Date Published: September 8, 2011
Publisher: Impact Journals LLC
Author(s): Lauren S. Fink, Michaela Roell, Emanuela Caiazza, Chad Lerner, Thomas Stamato, Silvana Hrelia, Antonello Lorenzini, Christian Sell.
Faithful repair of damaged DNA is a crucial process in maintaining cell viability and function. A multitude of factors and pathways guides this process and includes repair proteins and cell cycle checkpoint factors. Differences in the maintenance of genomic processes are one feature that may contribute to species-specific differences in lifespan. We predicted that 53BP1, a key transducer of the DNA damage response and cell cycle checkpoint control, is highly involved in maintaining genomic stability and may function differently in cells from different species. We demonstrate a difference in the levels and recruitment of 53BP1 in mouse and human cells following DNA damage. In addition, we show that unresolved DNA damage persists more in mouse cells than in human cells, as evidenced by increased numbers of micronuclei. The difference in micronuclei seems to be related to the levels of 53BP1 present in cells. Finally, we present evidence that unresolved DNA damage correlates with species lifespan. Taken together, these studies suggest a link between recruitment of 53BP1, resolution of DNA damage, and increased species lifespan.
Human cells exhibit a greatly enhanced stringency for growth control and greater genomic stability when compared to rodent cells . This higher stringency is thought to underlie the relative resistance of humans to malignancies. For example, it has been estimated that the difference in the spontaneous immortalization rate between rodents and human cells is 10−5 to 10−6 compared to 10−9 to 10−10. This difference is reflected experimentally in studies which introduced active oncogenes to drive malignant transformation in primary cells. Rodent cells required a single oncogenic mutation coupled with abrogation of the major cell cycle checkpoint regulator p53, while human cells required the addition of multiple oncogenic mutations targeting several intracellular pathways critical to cell cycle progression and survival, which include p53, pRb, and the Ras/MAPK pathway as well as cellular phosphatases . Substantial differences have been found at the cellular level that may contribute to the species-specific differences in genomic stability between rodent and human cells. For example, telomere biology differs significantly between rodent and human cells , and the spindle checkpoint has been found to be much less stable in rodent cells [5,6]. In addition, there is variation in essential DNA repair proteins such the Ku70/80 heterodimer and DNA end binding, although absolute DNA repair rates appear to be similar in rodents and humans as assessed by standard assays .
The DNA damage response and cell cycle checkpoints are closely linked, and there is substantial overlap of signaling factors in both processes. In addition to the differences our laboratory has observed in Ku80 levels between mouse and human cells , we have now demonstrated differences in the accumulation of 53BP1 between these two cell types. Both under normal conditions and in response to treatment with a DNA damaging agent, we find that there is a more robust 53BP1 foci response in human fibroblasts, which display higher numbers of 53BP1 foci over time, compared to mouse fibroblasts. This observation suggests that 53BP1 recruitment may be linked to the differences in genome stability that characterize rodent and human cells. This interpretation is consistent with previous studies that suggest that γH2AX serves to facilitate the accumulation of 53BP1 at damage foci to ensure proper cell cycle arrest . Consistent with this concept, it has previously been shown that 53BP1-deficient mouse fibroblasts cannot maintain G2/M arrest as efficiently after damage . Interestingly, it has also been reported that 53BP1 foci may act to shield unresolved DNA lesions occurring as a result of replication stress and that 53BP1 foci may be maintained into subsequent cell cycles; these foci were associated with sites of frequent chromosome breaks such as fragile sites . We noted that mouse cells contain more 53BP1 foci than human cells at baseline (labeled as control in Figure 3C), suggesting that mouse cells are more likely to harbor unresolved DNA lesions than human cells. However, following DNA damage, mouse cells mount a less robust response as evidenced by the reduced number of 53BP1 foci per cell, and presumably this reduced response leads to an increase in the number of cells that produce micronuclei relative to human cells. The link between micronuclei formation and 53BP1 levels is supported by the fact that a reduction in 53BP1 levels in human cells leads to a significant increase in micronuclei even in cells that have not been exposed to DNA damaging agents.