Telomere Dysfunction and DNA Damage Checkpoints in Diseases and Cancer of the Gastrointestinal Tract

Abstract

Aging occurs after the reproductive phase of life has been completed. The aging process is associated with a progressive decline in organ maintenance and an increasing risk of diseases, particularly cancer. A current hypothesis indicates that aging is not programmed, but rather is the result of stochastic interaction consisting of the injury and repair processes competing with other organismal functions—such as learning, motor activity, and reproduction—for energy resources that are necessary to maintain the integrity of molecular structures.1Kirkwood T.B. Understanding the odd science of aging.Cell. 2005; 120: 437-447Google Scholar According to this model, the failure of cellular and tissue maintenance and repair results from integrated actions among genes, the environment, and intrinsic defects of the organism. This finally leads to an accumulation of molecular damage. Among the various types of molecular damage, the accumulation of nuclear DNA damage seems to be an important factor of human aging.2Jiang H. Schiffer E. Song Z. et al.Proteins induced by telomere dysfunction and DNA damage represent biomarkers of human aging and disease.Proc Natl Acad Sci U S A. 2008; 105: 11299-11304Google Scholar Different mechanisms may contribute to this accumulation, including oxidative metabolites, such as reactive oxygen species (ROS), irradiation, and telomere shortening. The accumulation of molecular damage may not be linear, but might harbor exponential kinetics over a lifetime. Increasing levels of molecular damage ultimately leads to defects that limit the survival of cells by the induction of processes such as senescence, apoptosis, and autophagy. Nevertheless, the relative contribution of different cellular outcomes to the aging process remains to be clarified. Moreover, cellular defects can cause environmental alterations, including inflammatory reactions that can, in turn, exacerbate existing damage and influence the aging process. This review summarizes experimental data indicating that telomere dysfunction and DNA damage checkpoints can contribute to gastrointestinal (GI) diseases and cancers. In the 1930s, it was first postulated that chromosome ends are important for the maintenance of chromosomal stability. In the 1980s, it was demonstrated that the terminal DNA sequences of chromosomes, that were named telomeres, consist of simple DNA repeats (TTAGGG in humans).3Blackburn E.H. Gall J.G. A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena.J Mol Biol. 1978; 120: 33-53Google Scholar, 4Moyzis R.K. Buckingham J.M. Cram L.S. et al.A highly conserved repetitive DNA sequence, (TTAGGG)n, present at the telomeres of human chromosomes.Proc Natl Acad Sci U S A. 1988; 85: 6622-6626Google Scholar Experiments performed in yeast show that the addition of telomere repeats was sufficient to stabilize artificial chromosomes, thus proving the concept that telomeres are required for the maintenance of chromosome stability. Telomeres shorten with each round of cell division because of (i) the inability of the DNA polymerase to fully replicate the very end of linear DNA (“end-replication problem”) and (ii) the processing of telomeres during the cell cycle, which is required for the formation of 3-dimensional telomere structures, for example, T-loops or g-quadruplexes.5Griffith J.D. Comeau L. Rosenfield S. et al.Mammalian telomeres end in a large duplex loop.Cell. 1999; 97: 503-514Google Scholar, 6McElligott R. Wellinger R.J. The terminal DNA structure of mammalian chromosomes.Embo J. 1997; 16: 3705-3714Google Scholar In human fibroblasts, telomeres shorten by 50–100 base pairs per cell division.7Allsopp R.C. Vaziri H. Patterson C. et al.Telomere length predicts replicative capacity of human fibroblasts.Proc Natl Acad Sci U S A. 1992; 89: 10114-10118Google Scholar, 8Wright W.E. Shay J.W. The two-stage mechanism controlling cellular senescence and immortalization.Exp Gerontol. 1992; 27: 383-389Google Scholar Telomere shortening limits the proliferative capacity of human fibroblasts to 50–70 cell divisions7Allsopp R.C. Vaziri H. Patterson C. et al.Telomere length predicts replicative capacity of human fibroblasts.Proc Natl Acad Sci U S A. 1992; 89: 10114-10118Google Scholar before the cells finally undergo a permanent growth arrest, also known as replicative senescence. Critically short telomeres can no longer cap the chromosome ends; these “telomere-free ends” lead to the activation of DNA damage responses involving the formation of DNA damage foci, activation of ATR/ATM kinases, phosphorylation of downstream kinases (Chk1 and Chk2), and activation of the p53 signaling pathway (for review, see Nalapareddy et al9Nalapareddy K. Jiang H. Guachalla Gutierrez L.M. et al.Determining the influence of telomere dysfunction and DNA damage on stem and progenitor cell aging: what markers can we use?.Exp Gerontol. 2008; 43: 998-1004Google Scholar). These responses to telomere dysfunction are very similar to those responses triggered by DNA double-strand breaks, which makes it difficult to distinguish dysfunctional telomeres from intrachromosomal DNA damage and to determine the relative contribution of both types of damage to organismal aging.9Nalapareddy K. Jiang H. Guachalla Gutierrez L.M. et al.Determining the influence of telomere dysfunction and DNA damage on stem and progenitor cell aging: what markers can we use?.Exp Gerontol. 2008; 43: 998-1004Google Scholar The activation of DNA damage pathways leads to the induction of cell cycle arrest (senescence), apoptosis, and possibly autophagy (Figure 1). The induction of senescence depends on the function of p53 and its downstream target p21.10Brown J.P. Wei W. Sedivy J.M. Bypass of senescence after disruption of p21CIP1/WAF1 gene in normal diploid human fibroblasts.Science. 1997; 277: 831-834Google Scholar The deletion of either component sufficed to abrogate senescence arrest of late-passage fibroblasts with critically short telomeres. In addition, there is evidence that the p16/Rb checkpoint contributes to the senescence arrest.11Stein G.H. Drullinger L.F. Soulard A. et al.Differential roles for cyclin-dependent kinase inhibitors p21 and p16 in the mechanisms of senescence and differentiation in human fibroblasts.Mol Cell Biol. 1999; 19: 2109-2117Crossref Scopus (579) Google Scholar Cells with impaired p53 checkpoint function can continue to divide in the presence of dysfunctional telomeres. However, further telomere shortening ultimately leads to an increase of telomere dysfunction, genetic instability, and p53-independent cell death at a second checkpoint, which is called crisis.8Wright W.E. Shay J.W. The two-stage mechanism controlling cellular senescence and immortalization.Exp Gerontol. 1992; 27: 383-389Google Scholar In comparison with the senescence checkpoint, the molecular pathways that control the induction of crisis are less well defined. In contrast to the sequential induction of 2 mortality stages (senescence and crisis) in fibroblast cultures, telomere dysfunction in mice leads to a parallel activation of cell-cycle arrest and apoptosis in various stem cell compartments.12Lee H.W. Blasco M.A. Gottlieb G.J. et al.Essential role of mouse telomerase in highly proliferative organs.Nature. 1998; 392: 569-574Google Scholar, 13Schaetzlein S. Kodandaramireddy N.R. Ju Z. et al.Exonuclease-1 deletion impairs DNA damage signaling and prolongs lifespan of telomere-dysfunctional mice.Cell. 2007; 130: 863-877Google Scholar These data indicate that checkpoints induced by telomere dysfunction in stem and progenitor cells in vivo may potentially differ from those observed in fibroblast cultures. Normal human fibroblasts cannot bypass the senescence checkpoint spontaneously.14Forsyth N.R. Morales C.P. Damle S. et al.Spontaneous immortalization of clinically normal colon-derived fibroblasts from a familial adenomatous polyposis patient.Neoplasia. 2004; 6: 258-265Google Scholar Moreover, spontaneous escape from crisis is very rare and immortalizing clones that survive the crisis checkpoint always activate mechanisms of telomere stabilization involving either the activation of telomerase or alternative mechanisms that lead to the lengthening of telomeres (ALT).8Wright W.E. Shay J.W. The two-stage mechanism controlling cellular senescence and immortalization.Exp Gerontol. 1992; 27: 383-389Google Scholar, 15Opitz O.G. Suliman Y. Hahn W.C. et al.Cyclin D1 overexpression and p53 inactivation immortalize primary oral keratinocytes by a telomerase-independent mechanism.J Clin Invest. 2001; 108: 725-732Google Scholar Studies on overexpression of telomerase have proven that telomere dysfunction limits cell survival by induction of senescence or crisis: Overexpression and thereby activation of telomerase in primary human cells stabilized telomere length, prevented the induction of senescence16Bodnar A.G. Ouellette M. Frolkis M. et al.Extension of life-span by introduction of telomerase into normal human cells.Science. 1998; 279: 349-352Google Scholar and crisis,17Zhu J. Wang H. Bishop J.M. et al.Telomerase extends the lifespan of virus-transformed human cells without net telomere lengthening.Proc Natl Acad Sci U S A. 1999; 96: 3723-3728Google Scholar and enabled human cells to proliferate indefinitely. Taken together, telomere shortening and the activation of DNA damage checkpoints represent potent tumor suppressor mechanisms that limit the proliferative capacity of human cells.8Wright W.E. Shay J.W. The two-stage mechanism controlling cellular senescence and immortalization.Exp Gerontol. 1992; 27: 383-389Google Scholar However, these mechanisms can contribute to the decline of regenerative reserve and the impairment of tissue maintenance in chronic diseases and aging, and can also affect the GI tract (Figure 1). In concordance with this premise, studies in telomerase-knockout mice (Terc−/−) have revealed that the abrogation of checkpoint genes (Exonuclease-1, p21) can rescue stem cell function, organ maintenance, and increase the lifespan of telomere dysfunctional mice,13Schaetzlein S. Kodandaramireddy N.R. Ju Z. et al.Exonuclease-1 deletion impairs DNA damage signaling and prolongs lifespan of telomere-dysfunctional mice.Cell. 2007; 130: 863-877Google Scholar, 18Choudhury A.R. Ju Z. Djojosubroto M.W. et al.Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation.Nat Genet. 2007; 39: 99-105Google Scholar indicating that these checkpoints could represent targets for therapies that aim to improve organ maintenance and regenerative reserve during chronic diseases and aging. Telomerase is active during human embryogenesis. By contrast, most adult human tissues essentially lack telomerase expression and show significant telomere shortening during aging (for review, see Jiang et al19Jiang H. Ju Z. Rudolph K.L. Telomere shortening and ageing.Z Gerontol Geriatr. 2007; 40: 314-324Google Scholar). Low levels of telomerase activity are present in some stem and progenitor cells in humans. However, the level of telomerase does not suffice to maintain stable chromosomes in human stem cells during aging.20Vaziri H. Dragowska W. Allsopp R.C. et al.Evidence for a mitotic clock in human hematopoietic stem cells: loss of telomeric DNA with age.Proc Natl Acad Sci U S A. 1994; 91: 9857-9860Google Scholar Studies on telomerase-knockout mice have shown that telomere shortening predominantly affects maintenance of those organ systems with high rates of cell turnover, including the hematopoietic system and the intestinal epithelium. These data indicate that telomerase deletion impairs the function of adult stem cells that contribute to tissue maintenance of these organs.12Lee H.W. Blasco M.A. Gottlieb G.J. et al.Essential role of mouse telomerase in highly proliferative organs.Nature. 1998; 392: 569-574Google Scholar, 13Schaetzlein S. Kodandaramireddy N.R. Ju Z. et al.Exonuclease-1 deletion impairs DNA damage signaling and prolongs lifespan of telomere-dysfunctional mice.Cell. 2007; 130: 863-877Google Scholar, 18Choudhury A.R. Ju Z. Djojosubroto M.W. et al.Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation.Nat Genet. 2007; 39: 99-105Google Scholar This concept is supported by direct experimental evidence demonstrating that telomere shortening impairs the function of hematopoietic stem cells in transplantation experiments.18Choudhury A.R. Ju Z. Djojosubroto M.W. et al.Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation.Nat Genet. 2007; 39: 99-105Google Scholar, 21Allsopp R.C. Morin G.B. DePinho R. et al.Telomerase is required to slow telomere shortening and extend replicative lifespan of HSCs during serial transplantation.Blood. 2003; 102: 517-520Google Scholar In addition, there is experimental evidence that telomerase can control stem cell function through mechanisms that are independent of its role in telomere length control.22Sarin K.Y. Cheung P. Gilison D. et al.Conditional telomerase induction causes proliferation of hair follicle stem cells.Nature. 2005; 436: 1048-1052Google Scholar It is well known that telomere length decreases with age in most human tissues, including the GI tract.23O'Sullivan J. Risques R.A. Mandelson M.T. et al.Telomere length in the colon declines with age: a relation to colorectal cancer?.Cancer Epidemiol Biomarkers Prev. 2006; 15: 573-577Google Scholar Critical shortening induces telomere dysfunction, leading to the formation of chromosomal fusions. An increase in chromosomal fusions has been observed in late passage human fibroblasts as well as in telomerase-knockout mice (Terc−/−) with dysfunctional telomeres.24Blasco M.A. Lee H.W. Hande M.P. et al.Telomere shortening and tumor formation by mouse cells lacking telomerase RNA.Cell. 1997; 91: 25-34Google Scholar, 25Ducray C. Pommier J.P. Martins L. et al.Telomere dynamics, end-to-end fusions and telomerase activation during the human fibroblast immortalization process.Oncogene. 1999; 18: 4211-4223Google Scholar When cells with fused chromosomes enter the cell cycle, these fusions are often disrupted during mitosis; anaphase bridges represent a morphologic correlate of this process in vivo.26Rudolph K.L. Millard M. Bosenberg M.W. et al.Telomere dysfunction and evolution of intestinal carcinoma in mice and humans.Nat Genet. 2001; 28: 155-159Google Scholar The disruption of chromosomal fusions during anaphase results in chromosomal gains and losses in the daughter cells. In addition, new telomere-free ends are generated at the breakpoints, resulting in repetitive “fusion–bridge–breakage” cycles and evolution of chromosomal instability. Telomere shortening and the evolution of chromosomal instability could contribute to the increase in cancer initiation that occurs during aging27DePinho R.A. The age of cancer.Nature. 2000; 408: 248-254Google Scholar and in response to chronic disease.28El-Serag H.B. Rudolph K.L. Hepatocellular carcinoma: epidemiology and molecular carcinogenesis.Gastroenterology. 2007; 132: 2557-2576Google Scholar Studies on Terc−/− mice revealed an increase in tumor initiation in mice with dysfunctional telomeres.26Rudolph K.L. Millard M. Bosenberg M.W. et al.Telomere dysfunction and evolution of intestinal carcinoma in mice and humans.Nat Genet. 2001; 28: 155-159Google Scholar, 29Artandi S.E. Chang S. Lee S.L. et al.Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice.Nature. 2000; 406: 641-645Google Scholar, 30Farazi P.A. Glickman J. Jiang S. et al.Differential impact of telomere dysfunction on initiation and progression of hepatocellular carcinoma.Cancer Res. 2003; 63: 5021-5027Google Scholar, 31Rudolph K.L. Chang S. Lee H.W. et al.Longevity, stress response, and cancer in aging telomerase-deficient mice.Cell. 1999; 96: 701-712Google Scholar Of note, the deletion of the p53 checkpoint gene accelerated the initiation of epithelial cancer in these mice.29Artandi S.E. Chang S. Lee S.L. et al.Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice.Nature. 2000; 406: 641-645Google Scholar A possible explanation is that the loss of p53-dependent checkpoints (cell-cycle arrest and/or apoptosis) could contribute to the transformation of epithelial cells by increasing cellular survival and proliferation in the context of telomere dysfunction. In contrast to the increase in cancer initiation, telomere shortening suppresses the progression of cancers in Terc−/− mice.26Rudolph K.L. Millard M. Bosenberg M.W. et al.Telomere dysfunction and evolution of intestinal carcinoma in mice and humans.Nat Genet. 2001; 28: 155-159Google Scholar, 30Farazi P.A. Glickman J. Jiang S. et al.Differential impact of telomere dysfunction on initiation and progression of hepatocellular carcinoma.Cancer Res. 2003; 63: 5021-5027Google Scholar, 32Lechel A. Holstege H. Begus Y. et al.Telomerase deletion limits progression of p53-mutant hepatocellular carcinoma with short telomeres in chronic liver disease.Gastroenterology. 2007; 132: 1465-1475Abstract Full Text Full Text PDF Scopus (52) Google Scholar Tumor suppression was associated with the evolution of very high rates of chromosomal instability.32Lechel A. Holstege H. Begus Y. et al.Telomerase deletion limits progression of p53-mutant hepatocellular carcinoma with short telomeres in chronic liver disease.Gastroenterology. 2007; 132: 1465-1475Abstract Full Text Full Text PDF Scopus (52) Google Scholar These results support a model indicating that telomerase activation is an essential step reducing chromosomal instability in transformed tumor cells at the crisis stage, thus allowing tumor cell survival and tumor progression. According to this model, telomere dysfunction and telomerase reactivation have dual and sequential roles in tumorigenesis.1Telomere shortening induces chromosomal instability and cancer initiation and2Initiated tumors require telomerase activation in order to prevent high levels of chromosomal instability that induce genetic chaos and tumor cell death.Most human cancer cells are characterized by very short telomeres (shorter than the surrounding nontransformed tissue) and an overexpression of telomerase, indicating that a sequence of telomere shortening during tumor initiation and telomerase or ALT activation during tumor progression may occur during human carcinogenesis (Figure 2). Several studies have shown that gastroesophageal reflux disease (GERD) increases the proliferation of esophageal squamous cells.33Livstone E.M. Sheahan D.G. Behar J. Studies of esophageal epithelial cell proliferation in patients with reflux esophagitis.Gastroenterology. 1977; 73: 1315-1319Google Scholar, 34Zhang F. Altorki N.K. Wu Y.C. et al.Duodenal reflux induces cyclooxygenase-2 in the esophageal mucosa of rats: evidence for involvement of bile acids.Gastroenterology. 2001; 121: 1391-1399Abstract Full Text Full Text PDF Scopus (138) Google Scholar Severe GERD is associated with the generation of ROS that causes oxidative DNA damage in the esophageal squamous epithelium.34Zhang F. Altorki N.K. Wu Y.C. et al.Duodenal reflux induces cyclooxygenase-2 in the esophageal mucosa of rats: evidence for involvement of bile acids.Gastroenterology. 2001; 121: 1391-1399Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 35Olliver J.R. Hardie L.J. Dexter S. et al.DNA damage levels are raised in Barrett's oesophageal mucosa relative to the squamous epithelium of the oesophagus.Biomarkers. 2003; 8: 509-521Google Scholar, 36Theisen J. Peters J.H. Fein M. et al.The mutagenic potential of duodenoesophageal reflux.Ann Surg. 2005; 241: 63-68Google Scholar, 37Wetscher G.J. Hinder R.A. Bagchi D. et al.Reflux esophagitis in humans is mediated by oxygen-derived free radicals.Am J Surg. 1995; 170 (discussion 556–557): 552-556Google Scholar The GERD-induced increase in cellular proliferation and oxidative DNA damage would be expected to result in shortened telomeres if the squamous cells were not able to produce sufficient levels of telomerase to counterbalance the loss of telomeric DNA. In fact, GERD-induced injury and the regeneration of esophageal squamous cells do cause a progressive shortening of telomeres. The GERD-induced loss of telomeric DNA eventually reaches a critical level that triggers senescence. Thereby, it interferes with esophageal healing and promotes epithelial repair through alternative mechanisms such as intestinal metaplasia.38Souza R.F. Lunsford T. Ramirez R.D. et al.GERD is associated with shortened telomeres in the squamous epithelium of the distal esophagus.Am J Physiol Gastrointest Liver Physiol. 2007; 293: G19-G24Google Scholar Barrett's esophagus is such a hyperproliferative, columnar epithelial intestinal metaplasia and 1 hypothesis for its initiation is that it replaces normal squamous esophageal epithelium as an alternative mechanism of epithelial repair in chronic GERD.39Winters Jr., C. Spurling T.J. Chobanian S.J. et al.Barrett's esophagus A prevalent, occult complication of gastroesophageal reflux disease.Gastroenterology. 1987; 92: 118-124Google Scholar Through the mechanisms described that involve inflammation, generation of ROS, telomere attrition, and consequently the acquisition of genetic changes, Barrett's esophagus then predisposes to the development of esophageal adenocarcinoma. The progression of Barrett's esophagus to cancer thereby involves histopathologic stages of metaplasia, low- and high-grade dysplasia (intraepithelial neoplasia), and cancer.40Jankowski J.A. Wright N.A. Meltzer S.J. et al.Molecular evolution of the metaplasia-dysplasia-adenocarcinoma sequence in the esophagus.Am J Pathol. 1999; 154: 965-973Google Scholar Genetic changes, for example, in DNA ploidy, p16 and p53, already accumulate in precancerous diseases.41Rabinovitch P.S. Longton G. Blount P.L. et al.Predictors of progression in Barrett's esophagus III: baseline flow cytometric variables.Am J Gastroenterol. 2001; 96: 3071-3083Google Scholar, 42Reid B.J. Blount P.L. Rabinovitch P.S. Biomarkers in Barrett's esophagus.Gastrointest Endosc Clin N Am. 2003; 13: 369-397Google Scholar, 43Reid B.J. Prevo L.J. Galipeau P.C. et al.Predictors of progression in Barrett's esophagus II: baseline 17p (p53) loss of heterozygosity identifies a patient subset at increased risk for neoplastic progression.Am J Gastroenterol. 2001; 96: 2839-2848Google Scholar, 44Wong D.J. Paulson T.G. Prevo L.J. et al.p16(INK4a) lesions are common, early abnormalities that undergo clonal expansion in Barrett's metaplastic epithelium.Cancer Res. 2001; 61: 8284-8289Google Scholar It has been suggested that such changes occur during a process of clonal evolution and neoplastic progression, which is facilitated by factors that increase chromosomal and genetic instability.45Barrett M.T. Sanchez C.A. Prevo L.J. et al.Evolution of neoplastic cell lineages in Barrett oesophagus.Nat Genet. 1999; 22: 106-109Google Scholar In Barrett's esophagus, the evolution of chromosomal instability is related to telomere shortening involving a set of characteristic lesions (gains and losses) at specific chromosomal loci. Because telomere shortening and chromosomal instability occur early in the neoplastic progression of the Barrett's epithelium and are highly variable among patients, it is important to determine whether they identify a subset of patients that is at risk for more rapid neoplastic evolution. Furthermore, these processes may be relevant to a broader spectrum of precancerous lesions in humans, in which telomere shortening has been observed.46Meeker A.K. Hicks J.L. Iacobuzio-Donahue C.A. et al.Telomere length abnormalities occur early in the initiation of epithelial carcinogenesis.Clin Cancer Res. 2004; 10: 3317-3326Google Scholar The data on telomeres and chromosomal instability in GERD and Barrett's cancer stand in line with the described model, indicating that telomere dysfunction and telomerase reactivation have a sequential role in tumor initiation (induction of chromosomal instability by dysfunctional telomeres) and tumor progression (stabilizing of telomeres and chromosomal instability by ALT and/or telomerase reactivation). The development of Barrett's cancer involves a chronic inflammatory state suggesting the inflammatory signaling and ROS may influence cancer formation in the context of telomere dysfunction. However, carcinogenesis in esophageal squamous cells eg, in the upper esophagus and without a chronic inflammatory state seems also to follow the sequence of telomere shortening during tumor initiation and telomere stabilization by ALT and/or telomerase activation during tumor progression. Therefore, a chronic inflammatory process, such as by GERD, seems at least not to be a prerequisite for this process.47Goessel G. Quante M. Hahn W.C. et al.Creating oral squamous cancer cells: a cellular model of oral-esophageal carcinogenesis.Proc Natl Acad Sci U S A. 2005; 102: 15599-15604Google Scholar Liver cirrhosis represents the end stage of a variety of chronic liver diseases that develop after extended latencies of 20–40 years. Pathologists have long recognized that, compared to normal liver cells, hepatocyte proliferation is increased during chronic liver disease, but a decline in liver cell proliferation occurs during cirrhosis.48Delhaye M. Louis H. Degraef C. et al.Relationship between hepatocyte proliferative activity and liver functional reserve in human cirrhosis.Hepatology. 1996; 23: 1003-1011Google Scholar Meanwhile, several studies have shown that telomeres are shorter in cirrhosis compared to precirrhotic disease,49Wiemann S.U. Satyanarayana A. Tsahuridu M. et al.Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.FASEB J. 2002; 16: 935-942Google Scholar suggesting that chronic liver diseases accelerate both the rate of cell turnover and telomere shortening in the liver. Studies of Terc−/− mice have provided the first evidence that telomere shortening can limit the regenerative capacity of the liver in response to injury.50Rudolph K.L. Chang S. Millard M. et al.Inhibition of experimental liver cirrhosis in mice by telomerase gene delivery.Science. 2000; 287: 1253-1258Google Scholar, 51Satyanarayana A. Wiemann S.U. Buer J. et al.Telomere shortening impairs organ regeneration by inhibiting cell cycle re-entry of a subpopulation of cells.Embo J. 2003; 22: 4003-4013Google Scholar These studies have shown that telomere shortening and impaired liver regeneration induce premature activation of stellate cells and an accelerated formation of fibrosis and steatosis in response to chronic liver injury. These data support a new hypothesis, indicating that telomere shortening plays a causal role in cirrhosis progression by impairing liver regeneration and accelerating fibrosis. In agreement with this hypothesis, telomere shortening was detected in human cirrhosis independent of the underlying etiology of liver disease.49Wiemann S.U. Satyanarayana A. Tsahuridu M. et al.Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.FASEB J. 2002; 16: 935-942Google Scholar Telomere shortening was more pronounced in hepatocytes compared with cells in the fibrotic scar, indicating that proliferation of stellate cells occurs during the late stages of the disease when hepatocyte proliferation declines. Consistent with this assumption, senescence-associated β-galactosidase staining was observed in hepatocytes at the cirrhosis stage,49Wiemann S.U. Satyanarayana A. Tsahuridu M. et al.Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.FASEB J. 2002; 16: 935-942Google Scholar accompanied by an activation of DNA damage checkpoints, including the senescence-inducing cell-cycle inhibitor p21.52Lunz 3rd, J.G. Tsuji H. Nozaki I. et al.An inhibitor of cyclin-dependent kinase, stress-induced p21Waf-1/Cip-1, mediates hepatocyte mito-inhibition during the evolution of cirrhosis.Hepatology. 2005; 41: 1262-1271Google Scholar Taken together, these data suggest that telomere dysfunction and the induction of DNA damage pathways contribute to the progression of chronic liver disease and cirrhosis formation. Studies in Terc−/− mice have shown that deletion of DNA damage checkpoints can improve the organ maintenance and lifespan of telomere-dysfunctional mice (as discussed). Whether these approaches can improve liver regeneration in the context of telomere dysfunction without the acceleration of hepatocarcinogenesis remains to be tested in animal models. In that case, the development of DNA damage checkpoint inhibitors could represent a novel therapeutic approach for the treatment of advanced liver disease, especially in clinical cases where a liver transplantation is not a viable option. Recent work has shown that induction of senescence of stellate cells can also have protective effects limiting the progression of liver fibrosis.53Krizhanovsky V. Yon M. Dickins R.A. et al.Senescence of activated stellate cells limits liver fibrosis.Cell. 2008; 134: 657-667Google Scholar However, this mechanism might be limited to acute liver injury; in chronic liver diseases in humans, there was no evidence for senescence of stellate cells.49Wiemann S.U. Satyanarayana A. Tsahuridu M. et al.Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis.FASEB J. 2002; 16: 935-942Google Scholar Liver cirrhosis is the most unifying risk factor for the development of hepatocellular carcinoma (HCC). The risk of HCC development in a normal liver is extremely rare. Conversely, patients with liver cirrhosis have an annual incidence of HCC of up to 7%.28El-Serag H.B. Rudolph K.L. Hepatocellular carcinoma: ep