DNA repair deficiencies and cellular senescence are unrelated in xeroderma pigmentosum cell lines

Disorders of nucleotide excision repair: the genetic and molecular basis of heterogeneity

Mutations in genes on the nucleotide excision repair pathway are associated with diseases, such as xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy, that involve skin cancer and developmental and neurological symptoms. These mutations cause the defective repair of damaged DNA and increased transcription arrest but, except for skin cancer, the links between repair and disease have not been obvious. Widely different clinical syndromes seem to result from mutations in the same gene, even when the mutations result in complete loss of function. The mapping of mutations in recently solved protein structures has begun to clarify the links between the molecular defects and phenotypes, but the identification of additional sources of clinical variability is still necessary.

Clinical implications of the basic defects in Cockayne syndrome and xeroderma pigmentosum and the DNA lesions responsible for cancer, neurodegeneration and aging

Cancer, aging, and neurodegeneration are all associated with DNA damage and repair in complex fashions. Aging appears to be a cell and tissue-wide process linked to the insulin-dependent pathway in several DNA repair deficient disorders, especially in mice. Cancer and neurodegeneration appear to have complementary relationships to DNA damage and repair. Cancer arises from surviving cells, or even stem cells, that have down-regulated many pathways, including apoptosis, that regulate genomic stability in a multi-step process. Neurodegeneration however occurs in nondividing neurons in which the persistence of apoptosis in response to reactive oxygen species is, itself, pathological. Questions that remain open concern: sources and chemical nature of naturally occurring DNA damaging agents, especially whether mitochondria are the true source; the target tissues for DNA damage and repair; do the human DNA repair deficient diseases delineate specific pathways of DNA damage relevant to clinical outcomes; if naturally occurring reactive oxygen species are pathological in human repair deficient disease, would anti-oxidants or anti-apoptotic agents be feasible therapeutic agent?

Unique tissue-specific level of DNA nucleotide excision repair in primary human mammary epithelial cultures

DNA repair is essential for the maintenance of genomic integrity and stability. Nucleotide excision repair (NER) is a major pathway responsible for remediation of damage caused by UV light, bulky adducts, and cross-linking agents. We now show that NER capacity is differentially expressed in human tissues. We established primary cultures of peripheral blood lymphocytes (PBLs: N = 33) and foreskin fibroblasts (FF: N = 6), as well as adult breast tissue (N = 22) using a unique culture system, and measured their NER capacity using the unscheduled DNA synthesis (UDS) functional assay. Relative to FF, primary cultures of breast cells exhibited only 24.6 +/- 2.1% of NER capacity and PBLs only 8.9 +/- 1.2%. Cells from the breast therefore have a unique and distinctive DNA repair capacity. The NER capacities of all three cell types had similar coefficients of variation in the range of 10%-15%, which should be taken into account when running controls for this contextual assay. Unlike previous studies and speculation in the field, we found that NER was not affected by cell morphology, donor age, or proliferation as measured by the S phase index. While the NER capacity of the transformed lymphoblastoid cell line TK6 was within the range of our PBL samples, the breast tumor-derived MDA MB-231 cell line was four-fold higher than normal breast tissue. These studies show that analysis of baseline DNA repair in normal human cell types is critical as a basis for evaluation of the effects of "mutator" genes as etiological factors in the development of cancer.

Richard B. Setlow, a commentary on seminal contributions and scientific controversies

Richard B. Setlow inspired the field of DNA repair. His demonstration that photoproducts could be quantified within cells and their excision examined experimentally pioneered the identification of nucleotide excision repair. His early work was associated with the discovery of many founding phenomena of photobiology and DNA repair: the concept of excision repair itself, correlations between DNA repair, life span and aging, variations in repair among mammalian species, caffeine sensitization to UV damage, and the xeroderma pigmentosum (XP) repair deficiencies. We may now have mapped thoroughly the landscape of DNA repair that Dick helped open to exploration, but questions persist of how comprehensively we have explored all its canyons and mesas. Research into nontraditional species and kingdoms may yet provide unexpected surprises. The signal transduction pathways and mechanisms of DNA replication arrest in damaged mammalian cells remain a challenge. The importance of repair in vivo also provides many difficult research questions. One problem of current interest is the role of endogenous DNA damage and repair in human pathology, especially neurodegeneration exemplified by many XP patients. Cancer and neurodegeneration may represent converse responses of dividing and nondividing cells to mutagenic and lethal effects of DNA damaging agents. Cell death from endogenous oxidative DNA damage (apoptosis) may be antagonistic to malignant transformation in dividing cells but may cause neurodegeneration in nondividing neural tissue. Small reductions in the efficiency of repair, especially transcription-coupled repair, may overemphasize carcinogenesis in mice, while minimizing neurodegeneration, as compared to human patients.

Gene-specific DNA repair of pyrimidine dimers does not decline during cellular aging in vitro

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and one of the causes for this could be a decrease in DNA repair capacity. However, the level of total genomic repair has not been strongly correlated with aging. DNA repair of certain kinds of damage is known to be closely connected to the transcription process; thus, we chose to investigate the level of gene-specific repair of UV-induced damage using in vitro aging of human diploid skin fibroblasts and trabecular osteoblasts as model systems for aging. We find that the total genomic repair is not significantly affected during cellular aging of cultures of both human skin fibroblasts and trabecular osteoblasts. Gene-specific repair was analyzed during cellular aging in the dihydrofolate reductase housekeeping gene, the p53 tumor suppressor gene, and the inactive region X(754). There was no clear difference in the capacity of young and old cells to repair UV-induced pyrimidine dimers in any of the analyzed genes. Thus, in vitro senescent cells can sustain the ability to repair externally induced damage.

Elevated DNA excision repair capacity in the extraembryonic mesoderm of the midgestation mouse embryo

In order to determine whether there is differential cell-type-specific DNA repair we measured the nucleotide excision repair capacity of the four distinct cell lineages that comprise the extraembryonic yolk sac using the unscheduled DNA synthesis assay. Yolk sacs from mouse embryos at 11.5-12.5 days gestation were microdissected to yield purified trophoblast, parietal endoderm, mesoderm, and visceral endoderm, as well as fetal skin fibroblasts which were then grown as primary explants. At this midgestational stage of development, the yolk sac provides essential functions for the sustenance of the embryo while the complex process of organogenesis is proceeding in the liver, kidney, and gut. Trophoblast giant cells, parietal endoderm, and visceral endoderm all demonstrated low levels of unscheduled DNA synthesis consistent with levels measured in adult mouse skin fibroblasts. As has previously been documented, embryonic mouse skin fibroblasts were reproducibly 2- to 3-fold higher than adult mouse skin fibroblasts in levels of DNA excision repair. The extraembryonic mesoderm, however, displayed a statistically significant level of unscheduled DNA synthesis 10-fold higher than adult mouse skin fibroblasts or the other lineages of the midgestation yolk sac. Further, the S-indexes of these lineages were also determined to assess the possible relevance of differential repair to the proliferative status of the cells. These data demonstrate that DNA excision repair capacity is lineage-specific during embryogenesis in the mouse. These studies may begin to provide a context for understanding the perplexing developmental aspects such as the characteristic congenital abnormalities associated with the human heritable DNA repair deficiency diseases.

Prenatal diagnosis of xeroderma pigmentosum and Cockayne syndrome

In a study of fetal cells from a series of 12 pregnancies in ten families at risk for the ultraviolet light-sensitive, DNA repair-deficient diseases xeroderma pigmentosum (XP) and Cockayne syndrome (CS), we detected one XP and two CS homozygote fetuses. The diagnoses were confirmed by analysis of fetal skin fibroblasts or second amniotic samples after termination of the pregnancies. The measurement of ultraviolet light sensitivity and DNA repair depended on properties common to the seven excision repair-deficient XP complementation groups (A-G) and the two CS complementation groups (A, B). No XP variant families were included in the study, because the variant requires different testing techniques. Reliable and rapid diagnosis proved possible in all but one of the 12 pregnancies, supporting the use of these methods until the spectrum of mutations in the various XP and CS genes of the U.S. population is fully characterized and a DNA sequence-based diagnostic procedure becomes available.

Homologous recombination is elevated in some Werner-like syndromes but not during normal in vitro or in vivo senescence of mammalian cells

Werner syndrome (WS) is a recessive genetic condition associated with markedly reduced replicative lifespans of cells in culture, high chromosomal instability in vivo and in vitro, and premature appearance of many characteristics of normal aging, including an increased incidence of cancer. We have monitored plasmid homologous recombination frequencies in diploid fibroblasts from 6 Werner or Werner-like syndrome patients, following transfection with a plasmid substrate containing 2 overlapping fragments of the TN5 Neor gene. Plasmid DNA recovered from these cells was then assayed for homologous recombination by (a) transformation of recA- bacteria to Ampr (indicating total viable plasmid) or Neor (indicating viable recombinant plasmid), and (b) by limited-cycle polymerase chain reaction (PCR) to co-amplify a recombinant fragment containing the overlap region, and a control region of the same plasmid, without bacterial transformation. Bacterial assay data indicated that recombination rates in 3 of the 6 WS strains were significantly elevated above normal controls; 4 of 6 appeared elevated by PCR assay. The highest-recombination WS strain showed evidence of reduced degradation of transfected plasmid DNA. For this small sample of WS strains, clinical severity of WS was not well correlated with recombination rate as determined by either assay (Pearson r = 0.78, not significant, for PCR assay); elevated recombination may, however, define a subset of WS at greatest risk for cancer and/or atherosclerosis. PCR assay of a hyperoxia-resistant HeLa cell line, displaying substantially increased chromosome breakage, indicated increased recombination between direct-repeat fragments. Nevertheless, elevated recombination in WS strains is unlikely to be secondary to impaired replicative capacity characteristic of WS cells, or to defective repair of chromosome damage which is increased in WS, since recombination in non-WS strains was unaffected by passage level or repeated UV irradiation.

Enhanced expression of procollagenase in ataxia-telangiectasia and xeroderma pigmentosum fibroblasts

Ataxia-telangiectasia and xeroderma pigmentosum are human hereditary diseases in which patients are cancer prone and demonstrate increased sensitivity to DNA damage by ionizing and ultraviolet radiation, respectively. In culture, both ataxia-telangiectasia and xeroderma pigmentosum skin fibroblasts show increased synthesis and secretion of the extracellular matrix proteins fibronectin and collagen. To determine whether these differences in protein production result from fundamental abnormalities in regulation of genes associated with cellular interactions, we compared the effects of trifluoperazine and 12-O-tetradecanoylphorbol-13-acetate on expression of the extracellular matrix-degrading metalloproteinases, procollagenase and prostromelysin, by normal, ataxia-telangiectasia, and xeroderma pigmentosum fibroblasts. After trifluoperazine treatment the overall levels of these metalloproteinases were much greater in three ataxia-telangiectasia cell strains and in cells from xeroderma pigmentosum complementation groups A and D than in normal cells. In contrast, cells from xeroderma pigmentosum complementation group C produced only slightly more procollagenase than normal cells. 12-O-tetradecanoylphorbol-13-acetate also induced higher than normal levels of procollagenase in some ataxia-telangiectasia and xeroderma pigmentosum strains, but less than that induced by trifluoperazine. Because increased extracellular accumulation of matrix-degrading enzymes has long been implicated in metastatic progression, this altered expression of procollagenase and prostromelysin in ataxia-telangiectasia and xeroderma pigmentosum cells could play an important role in the pathogenesis of various tumors in individuals with these genetic diseases.

Cell proliferation, cell death and aging

An integrated view of the processes which most likely play a critical role in the aging process at the cellular level is proposed. Cells are continuously exposed to a variety of internal and external stressors, potentially dangerous for the maintenance of the functional integrity of the cell (UV and gamma radiation, heat, oxygen free radicals, glucose, bacteria, viruses). In the course of evolution a number of mechanisms [DNA repair, production of heat shock and other stress proteins, enzymatic and non-enzymatic antioxidant defence systems, poly(ADP-ribose) polymerase activation] have emerged which allow the cell to cope with such a variety of potentially harmful agents. These mechanisms are in fact interconnected and constitute a network of cellular defence systems. It is suggested that they play a physiological role, being involved in the control of gene expression. A failure of these mechanisms does not allow the cell to maintain homeostasis and has profound consequences as far as two of the major programs of the cell are concerned, i.e. cell proliferation and cell death. Recent data suggesting that these are two physiologically active phenomena tightly linked and regulated are examined. Thus, activation of cell cycle related genes and active inhibition of suicide genes appear to be a part of an integrated process. Conversely, deprivation of growth factors seems able to induce an active process of programmed cell death characterized by Ca++,Mg+(+)-dependent endonuclease activity and DNA fragmentation (apoptosis). Similar phenomena have been shown to accompany the terminal differentiation process in several cellular systems. The understanding of the factors which favour or prevent cell death (a phenomenon which has been recognized as one of the most important in fetal development and morphogenesis) will help to unravel and eventually to manipulate the aging process. In an evolutionary perspective, cell senescence appears to be the price paid to avoid unlimited capability of proliferation, i.e. cell transformation and cancer.

Cellular senescence revisited: a review

The field of cellular senescence (cytogerontology) is reviewed. The historical precedence for investigation in this field is summarized, and placed in the context of more recent studies of the regulation of cellular proliferation and differentiation. The now-classical embryonic lung fibroblast model is compared to models utilizing other cell types as well as cells from donors of different ages and phenotypes. Modulation of cellular senescence by growth factors, hormones, and genetic manipulation is contrasted, but newer studies in oncogene involvement are omitted. A current consensus would include the view that the life span of normal diploid cells in culture is limited, is under genetic control, and is capable of being modified. Finally, embryonic cells aging in vitro share certain characteristics with early passage cells derived from donors of increasing age.

DNA repair and replication in xeroderma pigmentosum and related disorders

Xeroderma pigmentosum (XP), ataxia telangiectasia (AT), and Cockayne syndrome (CS) are human diseases that exhibit increased sensitivity to environmental carcinogens [e.g., ultraviolet (UV) light, ionizing radiations, chemicals] because of genetic defects in the patient's capacity to repair and replicate damaged DNA accurately. The major defect in XP is a failure to repair UV damage to DNA; in AT, the failure is in repair or replication of double-strand breaks in DNA; in CS, the failure is in recovery of DNA replication after UV irradiation. Cancer is a major clinical feature of XP and AT, but not of CS. Each disease is complex, with multiple groups defined by complementation in cell-cell hybridization. Overlap is reported between some XP and CS groups. UV-sensitive hamster cell mutants are also known: most of these complement XP groups, and a human gene on chromosome 19 can correct the defects in hamster mutants, but not XP. XP group C is distinct from the other groups in exhibiting a strongly clustered mode of repair, as if only certain regions of the genome can be mended. This mode mainly occurs in confluent group C cells under conditions that permit much greater survival than in exponential growth, and therefore represents a more efficient mode of repair. These diseases all represent important examples of perturbation in the way carcinogen damage in DNA is metabolized, and further research aimed at identifying the kinds of molecular changes involved in the malignancy will be important.