DNA repair pathways correct DNA lesions to prevent mutations and preserve genomic integrity (Hoeijmakers, 2001
). These pathways are regarded as longevity assurance mechanisms important for suppressing tumor-causing mutations. Specific pathways are specialized for repairing specific damage, even though there is some functional overlap. For example, a variety of excision repair pathways correct ssDNA lesions including BER (Almeida and Sobol, 2007
; Barnes and Lindahl, 2004
), nucleotide excision repair (NER) (de Boer and Hoeijmakers, 2000
) and mismatch repair (MMR) (Kolodner and Marsischky, 1999
). Of these, BER is prominent for repairing ROS-induced DNA damage, NER is important for repairing UV light-induced lesions and MMR is critical for postreplication repair. The major pathways that repair DNA DSBs are homologous recombination (HR) and nonhomologous end joining (NHEJ). HR is an error free pathway that utilizes the sister chromatid as a template during S/G2
phase (West, 2003
) while NHEJ is considered to be an error prone pathway that joins ends together without a template and functions during both G1
and S phases (Lieber et al., 2003
). Both pathways are frequently used in mammalian cells (Jasin, 2000
) and disruption of either leads to gross chromosomal rearrangements (GCRs) that may cause cancer (Hoeijmakers, 2001
) and perhaps aging (Hasty et al., 2003
; Lombard et al., 2005
; Vijg and Dolle, 2002
). In addition, telomere length maintenance ensures chromosomal ends do not become open DSBs. These pathways are critical for ensuring sufficient health span for reproduction. Some data shows they decline as mammals age suggesting a causal factor for aging (Gorbunova et al., 2007
). For this review, we focus on pathways that repair or suppress DNA DSBs: HR, NHEJ and telomere maintenance.
HR repairs DNA DSBs by using a homologous template to ensure fidelity; therefore, HR occurs primarily during S or G2
). There are many proteins required for efficient HR and many more that influence its function. Central to HR is Rad51, a homolog to the prokaryotic recombination protein, RecA. The ends of a DSB are resected to expose single strands and Rad51 forms a nucleoprotein filament on these strands to induce invasion to a homologous template usually provided by the sister chromatid but potentially provided by either the homologous chromosome or by a repeat on the same or different chromosome. Invading strands serve as primers for DNA replication and subsequent intermediates may be resolved in a noncrossover plane such that the DSB is repaired without recombination or in a crossover plane such that the DSB is repaired in addition to recombination (). While HR repairs DNA DSBs with fidelity, it may also be mutagenic if it occurs between repeats on the same or a different chromosome to generate inversions, deletions or translocations. In addition, HR between homologous chromosomes may induce loss of heterozygosity by gene conversion since the ends are resected and since heteroduplexes are formed (Luo et al., 2000
). Thus, HR must be tightly monitored to prevent these mutations. Thus, it is possible an age-related decline in this regulation contributes to aging.
Fig. 1 Homologous recombination and gene conversion. (a) Homologous templates are aligned. This usually occurs between sister chromatids during DNA replication, but may also occur between chromosomes as shown here. The red chromosome has a DSB and a sequence (more ...)
There is some evidence that HR regulation changes with age. For example, the Drosophila
male germline shows an age-dependent increase in HR. However, there is little evidence for increased or decreased HR in mammals probably because the majority of cells in an adult mammal are postmitotic and HR does not function in G0
cells. Yet, the levels of nonallelic homologous recombination appear to increase with age since rearrangements in human blood cells are elevated from newborns to adults (Flores et al., 2007
). Thus, it is possible these rearrangements accumulate with age due to diminished HR regulation.
NHEJ repairs DNA DSBs by joining open ends together without the aid of a homologous template and can therefore occur during G1
in addition to S phase. In mammals, at least seven proteins are required: Ku70, Ku80, DNA-PKCS
, Artemis, Xrcc4, DNA Ligase IV (Lig4) and Xrcc4-like factor (Lieber et al., 2004
). Ku70 and Ku80 form a heterodimer called Ku that binds to DNA ends (Walker et al., 2001
) and together with a PI-3 kinase catalytic subunit, DNA-PKCS
, forms a holoenzyme referred to as DNA-PK (DNA dependent – protein kinase). Artemis opens hairpins and processes overhangs in a complex with DNA-PKCS
and these ends are ligated by the Xrcc4-Lig4 heterodimer in a complex with Xrcc4-like factor (Ahnesorg et al., 2006
; Buck et al., 2006
). Mice deleted for Ku or Xrcc4/Lig4 exhibit phenotypes that result from defective repair of DNA DSBs such as hypersensitivity to clastogenic agents and GCRs (Ferguson et al., 2000
; Gu et al., 1997
; Lim et al., 2000
; Nussenzweig et al., 1996
). NHEJ also repairs the DSBs formed during the assembly of V(D)J [Variable(Diverse)Joining] segments of antigen receptor genes; thus, NHEJ-deletion causes failed lymphocyte development resulting in severe combined immunodeficiency (scid).
There is evidence that NHEJ functionally declines with age suggesting a process that limits life span and contributes to aging (Gorbunova et al., 2007
). As rats age Ku levels diminish in the testis and Ku70 or Ku80 levels are differentially expressed in various tissues (Um et al., 2003
). Similarly as humans age, Ku nuclear localization and DNA binding is impaired in blood mononuclear cells (Doria et al., 2004
; Frasca et al., 1999
) and Ku70, but not Ku80, levels decline in lymphocytes (Ju et al., 2006
). By correlation, Ku levels decline and their cellular distribution is altered as human fibroblasts approach senescence (Seluanov et al., 2007b
). In keeping with these results, NHEJ function declines in the brains of aging rats (Ren and de Ortiz, 2002
; Vyjayanti and Rao, 2006
) and in Alzheimer's patients (Shackelford, 2006
) and becomes less efficient and more error-prone in senescent cells (Seluanov et al., 2004
). Thus, NHEJ declines with age supporting the possibility that defective NHEJ will lead to early aging.
Telomeres are important structures that cap and maintain chromosome ends by forming higher order structures called telomeric loops (de Lange, 2002
). This cap is composed of many TTAGGG repeats that end in a single-stranded overhang used by a telomere-specific enzyme, telomerase, as a template to extend and maintain telomere length. This means the natural ends of chromosomes are not the same as DNA DSBs. However, when telomeres erode the chromosomal end is much like a DSB and available for end joining by NHEJ such that chromosomes can become fused together (Artandi et al., 2000
; Chin et al., 1999
). Similar to HR, telomerase is important for proliferating cells and telomerase activity is restricted to only a few cells in an adult, the germ cells and stem cells (Flores et al., 2006
); thus, telomerase can only impact these cell types with age. Importantly, telomerase activity is insufficient to protect telomeres in stem cells suggesting that tissue renewal can become compromised with age due to eroded telomeres that may appear as DSBs.
Some species, but not all, express telomerase at very low levels in quiescent somatic cells in the adult permitting speculation that low telomerase levels may be important for determining the health of quiescent cells with age. However, telomerase expression may also promote cancer by enabling cellular proliferation since eroded telomeres induce anti-tumor responses that either kill the cell or stop proliferation. Recent data show telomerase levels in quiescent somatic cells are inversely proportional to species body mass with no correlation to life span (Seluanov et al., 2007a
). Thus, the larger the body mass the less telomerase is expressed in adult quiescent cells indicating that extra measures are needed to prevent cancer for species of increased body mass. This is likely the reason somatic cells express low levels of telomerase in mice but not humans. Therefore, human somatic cells that express low telomerase levels are cancer prone.
Besides telomerase, there are many proteins needed to maintain telomeres including the NHEJ proteins Ku70, Ku80 and DNA-PKCS
. These NHEJ proteins are proposed to maintain telomeres since they associate with telomeres (d'Adda di Fagagna et al., 2001
; Hsu et al., 1999
), suppress telomere fusions (Bailey et al., 1999
; Hsu et al., 2000
; Li et al., 2007
; Samper et al., 2000
) and impact telomere length maintenance (d'Adda di Fagagna et al., 2001
; Espejel et al., 2002
). Thus, Ku70, Ku80 and DNA-PKCS
are important for both DSB repair and telomere maintenance. Since these protein levels decline with age it is possible that telomere maintenance, as well as NHEJ, undergoes an age-dependent decline.