*2.2.1 Homologous recombination repair (HRR)*

With aging there is an increase in DNA double-strand breaks [66]. However, it is unknown whether this increase is a consequence of accumulation of unrepaired DSBs or progressively delayed repair events, possibly as a reflection of an inherently limited capacity to process DSBs [67]. To repair this kind of DNA damage, HRR, considered a highly reliable pathway, allows the cell to access and copy information from the intact DNA sequence into the sister chromatid. Notably, HRR is restricted to late S to G2 phases when chromosomes are aligned [68]. RAD51 and other members of the RAD52 epistasis group as RAD50, MRE11, and XRS2 are needed for HRR. The efficiency of HRR is enhanced by mediator proteins that promote the loading of RAD51 onto ssDNA, RAD52 among them [69]. HR-mediated repair efficiency declines precipitously during cellular aging together with a decline of RAD51, RAD51C, RAD52, NBS1, CTIP, and MRE11 levels [66, 70]. Furthermore, in human and mice oocytes, a decrease in expression of BRCA1 and ATM [71] and an impaired recruitment of RAD51 to DNA damage sites during aging [72] were observed, which could force cells to utilize the error-prone NHEJ pathway. At the same time, in older mice a lower activity of the ATM kinase that results in less p53 phosphorylation was reported, thus affecting apoptosis, cell-cycle arrest, and senescence [73]. In addition to the above, the decrease in the levels of PARP1 [36] and SIRT6 [38] not only affects BER pathway but also has a relevant role in HRR since supplementation of recombinant SIRT6 was able to partly restore HR activity [70]. This could be related to a higher binding of DBC1 to PARP1 inhibiting its enzymatic activity as well as the change in NAD+ levels [74]. Decreased NAD+ levels observed with age also reduce activity of other sirtuins as SIRT1 and SIRT7 together with PARP1, reducing NHEJ and HRR pathways [75]. Although HRR is essential, its activity must be carefully controlled in order to maintain genomic integrity [76]. Previously, it has been demonstrated that frequency of recombinant cells is highly variable among tissues, from very low levels in the brain and stomach to very frequent in the pancreas and spleen. Additionally, de novo recombination events indeed accumulate in mice colonic somatic stem cells with age [77].

#### *2.2.2 Nonhomologous end joined (NHEJ)*

In human cells, NHEJ is the major pathway for the repair of DSBs, where two ends of DNA with little or no sequence homology are brought together and repaired. NHEJ can act throughout most of the cell cycle but predominantly in G1 phase [68]. NHEJ is divided into two subpathways: the classical NHEJ pathway (c-NHEJ), in which DNA-PKcs, Ku70/Ku80 heterodimers, Artemis, XRCC4, XLF, and DNA Ligase 4 are involved, and the alternative NHEJ pathway (alt-NHEJ), comprised of the repair factors PARP1 and DNA ligase 3 [78]. Both NHEJ pathways are associated with changes in DNA sequence, where c-NHEJ causes deletions and insertions, whereas alt-NHEJ propitiates the loss of genetic information between microhomologies on chromosomes [79]. NHEJ becomes inefficient and more errorprone during cellular senescence, thus favoring genomic instability and higher

#### *The Role of DNA Repair in Cellular Aging Process DOI: http://dx.doi.org/10.5772/intechopen.84628*

 incidence of cancer in the elderly [80, 81]. Furthermore, NHEJ-mediated VDJ recombination in B lymphocytes is impaired, reducing class switch recombination efficiency and contributing to reduced humoral repertoire and impaired immunity with aging [82]. Frequency of microhomology-mediated end joining (MMEJ) increases as a compensatory mechanism; however, at the same time, it favors that more mistakes are generated [81]. Ku 70 and 80 proteins decreased their expression at least twofold in two lines of senescent human fibroblast; at the same time, their localization was changed concentrating them in the nucleus when compared with young cells where they are present in both the nucleus and cytoplasm [83]. Cytoplasmic Ku proteins could serve as a reserve (pool) that is recruited to the nucleus upon DNA damage; therefore in senescent cells these proteins are unavailable to repair new lesions [25]. Additionally, binding activity of the Ku 70/80 heterodimers to broken DNA ends also declines with aging [66]. Notably, mice and cells deleted for either Ku70 or Ku80 exhibited not solely NHEJ disruption but also altered BER [84]. On the other hand, decreased expression of XRCC4, DNA ligase 4, and DNA ligase 3 has been observed, and this implicates that during the aging process, NHEJ becomes more inefficient and inaccurate, leaving more damage sites repaired with a loss of additional genetic information [72]. Interestingly, aging increases DNA-PK activity phosphorylating HSP90α and decreasing its chaperone function in AMPK, which is critical for mitochondrial biogenesis and energy metabolism [85]. Consistently, DNA ligase 4 and Ku80 gene promoters were frequently observed as hypermethylated in elderly people, which could be associated with the silencing expression of both genes [86]. However, as mentioned for other DNA repair mechanisms, caloric restriction diet improves NHEJ activity possibly through SIRT1 and FOXO activity [87].
