DNA double-strand breaks measured by pulsed-field gel electrophoresis in irradiated lymphocytes from normal humans and those with Alzheimer's disease

Activated or Impaired: An Overview of DNA Repair in Neurodegenerative Diseases

As the population ages, age-related neurodegenerative diseases have become a major challenge in health science. Currently, the pathology of neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, is still not fully understood. Remarkably, emerging evidence indicates a role of genomic DNA damage and repair in various neurodegenerative disorders. Here, we summarized the current understanding of the function of DNA damage repair, especially base excision repair and double strand break repair pathways, in a variety of neurodegenerative diseases. We concluded that exacerbation of DNA lesions is found in almost all types of neurodegenerative diseases, whereas the activities of different DNA repair pathways demonstrate distinct trends, depending on disease type and even brain region. Specifically, key enzymes involved in base excision repair are likely impaired in Alzheimer's disease and amyotrophic lateral sclerosis but activated in Parkinson's disease, while nonhomologous end joining is likely downregulated in most types of neurodegenerative diseases. Hence, impairment of nonhomologous end joining is likely a common etiology for most neurodegenerative diseases, while defects in base excision repair are likely involved in the pathology of Alzheimer's disease and amyotrophic lateral sclerosis but are Parkinson's disease, based on current findings. Although there are still discrepancies and further studies are required to completely elucidate the exact roles of DNA repair in neurodegeneration, the current studies summarized here provide crucial insights into the pathology of neurodegenerative diseases and may reveal novel drug targets for corresponding neurodegenerative diseases.

A non-radioactive, PFGE-based assay for low levels of DNA double-strand breaks in mammalian cells

A PFGE method was adapted to measure DNA double-strand breaks (DSBs) in mammalian cells after low (0-25 Gy) doses of ionising radiation. Instead of radionuclide incorporation, DNA staining in the gel by SYBR-Gold was used, which lowered the background of DNA damage and could be applied to non-cycling cells. DSB level was defined as a product of a fraction of DNA released to the gel (FR) and a number of DNA fragments in the gel (DNA(fragm)) and expressed as a percentage above control value. The slope of the dose-response curve was two-fold higher compared to that with FR alone as DSB level indicator (31.4 versus 15.6% per Gy). Two alternative ways were proposed to determine the total amount of DNA, used for FR calculation: measurement of DNA content in a plug not subjected to electrophoresis, with the use of Pico-Green, or estimation of DNA released to the gel from a plug irradiated with 600 Gy of gamma-rays. The limit of DSB detection was 0.25 Gy for human G1-lymphocytes and 0.5-1 Gy for asynchronous cultures of human glioma M059 K and J or mouse lymphoma L5178Y-R and -S cells. Specificity of our PFGE assay to DSB was confirmed by the fact that no damage was detected after treatment of the cells with H(2)O(2), an inducer of single-strand DNA breaks (SSBs). On the contrary, the H(2)O(2) inflicted damage was detected by neutral comet assay, attaining 160% above control (equivalent to 2.5 Gy of X-radiation). DSB rejoining, measured in cells after X-irradiation with a dose of 10 Gy, generally proceeded faster than that measured previously after higher (30-50 Gy) doses of ionising radiation. Clearly seen were defects in DSB rejoining in radiosensitive M059 J and L5178Y-S cells compared to their radioresistant counterparts, M059 K and L5178Y-R. In some cell lines, a secondary post-irradiation increase in DSB levels was observed. The possibility is considered that these additional DSBs may accumulate during processing of non-DSB clustered DNA damage or/and represent early apoptotic events.

Alzheimer's disease: the two-hit hypothesis

There are many lines of evidence showing that oxidative stress and aberrant mitogenic changes have important roles in the pathogenesis of Alzheimer's disease (AD). However, although both oxidative stress and cell cycle-related abnormalities are early events, occurring before any cytopathology, the relation between these two events, and their role in pathophysiology was, until recently, unclear. However, on the basis of studies of mitogenic and oxidative stress signalling pathways in AD, we proposed a "two-hit hypothesis" which states that although either oxidative stress or abnormalities in mitotic signalling can independently serve as initiators, both processes are necessary to propagate disease pathogenesis. In this paper, we summarise evidence for oxidative stress and abnormal mitotic alterations in AD and explain the two-hit hypothesis by describing how both mechanisms are necessary and invariant features of disease.

Lack of correlation between residual radiation-induced DNA damage, in keratinocytes assayed directly from skin, and late radiotherapy reactions in breast cancer patients

PURPOSE

To study the relationship between the severity of late reactions to radiotherapy in breast cancer patients, and the extent of residual radiation-induced DNA damage, using a rapid assay of keratinocytes obtained directly from skin biopsies.

METHODS AND MATERIALS

A review was made of 32 patients with breast cancer, treated uniformly by radiotherapy between 1983 and 1988, following breast-conserving surgery. Their late radiotherapy reactions were scored (9-14 years post-radiotherapy) using a modified LENT SOMA scale, and a 5-mm buttock skin punch biopsy was obtained. Intact skin was irradiated at room temperature, and after allowing 24 h for repair, the tissue was disaggregated and the cells processed for pulsed field gel electrophoresis (PFGE). Residual DNA damage was expressed as the fraction of DNA released (FDR) following 150 Gy.

RESULTS

Studies using flow cytometry on disaggregated breast skin showed that over 90% of the cells were keratinocytes. The PFGE assay was robust with low background FDRs in unirradiated skin samples (mean 3.2%) and a wide range of FDRs following irradiation from 11.5% to 26.6%. No correlation was found between the FDR at 150 Gy (FDR 150) and any of the late reaction scores or retrospective acute reaction scores. There was, however, a borderline significant correlation for family history and FDR 150 (p = 0.059).

CONCLUSION

Rapid measurement of residual DNA damage in irradiated differentiated keratinocytes, the predominant cell population in skin biopsies, showed no correlation with the severity of symptomatic early or documented late reactions in a retrospectively studied group of 32 breast cancer patients.

On the role of DNA double-strand breaks in toxicity and carcinogenesis

DNA double-strand breaks are associated with various endogenous processes, such as transcription, recombination, replication, and with the process of active cell death, which aims to eliminate cells. In addition, DNA double-strand breaks can be induced by irradiation, exposure to chemicals, increased formation of reactive oxygen species, and, indirectly, during repair of other types of DNA damage or as a consequence of extranuclear lesions. In addition to the neutral filter elution of DNA, the recently introduced pulsed-field gel electrophoresis is capable of determining DNA double-strand breaks with higher accuracy and sensitivity and is expected to increase our knowledge on the frequency and the role of DNA breakage. Parallel determination of parameters for cytotoxicity is necessary to elucidate the causal primary lesion. Although the repair of DNA double-strand breaks is a complex task, cells are capable of repairing--with or without errors and up to a certain extent--and surviving this DNA lesion. Gene translocations, rearrangements, amplifications, and deletions arising during repair and misrepair of double-strand breaks may contribute to cell transformation and tumor development.

Relationship between DNA damage, rejoining and cell killing by radiation in mammalian cells

The prevailing hypothesis on the mechanism of radiation-induced cell killing identifies the genetic material deoxyribonucleic acid (DNA) as the most important subcellular target at biologically relevant doses. In this review we present new data and summarize the role of the DNA double-strand breaks (dsb) induced by ionizing radiation and DNA dsb rejoining as determinants of cellular radiosensitivity. When cells were irradiated at high dose-rate, two molecular end-points were identified which often correlated with radiosensitivity: (1) the apparent number of DNA dsb induced per Gy per DNA unit and (2) the half-time of the fast component of the DNA dsb rejoining kinetics. These two molecular determinants, not mutually exclusive, may be linked through a common factor such as the conformation of DNA.

DNA damage, micronucleus formation, and cell death from 125I decays in DNA

CHO cells were pulse-labeled with 125I-iododeoxyuridine, harvested 30 min or 5 h after labeling, and stored at -196 degrees C for accumulation of 125I decays. The 30- min groups yielded low-LET survival curves (large shoulder, D0 136 decays/cell); 5-h groups showed a high-LET pattern of cell killing (no shoulder, D0 45 decay/cell). Surprisingly, the shift in 125I action was abolished in cells exposed to HAT medium; both 30-min and 5-h cell groups exhibited high-LET-type killing (no shoulder, D0 52 decays/cell). The striking difference in cell death was not accompanied by any change in induction or repair of DNA DSBs, but the pattern of micronucleus formation (and by implication chromosome damage) did parallel 125I-induced cell death. These findings suggest that cell killing may not be directly linked to the absolute number of DNA DSBs and that damage to higher-order genome structures may be an important factor in radiation-induced cell death.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.

Comparative studies of induction and repair of DNA double-strand breaks in X-irradiated alveolar macrophages and resting peripheral blood lymphocytes using constant-field gel electrophoresis

Induction and repair of X-ray-induced DNA double-strand breaks (dsbs) was compared for normal broncho-alveolar macrophages and human peripheral blood lymphocytes, using CHO cells as a reference cell model. The cells, upon their separation, were processed in a similar manner. After X-irradiation, cell lysis and proteinase K treatment, the DNA samples were subjected to constant-field gel electrophoresis (CFGE) followed by fluorimetric densitometry for quantification of released DNA. Induction of dsbs after X-ray doses of 5-100 Gy was found to show no gross differences for all cell systems used. Repair of dsbs was studied after X-ray dose of 60 Gy for up to 24 h after irradiation. The repair curves obtained proved to be similar for bronchoalveolar macrophages and CHO cells (97% of all dsbs rejoined after 24 h). However, in blood lymphocytes from normal subjects and from bone marrow recipients, dsb repair proceeded rapidly only for 0.5-1 h post-irradiation, being followed by the gradual degradation of DNA at longer intervals. The kinetics of DNA degradation correlated with cytological features of pyknosis and necrosis.

Gene specific DNA repair of damage induced in familial Alzheimer disease cells by ultraviolet irradiation or by nitrogen mustard

We have measured gene specific DNA repair in a normal human fibroblast cell line, and in fibroblast lines from two patients with familial Alzheimer disease (AD). Cells were treated with either ultraviolet radiation (UV) or the chemotherapeutic alkylating agent, nitrogen mustard (HN2). DNA damage formation and repair were studied in the active dihydrofolate reductase (DHFR) gene for the main lesions introduced by each of these two types of DNA damaging agents. The gene specific repair of UV induced cyclobutane pyrimidine dimers in the human DHFR gene was 86% complete in the AD cells after 24 h of repair incubation. This repair efficiency was similar to what we and others have found in normal human fibroblasts. After treatment of the AD cells with HN2, we found the frequency of HN2 induced lesions in the DHFR gene to be similar to the frequency in the transcriptionally inactive delta-globin gene. The gene specific repair of HN2 induced lesions in the DHFR gene was completed within 8-24 h in the normal fibroblast line and in the familial AD line, and the repair kinetics were similar for both cell lines. These results indicate that familial AD fibroblasts have normal gene specific repair of both UV induced and HN2 induced DNA damage in active genes.

Possible factors in the etiology of Alzheimer's disease

Inherited cases of Alzheimer's disease (AD) comprise only a very small proportion of the total. The remainder are of unknown etiopathogenesis, but they are very probably multifactorial in origin. This article describes studies on four possible factors: aluminum; viruses--in particular, herpes simplex type I virus (HSV1); defective DNA repair; and head trauma. Specific problems associated with aluminum, such as inadvertent contamination and its insolubility, have led to some controversy over its usage. Nonetheless, the effects of aluminum on animals and neuronal cells in culture have been studied intensively. Changes in protein structure and location in the cell are described, including the finding in this laboratory of a change in tau resembling that in AD neurofibrillary tangles, and also the lack of appreciable binding of aluminum to DNA. As for HSV1, there has previously been uncertainty about whether HSV1 DNA is present in human brain. Work in this laboratory using polymerase chain reaction has shown that HSV1 DNA is present in many normal aged brains and AD brains, but is absent in brains from younger people. Studies on DNA damage and repair in AD and normal cells are described, and finally, the possible involvement of head trauma is discussed.

Chromosome studies in Alzheimer's disease patients: distribution of dicentric breakpoints in lymphocytes irradiated in vitro

A previous study showed a significant increase in dicentric frequency in lymphocytes irradiated in vitro from non-familial Alzheimer's disease patients compared to normal age-matched controls. This study examined the distribution of the chromosome breakpoints involved in the dicentric formation and found a non-specific increase in all chromosomes.