Frequency of 6-thioguanine-resistant T cells is inversely related to the declining T-cell activities in aging mice

Accumulation of mutations and somatic selection in aging neural stem/progenitor cells

Genomic instability within somatic stem cells may lead to the accumulation of mutations and contribute to cancer or other age-related phenotypes. However, determining the frequency of mutations that differ among individual stem cells is difficult from whole tissue samples because each event is diluted in the total population of both stem cells and differentiated tissue. Here the ability to expand neural stem/progenitor cells clonally permitted measurement of genomic alterations derived from a single initial cell. C57Bl/6 x DBA/2 hybrid mice were used and PCR analysis with strain-specific primers was performed to detect loss of heterozygosity on nine different chromosomes for each neurosphere. The frequency with which changes occurred in neurospheres derived from 2-month- and 2-year-old mice was compared. In 15 neurospheres derived from young animals both parental chromosomes were present for all nine chromosome pairs. In contrast, 16/17 neurospheres from old animals demonstrated loss of heterozygosity (LOH) on one or more chromosomes and seven exhibited a complete deletion of at least one chromosomal region. For chromosomes 9 and 19 there is a significant bias in the allele that is lost where in each case the C57Bl/6 allele is retained in 6/6 neurospheres exhibiting LOH. These data suggest that aging leads to a substantial mutational load within the neural stem cell compartment which can be expected to affect the normal function of these cells. Furthermore, the retention of specific alleles for chromosomes 9 and 19 suggests that a subset of mutational events lead to an allele-specific survival advantage within the neural stem cell compartment.

Accumulation of point mutations in mitochondrial DNA of aging mice

Mitochondrial DNA (mtDNA) exists in a highly genotoxic environment created by exposure to reactive oxygen species, somewhat deficient DNA repair, and the relatively low fidelity of polymerase gamma. Given the severity of the environment, it was anticipated that mutation accumulation in the mtDNA of aging animals should exceed that of nuclear genes by several orders of magnitude. We have analyzed fragments amplified from the D-loop region of mtDNA from 2 to 22-month-old mice. The amplified 432 bp fragments were cloned into plasmid vectors, and plasmid DNAs from individual clones were purified and sequenced. None of 110 fragments from young mice contained a mutation, while 9 of 87 clones originating from old animals contained base substitutions (chi square = 11.9, P<0.001). The estimated mutation frequency in mtDNA from old mice was 11.6+/-2.7 or 25.4+/-7.8 per 10(5) nucleotides (depending on assumptions of clonality), which exceeds existing estimates for mutation frequencies for nuclear genes by approximately 1000-fold. Our data suggest that at 22 months of age, which roughly corresponds to 3/4 of the mouse natural life span, most mtDNA molecules carry multiple point mutations.

Health span and life span in transgenic mice with modulated DNA repair

One way to better understand the contribution of DNA repair, DNA damage, and mutagenesis in aging would be to enhance DNA repair activity, lower DNA damage, and lower mutagenesis. Because the repair protein O6-methylguanine-DNA methyltransferase (MGMT) acts alone and stoichiometrically, the human MGMT (hMGMT) cDNA was selected to test the feasibility of enhancing DNA repair activity in transgenic mice. MGMT activity is largely responsible for ameliorating the deleterious effects of O6-methylguanine (O6mG) lesions in DNA in a direct reversal mechanism. A transgene was constructed consisting of a portion of the human transferrin (TF) promoter and hMGMT cDNA such that hMGMT is expressed in transgenic mouse brain and liver. Expression of hMGMT was associated with a significant reduction in the occurrence of an age-related hepatocellular carcinoma in male mice at 15 months of age. Longitudinal and cross-sectional studies were initiated to determine whether the reduced incidence of hepatocellular carcinoma would impact median or maximum life span. The cross-sectional study performed on 15-month-old male animals confirmed the reduced occurrence of spontaneous hepatocellular carcinoma. At 30 months of age, however, the occurrence of hepatocellular carcinoma in at least one transgenic line was similar to that for nontransgenic animals. The longitudinal study is ongoing; however, at present no significant differences in life span have been detected. Tissues expressing the MGMT transgene also displayed greater resistance to alkylation-induced tumor formation. These results suggest that transgenes can be used to direct enhanced DNA repair gene expression and that enhanced expression can protect animals from certain spontaneous and induced tumors.

DNA damage and mutation: contributors to the age-related alterations in T cell-mediated immune responses?

The genetic material of our cells is susceptible to damage by a wide variety of extrinsic and intrinsic entities. The amount of genetic damage accumulated in vivo will depend upon an individual's ability to defend against and/or repair DNA damage. T cells in vivo have been shown to accumulate DNA damage and mutations over time. The accumulation of such genetic damage will occur in T cells possessing a 'naive' or a 'memory' phenotype. Since T cells are required to undergo extensive clonal expansion upon antigenic stimulation, DNA damage and mutations may result in: a failure of T cells to proliferate, because of DNA damage-mediated cell cycle arrest; decreased rates of proliferation, as a consequence of selection in vivo against cells carrying certain mutations and/or apoptosis, triggered by critical levels of DNA damage. Thus, when T cells, containing critical levels of genetic damage, are required to undergo rapid clonal expansion in the presence of antigen, insufficient numbers of T cells may be produced and so the immune response would be sub-optimal. In this paper the possible contribution of DNA damage and/or mutation to the age-related alterations in T cell-mediated immune responses will be discussed.

Changes in fidelity levels of DNA polymerases alpha-1, alpha-2, and beta during ageing in rats

DNA polymerases (deoxynucleoside-triphosphate:DNA deoxynucleotidyltransferase EC 2.7.7.7.) were extracted from the regenerating livers of rats of various ages. The extracts were separated into three DNA polymerase fractions (alpha-1, alpha-2, and beta) by phosphocellulose column chromatography, and their fidelity levels were then monitored with the synthetic template-primer, poly (dA-dT), poly dA-dT10, or poly dC-poly dG. The fidelity levels of the three DNA polymerases from regenerating liver of rats younger than 20 months were high, while those of DNA polymerases from rats older than 20 months were significantly lower with similar profiles on all three template-primers. On the other hand, the fidelity levels of enzymes from 23- and 26-month-old rats were similar. These results indicate that the levels of error-prone DNA polymerases increase rapidly in the regenerating liver of rats from ages 20 to 23 months. This may due to the amplification of DNA polymerase gene mutations by an error-prone enzyme itself. However, the cells in which mutations in the functional gene occur may undergo cell death because the fidelity levels of the DNA polymerases in the older animals did not increase.

Age-associated changes in the template-reading fidelity of DNA polymerase alpha from regenerating rat liver

DNA polymerases (deoxynucleosidetriphosphate: DNA deoxynucleotidyltransferase EC 2.7.7.7.) were extracted from regenerating livers from young and aged rats. DNA polymerase alpha was separated and partially purified by DEAE-cellulose column chromatography, polyethyleneglycol precipitation, and phosphocellulose column chromatography, and fidelity levels were then monitored with the synthetic template-primer poly (dG-dC). The fidelity level of the DNA polymerase from regenerating liver a 4-month-old rat was very high, while that of the DNA polymerase from a 24-month-old rat was significantly decreased. To confirm this result, DNA was synthesized on poly (dG-dC) in a reaction mixture containing [32P]dTTP, and the synthetic polynucleotide was purified and digested with HhaI restriction endonuclease. After hydrolysis, the oligonucleotides were developed by two dimensional thin layer chromatography on PEI cellulose plates. Spots containing [32P]dTMP were observed when DNA polymerase from a 24 month-old rat was used, but none was found in polynucleotides synthesized using DNA polymerase from a 4 month-old rat. Nearest neighbor analysis suggested that dG-dT and dC-dT pairs were constructed by mis-incorporation due to DNA polymerase alpha.

Detection of 6-thioguanine-resistant spleen lymphocytes in different mouse strains by autoradiography

The variant frequencies for 6-thioguanine-resistant spleen cells in different mouse strains have been estimated by autoradiography for animals without chemical treatment and in cases of in vivo mutagen dosage with ethylnitrosourea and cyclophosphamide, respectively. In untreated mice, the following variant frequencies have been found: C57Bl/6J, 2.84 x 10(-5);NMRI, 3.04 x 10(-5);DBA/2J, 5.91 x 10(-5). The selective concentration of 6-thioguanine was 100 microM for strains NMRI and DBA, while in the case of C57Bl with this concentration, no variant cells could be counted and a selective concentration of 50 microM was chosen. Treatment with 70, 140, and 210 mg/kg ethylnitrosourea resulted in increased variant frequencies in cells isolated 8 or 15 days later. On the other hand, doses of 20, 60, and 120 mg/kg cyclophosphamide did not result in a clear dose-response relationship of variant frequency in cells isolated 1, 8, 15, 22, and 29 days after treatment. These data are discussed with respect to findings in human populations exposed occupationally to cyclophosphamide.

Cloned mouse lymphocytes permit analysis of somatic mutations that occur in vivo

As part of our mouse model of somatic mutation, we have begun to characterize spontaneously occurring hypoxanthine phosphoribosyltransferase (HPRT) -deficient mouse lymphocytes. Lymphocytes were cloned by in vitro exposure of spleen cells from male C57B1/6 mice to the mitogen concanavalin A, conditioned medium containing lymphocyte growth factors, and thioguanine (TG), in a limiting dilution assay. The 17 TG-resistant clones recovered were all highly deficient in HPRT activity and were found by analysis of surface antigens to be representative of the major subclasses of T lymphocytes. Southern analysis of lymphocyte genomic DNA detected alterations of the hprt gene in 12/17 of the HPRT-deficient lymphocyte clones. Of these 12, 2/17 were lacking the entire hprt locus, 7/17 lacked part of the locus, and 3/17 had other, unidentified alterations.

Factors that affect the frequency of thioguanine-resistant lymphocytes in mice following exposure to ethylnitrosourea

The frequency of thioguanine(TG)-resistant lymphocytes in mice treated with ethylnitrosourea (ENU) was followed for a period of 51 wk using our clonogenic assay [Jones et al, 1985a,b]. The effects of dose (0-58 mg/kg), time since treatment (2-51 wk), dose rate (5 weekly X 11.7 mg/kg versus 1 X 58 mg/kg), and age at time of treatment (3 vs 15 mo) on the frequency of TG-resistant, concanavalin A-responsive spleen cells were evaluated. The frequencies of TG-resistant spleen cells were generally dose responsive for 51 wk after exposure to ENU. They also were dependent upon the time that had elapsed since treatment with ENU, increasing to maximal values at 10 wk as previously reported [Jones et al, 1985a], and holding essentially stable at values of approximately 20% of the maximum frequency from week 15 until at least week 40 for the 3-month-old mice. Fractionation of 58 mg ENU/kg into 5 weekly doses did not affect the frequency of ENU-induced TG-resistant cells detected in the spleen but did increase the rate of appearance in the spleen, and the efficiency of induction by the unit dose, of TG-resistant cells. The mice exposed to ENU at 15 mo of age appeared to have a 4-fold reduction in the rate of increase in frequency of ENU-induced TG-resistant spleen cells. One set of control mice was found to have a 10-fold elevated frequency of TG-resistant cells in both the spleen and thymus, indicating that mutations can occur in stem cells of untreated animals.