or repaired properly, will threaten the survival of stem cells and culminate in organ failure, premature aging, or cancer formation. It is therefore of paramount importance that stem cells remain genomically stable throughout life. Given their unique biological and functional requirement, stem cells are thought to manage genotoxic stress somewhat differently from non-stem cells. The focus of this article is to review the current knowledge on how stem cells escape the barrage of oxidative and replicative DNA damage to stay in self-renewal. A clear statement on this subject should help us better understand tissue regeneration, aging, and cancer.

Heterogeneity of leukemia-initiating capacity of chronic myelogenous leukemia stem cells

Chronic myelogenous leukemia (CML) results from transformation of a long-term hematopoietic stem cell (LTHSC) by expression of the BCR-ABL fusion gene. However, BCR-ABL-expressing LTHSCs are heterogeneous in their capacity as leukemic stem cells (LSCs). Although discrepancies in proliferative, self-renewal, and differentiation properties of normal LTHSCs are being increasingly recognized, the mechanisms underlying heterogeneity of leukemic LTHSCs are poorly understood. Using a CML mouse model, we identified gene expression differences between leukemic and nonleukemic LTHSCs. Expression of the thrombopoietin (THPO) receptor MPL was elevated in leukemic LTHSC populations. Compared with LTHSCs with low MPL expression, LTHSCs with high MPL expression showed enhanced JAK/STAT signaling and proliferation in response to THPO in vitro and increased leukemogenic capacity in vivo. Although both G0 and S phase subpopulations were increased in LTHSCs with high MPL expression, LSC capacity was restricted to quiescent cells. Inhibition of MPL expression in CML LTHSCs reduced THPO-induced JAK/STAT signaling and leukemogenic potential. These same phenotypes were also present in LTHSCs from patients with CML, and patient LTHSCs with high MPL expression had reduced sensitivity to BCR-ABL tyrosine kinase inhibitor treatment but increased sensitivity to JAK inhibitors. Together, our studies identify MPL expression levels as a key determinant of heterogeneous leukemia-initiating capacity and drug sensitivity of CML LTHSCs and suggest that high MPL-expressing CML stem cells are potential targets for therapy.

Recombinant human thrombopoietin promotes hematopoietic reconstruction after severe whole body irradiation

Recombinant human thrombopoietin (rHuTPO) is a drug that is used clinically to promote megakaryocyte and platelet generation. Here, we report the mitigative effect of rHuTPO (administered after exposure) against severe whole body irradiation in mice. Injection of rHuTPO for 14 consecutive days following exposure significantly improved the survival rate of lethally irradiated mice. RHuTPO treatment notably increased bone marrow cell density and LSK cell numbers in the mice after sub-lethal irradiation primarily by promoting residual HSC proliferation. In lethally irradiated mice with hematopoietic cell transplantation, rHuTPO treatment increased the survival rate and enhanced hematopoietic cell engraftment compared with the placebo treatment. Our observations indicate that recombinant human TPO might have a therapeutic role in promoting hematopoietic reconstitution and HSC engraftment.

Immunosuppressive therapy for transplant-ineligible aplastic anemia patients

Aplastic anemia is a rare life-threatening bone marrow failure that is characterized by bicytopenia or pancytopenia in the peripheral blood and a hypoplastic or aplastic bone marrow. The patients are at risk of infection and hemorrhage due to neutropenia and thrombocytopenia and suffer from symptoms of anemia. The main treatment approaches are allogeneic stem cell transplantation and immunosuppression. Here, we review current standard immunosuppression and the attempts that have been made in the past two decades to improve results: review of recent developments also reveals that sometimes not only the advent of new drugs, good ideas and well-designed clinical trials decide the progress in the field but also marketing considerations of pharmaceutical companies. Aplastic anemia experts unfortunately had to face the situation that efficient drugs were withdrawn simply for marketing considerations. We will discuss the current options and challenges in first-line treatment and management of relapsing and refractory patients with an emphasis on adult patients. Some promising new approaches are currently under investigation in prospective, randomized trials.

L- and D-lactate enhance DNA repair and modulate the resistance of cervical carcinoma cells to anticancer drugs via histone deacetylase inhibition and hydroxycarboxylic acid receptor 1 activation

BACKGROUND

The consideration of lactate as an active metabolite is a newly emerging and attractive concept. Recently, lactate has been reported to regulate gene transcription via the inhibition of histone deacetylases (HDACs) and survival of cancer cells via hydroxycarboxylic acid receptor 1 (HCAR1). This study examined the role of L- and D-lactate in the DNA damage response in cervical cancer cells.

METHODS

Three cervical cancer cell lines were examined: HeLa, Ca Ski and C33A. The inhibitory activity of lactate on HDACs was analysed using Western blot and biochemical methods. The lactate-mediated stimulation of DNA repair and cellular resistance to neocarzinostatin, doxorubicin and cisplatin were studied using γ-H2AX, comet and clonogenic assays. HCAR1 and DNA repair gene expression was quantified by real-time PCR. DNA-PKcs activity and HCAR1 protein expression were evaluated via immunocytochemistry and Western blot, respectively. HCAR1 activation was investigated by measuring intracellular cAMP accumulation and Erk phosphorylation. HCAR1 expression was silenced using shRNA.

RESULTS

L- and D-lactate inhibited HDACs, induced histone H3 and H4 hyperacetylation, and decreased chromatin compactness in HeLa cells. Treating cells with lactate increased LIG4, NBS1, and APTX expression by nearly 2-fold and enhanced DNA-PKcs activity. Based on γ-H2AX and comet assays, incubation of cells in lactate-containing medium increased the DNA repair rate. Furthermore, clonogenic assays demonstrated that lactate mediates cellular resistance to clinically used chemotherapeutics. Western blot and immunocytochemistry showed that all studied cell lines express HCAR1 on the cellular surface. Inhibiting HCAR1 function via pertussis toxin pretreatment partially abolished the effects of lactate on DNA repair. Down-regulating HCAR1 decreased the efficiency of DNA repair, abolished the cellular response to L-lactate and decreased the effect of D-lactate. Moreover, HCAR1 shRNA-expressing cells produced significantly lower mRNA levels of monocarboxylate transporter 4. Finally, the enhancement of DNA repair and cell survival by lactate was suppressed by pharmacologically inhibiting monocarboxylate transporters using the inhibitor α-cyano-4-hydroxycinnamic acid (α-CHCA).

CONCLUSIONS

Our data indicate that L- and D-lactate present in the uterine cervix may participate in the modulation of cellular DNA damage repair processes and in the resistance of cervical carcinoma cells to anticancer therapy.

Autophagy confers DNA damage repair pathways to protect the hematopoietic system from nuclear radiation injury

Autophagy is essentially a metabolic process, but its in vivo role in nuclear radioprotection remains unexplored. We observed that ex vivo autophagy activation reversed the proliferation inhibition, apoptosis, and DNA damage in irradiated hematopoietic cells. In vivo autophagy activation improved bone marrow cellularity following nuclear radiation exposure. In contrast, defective autophagy in the hematopoietic conditional mouse model worsened the hematopoietic injury, reactive oxygen species (ROS) accumulation and DNA damage caused by nuclear radiation exposure. Strikingly, in vivo defective autophagy caused an absence or reduction in regulatory proteins critical to both homologous recombination (HR) and non-homologous end joining (NHEJ) DNA damage repair pathways, as well as a failure to induce these proteins in response to nuclear radiation. In contrast, in vivo autophagy activation increased most of these proteins in hematopoietic cells. DNA damage assays confirmed the role of in vivo autophagy in the resolution of double-stranded DNA breaks in total bone marrow cells as well as bone marrow stem and progenitor cells upon whole body irradiation. Hence, autophagy protects the hematopoietic system against nuclear radiation injury by conferring and intensifying the HR and NHEJ DNA damage repair pathways and by removing ROS and inhibiting apoptosis.