The capacity to remove 8-oxoG is enhanced in newborn neural stem/progenitor cells and decreases in juvenile mice and upon cell differentiation

Role of Hydrazine-Related Chemicals in Cancer and Neurodegenerative Disease

Hydrazine-related chemicals (HRCs) with carcinogenic and neurotoxic potential are found in certain mushrooms and plants used for food and in products employed in various industries, including aerospace. Their propensity to induce DNA damage (mostly O⁶-, N⁷- and 8-oxo-guanine lesions) resulting in multiple downstream effects is linked with both cancer and neurological disease. For cycling cells, unrepaired DNA damage leads to mutation and uncontrolled mitosis. By contrast, postmitotic neurons attempt to re-enter the cell cycle but undergo apoptosis or nonapoptotic cell death. Biomarkers of exposure to HRCs can be used to explore whether these substances are risk factors for sporadic amyotrophic laterals sclerosis and other noninherited neurodegenerative diseases, which is the focus of this paper.

Pyridoxine enhances chemo-responsiveness of breast cancer stem cells via redox reconditioning

A plethora of molecular strategies are employed by breast cancer stem cells (bCSCs) to evade chemotherapy-induced death signals, redox modulation being a crucial factor among those. Here, we observed that bCSCs are resistant to DNA damage and generate low ROS upon doxorubicin (Dox) treatment. Further exploration revealed inherently high NEIL2, a base excision repair (BER) enzyme that plays a key regulatory role in repairing DNA damage, in bCSCs. However, its role in modulating the redox status of bCSCs remains unexplored. In addition, Dox not only upregulates NEIL2 in bCSCs at both transcriptional and translational levels but also declines p300-induced acetylation thus activating NEIL2 and providing a protective effect against the stress inflicted by the genotoxic drug. However, when the redox status of bCSCs is altered by inducing high ROS, apoptosis of the resistant population is accomplished. Subsequently, when NEIL2 is suppressed in bCSCs, chemo-sensitization of the resistant population is enabled by redox reconditioning via impaired DNA repair. This signifies a possibility of therapeutically disrupting the redox balance in bCSCs to enhance their chemo-responsiveness. Our search for an inhibitor of NEIL2 revealed that vitamin B6, i.e., pyridoxine (PN), hinders NEIL2-mediated transcription-coupled repair process by not only decreasing NEIL2 expression but also inhibiting its association with RNA Pol II, thus stimulating DNA damage and triggering ROS. As a consequence of altered redox regulation, bCSCs become susceptible towards Dox, which then induces apoptosis via caspase cascade. These findings signify that PN enhances chemo-responsiveness of bCSCs via redox reconditioning.

DNA repair fidelity in stem cell maintenance, health, and disease

Stem cells are a sub population of cell types that form the foundation of our body, and have the potential to replicate, replenish and repair limitlessly to maintain the tissue and organ homeostasis. Increased lifetime and frequent replication set them vulnerable for both exogenous and endogenous agents-induced DNA damage compared to normal cells. To counter these damages and preserve genetic information, stem cells have evolved with various DNA damage response and repair mechanisms. Furthermore, upon experiencing irreparable DNA damage, stem cells mostly prefer early senescence or apoptosis to avoid the accumulation of damages. However, the failure of these mechanisms leads to various diseases, including cancer. Especially, given the importance of stem cells in early development, DNA repair deficiency in stem cells leads to various disabilities like developmental delay, premature aging, sensitivity to DNA damaging agents, degenerative diseases, etc. In this review, we have summarized the recent update about how DNA repair mechanisms are regulated in stem cells and their association with disease progression and pathogenesis.

Stem Cells and DNA Repair Capacity: Muse Stem Cells Are Among the Best Performers

Stem cells persist for long periods in the body and experience many intrinsic and extrinsic stresses. For this reason, they present a powerful and effective DNA repair system in order to properly fix DNA damage and avoid the onset of a degenerative process, such as neoplastic transformation or aging. In this chapter, we compare the DNA repair ability of pluripotent stem cells (ESCs, iPSCs, and Muse cells) and other adult stem cells. We also describe personal investigations showing a robust and effective capacity of Muse cells in sensing and repairing DNA following chemical and physical stress. Muse cells can repair DNA through base and nucleotide excision repair mechanisms, BER and NER, respectively. Furthermore, they present a pronounced capacity in repairing double-strand breaks by the nonhomologous end joining (NHEJ) process. The studies addressing the role of DNA damage repair in the biology of stem cells are of paramount importance for comprehension of their functions and, also, for setting up effective and safe stem cell-based therapy.

Defective mitochondrial respiration, altered dNTP pools and reduced AP endonuclease 1 activity in peripheral blood mononuclear cells of Alzheimer's disease patients

AIMS

Accurate biomarkers for early diagnosis of Alzheimer's disease (AD) are badly needed. Recent reports suggest that dysfunctional mitochondria and DNA damage are associated with AD development. In this report, we measured various cellular parameters, related to mitochondrial bioenergetics and DNA damage, in peripheral blood mononuclear cells (PBMCs) of AD and control participants, for biomarker discovery.

METHODS

PBMCs were isolated from 53 patients with AD of mild to moderate degree and 30 age-matched healthy controls. Tests were performed on the PBMCs from as many of these participants as possible. We measured glycolysis and mitochondrial respiration fluxes using the Seahorse Bioscience flux analyzer, mitochondrial ROS production using flow cytometry, dNTP levels by way of a DNA polymerization assay, DNA strand breaks using the Fluorometric detection of Alkaline DNA Unwinding (FADU) assay, and APE1 incision activity (in cell lysates) on a DNA substrate containing an AP site (to estimate DNA repair efficiency).

RESULTS

In the PBMCs of AD patients, we found reduced basal mitochondrial oxygen consumption, reduced proton leak, higher dATP level, and lower AP endonuclease 1 activity, depending on adjustments for gender and/or age.

CONCLUSIONS

This study reveals impaired mitochondrial respiration, altered dNTP pools and reduced DNA repair activity in PBMCs of AD patients, thus suggesting that these biochemical activities may be useful as biomarkers for AD.

Base excision DNA repair in the embryonic development of the sea urchin, Strongylocentrotus intermedius

In actively proliferating cells, such as the cells of the developing embryo, DNA repair is crucial for preventing the accumulation of mutations and synchronizing cell division. Sea urchin embryo growth was analyzed and extracts were prepared. The relative activity of DNA polymerase, apurinic/apyrimidinic (AP) endonuclease, uracil-DNA glycosylase, 8-oxoguanine-DNA glycosylase, and other glycosylases was analyzed using specific oligonucleotide substrates of these enzymes; the reaction products were resolved by denaturing 20% polyacrylamide gel electrophoresis. We have characterized the profile of several key base excision repair activities in the developing embryos (2 blastomers to mid-pluteus) of the grey sea urchin, Strongylocentrotus intermedius. The uracil-DNA glycosylase specific activity sharply increased after blastula hatching, whereas the specific activity of 8-oxoguanine-DNA glycosylase steadily decreased over the course of the development. The AP-endonuclease activity gradually increased but dropped at the last sampled stage (mid-pluteus 2). The DNA polymerase activity was high at the first cleavage division and then quickly decreased, showing a transient peak at blastula hatching. It seems that the developing sea urchin embryo encounters different DNA-damaging factors early in development within the protective envelope and later as a free-floating larva, with hatching necessitating adaptation to the shift in genotoxic stress conditions. No correlation was observed between the dynamics of the enzyme activities and published gene expression data from developing congeneric species, S. purpuratus. The results suggest that base excision repair enzymes may be regulated in the sea urchin embryos at the level of covalent modification or protein stability.

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.

Modulation of DNA base excision repair during neuronal differentiation

Neurons are terminally differentiated cells with a high rate of metabolism and multiple biological properties distinct from their undifferentiated precursors. Previous studies showed that nucleotide excision DNA repair is downregulated in postmitotic muscle cells and neurons. Here, we characterize DNA damage susceptibility and base excision DNA repair (BER) capacity in undifferentiated and differentiated human neural cells. The results show that undifferentiated human SH-SY5Y neuroblastoma cells are less sensitive to oxidative damage than their differentiated counterparts, in part because they have robust BER capacity, which is heavily attenuated in postmitotic neurons. The reduction in BER activity in differentiated cells correlates with diminished protein levels of key long patch BER components, flap endonuclease-1, proliferating cell nuclear antigen, and ligase I. Thus, because of their higher BER capacity, proliferative neural progenitor cells are more efficient at repairing DNA damage compared with their neuronally differentiated progeny.

Loss of Neil3, the major DNA glycosylase activity for removal of hydantoins in single stranded DNA, reduces cellular proliferation and sensitizes cells to genotoxic stress

7,8-Dihydro-8-oxoguanine (8-oxoG) is one of the most common oxidative base lesions in normal tissues induced by a variety of endogenous and exogenous agents. Hydantoins are products of 8-oxoG oxidation and as 8-oxoG, they have been shown to be mutagenic lesions. Oxidative DNA damage has been implicated in the etiology of various age-associated pathologies, such as cancer, cardiovascular diseases, arthritis, and several neurodegenerative diseases. The mammalian endonuclease VIII-like 3 (Neil3) is one of the four DNA glycosylases found to recognize and remove hydantoins in the first step of base excision repair (BER) pathway. We have generated mice lacking Neil3 and by using total cell extracts we demonstrate that Neil3 is the main DNA glycosylase that incises hydantoins in single stranded DNA in tissues. Using the neurosphere culture system as a model to study neural stem/progenitor (NSPC) cells we found that lack of Neil3 impaired self renewal but did not affect differentiation capacity. Proliferation was also reduced in mouse embryonic fibroblasts (MEFs) derived from Neil3(-/-) embryos and these cells were sensitive to both the oxidative toxicant paraquat and interstrand cross-link (ICL)-inducing agent cisplatin. Our data support the involvement of Neil3 in removal of replication blocks in proliferating cells.

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.

Mitochondrial DNA damage level determines neural stem cell differentiation fate

The mitochondrial DNA (mtDNA) of neural stem cells (NSCs) is vulnerable to oxidation damage. Subtle manipulations of the cellular redox state affect mtDNA integrity in addition to regulating the NSC differentiation lineage, suggesting a molecular link between mtDNA integrity and regulation of differentiation. Here we show that 8-oxoguanine DNA glycosylase (OGG1) is essential for repair of mtDNA damage and NSC viability during mitochondrial oxidative stress. Differentiating neural cells from ogg1(-/-) knock-out mice spontaneously accumulate mtDNA damage and concomitantly shift their differentiation direction toward an astrocytic lineage, similar to wt NSCs subjected to mtDNA damaging insults. Antioxidant treatments reversed mtDNA damage accumulation and separately increased neurogenesis in ogg1(-/-) cells. NSCs from a transgenic ogg1(-/-) mouse expressing mitochondrially targeted human OGG1 were protected from mtDNA damage during differentiation, and displayed elevated neurogenesis. The underlying mechanisms for this shift in differentiation direction involve the astrogenesis promoting Sirt1 via an increased NAD/NADH ratio in ogg1(-/-) cells. Redox manipulations to alter mtDNA damage level correspondingly activated Sirt1 in both cell types. Our results demonstrate for the first time the interdependence between mtDNA integrity and NSC differentiation fate, suggesting that mtDNA damage is the primary signal for the elevated astrogliosis and lack of neurogenesis seen during repair of neuronal injury.

The bright and the dark sides of DNA repair in stem cells

DNA repair is a double-edged sword in stem cells. It protects normal stem cells in both embryonic and adult tissues from genetic damage, thus allowing perpetuation of intact genomes into new tissues. Fast and efficient DNA repair mechanisms have evolved in normal stem and progenitor cells. Upon differentiation, a certain degree of somatic mutations becomes more acceptable and, consequently, DNA repair dims. DNA repair turns into a problem when stem cells transform and become cancerous. Transformed stem cells drive growth of a number of tumours (e.g., high grade gliomas) and being particularly resistant to chemo- and radiotherapeutic agents often cause relapses. The contribution of DNA repair to resistance of these tumour-driving cells is the subject of intense research, in order to find novel agents that may sensitize them to chemotherapy and radiotherapy.

Differential age-related changes in mitochondrial DNA repair activities in mouse brain regions

Aging in the brain is characterized by increased susceptibility to neuronal loss and functional decline, and mitochondrial DNA (mtDNA) mutations are thought to play an important role in these processes. Due to the proximity of mtDNA to the main sites of mitochondrial free radical generation, oxidative stress is a major source of DNA mutations in mitochondria. The base excision repair (BER) pathway removes oxidative lesions from mtDNA, thereby constituting an important mechanism to avoid accumulation of mtDNA mutations. The complexity of the brain implies that exposure and defence against oxidative stress varies among brain regions and hence some regions may be particularly prone to accumulation of mtDNA damages. In the current study we investigated the efficiency of the BER pathway throughout the murine lifespan in mitochondria from cortex and hippocampus, regions that are central in mammalian cognition, and which are severely affected during aging and in neurodegenerative diseases. A regional specific regulation of mitochondrial DNA repair activities was observed with aging. In cortical mitochondria, DNA glycosylase activities peaked at middle-age followed by a significant drop at old age. However, only minor changes were observed in hippocampal mitochondria during the whole lifespan of the animals. Furthermore, DNA glycosylase activities were lower in hippocampal than in cortical mitochondria. Mitochondrial AP endonuclease activity increased in old animals in both brain regions. Our data suggest an important regional specific regulation of mitochondrial BER during aging.

Repair system of 7, 8-dihydro-8-oxoguanine as a defense line against carcinogenesis

Reactive oxygen species (ROS) are essentially harmful for living organisms, including human beings. It is well known that ROS-induced damage of cellular components may lead to human diseases, such as inflammatory diseases, degenerative diseases, or cancer. In particular, oxidative DNA damage is premutagenic, and thus, the generation of DNA damage and the failure of its removal are critical events for tumorigenesis or carcinogenesis. To prevent this disadvantage, living organisms have defense mechanisms against ROS-induced gene instability. Studies of 8-oxo-Gua and its main repair enzyme, 8-oxoguanine DNA glycosylase 1 (OGG1), are informative and useful, because 8-oxo-Gua is commonly observed in DNA, and OGG1 enzymes exist in a wide variety of living organisms. The importance of OGG1 was confirmed by polymorphism analyses and studies using knockout mice. Moreover, analyses of the influences of environmental factors on DNA damage and repair systems have confirmed the effects of heavy metals on 8-oxo-Gua formation and OGG1 expression. These studies revealed that the 8-oxo-Gua repair system is crucial for the prevention of mutation-related diseases, such as cancer. In this review, the advances in this field during the last two decades are described.