Hematopoietic dysfunction in a mouse model for Fanconi anemia group D1

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), the protection of stalled replication forks, and the suppression of replicative gaps, but their relative contributions to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for fork protection and gap suppression but not HDR. In mice, the loss of fork protection/gap suppression does not compromise genome stability or shorten tumor latency. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection/gap suppression defects are also observed in Brca2 heterozygous cells, likely due to reduced RAD51 stabilization at stalled forks/gaps. Gaps arise from PRIMPOL activity, which is associated with 5-hydroxymethyl-2'-deoxyuridine sensitivity due to the formation of SMUG1-generated abasic sites and is exacerbated by poly(ADP-ribose) polymerase (PARP) inhibition. However, HDR proficiency has the major role in mitigating sensitivity to chemotherapeutics, including PARP inhibitors.

LepR+ niche cell-derived AREG compromises hematopoietic stem cell maintenance under conditions of DNA repair deficiency and aging

The cross talk between extrinsic niche-derived and intrinsic hematopoietic stem cell (HSC) factors controlling HSC maintenance remains elusive. Here, we demonstrated that amphiregulin (AREG) from bone marrow (BM) leptin receptor (LepR+) niche cells is an important factor that mediates the cross talk between the BM niche and HSCs in stem cell maintenance. Mice deficient of the DNA repair gene Brca2, specifically in LepR+ cells (LepR-Cre;Brca2fl/fl), exhibited increased frequencies of total and myeloid-biased HSCs. Furthermore, HSCs from LepR-Cre;Brca2fl/fl mice showed compromised repopulation, increased expansion of donor-derived, myeloid-biased HSCs, and increased myeloid output. Brca2-deficient BM LepR+ cells exhibited persistent DNA damage-inducible overproduction of AREG. Ex vivo treatment of wild-type HSCs or systemic treatment of C57BL/6 mice with recombinant AREG impaired repopulation, leading to HSC exhaustion. Conversely, inhibition of AREG by an anti-AREG-neutralizing antibody or deletion of the Areg gene in LepR-Cre;Brca2fl/fl mice rescued HSC defects caused by AREG. Mechanistically, AREG activated the phosphoinositide 3-kinases (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, promoted HSC cycling, and compromised HSC quiescence. Finally, we demonstrated that BM LepR+ niche cells from other DNA repair-deficient and aged mice also showed persistent DNA damage-associated overexpression of AREG, which exerts similar negative effects on HSC maintenance. Therefore, we identified an important factor that regulates HSCs function under conditions of DNA repair deficiency and aging.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Highlights

Gap suppression requires BRCA2 C-terminal RAD51 binding in mouse and human cells Brca2 heterozygosity in mice results in fork protection and gap suppression defects Gap suppression mitigates sensitivity to hmdU, but only when HDR is unperturbedHDR deficiency is the primary driver of chemotherapeutic sensitivity.

eTOC blurb

Lim et al . report that gap suppression as well as fork protection require BRCA2 stabilization of RAD51 filaments in human and mouse cells but have minimal impact on genome integrity, oncogenesis, and drug resistance. BRCA2 suppression of PRIMPOL-mediated replication gaps confers resistance to the nucleotide hmdU, incorporation of which leads to cytotoxic abasic sites.This effect is diminished when HDR is abrogated.

Summary

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), protection of stalled replication forks, and suppression of replicative gaps. The relative contributions of these pathways to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for both fork protection and gap suppression, but not HDR. Loss of fork protection and gap suppression do not compromise genome instability or shorten tumor latency in mice or cause replication stress in human mammary cells. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection and gap suppression defects are also observed in Brca2 heterozygous mouse cells, likely due to reduced RAD51 stabilization at stalled forks and gaps. Gaps arise from PRIMPOL activity, which is associated with sensitivity to 5-hydroxymethyl-2’-deoxyuridine due to the formation of abasic sites by SMUG1 glycosylase and is exacerbated by poly(ADP-ribose) polymerase inhibition. However, HDR deficiency ultimately modulates sensitivity to chemotherapeutics, including PARP inhibitors.

Hypomorphic Brca2 and Rad51c double mutant mice display Fanconi anemia, cancer and polygenic replication stress

The prototypic cancer-predisposition disease Fanconi Anemia (FA) is identified by biallelic mutations in any one of twenty-three FANC genes. Puzzlingly, inactivation of one Fanc gene alone in mice fails to faithfully model the pleiotropic human disease without additional external stress. Here we find that FA patients frequently display FANC co-mutations. Combining exemplary homozygous hypomorphic Brca2/Fancd1 and Rad51c/Fanco mutations in mice phenocopies human FA with bone marrow failure, rapid death by cancer, cellular cancer-drug hypersensitivity and severe replication instability. These grave phenotypes contrast the unremarkable phenotypes seen in mice with single gene-function inactivation, revealing an unexpected synergism between Fanc mutations. Beyond FA, breast cancer-genome analysis confirms that polygenic FANC tumor-mutations correlate with lower survival, expanding our understanding of FANC genes beyond an epistatic FA-pathway. Collectively, the data establish a polygenic replication stress concept as a testable principle, whereby co-occurrence of a distinct second gene mutation amplifies and drives endogenous replication stress, genome instability and disease.

Development of a mouse model for spontaneous oral squamous cell carcinoma in Fanconi anemia

Fanconi anemia (FA) patients frequently develop oral squamous cell carcinoma (OSCC). This cancer in FA patients is diagnosed within the first 3-4 decades of life, very often preceded by lesions that suffer a malignant transformation. In addition, they respond poorly to current treatments due to toxicity or multiple recurrences. Translational research on new chemopreventive agents and therapeutic strategies has been unsuccessful partly due to scarcity of disease models or failure to fully reproduce the disease. Here we report that Fanca gene knockout mice (Fanca^-/-) frequently display pre-malignant lesions in the oral cavity. Moreover, when these animals were crossed with animals having conditional deletion of Trp53 gene in oral mucosa (K14cre;Trp53^F2-10/F2-10), they spontaneously developed OSCC with high penetrance and a median latency of less than ten months. Tumors were well differentiated and expressed markers of squamous differentiation, such as keratins K5 and K10. In conclusion, Fanca and Trp53 genes cooperate to suppress oral cancer in mice, and Fanca^-/-;K14cre;Trp53^F2-10/F2-10 mice constitute the first animal model of spontaneous OSCC in FA.

Autologous K63 deubiquitylation within the BRCA1-A complex licenses DNA damage recognition

The BRCA1-A complex contains matching lysine-63 ubiquitin (K63-Ub) binding and deubiquitylating activities. How these functionalities are coordinated to effectively respond to DNA damage remains unknown. We generated Brcc36 deubiquitylating enzyme (DUB) inactive mice to address this gap in knowledge in a physiologic system. DUB inactivation impaired BRCA1-A complex damage localization and repair activities while causing early lethality when combined with Brca2 mutation. Damage response dysfunction in DUB-inactive cells corresponded to increased K63-Ub on RAP80 and BRCC36. Chemical cross-linking coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) and cryogenic-electron microscopy (cryo-EM) analyses of isolated BRCA1-A complexes demonstrated the RAP80 ubiquitin interaction motifs are occupied by ubiquitin exclusively in the DUB-inactive complex, linking auto-inhibition by internal K63-Ub chains to loss of damage site ubiquitin recognition. These findings identify RAP80 and BRCC36 as autologous DUB substrates in the BRCA1-A complex, thus explaining the evolution of matching ubiquitin-binding and hydrolysis activities within a single macromolecular assembly.

Different responses to DNA damage determine ageing differences between organs

Organs age differently, causing wide heterogeneity in multimorbidity, but underlying mechanisms are largely elusive. To investigate the basis of organ-specific ageing, we utilized progeroid repair-deficient Ercc1^{Δ /-} mouse mutants and systematically compared at the tissue, stem cell and organoid level two organs representing ageing extremes. Ercc1^{Δ /-} intestine shows hardly any accelerated ageing. Nevertheless, we found apoptosis and reduced numbers of intestinal stem cells (ISCs), but cell loss appears compensated by over-proliferation. ISCs retain their organoid-forming capacity, but organoids perform poorly in culture, compared with WT. Conversely, liver ages dramatically, even causing early death in Ercc1-KO mice. Apoptosis, p21, polyploidization and proliferation of various (stem) cells were prominently elevated in Ercc1^{Δ /-} liver and stem cell populations were either largely unaffected (Sox9+), or expanding (Lgr5+), but were functionally exhausted in organoid formation and development in vitro. Paradoxically, while intestine displays less ageing, repair in WT ISCs appears inferior to liver as shown by enhanced sensitivity to various DNA-damaging agents, and lower lesion removal. Our findings reveal organ-specific anti-ageing strategies. Intestine, with short lifespan limiting time for damage accumulation and repair, favours apoptosis of damaged cells relying on ISC plasticity. Liver with low renewal rates depends more on repair pathways specifically protecting the transcribed compartment of the genome to promote sustained functionality and cell preservation. As shown before, the hematopoietic system with intermediate self-renewal mainly invokes replication-linked mechanisms, apoptosis and senescence. Hence, organs employ different genome maintenance strategies, explaining heterogeneity in organ ageing and the segmental nature of DNA-repair-deficient progerias.

ATM modulates subventricular zone neural stem cell maintenance and senescence through Notch signaling pathway

Ataxia telangiectasia mutated (ATM) plays an essential role in DNA damage response and the maintenance of genomic stability. However, the role of ATM in regulating the function of adult neural stem cells (NSCs) remains unclear. Here we report that ATM deficiency led to accumulated DNA damage and decreased DNA damage repair capacity in neural progenitor cells. Moreover, we observed ATM ablation lead to the short-term increase of proliferation of neural progenitor cells, resulting in the depletion of the NSC pool over time, and this loss of NSC quiescence resulted in accelerated cell senescence. We further apply RNA sequencing to unravel that ATM knockout significantly affected Notch signaling pathway, furthermore, notch activation inhibit the abnormal increased proliferation of ATM^-/- NSCs. Taken together, these findings indicate that ATM can serve as a key regulator for the normal function of adult NSCs by maintaining their stemness and preventing cellular senescence primarily through Notch signaling pathway.

Transcription Factors, R-Loops and Deubiquitinating Enzymes: Emerging Targets in Myelodysplastic Syndromes and Acute Myeloid Leukemia

Simple Summary

The advent of DNA massive sequencing technologies has allowed for the first time an extensive look into the heterogeneous spectrum of genes and mutations underpinning myelodysplastic syndromes (MDSs) and acute myeloid leukemia (AML). In this review, we wish to explore the most recent advances and the rationale for the potential therapeutic interest of three main actors in myelo-leukemic transformation: transcription factors that govern myeloid differentiation; RNA splicing factors, which ensure proper mRNA maturation and whose mutations increase R-loops formation; and deubiquitinating enzymes, which contribute to genome stability in hematopoietic stem cells (HSCs).

Abstract

Myeloid neoplasms encompass a very heterogeneous family of diseases characterized by the failure of the molecular mechanisms that ensure a balanced equilibrium between hematopoietic stem cells (HSCs) self-renewal and the proper production of differentiated cells. The origin of the driver mutations leading to preleukemia can be traced back to HSC/progenitor cells. Many properties typical to normal HSCs are exploited by leukemic stem cells (LSCs) to their advantage, leading to the emergence of a clonal population that can eventually progress to leukemia with variable latency and evolution. In fact, different subclones might in turn develop from the original malignant clone through accumulation of additional mutations, increasing their competitive fitness. This process ultimately leads to a complex cancer architecture where a mosaic of cellular clones—each carrying a unique set of mutations—coexists. The repertoire of genes whose mutations contribute to the progression toward leukemogenesis is broad. It encompasses genes involved in different cellular processes, including transcriptional regulation, epigenetics (DNA and histones modifications), DNA damage signaling and repair, chromosome segregation and replication (cohesin complex), RNA splicing, and signal transduction. Among these many players, transcription factors, RNA splicing proteins, and deubiquitinating enzymes are emerging as potential targets for therapeutic intervention.

Should the BCRA1/2-mutations healthy carriers be valid candidates for hematopoietic stem cell donation?

It’s still not clear whether the mutational status of BRCA-mutated healthy hematopoietic stem cells (HSCs) donors could have an impact on the engraftment. Comparing the studies present in literature, we focused on the correlation between BRCA mutations and the development of hematological malignancies and Fanconi anemia (FA); then, we explored HSCs types, frequencies, and functions in the presence of BRCA mutations, as well as the reconstitution of hematopoiesis after chemotherapy and radiation treatments. The role of BRCA mutations in the FA showed a possible involvement in the onset of the disease; the mutation carriers, indeed, did not show any sign of the typical phenotype of the FA. BRCA mutational status can be considered as a risk factor for hematological malignancies, but only for secondary malignancies and/or in the presence of bone marrow stress factors. Currently we don’t know if a conditioning regimen could be compensated by BRCA mutated HSCs, even if murine models tried to show the possible differences between fully mutated, haploinsufficient and normal HSCs. Thus, given the downregulating effect of the mutations on hematopoiesis, it could be questionable to use the HSCs of a BRCA-mutated donor in the presence of another available donor with the same compatibility.

Animal models of Fanconi anemia: A developmental and therapeutic perspective on a multifaceted disease

Fanconi anemia (FA) is a genetic disorder characterized by developmental abnormalities, progressive bone marrow failure, and increased susceptibility to cancer. FA animal models have been useful to understand the pathogenesis of the disease. Herein, we review FA developmental models that have been developed to simulate human FA, focusing on zebrafish and mouse models. We summarize the recapitulated phenotypes observed in these in vivo models including bone, gametogenesis and sterility defects, as well as marrow failure. We also discuss the relevance of aldehydes in pathogenesis of FA, emphasizing on hematopoietic defects. In addition, we provide a summary of potential therapeutic agents, such as aldehyde scavengers, TGFβ inhibitors, and gene therapy for FA. The diversity of FA animal models makes them useful for understanding FA etiology and allows the discovery of new therapies.

How Hematopoietic Stem Cells Respond to Irradiation: Similarities and Differences between Low and High Doses of Ionizing Radiations

In this review, we will specifically address the newest insights on the effect of low doses of ionizing radiations on the hematopoietic stem cells, which are prone to long-term deleterious effects. Impact of high doses of irradiation on hematopoietic cells has been widely studied over the years, in line with the risk of accidental or terrorist exposure to irradiation and with a particular attention to the sensitivity of the hematopoietic system. Recently, more studies have focused on lower doses of irradiation on different tissues, due to the increasing exposure caused by medical imaging, radiotherapy or plane travelling for instance. Hence, we will delineate similarities and discrepancies in HSC response to high and low doses of irradiation from these studies.

USP15 Deubiquitinase Safeguards Hematopoiesis and Genome Integrity in Hematopoietic Stem Cells and Leukemia Cells

Altering ubiquitination by disruption of deubiquitinating enzymes (DUBs) affects hematopoietic stem cell (HSC) maintenance. However, comprehensive knowledge of DUB function during hematopoiesis in vivo is lacking. Here, we systematically inactivate DUBs in mouse hematopoietic progenitors using in vivo small hairpin RNA (shRNA) screens. We find that multiple DUBs may be individually required for hematopoiesis and identify ubiquitin-specific protease 15 (USP15) as essential for HSC maintenance in vitro and in transplantations and Usp15 knockout (KO) mice in vivo. USP15 is highly expressed in human hematopoietic tissues and leukemias. USP15 depletion in murine progenitors and leukemia cells impairs in vitro expansion and increases genotoxic stress. In leukemia cells, USP15 interacts with and stabilizes FUS (fused in sarcoma), a known DNA repair factor, directly linking USP15 to the DNA damage response (DDR). Our study underscores the importance of DUBs in preserving normal hematopoiesis and uncovers USP15 as a critical DUB in safeguarding genome integrity in HSCs and leukemia cells.

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In vivo shRNAs screens for deubiquitinases identify regulators of murine hematopoiesis

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Usp15 deletion compromises HSC maintenance and reconstitution potential in vivo

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USP15 loss affects genome integrity and growth of mHSPCs and human leukemia cells

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In human leukemia cells, USP15 stabilizes its interactor, FUS, a DNA repair factor

Clinical and Molecular Characterization of Fanconi Anemia Patients in Turkey

Fanconi anemia (FA) is a rare multigenic chromosomal instability syndrome that predisposes patients to life-threatening bone marrow failure, congenital malformations, and cancer. Functional loss of interstrand cross-link (ICL) DNA repair system is held responsible, though the mechanism is not yet fully understood. The clinical and molecular findings of 20 distinct FA cases, ages ranging from perinatal stage to 32 years, are presented here. Pathogenic variants in FANCA were found responsible in 75%, FANCC, FANCE, FANCJ/BRIP1, FANCL in 5%, and FANCD1/BRCA2 and FANCN/PALB2 in 2.5% of the subjects. Altogether, 25 different variants in 7 different FA genes, including 10 novel mutations in FANCA, FANCN/PALB2, FANCE, and FANCJ/BRIP1, were disclosed. Two compound heterozygous germline cases were mosaic for one allele, revealing that the incidence of reverse mutations may not be uncommon in FA. Another case with de novo FANCD1/BRCA2 and paternally inherited FANCN/PALB2 pathogenic alleles at first glance suggested a digenic inheritance, because the presence of a second pathogenic variant in the unexamined regions of FANCD1/BRCA2 and FANCN/PALB2 were exluded by sequencing and deletion/duplication analysis. A better understanding of the complexity of the FA genotype may provide further access to undiscovered ICL components and apparently dispensable cellular pathways where FA proteins may play important roles.

Clonal hematopoiesis in the inherited bone marrow failure syndromes

Inherited bone marrow failure syndromes (IBMFSs) are characterized by ineffective hematopoiesis and increased risk for developing myeloid malignancy. The pathophysiologies of different IBMFSs are variable and can relate to defects in diverse biological processes, including DNA damage repair (Fanconi anemia), telomere maintenance (dyskeratosis congenita), and ribosome biogenesis (Diamond-Blackfan anemia, Shwachman-Diamond syndrome). Somatic mutations leading to clonal hematopoiesis have been described in IBMFSs, but the distinct mechanisms by which mutations drive clonal advantage in each disease and their associations with leukemia risk are not well understood. Clinical observations and laboratory models of IBMFSs suggest that the germline deficiencies establish a qualitatively impaired functional state at baseline. In this context, somatic alterations can promote clonal hematopoiesis by improving the competitive fitness of specific hematopoietic stem cell clones. Some somatic alterations relieve baseline fitness constraints by normalizing the underlying germline deficit through direct reversion or indirect compensation, whereas others do so by subverting senescence or tumor-suppressor pathways. Clones with normalizing somatic mutations may have limited transformation potential that is due to retention of functionally intact fitness-sensing and tumor-suppressor pathways, whereas those with mutations that impair cellular elimination may have increased risk for malignant transformation that is due to subversion of tumor-suppressor pathways. Because clonal hematopoiesis is not deterministic of malignant transformation, rational surveillance strategies will depend on the ability to prospectively identify specific clones with increased leukemic potential. We describe a framework by which an understanding of the processes that promote clonal hematopoiesis in IBMFSs may inform clinical surveillance strategies.

TALEN mediated gene editing in a mouse model of Fanconi anemia

The promising ability to genetically modify hematopoietic stem and progenitor cells by precise gene editing remains challenging due to their sensitivity to in vitro manipulations and poor efficiencies of homologous recombination. This study represents the first evidence of implementing a gene editing strategy in a murine safe harbor locus site that phenotypically corrects primary cells from a mouse model of Fanconi anemia A. By means of the co-delivery of transcription activator-like effector nucleases and a donor therapeutic FANCA template to the Mbs85 locus, we achieved efficient gene targeting (23%) in mFA-A fibroblasts. This resulted in the phenotypic correction of these cells, as revealed by the reduced sensitivity of these cells to mitomycin C. Moreover, robust evidence of targeted integration was observed in murine wild type and FA-A hematopoietic progenitor cells, reaching mean targeted integration values of 21% and 16% respectively, that were associated with the phenotypic correction of these cells. Overall, our results demonstrate the feasibility of implementing a therapeutic targeted integration strategy into the mMbs85 locus, ortholog to the well-validated hAAVS1, constituting the first study of gene editing in mHSC with TALEN, that sets the basis for the use of a new safe harbor locus in mice.

A Fanci knockout mouse model reveals common and distinct functions for FANCI and FANCD2

Fanconi Anemia (FA) clinical phenotypes are heterogenous and rely on a mutation in one of the 22 FANC genes (FANCA-W) involved in a common interstrand DNA crosslink-repair pathway. A critical step in the activation of FA pathway is the monoubiquitination of FANCD2 and its binding partner FANCI. To better address the clinical phenotype associated with FANCI and the epistatic relationship with FANCD2, we created the first conditional inactivation model for FANCI in mouse. Fanci ^−/− mice displayed typical FA features such as delayed development in utero, microphtalmia, cellular sensitivity to mitomycin C, occasional limb abnormalities and hematological deficiencies. Interestingly, the deletion of Fanci leads to a strong meiotic phenotype and severe hypogonadism. FANCI was localized in spermatocytes and spermatids and in the nucleus of oocytes. Both FANCI and FANCD2 proteins co-localized with RPA along meiotic chromosomes, albeit at different levels. Consistent with a role in meiotic recombination, FANCI interacted with RAD51 and stimulated D-loop formation, unlike FANCD2. The double knockout Fanci^−/− Fancd2^−/− also showed epistatic relationship for hematological defects while being not epistatic with respect to generating viable mice in crosses of double heterozygotes. Collectively, this study highlights common and distinct functions of FANCI and FANCD2 during mouse development, meiotic recombination and hematopoiesis.

DNA damage tolerance in stem cells, ageing, mutagenesis, disease and cancer therapy

The DNA damage response network guards the stability of the genome from a plethora of exogenous and endogenous insults. An essential feature of the DNA damage response network is its capacity to tolerate DNA damage and structural impediments during DNA synthesis. This capacity, referred to as DNA damage tolerance (DDT), contributes to replication fork progression and stability in the presence of blocking structures or DNA lesions. Defective DDT can lead to a prolonged fork arrest and eventually cumulate in a fork collapse that involves the formation of DNA double strand breaks. Four principal modes of DDT have been distinguished: translesion synthesis, fork reversal, template switching and repriming. All DDT modes warrant continuation of replication through bypassing the fork stalling impediment or repriming downstream of the impediment in combination with filling of the single-stranded DNA gaps. In this way, DDT prevents secondary DNA damage and critically contributes to genome stability and cellular fitness. DDT plays a key role in mutagenesis, stem cell maintenance, ageing and the prevention of cancer. This review provides an overview of the role of DDT in these aspects.

Advances in Gene Therapy for Fanconi Anemia

Fanconi anemia (FA) is a rare inherited disease that is associated with bone marrow failure and a predisposition to cancer. Previous clinical trials emphasized the difficulties that accompany the use of gene therapy to treat bone marrow failure in patients with FA. Nevertheless, the discovery of new drugs that can efficiently mobilize hematopoietic stem cells (HSCs) and the development of optimized procedures for transducing HSCs, using safe, integrative vectors, markedly improved the efficiency by which the phenotype of hematopoietic repopulating cells from patients with FA can be corrected. In addition, these achievements allowed the demonstration of the in vivo proliferation advantage of gene-corrected FA repopulating cells in immunodeficient mice. Significantly, new gene therapy trials are currently ongoing to investigate the progressive restoration of hematopoiesis in patients with FA by gene-corrected autologous HSCs. Further experimental studies are focused on the ex vivo transduction of unpurified FA HSCs, using new pseudotyped vectors that have HSC tropism. Because of the resistance of some of these vectors to serum complement, new strategies for in vivo gene therapy for FA HSCs are in development. Finally, because of the rapid advancements in gene-editing techniques, correction of CD34⁺ cells isolated from patients with FA is now feasible, using gene-targeting strategies. Taken together, these advances indicate that gene therapy can soon be used as an efficient and safe alternative for the hematopoietic treatment of patients with FA.

CD34+ Stem Cells: Promising Roles in Cardiac Repair and Regeneration

Cell therapy has received significant attention as a novel therapeutic approach to restore cardiac function after injury. CD34-positive (CD34⁺) stem cells have been investigated for their ability to promote angiogenesis and contribute to the prevention of remodelling after infarct. However, there are significant differences between murine and human CD34⁺ cells; understanding these differences might benefit the therapeutic use of these cells. Herein we discuss the function of the CD34 cell and highlight the similarities and differences between murine and human CD34 cell function, which might explain some of the differences between the animal and human evolutions. We also summarize the studies that report the application of murine and human CD34⁺ cells in preclinical studies and clinical trials and current limitations with the application of cell therapy for cardiac repair. Finally, to overcome these limitations we discuss the application of novel humanized rodent models that can bridge the gap between preclinical and clinical studies as well as rejuvenation strategies for improving the quality of old CD34⁺ cells for future clinical trials of autologous cell transplantation.

Holding All the Cards-How Fanconi Anemia Proteins Deal with Replication Stress and Preserve Genomic Stability

Fanconi anemia (FA) is a hereditary chromosomal instability disorder often displaying congenital abnormalities and characterized by a predisposition to progressive bone marrow failure (BMF) and cancer. Over the last 25 years since the discovery of the first linkage of genetic mutations to FA, its molecular genetic landscape has expanded tremendously as it became apparent that FA is a disease characterized by a defect in a specific DNA repair pathway responsible for the correction of covalent cross-links between the two complementary strands of the DNA double helix. This pathway has become increasingly complex, with the discovery of now over 20 FA-linked genes implicated in interstrand cross-link (ICL) repair. Moreover, gene products known to be involved in double-strand break (DSB) repair, mismatch repair (MMR), and nucleotide excision repair (NER) play roles in the ICL response and repair of associated DNA damage. While ICL repair is predominantly coupled with DNA replication, it also can occur in non-replicating cells. DNA damage accumulation and hematopoietic stem cell failure are thought to contribute to the increased inflammation and oxidative stress prevalent in FA. Adding to its confounding nature, certain FA gene products are also engaged in the response to replication stress, caused endogenously or by agents other than ICL-inducing drugs. In this review, we discuss the mechanistic aspects of the FA pathway and the molecular defects leading to elevated replication stress believed to underlie the cellular phenotypes and clinical features of FA.

Minimal conditioning in Fanconi anemia promotes multi-lineage marrow engraftment at 10-fold lower cell doses

BACKGROUND

Gene therapy approaches for the treatment of Fanconi anemia (FA) hold promise for patients without a suitably matched donor for an allogeneic bone marrow transplant. However, significant limitations include the collection of sufficient stem cell numbers from patients, the fragility of these cells during ex vivo manipulation, and clinically meaningful engraftment following transplantation. With these challenges in mind, we were interested in determining (i) whether gene-corrected cells at progressively lower numbers can successfully engraft in FA; (ii) whether low-dose conditioning facilitates this engraftment; and (iii) whether these cells can be selected for post-transplant.

METHODS

Utilizing a well characterized mouse model of FA, we infused donor bone marrow from healthy heterozygote littermates that are unaffected carriers of the FANCA mutation to mimic a gene-corrected product, after administering low-dose conditioning. Once baseline engraftment was observed, we administered a second, very-low selective dose to determine whether gene-corrected cells could be selected for in vivo.

RESULTS

We demonstrate that upfront low-dose conditioning greatly increases successful engraftment of hematopoietic corrected cells in a pre-clinical animal model of FA. Additionally, without conditioning, cells can still engraft and demonstrate a selective advantage in vivo over time following transplantation, and these corrected cells can be directly selected for in vivo after engraftment.

CONCLUSIONS

Minimal conditioning prior to bone marrow transplant in Fanconi anemia promotes the multi-lineage engraftment of 10-fold fewer cells compared to nonconditioned controls. These data provide important insights into the potential of minimally toxic conditioning protocols for FA gene therapy applications.

Replication stress in hematopoietic stem cells in mouse and man

Life-long blood regeneration relies on a rare population of self-renewing hematopoietic stem cells (HSCs). These cells' nearly unlimited self-renewal potential and lifetime persistence in the body signifies the need for tight control of their genome integrity. Their quiescent state, tightly linked with low metabolic activity, is one of the main strategies employed by HSCs to preserve an intact genome. On the other hand, HSCs need to be able to quickly respond to increased blood demands and rapidly increase their cellular output in order to fight infection-associated inflammation or extensive blood loss. This increase in proliferation rate, however, comes at the price of exposing HSCs to DNA damage inevitably associated with the process of DNA replication. Any interference with normal replication fork progression leads to a specialized molecular response termed replication stress (RS). Importantly, increased levels of RS are a hallmark feature of aged HSCs, where an accumulating body of evidence points to causative relationships between RS and the aging-associated impairment of the blood system's functional capacity. In this review, we present an overview of RS in HSCs focusing on its causes and consequences for the blood system of mice and men.

Distinct Brca1 Mutations Differentially Reduce Hematopoietic Stem Cell Function

BRCA1 is a well-known DNA repair pathway component and a tissue-specific tumor suppressor. However, its role in hematopoiesis is uncertain. Here, we report that a cohort of patients heterozygous for BRCA1 mutations experienced more hematopoietic toxicity from chemotherapy than those with BRCA2 mutations. To test whether this reflects a requirement for BRCA1 in hematopoiesis, we generated mice with Brca1 mutations in hematopoietic cells. Mice homozygous for a null Brca1 mutation in the embryonic hematopoietic system (Vav1-iCre;Brca1^{F22-24/F22-24}) developed hematopoietic defects in early adulthood that included reduced hematopoietic stem cells (HSCs). Although mice homozygous for a huBRCA1 knockin allele (Brca1^BRCA1/BRCA1) were normal, mice with a mutant huBRCA1/5382insC allele and a null allele (Mx1-Cre;Brca1^{F22-24/5382insC}) had severe hematopoietic defects marked by a complete loss of hematopoietic stem and progenitor cells. Our data show that Brca1 is necessary for HSC maintenance and normal hematopoiesis and that distinct mutations lead to different degrees of hematopoietic dysfunction.

Aging, metabolism and stem cells: Spotlight on muscle stem cells

All tissues and organs undergo a progressive regenerative decline as they age. This decline has been mainly attributed to loss of stem cell number and/or function, and both stem cell-intrinsic changes and alterations in local niches and/or systemic environment over time are known to contribute to the stem cell aging phenotype. Advancing in the molecular understanding of the deterioration of stem cell cells with aging is key for targeting the specific causes of tissue regenerative dysfunction at advanced stages of life. Here, we revise exciting recent findings on why stem cells age and the consequences on tissue regeneration, with a special focus on regeneration of skeletal muscle. We also highlight newly identified common molecular pathways affecting diverse types of aging stem cells, such as altered proteostasis, metabolism, or senescence entry, and discuss the questions raised by these findings. Finally, we comment on emerging stem cell rejuvenation strategies, principally emanating from studies on muscle stem cells, which will surely burst tissue regeneration research for future benefit of the increasing human aging population.

Effect of aging on stem cells

Pluripotent stem cells have the remarkable self-renewal ability and are capable of differentiating into multiple diverse cells. There is increasing evidence that the aging process can have adverse effects on stem cells. As stem cells age, their renewal ability deteriorates and their ability to differentiate into the various cell types is altered. Accordingly, it is suggested aging-induced deterioration of stem cell functions may play a key role in the pathophysiology of the various aging-associated disorders. Understanding the role of the aging process in deterioration of stem cell function is crucial, not only in understanding the pathophysiology of aging-associated disorders, but also in future development of novel effective stem cell-based therapies to treat aging-associated diseases. This review article first focuses on the basis of the various aging disease-related stem cell dysfunction. It then addresses the several concepts on the potential mechanism that causes aging-related stem cell dysfunction. It also briefly discusses the current potential therapies under development for aging-associated stem cell defects.

Interaction with PALB2 Is Essential for Maintenance of Genomic Integrity by BRCA2

Human breast cancer susceptibility gene, BRCA2, encodes a 3418-amino acid protein that is essential for maintaining genomic integrity. Among the proteins that physically interact with BRCA2, Partner and Localizer of BRCA2 (PALB2), which binds to the N-terminal region of BRCA2, is vital for its function by facilitating its subnuclear localization. A functional redundancy has been reported between this N-terminal PALB2-binding domain and the C-terminal DNA-binding domain of BRCA2, which undermines the relevance of the interaction between these two proteins. Here, we describe a genetic approach to examine the functional significance of the interaction between BRCA2 and PALB2 by generating a knock-in mouse model of Brca2 carrying a single amino acid change (Gly25Arg, Brca2G25R) that disrupts this interaction. In addition, we have combined Brca2G25R homozygosity as well as hemizygosity with Palb2 and Trp53 heterozygosity to generate an array of genotypically and phenotypically distinct mouse models. Our findings reveal defects in body size, fertility, meiotic progression, and genome stability, as well as increased tumor susceptibility in these mice. The severity of the phenotype increased with a decrease in the interaction between BRCA2 and PALB2, highlighting the significance of this interaction. In addition, our findings also demonstrate that hypomorphic mutations such as Brca2G25R have the potential to be more detrimental than the functionally null alleles by increasing genomic instability to a level that induces tumorigenesis, rather than apoptosis.

Aging of hematopoietic stem cells: DNA damage and mutations?

Aging in the hematopoietic system and the stem cell niche contributes to aging-associated phenotypes of hematopoietic stem cells (HSCs), including leukemia and aging-associated immune remodeling. Among others, the DNA damage theory of aging of HSCs is well established, based on the detection of a significantly larger amount of γH2AX foci and a higher tail moment in the comet assay, both initially thought to be associated with DNA damage in aged HSCs compared with young cells, and bone marrow failure in animals devoid of DNA repair factors. Novel data on the increase in and nature of DNA mutations in the hematopoietic system with age, the quality of the DNA damage response in aged HSCs, and the nature of γH2AX foci question a direct link between DNA damage and the DNA damage response and aging of HSCs, and rather favor changes in epigenetics, splicing-factors or three-dimensional architecture of the cell as major cell intrinsic factors of HSCs aging. Aging of HSCs is also driven by a strong contribution of aging of the niche. This review discusses the DNA damage theory of HSC aging in the light of these novel mechanisms of aging of HSCs.

Aging, Clonality, and Rejuvenation of Hematopoietic Stem Cells

Aging is associated with reduced organ function and increased disease incidence. Hematopoietic stem cell (HSC) aging driven by both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration, aging-associated immune remodeling, and increased leukemia incidence. Compromised DNA damage responses and the increased production of reactive oxygen species (ROS) have been previously causatively attributed to HSC aging. However, recent paradigm-shifting concepts, such as global epigenetic and cytoskeletal polarity shifts, cellular senescence, as well as the clonal selection of HSCs upon aging, provide new insights into HSC aging mechanisms. Rejuvenating agents that can reprogram the epigenetic status of aged HSCs or senolytic drugs that selectively deplete senescent cells provide promising translational avenues for attenuating hematopoietic aging and, potentially, alleviating aging-associated immune remodeling and myeloid malignancies.

Safe and Efficient Gene Therapy for Pyruvate Kinase Deficiency

Pyruvate kinase deficiency (PKD) is a monogenic metabolic disease caused by mutations in the PKLR gene that leads to hemolytic anemia of variable symptomatology and that can be fatal during the neonatal period. PKD recessive inheritance trait and its curative treatment by allogeneic bone marrow transplantation provide an ideal scenario for developing gene therapy approaches. Here, we provide a preclinical gene therapy for PKD based on a lentiviral vector harboring the hPGK eukaryotic promoter that drives the expression of the PKLR cDNA. This therapeutic vector was used to transduce mouse PKD hematopoietic stem cells (HSCs) that were subsequently transplanted into myeloablated PKD mice. Ectopic RPK expression normalized the erythroid compartment correcting the hematological phenotype and reverting organ pathology. Metabolomic studies demonstrated functional correction of the glycolytic pathway in RBCs derived from genetically corrected PKD HSCs, with no metabolic disturbances in leukocytes. The analysis of the lentiviral insertion sites in the genome of transplanted hematopoietic cells demonstrated no evidence of genotoxicity in any of the transplanted animals. Overall, our results underscore the therapeutic potential of the hPGK-coRPK lentiviral vector and provide high expectations toward the gene therapy of PKD and other erythroid metabolic genetic disorders.

Interplay between Fanconi anemia and homologous recombination pathways in genome integrity

The Fanconi anemia (FA) pathway plays a central role in the repair of DNA interstrand crosslinks (ICLs) and regulates cellular responses to replication stress. Homologous recombination (HR), the error-free pathway for double-strand break (DSB) repair, is required during physiological cell cycle progression for the repair of replication-associated DNA damage and protection of stalled replication forks. Substantial crosstalk between the two pathways has recently been unravelled, in that key HR proteins such as the RAD51 recombinase and the tumour suppressors BRCA1 and BRCA2 also play important roles in ICL repair. Consistent with this, rare patient mutations in these HR genes cause FA pathologies and have been assigned FA complementation groups. Here, we focus on the clinical and mechanistic implications of the connection between these two cancer susceptibility syndromes and on how these two molecular pathways of DNA replication and repair interact functionally to prevent genomic instability.

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.

BRCA2 minor transcript lacking exons 4-7 supports viability in mice and may account for survival of humans with a pathogenic biallelic mutation

The breast cancer gene, BRCA2, is essential for viability, yet patients with Fanconi anemia-D1 subtype are born alive with biallelic mutations in this gene. The hypomorphic nature of the mutations is believed to support viability, but this is not always apparent. One such mutation is IVS7+2T>G, which causes premature protein truncation due to skipping of exon 7. We previously identified a transcript lacking exons 4-7, which restores the open-reading frame, encodes a DNA repair proficient protein and is expressed in IVS7+2T>G carriers. However, because the exons 4-7 encoded region contains several residues required for normal cell-cycle regulation and cytokinesis, this transcript's ability to support viability can be argued. To address this, we generated a Brca2 knock-in mouse model lacking exons 4-7 and demonstrated that these exons are dispensable for viability as well as tumor-free survival. This study provides the first in vivo evidence of the functional significance of a minor transcript of BRCA2 that can play a major role in the survival of humans who are homozygous for a clearly pathogenic mutation. Our results highlight the importance of assessing protein function restoration by premature truncating codon bypass by alternative splicing when evaluating the functional significance of variants such as nonsense and frame-shift mutations that are assumed to be clearly pathogenic. Our findings will impact not only the assessment of variants that map to this region, but also influence counseling paradigms and treatment options for such mutation carriers.

Fancb deficiency impairs hematopoietic stem cell function

Fanconi anemia (FA) is a genetic disorder characterized by bone marrow failure, variable congenital malformations and a predisposition to malignancies. FANCB (also known as FAAP95), is the only X-linked FA gene discovered thus far. In the present study, we investigated hematopoiesis in adult Fancb deficient (Fancb^−/y) mice and found that Fancb^−/y mice have decreased hematopoietic stem cell (HSC) quiescence accompanied by reduced progenitor activity in vitro and reduced repopulating capacity in vivo. Like other FA mouse models previously reported, the hematopoietic system of Fancb^−/y mice is hypersensitive to DNA cross-linking agent mitomycin C (MMC), which induces bone marrow failure in Fancb^−/y mice. Furthermore, Fancb^−/y BM exhibits slower recovery kinetics and less tolerance to myelotoxic stress induced by 5-fluorouracil than wild-type littermates. RNA-seq analysis reveals altered expression of genes involved in HSC function and cell cycle regulation in Fancb^−/y HSC and progenitor cells. Thus, this Fancb^−/y mouse model provides a novel approach for studying the critical role of the FA pathway not only in germ cell development but also in the maintenance of HSC function.

Stem Cell-Specific Mechanisms Ensure Genomic Fidelity within HSCs and upon Aging of HSCs

Whether aged hematopoietic stem and progenitor cells (HSPCs) have impaired DNA damage repair is controversial. Using a combination of DNA mutation indicator assays, we observe a 2- to 3-fold increase in the number of DNA mutations in the hematopoietic system upon aging. Young and aged hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) do not show an increase in mutation upon irradiation-induced DNA damage repair, and young and aged HSPCs respond very similarly to DNA damage with respect to cell-cycle checkpoint activation and apoptosis. Both young and aged HSPCs show impaired activation of the DNA-damage-induced G1-S checkpoint. Induction of chronic DNA double-strand breaks by zinc-finger nucleases suggests that HSPCs undergo apoptosis rather than faulty repair. These data reveal a protective mechanism in both the young and aged hematopoietic system against accumulation of mutations in response to DNA damage.

Loss of Faap20 Causes Hematopoietic Stem and Progenitor Cell Depletion in Mice Under Genotoxic Stress

20-kDa FANCA-associated protein (FAAP20) is a recently identified protein that associates with the Fanconi anemia (FA) core complex component, FANCA. FAAP20 contains a conserved ubiquitin-binding zinc-finger domain and plays critical roles in the FA-BRCA pathway of DNA repair and genome maintenance. The function of FAAP20 in animals has not been explored. Here, we report that deletion of Faap20 in mice led to a mild FA-like phenotype with defects in the reproductive and hematopoietic systems. Specifically, hematopoietic stem and progenitor cells (HSPCs) from Faap20(-) (/) (-) mice showed defects in long-term multilineage reconstitution in lethally irradiated recipient mice, with milder phenotype as compared to HSPCs from Fanca(-) (/) (-) or Fancc(-) (/) (-) mice. Faap20(-) (/) (-) mice are susceptible to mitomycin C (MMC)-induced pancytopenia. That is, acute MMC stress induced a significant progenitor loss especially the erythroid progenitors and megakaryocyte-erythrocyte progenitors in Faap20(-) (/) (-) mice. Furthermore, Faap20(-) (/) (-) HSPCs displayed aberrant cell cycle pattern during chronic MMC treatment. Finally, using Faap20(-) (/) (-) Fanca(-) (/) (-) double-knockout mice, we demonstrated a possible dominant effect of FANCA in the interaction between FAAP20 and FANCA. This novel Faap20 mouse model may be valuable in studying the regulation of the FA pathway during bone marrow failure progress in FA patients.

Oxidative stress, bone marrow failure, and genome instability in hematopoietic stem cells

Reactive oxygen species (ROS) can be generated by defective endogenous reduction of oxygen by cellular enzymes or in the mitochondrial respiratory pathway, as well as by exogenous exposure to UV or environmental damaging agents. Regulation of intracellular ROS levels is critical since increases above normal concentrations lead to oxidative stress and DNA damage. A growing body of evidence indicates that the inability to regulate high levels of ROS leading to alteration of cellular homeostasis or defective repair of ROS-induced damage lies at the root of diseases characterized by both neurodegeneration and bone marrow failure as well as cancer. That these diseases may be reflective of the dynamic ability of cells to respond to ROS through developmental stages and aging lies in the similarities between phenotypes at the cellular level. This review summarizes work linking the ability to regulate intracellular ROS to the hematopoietic stem cell phenotype, aging, and disease.

Concise review: hematopoietic stem cell aging and the prospects for rejuvenation

Because of the continuous increases in lifetime expectancy, the incidence of age-related diseases will, unless counteracted, represent an increasing problem at both the individual and socioeconomic levels. Studies on the processes of blood cell formation have revealed several shortcomings as a consequence of chronological age. They include a reduced ability to mount adaptive immune responses and a blood cell composition skewed toward myeloid cells, with the latter coinciding with a dramatically increased incidence of myelogenous diseases, including cancer. Conversely, the dominant forms of acute leukemia affecting children associate with the lymphoid lineages. A growing body of evidence has suggested that aging of various organs and cellular systems, including the hematopoietic system, associates with a functional demise of tissue-resident stem cell populations. Mechanistically, DNA damage and/or altered transcriptional landscapes appear to be major drivers of the hematopoietic stem cell aging state, with recent data proposing that stem cell aging phenotypes are characterized by at least some degree of reversibility. These findings suggest the possibility of rejuvenating, or at least dampening, stem cell aging phenotypes in the elderly for therapeutic benefit.

Generation of iPSCs from genetically corrected Brca2 hypomorphic cells: implications in cell reprogramming and stem cell therapy

Fanconi anemia (FA) is a complex genetic disease associated with a defective DNA repair pathway known as the FA pathway. In contrast to many other FA proteins, BRCA2 participates downstream in this pathway and has a critical role in homology-directed recombination (HDR). In our current studies, we have observed an extremely low reprogramming efficiency in cells with a hypomorphic mutation in Brca2 (Brca2(Δ) (27/) (Δ27)), that was associated with increased apoptosis and defective generation of nuclear RAD51 foci during the reprogramming process. Gene complementation facilitated the generation of Brca2(Δ) (27/) (Δ27) induced pluripotent stem cells (iPSCs) with a disease-free FA phenotype. Karyotype analyses and comparative genome hybridization arrays of complemented Brca2(Δ) (27/) (Δ27) iPSCs showed, however, the presence of different genetic alterations in these cells, most of which were not evident in their parental Brca2(Δ) (27/) (Δ27) mouse embryonic fibroblasts. Gene-corrected Brca2(Δ) (27/) (Δ27) iPSCs could be differentiated in vitro toward the hematopoietic lineage, although with a more limited efficacy than WT iPSCs or mouse embryonic stem cells, and did not engraft in irradiated Brca2(Δ) (27/) (Δ27) recipients. Our results are consistent with previous studies proposing that HDR is critical for cell reprogramming and demonstrate that reprogramming defects characteristic of Brca2 mutant cells can be efficiently overcome by gene complementation. Finally, based on analysis of the phenotype, genetic stability, and hematopoietic differentiation potential of gene-corrected Brca2(Δ) (27/) (Δ) (27) iPSCs, achievements and limitations in the application of current reprogramming approaches in hematopoietic stem cell therapy are also discussed.

Exonuclease 1 is a critical mediator of survival during DNA double strand break repair in nonquiescent hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) populations require DNA repair pathways to maintain their long-term survival and reconstitution capabilities, but mediators of these processes are still being elucidated. Exonuclease 1 (Exo1) participates in homologous recombination (HR) and Exo1 loss results in impaired 5' HR end resection. We use cultured Exo1(mut) fibroblasts and bone marrow to demonstrate that loss of Exo1 function results in defective HR in cycling cells. Conversely, in Exo1(mut) mice HR is not required for maintenance of quiescent HSCs at steady state, confirming the steady state HSC reliance on nonhomologous end joining (NHEJ). Exo1(mut) mice sustained serial repopulation, displayed no defect in competitive repopulation or niche occupancy, and exhibited no increased sensitivity to whole body ionizing radiation. However, when Exo1(mut) HSCs were pushed into cell cycle in vivo with 5-fluorouracil or poly IC, the hematopoietic population became hypersensitive to IR, resulting in HSC defects and animal death. We propose Exo1-mediated HR is dispensable for stem cell function in quiescent HSC, whereas it is essential to HSC response to DNA damage processing after cell cycle entry, and its loss is not compensated by intact NHEJ. In HSCs, the maintenance of stem cell function after DNA damage is dependent on the DNA repair capacity, segregated by active versus quiescent points in cell cycle.

Modelling Fanconi anemia pathogenesis and therapeutics using integration-free patient-derived iPSCs

Fanconi anaemia (FA) is a recessive disorder characterized by genomic instability, congenital abnormalities, cancer predisposition and bone marrow (BM) failure. However, the pathogenesis of FA is not fully understood partly due to the limitations of current disease models. Here, we derive integration free-induced pluripotent stem cells (iPSCs) from an FA patient without genetic complementation and report in situ gene correction in FA-iPSCs as well as the generation of isogenic FANCA-deficient human embryonic stem cell (ESC) lines. FA cellular phenotypes are recapitulated in iPSCs/ESCs and their adult stem/progenitor cell derivatives. By using isogenic pathogenic mutation-free controls as well as cellular and genomic tools, our model serves to facilitate the discovery of novel disease features. We validate our model as a drug-screening platform by identifying several compounds that improve hematopoietic differentiation of FA-iPSCs. These compounds are also able to rescue the hematopoietic phenotype of FA patient BM cells.

DNA damage response in adult stem cells

This review discusses the processes of DNA-damage-response and DNA-damage repair in stem and progenitor cells of several tissues. The long life-span of stem cells suggests that they may respond differently to DNA damage than their downstream progeny and, indeed, studies have begun to elucidate the unique stem cell response mechanisms to DNA damage. Because the DNA damage responses in stem cells and progenitor cells are distinctly different, stem and progenitor cells should be considered as two different entities from this point of view. Hematopoietic and mammary stem cells display a unique DNA-damage response, which involves active inhibition of apoptosis, entry into the cell-cycle, symmetric division, partial DNA repair and maintenance of self-renewal. Each of these biological events depends on the up-regulation of the cell-cycle inhibitor p21. Moreover, inhibition of apoptosis and symmetric stem cell division are the consequence of the down-regulation of the tumor suppressor p53, as a direct result of p21 up-regulation. A deeper understanding of these processes is required before these findings can be translated into human anti-aging and anti-cancer therapies. One needs to clarify and dissect the pathways that control p21 regulation in normal and cancer stem cells and define (a) how p21 blocks p53 functions in stem cells and (b) how p21 promotes DNA repair in stem cells. Is this effect dependent on p21s ability to inhibit p53? Such molecular knowledge may pave the way to methods for maintaining short-term tissue reconstitution while retaining long-term cellular and genomic integrity.

Cellular mechanisms of somatic stem cell aging

Tissue homeostasis and regenerative capacity rely on rare populations of somatic stem cells endowed with the potential to self-renew and differentiate. During aging, many tissues show a decline in regenerative potential coupled with a loss of stem cell function. Cells including somatic stem cells have evolved a series of checks and balances to sense and repair cellular damage to maximize tissue function. However, during aging the mechanisms that protect normal cell function begin to fail. In this review, we will discuss how common cellular mechanisms that maintain tissue fidelity and organismal lifespan impact somatic stem cell function. We will highlight context-dependent changes and commonalities that define aging, by focusing on three age-sensitive stem cell compartments: blood, neural, and muscle. Understanding the interaction between extrinsic regulators and intrinsic effectors that operate within different stem cell compartments is likely to have important implications for identifying strategies to improve health span and treat age-related degenerative diseases.

Mechanisms that regulate stem cell aging and life span

Mammalian aging is associated with reduced tissue regeneration, increased degenerative disease, and cancer. Because stem cells regenerate many adult tissues and contribute to the development of cancer by accumulating mutations, age-related changes in stem cells likely contribute to age-related morbidity. Consistent with this, stem cell function declines with age in numerous tissues as a result of gate-keeping tumor suppressor expression, DNA damage, changes in cellular physiology, and environmental changes in tissues. It remains unknown whether declines in stem cell function during aging influence organismal longevity. However, mechanisms that influence longevity also modulate age-related morbidity, partly through effects on stem cells.

Learning from a paradox: recent insights into Fanconi anaemia through studying mouse models

Fanconi anaemia (FA) is a rare autosomal recessive or X-linked inherited disease characterised by an increased incidence of bone marrow failure (BMF), haematological malignancies and solid tumours. Cells from individuals with FA show a pronounced sensitivity to DNA interstrand crosslink (ICL)-inducing agents, which manifests as G2-M arrest, chromosomal aberrations and reduced cellular survival. To date, mutations in at least 15 different genes have been identified that cause FA; the products of all of these genes are thought to function together in the FA pathway, which is essential for ICL repair. Rapidly following the discovery of FA genes, mutant mice were generated to study the disease and the affected pathway. These mutant mice all show the characteristic cellular ICL-inducing agent sensitivity, but only partially recapitulate the developmental abnormalities, anaemia and cancer predisposition seen in individuals with FA. Therefore, the usefulness of modelling FA in mice has been questioned. In this Review, we argue that such scepticism is unjustified. We outline that haematopoietic defects and cancer predisposition are manifestations of FA gene defects in mice, albeit only in certain genetic backgrounds and under certain conditions. Most importantly, recent work has shown that developmental defects in FA mice also arise with concomitant inactivation of acetaldehyde metabolism, giving a strong clue about the nature of the endogenous lesion that must be repaired by the functional FA pathway. This body of work provides an excellent example of a paradox in FA research: that the dissimilarity, rather than the similarity, between mice and humans can provide insight into human disease. We expect that further study of mouse models of FA will help to uncover the mechanistic background of FA, ultimately leading to better treatment options for the disease.

Myelodysplastic syndrome: an inability to appropriately respond to damaged DNA?

Myelodysplastic syndrome (MDS) is considered a hematopoietic stem cell disease that is characterized by abnormal hematopoietic differentiation and a high propensity to develop acute myeloid leukemia. It is mostly associated with advanced age, but also with prior cancer therapy and inherited syndromes related to abnormalities in DNA repair. Recent technologic advances have led to the identification of a myriad of frequently occurring genomic perturbations associated with MDS. These observations suggest that MDS and its progression to acute myeloid leukemia is a genomic instability disorder, resulting from a stepwise accumulation of genetic abnormalities. The notion is now emerging that the underlying mechanism of this disease could be a defect in one or more pathways that are involved in responding to or repairing damaged DNA. In this review, we discuss these pathways in relationship to a large number of studies performed with MDS patient samples and MDS mouse models. Moreover, in view of our current understanding of how DNA damage response and repair pathways are affected by age in hematopoietic stem cells, we also explore how this might relate to MDS development.

The role of DNA repair in the pluripotency and differentiation of human stem cells

All living cells utilize intricate DNA repair mechanisms to address numerous types of DNA lesions and to preserve genomic integrity, and pluripotent stem cells have specific needs due to their remarkable ability of self-renewal and differentiation into different functional cell types. Not surprisingly, human stem cells possess a highly efficient DNA repair network that becomes less efficient upon differentiation. Moreover, these cells also have an anaerobic metabolism, which reduces the mitochondria number and the likelihood of oxidative stress, which is highly related to genomic instability. If DNA lesions are not repaired, human stem cells easily undergo senescence, cell death or differentiation, as part of their DNA damage response, avoiding the propagation of stem cells carrying mutations and genomic alterations. Interestingly, cancer stem cells and typical stem cells share not only the differentiation potential but also their capacity to respond to DNA damage, with important implications for cancer therapy using genotoxic agents. On the other hand, the preservation of the adult stem cell pool, and the ability of cells to deal with DNA damage, is essential for normal development, reducing processes of neurodegeneration and premature aging, as one can observe on clinical phenotypes of many human genetic diseases with defects in DNA repair processes. Finally, several recent findings suggest that DNA repair also plays a fundamental role in maintaining the pluripotency and differentiation potential of embryonic stem cells, as well as that of induced pluripotent stem (iPS) cells. DNA repair processes also seem to be necessary for the reprogramming of human cells when iPS cells are produced. Thus, the understanding of how cultured pluripotent stem cells ensure the genetic stability are highly relevant for their safe therapeutic application, at the same time that cellular therapy is a hope for DNA repair deficient patients.

Cyclophosphamide promotes engraftment of gene-modified cells in a mouse model of Fanconi anemia without causing cytogenetic abnormalities

A major hurdle for hematopoietic stem cell (HSC) gene therapy for inherited bone marrow disorders, including Fanconi anemia (FA), is adequate engraftment of gene-modified cells. A phenotypic defect in DNA repair renders FA patients sensitive to alkylating agents such as cyclophosphamide (Cy); however, at lower doses, Cy is well tolerated in the FA transplant setting. We tested whether non-alkylating agents could replace Cy for pretransplant conditioning to enhance engraftment of FANCA gene-modified hematopoietic cells. We compared Cy preconditioning with fludarabine (Flu) or cytarabine (AraC) or no conditioning as a control in fanca ( -/- ) mutant mice receiving gene-modified bone marrow cells. Only mice conditioned with Cy exhibited appreciable engraftment of gene-modified cells by PCR and resistance to mitomycin C (MMC). Cy administration following transplantation increased gene marking levels in all animals treated, but highest gene marking and corresponding MMC resistance were observed in mice receiving Cy pre- and posttransplantation. Importantly, no cytogenetic abnormalities were observed in Cy-treated mice. We conclude that Cy is an effective and superior preparative regimen with respect to engraftment of lentivirus-transduced cells and functional correction in fanca ( -/- ) mice. Thus, appropriately dosed Cy may provide a suitable conditioning regimen for FA patients undergoing HSC gene therapy.

Disrupted Signaling through the Fanconi Anemia Pathway Leads to Dysfunctional Hematopoietic Stem Cell Biology: Underlying Mechanisms and Potential Therapeutic Strategies

Fanconi anemia (FA) is the most common inherited bone marrow failure syndrome. FA patients suffer to varying degrees from a heterogeneous range of developmental defects and, in addition, have an increased likelihood of developing cancer. Almost all FA patients develop a severe, progressive bone marrow failure syndrome, which impacts upon the production of all hematopoietic lineages and, hence, is thought to be driven by a defect at the level of the hematopoietic stem cell (HSC). This hypothesis would also correlate with the very high incidence of MDS and AML that is observed in FA patients. In this paper, we discuss the evidence that supports the role of dysfunctional HSC biology in driving the etiology of the disease. Furthermore, we consider the different model systems currently available to study the biology of cells defective in the FA signaling pathway and how they are informative in terms of identifying the physiologic mediators of HSC depletion and dissecting their putative mechanism of action. Finally, we ask whether the insights gained using such disease models can be translated into potential novel therapeutic strategies for the treatment of the hematologic disorders in FA patients.