Repair of endogenous DNA base lesions modulate lifespan in mice

Abstract

The accumulation of DNA damage is thought to contribute to the physiological decay associated with the aging process. Here, we report the results of a large-scale study examining longevity in various mouse models defective in the repair of DNA alkylation damage, or defective in the DNA damage response. We find that the repair of spontaneous DNA damage by alkyladenine DNA glycosylase (Aag/Mpg)-initiated base excision repair and O(6)-methylguanine DNA methyltransferase (Mgmt)-mediated direct reversal contributes to maximum life span in the laboratory mouse. We also uncovered important genetic interactions between Aag, which excises a wide variety of damaged DNA bases, and the DNA damage sensor and signaling protein, Atm. We show that Atm plays a role in mediating survival in the face of both spontaneous and induced DNA damage, and that Aag deficiency not only promotes overall survival, but also alters the tumor spectrum in Atm(-/-) mice. Further, the reversal of spontaneous alkylation damage by Mgmt interacts with the DNA mismatch repair pathway to modulate survival and tumor spectrum. Since these aging studies were performed without treatment with DNA damaging agents, our results indicate that the DNA damage that is generated endogenously accumulates with age, and that DNA alkylation repair proteins play a role in influencing longevity.

Keywords: AAG/MPG; Aging; Base excision repair; DNA adducts; DNA glycosylase; Mgmt.

Figure 1. DNA alkylation repair contributes to longevity

A) Survival curves of wild type (black lines, n=37), Aag^−/− (red line, n=29), Mgmt^−/− (blue line, n=50) and Aag^−/− Mgmt^−/− (purple line, n=31). Pair-wise comparisons: Aag^−/− to wild type, p=0.1003; Mgmt^−/− to wild type, p=0.4752; Aag^−/− Mgmt^−/− to wild type, p=0.04, all Log-rank (Mantel-Cox) test. B) Histopathological classification of pathologies found in WT (n=18), Aag^−/− (n=24), Mgmt^−/− (n=35), and Aag^−/− Mgmt^−/− (n=31) mice.

Figure 2. Atm plays a role in the response to endogenous/spontaneous DNA alkylation damage

A) Survival curves of wild type (black lines, n=37), Atm^−/− (green line, n=19), Aag^−/− Atm^−/− (green/red line, n=22) and Mgmt^−/− Atm^−/− (cyan line, n=21). All pair-wise comparisons to wild type, p<0.0001. Pair-wise comparison between Atm^−/− and Aag^−/− Atm^−/−, p=0.0193, and between Atm^−/− and Mgmt^−/− Atm^−/−, p=0.3423, all comparisons Log-rank (Mantel-Cox) test. B) Aag deficiency protects against lymphoma in Atm^−/− animals. Graph shows the incidence of age-related pathologies observed in Aag and Atm genotypic combinations. Wild-type (n=18); Aag^−/− (n=24); Mgmt−/− (n=35); Atm^−/− (n=15); Mgmt^−/− Atm^−/− (n=7); Aag^−/− Atm^−/− (n=10).

Figure 3. Aag and Mgmt mutations do not affect longevity of p53 mutant animals

A) Survival curves of wild type (black lines, n=37), p53^−/− (light green line, n=21), Aag^−/− p53^−/− (light red line, n=42), and Mgmt^−/− p53^−/− (light blue line, n=18). All pair-wise comparisons to wild type, p<0.0001, Log-rank (Mantel-Cox) test. B) Aag or Mgmt deficiency does not shift tumor spectrum in p53^−/− mice. Graph shows the incidence of age-related pathologies observed in Aag and p53 genotypic combinations. Wild-type (n=18); p53^−/− (n=8); Aag^−/− p53^−/− (n=14); Mgmt^−/− p53^−/− (n=9).

Figure 4. Interaction between Mgmt deficiency and the mismatch repair pathway

Survival curves of wild type (black lines, n=37), Mgmt^−/− (blue line, n=50), Exo1^−/− (purple line, n=19), Mgmt^−/− Exo1^−/− (dashed lilac line, n=30), Msh6^−/− (orange line, n=10), and Mgmt^−/− Msh6^−/− (dashed light orange line, n=19). Pair-wise comparison between Mgmt^−/− and Mgmt^−/− Exo1^−/−, p=0.0008 and between Mgmt^−/− and Mgmt^−/− Msh6^−/−, p<0.0001, Log-rank (Mantel-Cox) test. B) The Mgmt null mutation leads to a decrease in the incidence of lymphomas in Exo1^−/− animals but not in Msh6^−/− animals. Wild-type (n=18); Mgmt^−/− (n=33), Exo1^−/− (n=17), Msh6^−/− (n=10), Mgmt^−/− Msh6^−/− (n=19), Mgmt^−/− Exo1^−/− (n=29).

Figure 5. Atm and Aag interact in response to induced alkylation damage

A) Ex vivo alkylation sensitivity of BM cells to methyl methanesulfonate (MMS). BM cells were derived from wild type (closed squares), Aag^−/− (closed triangle), Atm^−/− (open squares) and Aag^−/− Atm^−/− (open triangle) mice. Experiments were done a minimum of three times each, data are mean ± SEM.. B) Synergistic interaction between Mgmt and Atm in response to MNU treatment. Ex vivo alkylation sensitivity of BM cells to methyl nitrosourea (MNU). BM cells were derived from wild type (closed squares), Mgmt^−/− (closed circles), Atm^−/− (open squares) and Mgmt^−/− Atm^−/− (open circles) mice. Experiments were done a minimum of three times each, data are mean ± SEM.