X-Message-Number: 25559
Date: Sun, 16 Jan 2005 05:25:59 -0800 (PST)
From: Doug Skrecky <>
Subject: DNA repair and human aging

[Keyword to look for: phosphorylated KU 80]

Ageing Res Rev. 2004 Apr;3(2):143-51.
Role of immune responsiveness and DNA repair capacity genes in ageing.
  The genetic factors that determine immune responsiveness and DNA repair
capacity are reviewed as major elements influencing the life span. Within
this framework two sets of new data obtained in mice and humans are
reported and discussed. As to the first set, the role of immune response
genes was investigated in Biozzi mice genetically selected for high (H) or
low (L) antibody response. After 15-20 generations of assortative mating,
H and L mice exhibited almost complete intraline homozygosity and
interline polymorphism including distinct H-2 haplotypes, such as q in H
and s in L mice. In order to analyze the impact of quantitative trait loci
(QTL) on the antibody response as well as on the DNA repair capacity in
cells of the immune system independently of the selected H-2 haplotype,
congenic Hq and Hs as well as Lq and Ls mice were produced and analysed.
Both the antibody response and DNA repair capacity were found to be
independent of the H-2 haplotype and determined by QTL. As to the second
set of data, DNA repair was also studied in irradiated peripheral blood
mononuclear cells (PBMC) from ageing humans. The levels of ku 70, ku 80,
DNA-PKcs, phosphorylated ku 80 as well as the DNA-binding activity of the
ku70/ku 80 heterodimer were determined in the cytoplasmic and nuclear
extracts obtained, before and after irradiation, from young and elderly
subjects. The results of this study suggest that the decreased DNA repair
capacity in PBMC from elderly subjects may be related to impaired
migration of the phosphorylated ku 80 from the cytoplasm to the nucleus.
This finding helps to elucidate questions related to the impairment of
DNA repair during ageing.

Mech Ageing Dev. 2003 Apr;124(4):517-24
DNA damage recognition and repair capacities in human naive and memory T
cells from peripheral blood of young and elderly subjects.
  T cells accumulate genetic damage over time but naive cells display
higher genomic stability and longer lifespan as compared to memory cells.
We found in naive and memory T cells from young and elderly subjects
that DNA damage in unirradiated cells is higher in memory than in nai;ve
T cells, and is increased by radiation in both cell types. Repair of the
radiation-induced DNA damage was much higher in naive than in memory T
cells from young subjects but null in both cell types from elderly
subjects. Molecular mechanisms involved in DNA damage recognition and
repair were analyzed in both cell subsets from young subjects. The
intracellular distribution and amount of the DNA-dependent protein kinase
(DNA-PK) complex components (ku 70, ku 80, DNA-PKcs), which are involved
in the recognition and repair of DNA breaks caused by ionizing
radiations, V(D)J recombination and isotype switching, was assessed in
naive and memory T cells from young subjects. While the expression of ku
70 and ku 80 was at comparable levels in both T cell subsets, DNA-PKcs,
phosphorylated ku 80, and DNA-binding of ku 70/80 were mostly evident in
naive but negligible or absent in memory T cells. These findings may
account for the higher genomic stability and longer lifespan of naive as
compared to memory human T cells from young subjects.

Annu Rev Med. 2004 Oct 12; [Epub ahead of print]
DNA Repair Defects in Stem Cell Function and Aging.
  Cellular DNA is under constant challenge by exogenous and endogenous
genotoxic stress, which results in both transient and accumulated DNA
damage and genomic instability. All cells are equipped with DNA damage
response pathways that trigger DNA repair, cell cycle arrest, and, if
need be, apoptosis, to eliminate DNA damage or damaged cells. The
consequences of these processes for stem cells can be profound:
diminution in stem cell pools, or, because of altered gene expression, an
increased chance for stem cell differentiation or malignant
transformation. Furthermore, a number of DNA repair abnormalities are
linked to premature aging syndromes, and these are associated with
defects in the stem cell population. The specific DNA repair systems for
which there are data regarding the impact of repair defects on stem cell
function include O6-alkylguanine DNA alkyltransferase, nucleotide
excision repair, base excision repair, mismatch repair, non-homologous
DNA end-joining Fanconi's anemia protein complex, and homologous
recombination. It has recently become clear that deficiencies of these
processes are associated not only with cancer and/or aging but also with
stem cell defects. This discovery raises the possibility of a link
between aging and stem cell dysfunction. In this review, we provide
evidence for a link between DNA repair systems and the maintenance and
longevity of stem cells.

Proc Natl Acad Sci U S A. 2004 May 18;101(20):7624-9. Epub 2004 Apr 28.
DNA end joining becomes less efficient and more error-prone during
cellular senescence.
  Accumulation of somatic mutations is thought to contribute to the aging
process. Genomic instability has been shown to increase during aging,
suggesting an aberrant function of DNA double-strand break (DSB) repair.
Surprisingly, DSB repair has not been examined with respect to cellular
senescence. Therefore, we have studied the ability of young, presenescent,
and senescent normal human fibroblasts to repair DSBs in transfected DNA
by using a fluorescent reporter substrate. We have found that the
efficiency of end joining is reduced up to 4.5 fold in presenescent and
senescent cells, relative to young cells. Sequence analysis of end
junctions showed that the frequency of precise ligation was higher in
young cells, whereas end joining in old cells was associated with
extended deletions. These results indicate that end joining becomes
inefficient and more error-prone during cellular senescence. Furthermore,
the ability to use microhomologies for end joining was compromised in
senescent cells, suggesting that young and senescent cells may use
different end joining pathways. We hypothesize that inefficient and
aberrant end joining is a likely mechanism underlying the age-related
genomic instability and higher incidence of cancer in the elderly.

Exp Gerontol. 2004 Apr;39(4):507-15.
Age related microsatellite instability in T cells from healthy individuals.
  Many immune functions decline with age and may jeopardize the elderly,
as illustrated, for example by the significantly higher mortality rate
from influenza in old age. Although innate and humoral immunity are
affected by aging, it is the T cell compartment, which manifests most
alterations. The mechanisms behind these alterations are still unclear,
and several explanations have been offered including thymic involution
and Telomere attrition leading to cell senescence. Age related
accumulation of mutations has been documented and could serve as an
additional mechanism of T cell dysfunction. One effective repair
mechanism capable of rectifying errors in DNA replications is the
mismatch repair (MMR) system. We previously reported a comparative
examination of individual DNA samples from blood cells obtained at 10
year intervals from young and old subjects. We showed significantly
higher rates of microsatellite instability (MSI), an indicator of MMR
dysfunction in older subjects, compared to young. In the present study we
confirm this result, using direct automated sequencing and in addition,
we demonstrate that as CD8 lymphocytes from aged individuals, undergo
repeated population doublings (PDs) in culture, they develop MSI. CD4
clones that also undergo repeated PDs in culture develop significant MSI
as well. Elucidation of this previously unexplored facet of lymphocyte
dynamics in relation to aging may help identify novel mechanisms of
immunosenescence and pathways that  could serve as targets for
interventions to restore immune function.

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