Trisomy 21 and accelerated aging: DNA-repair parameters in peripheral lymphocytes of Down's syndrome patients

Total cell N-glycosylation is altered during differentiation of induced pluripotent stem cells to neural stem cells and is disturbed by trisomy 21

Down syndrome (DS), a genetic condition caused by trisomy 21 (T21), manifests various neurological symptoms, including intellectual disability, early neurodegeneration, and early-onset dementia. N-glycosylation is a protein modification that plays a critical role in numerous neurobiological processes and whose dysregulation is associated with a range of neurological disorders. However, whether N-glycosylation of neural glycoproteins is affected in DS has not been studied. To better understand how T21 affects N-glycosylation during neural differentiation, we utilized an isogenic in vitro induced pluripotent stem cell (iPSC) model of T21 in which both T21 and euploid disomic karyotype (D21) clones were obtained from a single individual with mosaic DS. We comprehensively characterized and compared the total N-glycomes of iPSCs and their neural stem cell (NSC) derivatives. N-glycomics analysis of whole cell lysates was performed using liquid chromatography coupled with tandem mass spectrometry to determine N-glycan structures. Our results show that neural differentiation of iPSCs to NSCs is characterized by an increase in the abundance of complex N-glycans at the expense of minimally processed mannosidic N-glycans. Moreover, we found differences in N-glycosylation patterns between D21 and T21 cells. Notably, the abundance of pseudohybrid N-glycans was significantly higher in T21 cells which also exhibited a significantly lower abundance of a specific hybrid monoantennary fucosylated N-glycan (H6N3F1). Overall, our data define the total N-glycome of both D21 and T21 iPSCs and NSCs and show that T21 already impacts N-glycosylation patterns in the stem cell state in a manner consistent with aberrantly premature neural differentiation of T21 cells.

Analysis of the circumstances associated with death and predictors of mortality in Spanish adults with Down syndrome, 1997-2014

OBJECTIVES

Characterise the circumstances associated with death during admission of adults with Down syndrome (DS) and to identify predictors of mortality.

PATIENTS AND METHODS

Observational study based on data on all emergent admissions of adults with DS to hospitals of the Spanish National Health System between 1997 and 2014. We analysed epidemiological and clinical variables.

RESULTS

We analysed admissions of 11,594 adults with DS, mean age 47 years. 1715 patients died (15%), being the highest mortality (35%) in individuals aged 50-59. A past medical history of cerebrovascular disease (aOR 2.95 [2.30-3.77]) or cancer (aOR 2.79 [2.07-3.75]), gross aspiration's admission (aOR 2.59 [2.20-3.04]), immobility (aOR 2.31 [1.46-3-62]), and readmission within 30 days (aOR 2.43 [2.06-2.86]) were identified as predictors of mortality.

CONCLUSIONS

Adults with DS have a high in-hospital mortality rate. The main predictors of death were cerebrovascular disease, cancer, early readmission, and conditions commonly associated with advanced dementia.

Dose imbalance of DYRK1A kinase causes systemic progeroid status in Down syndrome by increasing the un-repaired DNA damage and reducing LaminB1 levels

BACKGROUND

People with Down syndrome (DS) show clinical signs of accelerated ageing. Causative mechanisms remain unknown and hypotheses range from the (essentially untreatable) amplified-chromosomal-instability explanation, to potential actions of individual supernumerary chromosome-21 genes. The latter explanation could open a route to therapeutic amelioration if the specific over-acting genes could be identified and their action toned-down.

METHODS

Biological age was estimated through patterns of sugar molecules attached to plasma immunoglobulin-G (IgG-glycans, an established "biological-ageing-clock") in n = 246 individuals with DS from three European populations, clinically characterised for the presence of co-morbidities, and compared to n = 256 age-, sex- and demography-matched healthy controls. Isogenic human induced pluripotent stem cell (hiPSCs) models of full and partial trisomy-21 with CRISPR-Cas9 gene editing and two kinase inhibitors were studied prior and after differentiation to cerebral organoids.

FINDINGS

Biological age in adults with DS is (on average) 18.4-19.1 years older than in chronological-age-matched controls independent of co-morbidities, and this shift remains constant throughout lifespan. Changes are detectable from early childhood, and do not require a supernumerary chromosome, but are seen in segmental duplication of only 31 genes, along with increased DNA damage and decreased levels of LaminB1 in nucleated blood cells. We demonstrate that these cell-autonomous phenotypes can be gene-dose-modelled and pharmacologically corrected in hiPSCs and derived cerebral organoids. Using isogenic hiPSC models we show that chromosome-21 gene DYRK1A overdose is sufficient and necessary to cause excess unrepaired DNA damage.

INTERPRETATION

Explanation of hitherto observed accelerated ageing in DS as a developmental progeroid syndrome driven by DYRK1A overdose provides a target for early pharmacological preventative intervention strategies.

FUNDING

Main funding came from the "Research Cooperability" Program of the Croatian Science Foundation funded by the European Union from the European Social Fund under the Operational Programme Efficient Human Resources 2014-2020, Project PZS-2019-02-4277, and the Wellcome Trust Grants 098330/Z/12/Z and 217199/Z/19/Z (UK). All other funding is described in details in the "Acknowledgements".

The Paradox of Myeloid Leukemia Associated with Down Syndrome

Children with Down syndrome constitute a distinct genetic population who has a greater risk of developing acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) compared to their non-Down syndrome counterparts. The risk for developing solid tumors is also distinct from the non-Down syndrome population. In the case of myeloid leukemias, the process of leukemogenesis in Trisomy 21 begins in early fetal life where genetic drivers including GATA1 mutations lead to the development of the preleukemic condition, transient abnormal myelopoiesis (TAM). Various other mutations in genes encoding cohesin, epigenetic regulators and RAS pathway can result in subsequent progression to Myeloid Leukemia associated with Down Syndrome (ML-DS). The striking paradoxical feature in the Down syndrome population is that even though there is a higher predisposition to developing AML, they are also very sensitive to chemotherapy agents, particularly cytarabine, thus accounting for the very high cure rates for ML-DS compared to AML in children without Down syndrome. Current clinical trials for ML-DS attempt to balance effective curative therapies while trying to reduce treatment-associated toxicities including infections by de-intensifying chemotherapy doses, if possible. The small proportion of patients with relapsed ML-DS have an extremely poor prognosis and require the development of new therapies.

Characteristics of adults with Down syndrome hospitalised in Spanish internal medicine departments during 2005-2014

INTRODUCTION AND OBJECTIVES

The clinical problems of adults with Down syndrome seem to differ from those of the general population. To better understand these differences, we list the demographic and clinical characteristics of adults with Down syndrome admitted to Spanish internal medicine departments during 2005-2014.

PATIENTS AND METHODS

We conducted an observational retrospective study using data collected from the minimum basic data set on hospitalisation episodes of adults with Down syndrome in the internal medicine departments of Spain's National Health System from 2005 to 2014. We analysed the patients' epidemiological, clinical and societal data.

RESULTS

A total of 7548 hospitalisation episodes from 3786 patients were recorded. Some 56.6% of the patients were male with a mean age (±SD) of 47±13 years, and 715 of the patients died (18.9%). The age-adjusted mortality was 26.6%, and the mean stay was 9.6±12 days. The hospitalisation was for respiratory disease in 3684 episodes (48.8%) and for cardiac origin in 760 (10%). The most common comorbidities were hypothyroidism (27.1%, 2043 episodes), epilepsy (24.1%, 1819 episodes) and dementia (15.4%, 1162 episodes).

CONCLUSIONS

The hospitalisation of adults with Down syndrome in internal medicine departments has increased in the past decade. Although the reasons for hospitalisation, mean stay and cost per episode for this population are similar to those of the general population treated by internal medicine departments, the age-adjusted hospital mortality was significantly greater.

Down Syndrome, Ageing and Epigenetics

During the past decades, life expectancy of subjects with Down syndrome (DS) has greatly improved, but age-specific mortality rates are still important and DS subjects are characterized by an acceleration of the ageing process, which affects particularly the immune and central nervous systems. In this chapter, we will first review the characteristics of the ageing phenomenon in brain and in immune system in DS and we will then discuss the biological hallmarks of ageing in this specific population. Finally, we will also consider in detail the knowledge on epigenetics in DS, particularly DNA methylation.

Epigenetic and redox biomarkers: Novel insights from the MARK-AGE study

Ageing is a multifactorial process that affects most, if not all, of the body's tissues and organs and can be defined as the accumulation of physical and psychological changes in a human being over time. The rate of ageing differs between individuals of the same chronological age, meaning that 'biological age' of a person may be different from 'chronological age'. Furthermore, ageing represents a very potent risk factor for diseases and disability in humans. Therefore, establishment of markers of biological ageing is important for preventing age-associated diseases and extending health span. MARK-AGE, a large-scale European study, aimed at identifying a set of biomarkers which, as a combination of parameters with appropriate weighting, would measure biological age better than any marker in isolation. But beyond the identification of useful biomarkers, MARK-AGE provided new insights in age-associated specific cellular processes, such as DNA methylation, oxidative stress and the regulation of zinc homeostasis.

NAD+ supplementation normalizes key Alzheimer's features and DNA damage responses in a new AD mouse model with introduced DNA repair deficiency

Emerging findings suggest that compromised cellular bioenergetics and DNA repair contribute to the pathogenesis of Alzheimer's disease (AD), but their role in disease-defining pathology is unclear. We developed a DNA repair-deficient 3xTgAD/Polβ+/- mouse that exacerbates major features of human AD including phosphorylated Tau (pTau) pathologies, synaptic dysfunction, neuronal death, and cognitive impairment. Here we report that 3xTgAD/Polβ+/- mice have a reduced cerebral NAD+/NADH ratio indicating impaired cerebral energy metabolism, which is normalized by nicotinamide riboside (NR) treatment. NR lessened pTau pathology in both 3xTgAD and 3xTgAD/Polβ+/- mice but had no impact on amyloid β peptide (Aβ) accumulation. NR-treated 3xTgAD/Polβ+/- mice exhibited reduced DNA damage, neuroinflammation, and apoptosis of hippocampal neurons and increased activity of SIRT3 in the brain. NR improved cognitive function in multiple behavioral tests and restored hippocampal synaptic plasticity in 3xTgAD mice and 3xTgAD/Polβ+/- mice. In general, the deficits between genotypes and the benefits of NR were greater in 3xTgAD/Polβ+/- mice than in 3xTgAD mice. Our findings suggest a pivotal role for cellular NAD+ depletion upstream of neuroinflammation, pTau, DNA damage, synaptic dysfunction, and neuronal degeneration in AD. Interventions that bolster neuronal NAD+ levels therefore have therapeutic potential for AD.

Defective DNA repair and increased chromatin binding of DNA repair factors in Down syndrome fibroblasts

Down syndrome (DS) is characterized by genetic instability, neurodegeneration, and premature aging. However, the molecular mechanisms leading to this phenotype are not yet well understood. Here, we report that DS fibroblasts from both fetal and adult donors show the presence of oxidative DNA base damage, such as dihydro-8-oxoguanine (8-oxodG), and activation of a DNA damage response (DDR), already during unperturbed growth conditions. DDR with checkpoint activation was indicated by histone H2AX and Chk2 protein phosphorylation, and by increased p53 protein levels. In addition, both fetal and adult DS fibroblasts were more sensitive to oxidative DNA damage induced by potassium bromate, and were defective in the removal of 8-oxodG, as compared with age-matched cells from control healthy donors. The analysis of core proteins participating in base excision repair (BER), such as XRCC1 and DNA polymerase β, showed that higher amounts of these factors were bound to chromatin in DS than in control cells, even in the absence of DNA damage. These findings occurred in concomitance with increased levels of phosphorylated XRCC1 detected in DS cells. These results indicate that DS cells exhibit a BER deficiency, which is associated with prolonged chromatin association of core BER factors.

DNA polymerase β deficiency leads to neurodegeneration and exacerbates Alzheimer disease phenotypes

We explore the role of DNA damage processing in the progression of cognitive decline by creating a new mouse model. The new model is a cross of a common Alzheimer's disease (AD) mouse (3xTgAD), with a mouse that is heterozygous for the critical DNA base excision repair enzyme, DNA polymerase β. A reduction of this enzyme causes neurodegeneration and aggravates the AD features of the 3xTgAD mouse, inducing neuronal dysfunction, cell death and impairing memory and synaptic plasticity. Transcriptional profiling revealed remarkable similarities in gene expression alterations in brain tissue of human AD patients and 3xTg/Polβ(+/-) mice including abnormalities suggestive of impaired cellular bioenergetics. Our findings demonstrate that a modest decrement in base excision repair capacity can render the brain more vulnerable to AD-related molecular and cellular alterations.

Validation and application of health utilities index in Chinese subjects with down syndrome

Objectives

The objectives of the study were (1) to validate the Chinese version of Health Utilities Index (HUI-Ch); (2) to examine the Health-related Quality of Life (HRQoL) of Chinese subjects with Down syndrome (DS); and (3) to study the impact of chronic health conditions on HRQoL of Chinese with DS.

Methods

The multiple choice questionnaire for scoring Health Utilities Index Mark 2 (HUI2) and Health Utilities Index Mark 3 (HUI3) was translated and validated. In addition to the HRQoL scores from HUI2 and HUI3, proxy-data on socio-demographics, and 10 common chronic health conditions for people with DS were collected and analyzed. Data analysis involves multiple imputation and multiple regression analysis to predict variations in HRQoL in relation to different factors. Lastly, a gradient interval was constructed on the number of chronic health conditions in relation to HRQoL.

Results

HUI-Ch was validated according to standard guidelines. People with DS were found to have a lower HRQoL as compared to the general population, with the majority categorized as moderate or severe on the scale. Behavioral and hearing problems on HUI2, and hearing problems on HUI3 were found to be statistically significant predictors of a lower HRQoL score. A significant gradient relationship existed showing when the number of health problems increased, the HRQoL scores decreased.

Conclusions

HUI-Ch is a valid instrument to assess HRQoL. It can have broad application in Chinese subjects with DS including the study of the impact of different chronic health conditions on their quality of life. The quantifiable nature of HUI-Ch will facilitate longitudinal study on the well-being of subjects with DS and evaluation of effectiveness of intervention programs in the near future.

Electronic supplementary material

The online version of this article (doi:10.1186/s12955-014-0144-x) contains supplementary material, which is available to authorized users.

Are GATA1 mutations occurring at random in Down syndrome transient leukemia?

The somatic mutation theory of cancer proposes that cancer begins with a somatic mutation occurring at random in a single cell that then passes the mutation to its progeny, generating a clone of premalignant cells. This clone leads to a full malignant tumor through additional mutations and selection processes. Strikingly, the best-documented human model of early oncogenesis, i.e., transient myeloproliferative disorder followed by acute megakaryoblastic leukemia (AMKL) in infants with Down syndrome (DS, or trisomy 21), exhibits important discrepancies with the SMT. Somatic mutations in megakaryocytic precursors occur at least 100,000 times more frequently in the GATA1 gene in fetuses with DS compared to the general population. Further, mutations are limited to GATA1 only; the general mutation rate does not significantly differ between individuals with DS and euploid individuals. Importantly, the mutations are also lineage-specific, occurring only in the megakaryocytic lineage, and proliferative anomalies of the megakaryocytic lineage are observed before the occurrence of GATA1 mutations. Thus, GATA1 mutations in fetuses with DS cannot be random events occurring in normal cells. Here, transcription-associated mutagenesis is proposed as the mechanism by which the earliest mutations of AMKL occur in DS. Transcription-associated mutagenesis is observed in non-dividing cells when a gene is over-expressed. The over-expression of GATA1 in the megakaryocytic lineage in DS fetal liver cells is proposed to be the cause of targeted GATA1 somatic mutations. As transcription-associated mutagenesis is a universal process, this mechanism may also apply to early oncogenesis in other situations, including after birth and following exposure to a carcinogenic agent. Thus, this hypothesis represents a new avenue for understanding and exploring oncogenesis in the context of DS and in other disease states.

The triple challenges associated with age-related comorbidities in Down syndrome

BACKGROUND

Major increases in the survival of people with Down syndrome during the last two generations have resulted in extended periods of adulthood requiring specialist care, which in turn necessitates greater understanding of the nature, timing and impact of comorbidities associated with the disorder.

METHOD

The prevalence of five comorbidities reported as common in adults with Down syndrome, visual impairment, hearing impairment, epilepsy, thyroid disorders and dementia was assessed by decade of life.

RESULTS

From early adulthood, people with Down syndrome are at enhanced risk of developing new comorbidities and they may present with multiple conditions. Three specific challenges are identified and discussed: are comorbidities detected in a timely manner, is the clinical progress of the disorder adequately understood, and who is responsible for the provision of care?

CONCLUSIONS

Further detailed investigations into the development and treatment of comorbidities across the lifespan are needed for a successful longitudinal approach to healthcare in people with Down syndrome. Implementation of this approach will better inform healthcare providers to ensure continuity of care with advancing age.

Tumorigenesis in Down's syndrome: big lessons from a small chromosome

If assessed by a number of criteria for cancer predisposition, Down's syndrome (DS) should be an overwhelmingly cancer-prone condition. Although childhood leukaemias occur more frequently in DS, paradoxically, individuals with DS have a markedly lower incidence of most solid tumours. Understanding the mechanisms that are capable of overcoming such odds could potentially open new routes for cancer prevention and therapy. In this Opinion article, we discuss recent reports that suggest unique and only partially understood mechanisms behind this paradox, including tumour repression, anti-angiogenic effects and stem cell ageing and availability.

Down syndrome as a model of DNA polymerase beta haploinsufficiency and accelerated aging

Down syndrome is a condition of intellectual disability characterized by accelerated aging. As with other aging syndromes, evidence accumulated over the past several decades points to a DNA repair defect inherent in Down syndrome. This evidence has led us to suggest that Down syndrome results in reduced DNA base excision repair (BER) capacity, and that this contributes to the genomic instability and the aging phenotype of Down syndrome. We propose important roles for microRNA and/or folate metabolism and oxidative stress in the dysregulation of BER in Down syndrome. Further, we suggest these pathways are involved in the leukemogenesis of Down syndrome. We have reviewed the role of BER in the processing of oxidative stress, and the impact of folate depletion on BER capacity. Further, we have reviewed the role that loss of BER, specifically DNA polymerase beta, plays in accelerating the rate of aging. Like that seen in the DNA polymerase beta heterozygous mouse, the aging phenotype of Down syndrome is subtle, unlike the aging phenotypes seen in the classical progeroid syndromes and mouse models of aging. As such, Down syndrome may provide a model for elucidating some of the basic mechanisms of aging.

Mitochondrial dysfunction and Down's syndrome: is there a role for coenzyme Q(10) ?

Structural changes and abnormal function of mitochondria have been documented in Down's syndrome (DS) cells, patients, and animal models. DS cells in culture exhibit a wide array of functional mitochondrial abnormalities including reduced mitochondrial membrane potential, reduced ATP production, and decreased oxido-reductase activity. New research has also brought to central stage the prominent role of oxidative stress in this condition. This review focuses on recent advances in the field with a particular emphasis on novel translational approaches involving the utilization of coenzyme Q(10) (CoQ(10) ) to treat a variety of clinical phenotypes associated with DS that are linked to increased oxidative stress and energy deficits. CoQ(10) has already provided promising results in several different conditions associated with altered energy metabolism and oxidative stress in the CNS. Two studies conducted in Ancona investigated the effect of CoQ(10) treatment on DNA damage in DS patients. Although the effect of CoQ(10) was evidenced only at single cell level, the treatment affected the distribution of cells according to their content in oxidized bases. In fact, it produced a strong negative correlation linking cellular CoQ(10) content and the amount of oxidized purines. Results suggest that the effect of CoQ(10) treatment in DS not only reflects antioxidant efficacy, but likely modulates DNA repair mechanisms.

Prolonged coenzyme Q10 treatment in Down syndrome patients: effect on DNA oxidation

Oxidative stress is known to play a relevant role in Down syndrome (DS) and its effects are documented from embryonic life. Oxidative DNA damage has been shown to be significantly elevated in Down syndrome patients, and this has been indicated as an early event promoting neurodegeneration and Alzheimer type dementia. The aim of this study was to investigate the efficacy of coenzyme Q(10) (CoQ(10)) in delaying the effect of oxidative damage in these patients. In our previous study we demonstrated a mild protective effect of CoQ(10) on DNA, although the treatment was unable to modify the overall extent of oxidative damage at the patient level. Possible limitations of the previous study were: time of treatment (6 months) or spectrum of DNA lesions detected. In order to overcome these limitations we planned a continuation of the trial aimed at evaluating the effects of CoQ(10) following a prolonged treatment. Our results highlight an age-specific reduction in the percentage of cells showing the highest amount of oxidized bases, indicating a potential role of CoQ(10) in modulating DNA repair mechanisms.

Mutational spectrum at GATA1 provides insights into mutagenesis and leukemogenesis in Down syndrome

Down syndrome (DS) children have a unique genetic susceptibility to develop leukemia, in particular, acute megakaryocytic leukemia (AMkL) associated with somatic GATA1 mutations. The study of this genetic susceptibility with the use of DS as a model of leukemogenesis has broad applicability to the understanding of leukemia in children overall. On the basis of the role of GATA1 mutations in DS AMkL, we analyzed the mutational spectrum of GATA1 mutations to begin elucidating possible mechanisms by which these sequence alterations arise. Mutational analysis revealed a predominance of small insertion/deletion, duplication, and base substitution mutations, including G:C>T:A, G:C>A:T, and A:T>G:C. This mutational spectrum points to potential oxidative stress and aberrant folate metabolism secondary to genes on chromosome 21 (eg, cystathionine-beta-synthase, superoxide dismutase) as potential causes of GATA1 mutations. Furthermore, DNA repair capacity evaluated in DS and non-DS patient samples provided evidence that the base excision repair pathway is compromised in DS tissues, suggesting that inability to repair DNA damage also may play a critical role in the unique susceptibility of DS children to develop leukemia. A model of leukemogenesis in DS is proposed in which mutagenesis is driven by cystathionine-beta-synthase overexpression and altered folate homeostasis that becomes fixed as the ability to repair DNA damage is compromised.

Widespread impairment of cell proliferation in the neonate Ts65Dn mouse, a model for Down syndrome

OBJECTIVES

Among the many pathological aspects of Down syndrome, brain hypoplasia and mental retardation have been recently ascribed to defective proliferation of neural precursors during central nervous system development. By analogy, other features of Down syndrome, such as heart defects, gastrointestinal abnormalities, craniofacial dystrophy and reduced growth rate could be related, at least in theory, to similar proliferation impairment in peripheral tissues.

MATERIALS AND METHODS

In order to test this hypothesis, we evaluated cell proliferation in peripheral tissues of the Ts65Dn mouse, one of the animal models most commonly used to investigate Down syndrome.

RESULTS

In fibroblast cultures from neonatal Ts65Dn mice, we found that cell proliferation was notably impaired. While length of the cell cycle was similar in fibroblasts from Ts65Dn and control mice, the number of actively proliferating cells was significantly smaller in Ts65Dn mice. Moreover, fibroblasts from Ts65Dn animals exhibited limited population-doubling capacity, decreased proliferative lifespan and premature senescence. Analysis of cell proliferation in the skin of neonates, in vivo, showed that in Ts65Dn mice, cell proliferation was significantly reduced compared to control mice.

CONCLUSIONS

Our results suggest that defective proliferation may be a generalized feature of trisomic mice. In view of the genetic and phenotypic similarities between Ts65Dn mice and individuals with Down syndrome, proliferation impairment in various organs may also occur in subjects with Down syndrome. Thus, perturbation of a basic developmental function, cell proliferation, may be a critical determinant that contributes to the many aspects of pathology of this condition.

Oxidative stress: A bridge between Down's syndrome and Alzheimer's disease

Besides the genetic, biochemical and neuropathological analogies between Down's syndrome (DS) and Alzheimer's disease (AD), there is ample evidence of the involvement of oxidative stress (OS) in the pathogenesis of both disorders. The present paper reviews the publications on DS and AD in the past 10 years in light of the "gene dosage" and "two-hit" hypotheses, with regard to the alterations caused by OS in both the central nervous system and the periphery, and the main pipeline of antioxidant therapeutic strategies. OS occurs decades prior to the signature pathology and manifests as lipid, protein and DNA oxidation, and mitochondrial abnormalities. In clinical settings, the assessment of OS has traditionally been hampered by the use of assays that suffer from inherent problems related to specificity and/or sensitivity, which explains some of the conflicting results presented in this work. For DS, no scientifically proven diet or drug is yet available, and AD trials have not provided a satisfactory approach for the prevention of and therapy against OS, although most of them still need evidence-based confirmation. In the future, a balanced up-regulation of endogenous antioxidants, together with multiple exogenous antioxidant supplementation, may be expected to be one of the most promising treatment methods.

Mechanisms of Disease: DNA repair defects and neurological disease

In this Review, familial and sporadic neurological disorders reported to have an etiological link with DNA repair defects are discussed, with special emphasis placed on the molecular link between the disease phenotype and the precise DNA repair defect. Of the 15 neurological disorders listed, some of which have symptoms of progeria, six--spinocerebellar ataxia with axonal neuropathy-1, Huntington's disease, Alzheimer's disease, Parkinson's disease, Down syndrome and amyotrophic lateral sclerosis--seem to result from increased oxidative stress, and the inability of the base excision repair pathway to handle the damage to DNA that this induces. Five of the conditions (xeroderma pigmentosum, Cockayne's syndrome, trichothiodystrophy, Down syndrome, and triple-A syndrome) display a defect in the nucleotide excision repair pathway, four (Huntington's disease, various spinocerebellar ataxias, Friedreich's ataxia and myotonic dystrophy types 1 and 2) exhibit an unusual expansion of repeat sequences in DNA, and four (ataxia-telangiectasia, ataxia-telangiectasia-like disorder, Nijmegen breakage syndrome and Alzheimer's disease) exhibit defects in genes involved in repairing double-strand breaks. The current overall picture indicates that oxidative stress is a major causative factor in genomic instability in the brain, and that the nature of the resulting neurological phenotype depends on the pathway through which the instability is normally repaired.

Haploinsufficiency in DNA polymerase beta increases cancer risk with age and alters mortality rate

This study uses a base excision repair (BER)-deficient model, the DNA polymerase beta heterozygous mouse, to investigate the effect of BER deficiency on tumorigenicity and aging. Aged beta-pol(+/-) mice express 50% less beta-pol transcripts and protein (P < 0.05) than aged beta-pol(+/+) mice, showing maintenance of the heterozygous state over the life span of the mouse. This reduction in beta-pol expression was not associated with an increase in mutation rate but was associated with a 100% increase in the onset of hypoploidy. Aged beta-pol(+/-) mice exhibited a 6.7-fold increase in developing lymphoma (P < 0.01). Accordingly, 38% of beta-pol(+/-) mice exhibited lymphoid hyperplasia, whereas none of the beta-pol(+/+) exhibited this phenotype. beta-pol(+/-) mice were also more likely to develop adenocarcinoma (2.7-fold increase; P < 0.05) and more likely to develop multiple tumors, as 20% of the beta-pol(+/-) animals died bearing multiple tumors compared with only 5% of the beta-pol(+/+) animals (P < 0.05). In spite of accelerated tumor development, no gross effect of beta-pol heterozygosity was seen with respect to life span. However, the survival curves for the beta-pol(+/+) and beta-pol(+/-) mice are not identical. A maximum likelihood estimation analysis showed a modest but significant (P < 0.05) acceleration of the age-dependent mortality rate in beta-pol(+/-) mice. Thus, the beta-pol(+/-) mouse represents a model in which mortality rate and tumor development are accelerated and provides evidence supporting the role of genomic maintenance in both aging and carcinogenesis.

Age-dependent oxidative stress-induced DNA damage in Down’s lymphocytes

The aim of the present study was to investigate the oxidative status of lymphocytes from children (n=7) and adults (n=18) with Down's syndrome (DS). The basal oxidative condition, the vulnerability to in vitro hydrogen peroxide exposure, and the repair capacity were measured by means of the damage-specific alkaline comet assay. Significantly and age-independently elevated numbers of single strand breaks and oxidized bases (pyrimidines and purines) were found in the nuclear DNA of the lymphocytes in the DS group in the basal condition. These results may support the role of an increased level of endogenous oxidative stress in DS and are similar to those previously demonstrated in Alzheimer's disease. In the in vitro oxidative stress-induced state, a markedly higher extent of DNA damage was observed in DS children as compared with age- and gender-matched healthy controls, suggesting that young trisomic lymphocytes are more sensitive to oxidative stress than normal ones. However, the repair ability itself was not found to be deteriorated in either DS children or DS adults.

Relevance of drug uptake, cellular distribution and cell membrane fluidity to the enhanced sensitivity of Down's syndrome fibroblasts to anticancer antibiotic-mitoxantrone

Sensitivity of human fibroblasts derived from Down's syndrome (DS) individuals (S-240, T-158, T-74, T-164) and normal donors (S-126, WA-1) to anticancer antibiotic-mitoxantrone (1,4-dihydroxy-5,8-bis((2-((2-hydroxy-ethyl)amino)ethyl)amino)-9,10-anthracenedione dihydrochloride; MIT) and its relationship to the transport rate, cellular distribution and interaction with cell membrane were studied. The survival assay showed that MIT was more toxic to trisomic fibroblast lines than to normal cells. Studies of transport kinetics indicated that the amount of drug taken up and extruded by DS cells was diminished, compared to control cells. In contrast, the cellular level of MIT associated with DNA was greater in trisomic than in normal cells. The fluorescence anisotropy measurements of TMA-DPH and 12-AS demonstrated that the fluidity of the polar region of the outer lipid monolayer of DS cell membrane was decreased in comparison with normal cells. MIT treatment decreased fluidity of the inner hydrophobic region of plasma membrane, but only slightly influenced the fluidity of the outer surface of the cell membrane. Finally, we concluded that lowered membrane fluidity, diminished amount of MIT extruded by cells and the enhanced level of the drug associated with DNA could be responsible for the enhanced sensitivity of DS fibroblasts to the MIT treatment.

Thermosensitivity of red blood cells from Down's syndrome individuals

Biochemical disturbances of the reactive oxygen species metabolism revealed in subjects with Down's syndrome (DS), and the findings indicating that heat-induced cell alterations have been, at least, partly mediated by reactive oxygen species, made the elucidation of the response of trisomic cells to elevated temperatures of special interest. Kinetic analysis of cell-survival curves, accompanied by the flow cytometry and the scanning electron microscopy (SEM) examinations, and their relationship with the cell membrane fluidity, were undertaken. At each temperature (48-54 degrees C), Dq parameters, representing the ability to accumulate sublethal damages, were similar for both cell groups. D0 parameters (inverse leakage rates; D0 = 1/k) were greater for DS cells at each temperature below 54 degrees C. The haemolysis sensitivity ratio (HSR) showed that DS erythrocytes were, in average, 1.60 times more resistant to heat injury than those from normal subjects. Activation energies of haemolysis, calculated according to the Arrhenius equation, were similar both for normal (290.8 +/- 6.5 [kJ/mol]) and DS erythrocytes (288.0 +/- 5.5 [kJ/mol]). Flow cytometry studies showed that the scattering properties of intact DS erythrocytes (reflecting size, volume, shape and cell membrane surface morphology) were different than those of normal cells. Scanning electron micrographs and scattering diagrams obtained for cells submitted to heat stress (51 degrees C) confirmed that DS erythrocytes were more resistant, to a certain extent, to heat-induced disruption than normal cells. The steady-state fluorescence anisotropy of TMA-DPH (1-(4-trimethyl-ammoniumphenyl)-6-phenyl-1,3,5-hexatriene) showed that untreated DS erythrocytes had substantially lower fluidity (r = 0.356 +/- 0.008) of the outer monolayer of cell membranes as compared to normal cells (r = 0.324 +/- 0.011). The increase of the cell membrane fluidity during exposure to heat was observed. The greatest elevation of cell membrane fluidity occurred during the preleakage period, immediately upon the heat treatment and was considered as a rate-limiting step of heat-induced haemolysis.

Damage and repair of nerve cell DNA in toxic stress

It is generally agreed that ALS/PDC is triggered by a disappearing environmental factor peculiar to the lifestyle of people of the western Pacific (i.e., Guam, Irian Jaya, Indonesia, and the Kii Peninsula of Japan). A strong candidate is the cycad plant genotoxin cycasin, the beta-D-glucoside of methylazoxymethanol (MAM). We propose that prenatal or postnatal exposure to low levels of cycasin/MAM may damage neuronal DNA, compromise DNA repair, perturb neuronal gene expression, and irreversibly alter cell function to precipitate a slowly evolving disease ("slow-toxin" hypothesis). In support of our hypothesis, we have demonstrated the following: 1. DNA from postmitotic rodent central nervous system neurons is particularly sensitive to damage by MAM. 2. MAM reduces DNA repair in human and rodent neurons, whereas DNA-repair inhibitors potentiate MAM-induced DNA damage and toxicity in mature rodent nervous tissue. 3. Human neurons (SY5Y neuroblastoma) that are deficient in DNA repair are susceptible to MAM-induced cytotoxicity and DNA damage, whereas overexpression of DNA repair in similar cells is protective. 4. MAM alters gene expression in SY5Y human neuroblastoma cells and, in the presence of DNA damage and reduced DNA repair, enhances glutamate-modulated expression of tau mRNA in rat primary neurons; the corresponding protein (TAU) is elevated in ALS/PDC and Alzheimer's disease. These findings support a direct relationship between MAM-induced DNA damage and neurotoxicity and suggest the genotoxin may operate in a similar manner in vivo. More broadly, a combination of genotoxin-induced DNA damage (via exogenous and/or endogenous agents) and disturbed DNA repair may be important contributing factors in the slow and progressive degeneration of neurons that is characteristic of sporadic neurodegenerative disease. Preliminary studies demonstrate that DNA repair is reduced in the brain of subjects with western Pacific ALS/PDC, ALS, and Alzheimer's disease, which would increase the susceptibility of brain tissue to DNA damage by endogenous/exogenous genotoxins. Interindividual differences in the extent of prior exposure to DNA-damaging agents and/or the efficiency of its repair might produce population variety in the rate of damage accumulation and explain the susceptibility of certain individuals to sporadic neurodegenerative disease. Studies are underway using DNA-repair proficient and deficient neuronal cell cultures and mutant mice to explore gene-environment interplay with respect to MAM treatment, DNA damage, and DNA repair, and the age-related appearance of neurobehavioral and neuropathological compromise.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.