Telomere dysfunction and its role in haematological cancer

Observations in human tumours, as well as mouse models, have indicated that telomere dysfunction may be a key event driving genomic instability and disease progression in many solid tumour types. In this scenario, telomere shortening ultimately results in telomere dysfunction, fusion and genomic instability, creating the large-scale rearrangements that are characteristic of these tumours. It is now becoming apparent that this paradigm may also apply to haematological malignancies; indeed these conditions have provided some of the most convincing evidence of telomere dysfunction in any malignancy. Telomere length has been shown in several malignancies to provide clinically useful prognostic information, implicating telomere dysfunction in disease progression. In these malignancies extreme telomere shortening, telomere dysfunction and fusion have all been documented and correlate with the emergence of increased genomic complexity. Telomeres may therefore represent both a clinically useful prognostic tool and a potential target for therapeutic intervention.     

Telomere dysfunction causes alveolar stem cell failure

Telomere syndromes have their most common manifestation in lung disease that is recognized as idiopathic pulmonary fibrosis and emphysema. In both conditions, there is loss of alveolar integrity, but the underlying mechanisms are not known. We tested the capacity of alveolar epithelial and stromal cells from mice with short telomeres to support alveolar organoid colony formation and found that type 2 alveolar epithelial cells (AEC2s), the stem cell-containing population, were limiting. When telomere dysfunction was induced in adult AEC2s by conditional deletion of the shelterin component telomeric repeat-binding factor 2, cells survived but remained dormant and showed all the hallmarks of cellular senescence. Telomere dysfunction in AEC2s triggered an immune response, and this was associated with AEC2-derived up-regulation of cytokine signaling pathways that are known to provoke inflammation in the lung. Mice uniformly died after challenge with bleomycin, underscoring an essential role for telomere function in AEC2s for alveolar repair. Our data show that alveoloar progenitor senescence is sufficient to recapitulate the regenerative defects, inflammatory responses, and susceptibility to injury that are characteristic of telomere-mediated lung disease. They suggest alveolar stem cell failure is a driver of telomere-mediated lung disease and that efforts to reverse it may be clinically beneficial.Mutations in telomerase and telomere genes cause abnormal telomere shortening. Clinically, this molecular abnormality manifests in a spectrum of telomere syndromes that recapitulate features of age-associated pathology (1). In highly proliferative compartments, such as the bone marrow, telomere dysfunction causes stem cell exhaustion, and hematopoietic stem cell transplantation can reverse this pathology (1). More commonly, short telomeres predispose to adult-onset disease in the lung, a tissue that has slow cell turnover (1). Idiopathic pulmonary fibrosis and emphysema are the most prevalent clinical manifestations of human telomere syndromes and account for more than 80% of presentations (1, 2). The alveolar structures are preferentially affected in these disorders, and their pathology is marked by inflammation and mesenchymal abnormalities (3, 4). Affected patients are also exquisitely sensitive to pulmonary-toxic drugs, which are fatal even when there is no detectable baseline lung disease (1, 5).The mechanisms by which telomere defects provoke lung disease are not understood, but a number of observations have pointed to lung-intrinsic factors and epithelial dysfunction as candidate events (6–10). For example, in telomerase-null mice, DNA damage preferentially accumulates in the air-exposed epithelium after environmentally induced injury, such as with cigarette smoke (7). The additive effect of environmental injury and telomere dysfunction has been suggested to contribute to the susceptibility to emphysema seen in these mice (7). Moreover, humans that carry mutations in the surfactant protein C gene, SFTPC, which is expressed exclusively in type 2 alveolar epithelial cells (AEC2s), develop lung disease phenotypes similar to those seen in telomerase mutation carriers (10–12). Pulmonary fibrosis and emphysema patients have also been noted to have abnormally short telomeres in AEC2s (6, 7, 13). These observations, along with AEC2s’ regenerative capacity (14–16), led us to hypothesize that telomere dysfunction is sufficient to provoke AEC2 failure and that this event drives lung disease pathogenesis.One hurdle to modeling the consequences of telomere dysfunction in a cell type-specific manner is that laboratory mice have very long telomeres (17). In the absence of telomerase, telomere dysfunction can be generated only after several generations of breeding, precluding cell type-specific studies (18). To overcome this limitation, we designed two experimental systems. First we examined the role of telomere shortening in purified AEC2s in a stem cell assay ex vivo. For an in vivo system, we generated a model in which telomere dysfunction can be induced by deleting telomeric repeat-binding factor 2 (Trf2) (19, 20) exclusively in adult AEC2s. Trf2 functions to suppress the DNA damage response, and its loss leads to telomere dysfunction by uncapping, thus allowing cell type-specific studies within a single generation (19, 20). The latter surrogate model allowed us to test the consequences of acquired DNA damage and telomere dysfunction in the adult lung. We show here, in both late-generation telomerase-null mice and in a conditional mutant model, that telomere dysfunction restricted to AEC2s impairs stem cell function by inducing senescence. This program recapitulates the inflammatory responses and susceptibility to injury that are hallmarks of telomere-mediated lung disease.     

Endothelial dysfunction in hypertension

Endothelial cells release both relaxing and contracting factors that modulate vascular smooth muscle tone and also participate in the pathophysiology of essential hypertension. Endothelium-dependent vasodilation is regulated primarily by nitric oxide but also by an unidentified endothelium-derived hyperpolarizing factor and by prostacyclin. Endothelium-derived contracting factors include endothelin-1, vasoconscrictor prostanoids, angiotensin II and superoxide anions. Under physiological conditions, there is a balanced release of relaxing and contracting factors. The balance can be altered in cardiovascular diseases such as hypertension, atherosclerosis, diabetes and other conditions, thereby contributing to further progression of vascular and end-organ damage. In particular, endothelial dysfunction leading to decreased bioavailability of nitric oxide impairs endothelium-dependent vasodilation in patients with essential hypertension and may also be a determinant for the premature development of atherosclerosis. Different mechanisms of reduced nitric oxide activity have been shown both in hypertensive states and several cardiovascular diseases, and endothelial dysfunction is likely to occur prior to vascular dysfunction. Thus, the strategies currently used to improve endothelial dysfunction may result in decreased morbidity and mortality in hypertensive patients.     

Diastolic dysfunction in hypertension

Heart failure is one of the most common causes of cardiovascular morbidity and mortality, and hypertension is the most common cause of cardiac failure. Recent studies have shown that isolated diastolic dysfunction very often accompanies hypertensive heart disease. Ventricular diastolic function may be divided into an active relaxation phase and a passive compliance period. These two components have been investigated invasively, and they remain the gold standards for the study of diastolic function. However, in the routine clinical setting, echocardiographic and Doppler techniques are most useful for evaluating ventricular filling. Thus, analysis of E and A waves of mitral flow have provided important and useful information. Unfortunately, these indices depend on too many factors. Newer indices obtained from ventricular time intervals, tissue Doppler imaging, and color M-mode echocardiography have enhanced the means to assess diastolic function. In addition, new methods including MRI and cine CT have also provided better understanding of left ventricular filling in hypertension. Using these techniques, diastolic dysfunction has been found to be common in patients with hypertension, even before left ventricular hypertrophy is demonstrable and before hypertension in young, normotensive male offspring of hypertensive parents has developed. Furthermore, it has been made clear recently that myocardial ischemia and fibrosis are two important factors associated with diastolic dysfunction in hypertension.     

Telomere dysfunction alters the chemotherapeutic profile of transformed cells

Telomerase inhibition has been touted as a novel cancer-selective therapeutic goal based on the observation of high telomerase levels in most cancers and the importance of telomere maintenance in long-term cellular growth and survival. Here, the impact of telomere dysfunction on chemotherapeutic responses was assessed in normal and neoplastic cells derived from telomerase RNA null (mTERC(-/-)) mice. Telomere dysfunction, rather than telomerase per se, was found to be the principal determinant governing chemosensitivity specifically to agents that induced double-stranded DNA breaks (DSB). Enhanced chemosensitivity in telomere dysfunctional cells was linked to therapy-induced fragmentation and multichromosomal fusions, whereas telomerase reconstitution restored genomic integrity and chemoresistance. Loss of p53 function muted the cytotoxic effects of DSB-inducing agents in cells with telomere dysfunction. Together, these results point to the combined use of DSB-inducing agents and telomere maintenance inhibition as an effective anticancer therapeutic approach particularly in cells with intact p53-dependent checkpoint responses.     

Telomere dysfunction and cell cycle checkpoints in hematopoietic stem cell aging

Stem cells are believed to be closely associated with tissue degeneration during aging. Studies of human genetic diseases and gene-targeted animal models have provided evidence that functional decline of telomeres and deregulation of cell cycle checkpoints contribute to the aging process of tissue stem cells. Telomere dysfunction can induce DNA damage response via key cell cycle checkpoints, leading to cellular senescence or apoptosis depending on the tissue type and developmental stage of a specific stem cell compartment. Telomerase mutation and telomere shortening have been observed in a variety of hematological disorders, such as dyskeratosis congenital, aplastic anemia, myelodysplastic syndromes and leukemia, in which the hematopoietic stem cells (HSC) are a major target during the pathogenesis. Moreover, telomere dysfunction is able to induce both cell-intrinsic checkpoints and environmental factors limiting the self-renewal capacity and differentiation potential of HSCs. Crucial components in the cascade of DNA damage response, including ataxia telangiectasia mutated, CHK2, p53, p21 and p16/p19^ARF, play important roles in HSC maintenance and self-renewal in the scenarios of both sufficient telomere reserve and dysfunctional telomere. Therefore, a further understanding of the molecular mechanisms underlying HSC aging may help identity new therapeutic targets for stem cell-based regenerative medicine.     

Telomere dysfunction as a cause of genomic instability in Werner syndrome

Werner syndrome (WS) is a rare human premature aging disease caused by mutations in the gene encoding the RecQ helicase WRN. In addition to the aging features, this disorder is marked by genomic instability, associated with an elevated incidence of cancer. Several lines of evidence suggest that telomere dysfunction is associated with the aging phenotype of the syndrome; however, the origin of the genomic instability observed in WS cells and the reason for the high incidence of cancer in WS have not been established. We previously proposed that WRN helicase activity was necessary to prevent dramatic telomere loss during DNA replication. Here we demonstrate that replication-associated telomere loss is responsible for the chromosome fusions found in WS fibroblasts. Moreover, using metaphase analysis we show that telomere elongation by telomerase can significantly reduce the appearance of new chromosomal aberrations in cells lacking WRN, similar to complementation of WS cells with WRN. Our results suggest that the genome instability in WS cells depends directly on telomere dysfunction, linking chromosome end maintenance to chromosomal aberrations in this disease.     

Endothelial dysfunction in arterial hypertension

Systemic arterial pressure is a dynamic and reactive physiological parameter depending on a great many factors. The endothelial cells of the vascular system are responsible for many biochemical reactions maintaining vascular homeostasis and therefore arterial pressure. Arterial hypertension, atherosclerosis and endothelial dysfunction constitute risk factors increasing morbidity and mortality of cardiovascular origin. These three elements are closely related and frequently act simultaneously damaging different organs. In this paper we review the physiology of the endothelium and the probable consequences of endothelial dysfunction on the pathophysiology of arterial pressure.     

Endothelial dysfunction in hypertension.

Hypertension is an important risk factor for the development of atherosclerosis and endothelial dysfunction may be a key mechanism. The nitric oxide mediated vasodilator function of the endothelium has been widely studied as a test of endothelial integrity in patients with hypertension and impaired basal and muscarinic agonist stimulated components of nitric oxide mediated vascular tone have been found in coronary, forearm and cutaneous resistance vessels and in coronary and forearm conduit vessels. The underlying abnormalities of these changes are unknown but it is likely to be a secondary phenomenon due to increased blood pressure. However, endothelial dysfunction as assessed by response to muscarinic agonists does not occur in all patients and antihypertensive therapy so far has been mostly unable to reverse it. Although widely used the response to acetylcholine has some shortcomings and it remains to be established what parameter best reflects endothelial dysfunction. The relationship of such abnormalities to the present or future atherosclerosis needs to be defined.     

线粒体功能障碍与高血压

高血压发病率在全球范围内呈逐年上升趋势,全面深入研究高血压发病机制已成为医学界共识。与线粒体功能障碍有关的供能不足、氧化损伤、信号传导异常和线粒体基因突变是高血压产生的危险因素。了解线粒体功能损伤与高血压的关系将为高血压的研究和治疗提供新的思路。本文从线粒体功能、线粒体功能障碍以及线粒体功能障碍与高血压的关系三个角度做详细阐述。     

肺血管内皮功能紊乱与肺动脉高压

肺动脉高压是以肺血管阻力进行性增加并伴有不可逆的血管重塑为特征的疾病.越来越多的研究表明,肺血管内皮功能紊乱在肺动脉高压形成的起始及发展阶段都起着重要作用.本文就肺血管内皮功能紊乱在肺动脉高压发病机制中的研究进展作一综述.     

Treatment of diastolic dysfunction in hypertension

Diastolic dysfunction is present in half of patients with hypertension and has been shown to be associated with increased cardiovascular morbidity and mortality, as well as the development of heart failure. With the high prevalence of hypertension and its associated complications, treatment of diastolic dysfunction in hypertension is an important and desirable goal. Angiotensin converting enzyme inhibitors and angiotensin receptor blockers have been shown to be effective in improvement of measures of diastolic function and are recommended as first-line agents in the control of hypertension in patients with diastolic heart failure. Beta-blockers, calcium channel blockers, and diuretics have also shown some efficacy in improved indices of diastolic filling. However, the independent impact of these pharmacologic interventions on prognosis and outcome in diastolic dysfunction has yet to be clarified. The Irbesartan in Heart Failure with Preserved Ejection Fraction (I-PRESERVE) study, Candesartan in Heart Failure: Assessment in Reduction of Mortality and Morbidity (CHARM-Preserved) trial and the Losartan Intervention For End-point Reduction in Hypertension (LIFE) Study all failed to show improved morbidity and mortality with these drugs although, the LIFE study showed reduced heart failure hospitalization in hypertensive patients with normal in-treatment diastolic function. The Trial Of Preserved Cardiac function heart failure with an Aldosterone anTagonist (TOPCAT) is an on-going large, international study evaluating the effect of spironolactone on cardiovascular mortality, aborted cardiac arrest, or hospitalization for diastolic heart failure. This and other studies will provide further insight into the pathophysiology and management of patients with diastolic dysfunction.     

Telomere dysfunction in peripheral blood mononuclear cells from patients with primary biliary cirrhosis

BackgroundChromosomal instability in peripheral blood mononuclear cells has a role in the onset of primary biliary cirrhosis. We hypothesized that patients with primary biliary cirrhosis may harbour telomere dysfunction, with consequent chromosomal instability and cellular senescence.AimTo evaluate the clinical significance of telomerase activity and telomere length in peripheral blood mononuclear cells from patients with primary biliary cirrhosis.Study designIn this population-based case control study, 48 women with primary biliary cirrhosis (25 with cirrhosis), 12 with chronic hepatitis C matched by age and severity of disease, and 55 age-matched healthy women were identified. Mononuclear cells from the peripheral blood of patients and controls were isolated. Telomere length and telomerase activity were measured.ResultsTelomere length and telomerase activity did not differ between cases (5.9 ± 1.5 kb) and controls (6.2 ± 1.4 kb, p_c = 0.164). Telomere shortening and advanced-stage disease strongly correlated with telomerase activity. Patients with advanced disease retained significantly less telomerase activity than those with early-stage disease (0.6 ± 0.9 OD vs. 1.5 ± 3.7 OD, p = 0.03). Telomere loss correlated with age, suggesting premature cellular ageing in patients with primary biliary cirrhosis.ConclusionOur data strongly support the telomere hypothesis of human cirrhosis, indicating that telomere shortening and telomerase activity represent a molecular mechanism in the evolution of human cirrhosis in a selected population of patients.