Mutation Research/DNAging最新文献

Oral zinc supplementation is able to correct zinc deficiency and some immune defects present in Down's syndrome (DS), while other beneficial effects can be predicted because of the broad spectrum of biochemical pathways and the great variety of enzymes which depend on zinc bio-availability. To test if the maintenance of DNA integrity is also affected by zinc supplementation, DNA damage and repair after γ-radiation was studied by alkaline elution assay in phytohemagglutinin-stimulated lymphocytes from Down's syndrome children before and after an oral zinc supplementation given for 4 months to correct their immune defects.

In comparison with lymphocytes from normal children the DNA damage induction after ionizing radiation in DS lymphocytes both before and after zinc supplementation was normal. On the other hand, the rate of DNA repair in DS was highly and significantly accelerated before zinc treatment. After supplemenation with zinc sulfate, the DNA repair rate was consistently slowed down becoming similar to that of control subjects.

This is the first demonstration that a nutritional intervention in humans is apparently able to modify the biochemical steps which control the rate of DNA repair.

Outbred LIO rats were given subcutaneous injections (3.2 mg) of a synthetic analogue of thymidine, 5-bromo-2′-deoxyuridine (BrdUrd) on days 1, 3, 7 and 21 postnatally. At 3 months, the treated males and females were mated to generate F₁ progeny. The mean life span decreased by 31.6% and 9.1% in male rats and by 21.1% and 7.2% in female rats exposed to BrdUrd and in their offspring, respectively. Exposure to BrdUrd increased the aging rate of the rats and of their progeny. Age-related changes in the length of the estruc cycle and in the incidence of persistent estrus and/or anestrus were observed earlier in female rats exposed neonatally to BrdUrd and in their offspring compared to controls; also, developmental stigmas were observed in the offspring of rats exposed neonatally. The incidence of total and malignant tumors was increased in rats that had received BrdUrd as well as in their progeny. Our observations on the decrease in mean and maximum life span, the increase in aging rate, the acceleration of age-related changes in female reproductive system function, and the increase in tumor incidence and decrease in tumor latency in rats exposed to BrdUrd in early life suggest that this system could serve as a model of accelerated aging. These effects persist at least to the next generation.

The rates of formation and excision of UVC light-induced cyclobutane-type pyrimidine photodimers were determined in cultures of foreskin-derived normal human fibroblasts in mitotic (MF) and mitomycin-C (MMC)-induced postmitotic fibroblasts (PMF). Characteristic morphological changes support the notion that MMC accelerates the differentiation pathway from MF to PMF. In cultures treated with aphidicolin, I am able to show that this inhibitor of α and/or δ polymerases significantly inhibits the repair of pyrimidine photodimers in foreskin-derived mitotic and MMC-induced postmitotic fibroblasts in low serum cultures (0.5%) following UVC irradiation. Over the concentration range of 0–2 μg/ml of aphidicolin, there is a strong concentration-dependent inhibition of repair in cells treated with 10 J/m² of UVC and incubated with aphidicolin during the post-incubation time (0–24 h). The results demonstrate that pyrimidine photodimers are repaired in low serum cultures by an α- and/or δ-polymerase-dependent pathway. These data also imply that the fibroblast differentiation system is a very useful tool to unravel the complex mechanisms of UV-induced DNA damage and repair.

All eukaryotic cells rely on mitochondrial respiration as their major source of metabolic energy (ATP). However, the mitochondria are also the main cellular source of oxygen radicals and the mutation rate of mtDNA is much higher than for chromosomal DNA. Damage to mtDNA is of great importance because it will often impair cellular energy production. However, damaged mitochondria can still replicate because the enzymes for mitochondrial replication are encoded entirely in the cell nucleus. For these reasons, it has been suggested that accumulation of defective mitochondria may be an important contributor to loss of cellular homoeostasis underlying the ageing process.

We describe a mathematical model which treats the dynamics of a population of mitochondria subject to radical-induced DNA mutations. The model confirms the existence of an upper threshold level for mutations beyond which the mitochondrial population collapses. This threshold depends strongly on the division rate of the mitochondria. The model also reproduces and explains (i) the decrease in mitochondrial population with age, (ii) the increase in the fraction of damaged mitochondria in old cells, (iii) the increase in radical production per mitochondrion, and (iv) the decrease in ATP production per mitochondrion.

Spontaneous baseline frequencies of chromosome aberrations, micronucleus counts and cell division were analysed in peripheral lymphocytes of 127 normal healthy individuals in vitro. Cells were subjected to culture for 48 h in serum and PHA supplemented culture medium RPMI 1640. 100 metaphases were observed for chromosome aberrations and 1000 cells each for micronucleus counts and mitotic index. Regression analyses were carried out to see the effect of age on spontaneous abnormalities. The correlation of aberrations, micronucleus formation and mitotic index with donor's age is highly significant. The elevation of abnormalities and depression of mitotic index were linear to the increase of donor's age, with a higher frequency in males. Aged males and females from the age range of 40–70 years showed larger numbers of aberrations. Individuals with the smoking habit possessed higher frequencies of abnormalities than non-smokers.

Data on aneuploidy from a prospective study on a large number of lymphocyte metaphases (over 1000 in 72-h and 100 in 48-h cultures) per individual from eight healthy donors of various ages are reported. Chromosome losses were dependent on culture time, being significantly more frequent in 72-h than in 48-h cultures. All donors exhibited various degrees of aneuploidy which increased with age in women. This increase resulted essentially from X chromosome losses, as previously reported. Although the rate of aneuploidy limited to autosomes was similar in newborns and in adults, the distributions of the missing autosomes were different. In the two newborns studied, autosome aneuploidy was random. In the adults, a significant inverse correlation with autosome lenghts was observed. The inverse correlation between chromosome lenghts and losses may be explained by selective pressure against monosomic cells in the adults.

DNA methylation is known to change with age in several mammalian species. Here we have examined the effect of dietary energy restriction on this age-associated change in liver DNA of C3H/SHN mice. The total 5-methyldeoxycytidine level in the genome decreased slightly soon after energy restriction started. The effect, however, diminished with time and no appreciable difference was detected at middle and old ages. The degree of methylation at the c-myc gene, on the other hand, was not affected by energy restriction at early periods, but the age-dependent alterations at later ages were repressed. This is a new finding to show that DNA methylation is one of the molecular indices of aging affected by energy restriction. It suggests an importance of DNA methylation in the aging process.

The caffeine effects on chromosomal aberration frequency and mean G₂ duration were studied in human lymphocytes in vitro from three age groups of normal donors (I: 1–5 years old; II: 30–40 years old; III: 60–70 years old). Under control conditions, the three age group showed a similar frequency of chromosomal aberrations. All three age groups exhibited a linear dose response for aberrations with caffeine treatments. However, lymphocytes from aged individuals (groups II and III) showed higher chromosomal aberration frequencies and longer G₂ duration than cells from young individuals (group I). The caffeine effect in reducing G₂ length was rather similar in every age group. The reversion of caffeine effects by adenosine or niacinamide in lymphocytes from older individuals was higher than in cells from group I. The different caffeine effects and G₂ values between lymphocytes from old and young individuals are most likely due to a higher number of DNA lesions reaching G₂ phase and/or a decrease of the G₂ repair capability of lymphocytes from older individuals.

To investigate the effects of age on DNA repair of alkylation damage, C57BL/6NNia mice ranging from 9 months to 29 months of age were injected by the intraperitoneal route with single doses of N-methyl-N-nitrosourea (MNU). The rates of removal of 7-methylguanine (m⁷Gua) in nuclear DNA from kidney were determined at various intervals from 1 to 288 h after injection of either 25 mg or 50 mg MNU per kg body weight. Reversed phase HPLC with electrochemical detection was used to monitor adduct disappearance from DNA hydrolysates. The kinetics of m⁷Gua removal from DNA were at least biphasic. Evidence was obtained that there was a rapid removal of m⁷Gua occuring in the first 24 h after MNU administration, followed by a slow phase of removal with a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ greater than 150 h. We assume that these two phases of m⁷Gua removal correspond to active repair of DNA by N-alkylgylcosylases and to passive elimination via sponteneous hydrolysis, respectively. Young and old kidney tissues all exhibited significant repair of m⁷Gua (55–73% of the induced adducts were removed in the first 24 h), but a substantial fraction of m⁷Gua was removed slowly, indicating that there are methylated bases which were refractory to repair processes. At both doses of MNU studied, old tissues showed active repair of m⁷Gua that, within the limits of detection, had similar initial rates of removal as young tissues. However, old kidney did not remove this adduct with the same overall efficiency as young kidney. Therefore, the amount of m⁷Gua in the repair-resistant fraction was greater in the senescent tissues. The biochemical mechanisms responsible for the less efficient DNA repair in senescent kidney are not known, but we suggest that such differences are due in part to structural alterations in the chromatin.

Brain DNA from 20 humans ranging in age from neonatal to 100 years was analyzed by the nuclease P1-enhanced version of the ³²P-postlabeling assay for bulky covalently modified nucleotides. A reproducible pattern of three ³²P-labeled spots was obtained by thin-layer chromatography followed by autoradiography. Two of these spots increased with age (Mann-Whitney U-test; P<0.001; comparison of ages ≤ 60 years and ages > 60 years). Thus, these spots met the definition of I-compounds. Rat brain DNA exhibited the same two I-spots, whose intensities also increased with animal age (1, 4, and 10 months). In humans, considerable individual variation of brain I-compound levels was observed, especially at ages > 60 years, presumably reflecting environmental, life-style, or genetic factors. This variation was not noted for brain DNA of laboratory rats. Thus, human brain DNA undergoes progressive covalent modifications with aging.