Telomere lengths and the complex processes of telomere homeostasis are being researched intensively from a number of viewpoints. However, it has been some time since I have discussed issues related to telomere lengths in this blog. In this Part 1 blog posting, I focus on research relating telomere lengths to disease processes. I address a couple of specific questions: Are shorter telomere lengths predictive of cancers and other disease processes? And, are disease processes or unhealthful body conditions characterized by shorter telomere lengths? As usual the applicable literature is vast and I have had to be highly selective in what I cover. I include only selected controlled human population studies here, saving laboratory studies for subsequent blog entries.
In a Part 2 post, I will review research concerned with lifestyle, dietary, and other factors that appear to be associated with telomere shortening and lengthening. A Part 3 post will be concerned with the molecular biology of telomere length management focused on such subjects as the role of HSP-90 and P23 in regulation of telomere lengthening by telomerase, and implications for healthy aging.
Telomere lengths as predictors of cancers
For some time it has been known that cancer patients tend to have shorter telomere lengths in certain non-cancerous cells (e.g. blood lymphocytes) than correspondingly matched individuals without cancer. It has also been widely thought that abnormally short telomere lengths may increase cancer susceptibilities and, further, their presence could serve as biomarker predictors of cancers.
For example, consider the 2003 paper Telomere dysfunction: a potential cancer predisposition factor. “BACKGROUND: Genetic instability associated with telomere dysfunction (i.e., short telomeres) is an early event in tumorigenesis. We investigated the association between telomere length and cancer risk in four ongoing case-control studies. — METHODS: All studies had equal numbers of case patients and matched control subjects (92 for head and neck cancer, 135 for bladder cancer, 54 for lung cancer, and 32 for renal cell carcinoma). Telomere length was measured in peripheral blood lymphocytes from study participants. Genetic instability was assessed with the comet assay. Patient and disease characteristics were collected and analyzed for associations with risk for these cancers. All statistical tests were two-sided. — RESULTS: Telomeres were statistically significantly shorter in patients with head and neck cancer (6.5 kilobases [kb]) than in control subjects (7.4 kb) (difference = 0.9 kb, 95% confidence interval [CI] = 0.5 to 1.2 kb; P<.001). Nine percent of patients with head and neck cancer were in the longest quartile of telomere length, whereas 59% were in the shortest quartile. Similar patterns were observed for lung, renal cell, and bladder cancer. — CONCLUSION: Short telomeres appear to be associated with increased risks for human bladder, head and neck, lung, and renal cell cancers.” Note that the measurements showing shorter telomeres were in patients who already had cancers. The authors assumed that these patients started out with shorter telomeres and therefore concluded that shorter telomeres could be predictive of cancer risk. An alternative hypothesis is that telomere shortening resulted from the cancer process itself in which case the author’s conclusion would not follow. It would be like saying “Wet puddles on the ground are associated with increased risk of rain.”
Two 2010 studies and a 2009 study cast doubt on the hypothesis that short telomeres are predictive biomarkers for cancers. And one important 2010 study tends to support the same proposition.
The 2010 report Telomere length in blood cells and breast cancer risk: investigations in two case-control studies states “Telomere dysfunction, which leads to genomic instability, is hypothesized to play a causal role in the development of breast cancer. However, the few epidemiologic studies that assessed the relationship between telomere length in blood cells and breast cancer risk have been inconsistent. We conducted two case-control studies to further understand the role of telomere length and breast cancer risk. Overall telomere lengths were measured by telomere quantitative fluorescent in situ hybridization (TQ-FISH) and telomere quantitative real-time PCR (TQ-PCR). The associations between telomere length in blood leukocytes and risk of breast cancer were examined in two breast cancer case-control studies that were conducted at Roswell Park Cancer Institute (RPCI) and Lombardi Comprehensive Cancer Center (LCCC). Using the 50th percentile value in controls as a cut point, women who had shorter telomere length were not at significantly increased risk of breast cancer compared with women who had longer telomere length in the RPCI study (odds ratio [OR] = 1.34, 95% confidence interval [CI] = 0.84-2.12), in the LCCC study (OR = 1.18, 95% CI = 0.73-1.91), or in the combined RPCI and LCCC studies (OR = 1.23, 95% CI = 0.89-1.71). There was no significant dose-response relationship across quartiles of telomere length and no significant difference when comparing women in the lowest to highest quartile of telomere length. Overall telomere length in blood leukocytes was not significantly associated with the risk of breast cancer.”
The second 2010 study casting doubt on the value of telomere lengths for assessing cancer risk has a similar-sounding publication title but is quite different: Telomere length in prospective and retrospective cancer case-control studies. “Previous studies have reported that shorter mean telomere length in lymphocytes was associated with increased susceptibility to common diseases of aging, and may be predictive of cancer risk. However, most analyses have examined retrospectively collected case-control studies. Mean telomere length was measured using high-throughput quantitative real-time PCR. Blood for DNA extraction was collected after cancer diagnosis in the East Anglian SEARCH Breast (2,243 cases and 2,181 controls) and SEARCH Colorectal (2,249 cases and 2,161 controls) studies. Prospective case-control studies were conducted for breast cancer (199 cases) and colorectal cancer (185 cases), nested within the EPIC-Norfolk cohort. Blood was collected at least 6 months prior to diagnosis, and was matched to DNA from two cancer-free controls per case. In the retrospective SEARCH studies, the age-adjusted odds ratios for shortest (Q4) versus longest (Q1) quartile of mean telomere length was 15.5 [95% confidence intervals (CI), 11.6-20.8; p-het = 5.7 x 10(-75)], with a “per quartile” P-trend = 2.1 x 10(-80) for breast cancer; and 2.14 (95% CI, 1.77-2.59; p-het = 7.3 x 10(-15)), with a per quartile P-trend = 1.8 x 10(-13) for colorectal cancer. In the prospective EPIC study, the comparable odds ratios (Q4 versus Q1) were 1.58 (95% CI, 0.75-3.31; p-het = 0.23) for breast cancer and 1.13 (95% CI, 0.54-2.36; p-het = 0.75) for colorectal cancer risk. Mean telomere length was shorter in retrospectively collected cases than in controls but the equivalent association was markedly weaker in the prospective studies. This suggests that telomere shortening largely occurs after diagnosis, and therefore, might not be of value in cancer prediction.”
The 2009 report The association between leukocyte telomere length and cigarette smoking, dietary and physical variables, and risk of prostate cancer states “In comparison to normal tissues, telomeres are shorter in high-grade intraepithelial neoplasia and prostate cancer. We examined prostate cancer risk associated with relative telomere length as determined by quantitative PCR on prediagnostic buffy coat DNA isolated from 612 advanced prostate cancer cases and 1049 age-matched, cancer-free controls from the PLCO Cancer Screening Trial. Telomere length was analyzed as both a continuous and a categorical variable with adjustment for potential confounders. Statistically significant inverse correlations between telomere length, age and smoking status were observed in cases and controls. Telomere length was not associated with prostate cancer risk (at the median, OR = 0.85, 95% CI: 0.67, 1.08); associations were similar when telomere length was evaluated as a continuous variable or by quartiles. The relationships between telomere length and inflammation-related factors, diet, exercise, body mass index, and other lifestyle variables were explored since many of these have previously been associated with shorter telomeres. Healthy lifestyle factors (i.e., lower BMI, more exercise, tobacco abstinence, diets high in fruit and vegetables) tended to be associated with greater telomere length. — This study found no statistically significant association between leukocyte telomere length and advanced prostate cancer risk.”
On the other side of the scale, the study described in the July 2010 JAMA report Telomere Length and Risk of Incident Cancer and Cancer Mortality measured telomere lengths at baseline in a cohort population long before incidences of cancer. There is therefore no possibility of telomere shortening occurring after incidence of cancer. “Objective: To determine the association between baseline telomere length and incident cancer and cancer mortality. Design, Setting, and Participants: Leukocyte telomere length was measured by quantitative polymerase chain reaction in 787 participants free of cancer at baseline in 1995 from the prospective, population-based Bruneck Study in Italy. Main Outcome Measures: Incident cancer and cancer mortality over a follow-up period of 10 years (1995-2005 with a follow-up rate of 100%). Results: A total of 92 of 787 participants (11.7%) developed cancer (incidence rate, 13.3 per 1000 person-years). Short telomere length at baseline was associated with incident cancer independently of standard cancer risk factors (multivariable hazard ratio [HR] per 1-SD decrease in loge-transformed telomere length, 1.60; 95% confidence interval [CI], 1.30-1.98; P < .001). Compared with participants in the longest telomere length group, the multivariable HR for incident cancer was 2.15 (95% CI, 1.12-4.14) in the middle length group and 3.11 (95% CI, 1.65-5.84) in the shortest length group (P < .001). Incidence rates were 5.1 (95% CI, 2.9-8.7) per 1000 person-years in the longest telomere length group, 14.2 (95% CI, 10.0-20.1) per 1000 person-years in the middle length group, and 22.5 (95% CI, 16.9-29.9) per 1000 person-years in the shortest length group. The association equally applied to men and women and emerged as robust under a variety of circumstances. Furthermore, short telomere length was associated with cancer mortality (multivariable HR per 1-SD decrease in loge-transformed telomere length, 2.13; 95% CI, 1.58-2.86; P < .001) and individual cancer subtypes with a high fatality rate. Conclusion: In this study population, there was a statistically significant inverse relationship between telomere length and both cancer incidence and mortality.”
The 2007 publication Leukocyte Telomere Length Predicts Cancer Risk in Barrett’s Esophagus describes a prospective study where telomere length is measured initially in patients with a pre-cancerous condition, Barrett’s esophagus. “Purpose: Leukocyte telomere length has gained attention as a marker of oxidative damage and age-related diseases, including cancer. We hypothesize that leukocyte telomere length might be able to predict future risk of cancer and examined this in a cohort of patients with Barrett’s esophagus, who are at increased risk of esophageal adenocarcinoma and thus were enrolled in a long-term cancer surveillance program. — Patients and Methods: In this prospective study, telomere length was measured by quantitative PCR in baseline blood samples in a cohort of 300 patients with Barrett’s esophagus followed for a mean of 5.8 years. Leukocyte telomere length hazard ratios (HR) for risk of esophageal adenocarcinoma were calculated using multivariate Cox models. — Results: Shorter telomeres were associated with increased esophageal adenocarcinoma risk (age-adjusted HR between top and bottom quartiles of telomere length, 3.45; 95% confidence interval, 1.35-8.78; P = 0.009). This association was still significant when individually or simultaneously adjusted for age, gender, nonsteroidal anti-inflammatory drug (NSAID) use, cigarette smoking, and waist-to-hip ratio (HR, 4.18; 95% confidence interval, 1.60-10.94; P = 0.004). The relationship between telomere length and cancer risk was particularly strong among NSAID nonusers, ever smokers, and patients with low waist-to-hip ratio. — Conclusion: Leukocyte telomere length predicts risk of esophageal adenocarcinoma in patients with Barrett’s esophagus independently of smoking, obesity, and NSAID use. These results show the ability of leukocyte telomere length to predict the risk of future cancer and suggest that it might also have predictive value in other cancers arising in a setting of chronic inflammation.”
Shorter telomeres in cancer patients
Most cancer cells express telomerase and this serves to keep the telomeres in cancer cells long enough so that these cells do not senesce and become virtually immortal. However, studies have established that patients with several kinds of cancer have shorter telomeres than their normal counterparts. Some representative study reports bringing out this point are:
(2008) Short telomeres, telomerase reverse transcriptase gene amplification, and increased telomerase activity in the blood of familial papillary thyroid cancer patients. “RESULTS: RTL (relative telomere length), measured by quantitative PCR, was significantly (P < 0.0001) shorter in the blood of FPTC (familial papillary thyroid cancer) patients, compared with sporadic PTCs, healthy subjects, nodular goiter subjects, and unaffected siblings. Also by fluorescence in situ hybridization analysis, the results confirmed shorter telomere lengths in FPTC patients — CONCLUSION: Our study demonstrates that patients with FPTC display an imbalance of the telomere-telomerase complex in the peripheral blood, characterized by short telomeres, hTERT gene amplification, and expression.”
The study described in the 2007 paper Telomere length, cigarette smoking, and bladder cancer risk in men and women compares telomere lengths in bladder cancer cases and healthy controls. “Truncated telomeres are among the defining characteristics of most carcinomas. — Using quantitative real-time PCR, we measured relative telomere length in bladder cancer cases and healthy controls and evaluated the association between telomere length, cigarette smoking, and bladder cancer risk in a case-control study nested within the Health Professionals Follow-up Study and a case-control study nested within the Nurses’ Health Study. Telomeres were significantly shorter in bladder cancer cases (n = 184) than in controls (n = 192). The mean relative telomere length in cases was 0.23 (SD, 0.16) versus 0.27 (SD, 0.15) in controls (P = 0.001). The adjusted odds ratio for bladder cancer was 1.88 (95% confidence interval, 1.05, 3.36) for individuals in the quartile with the shortest telomeres as compared with individuals in the quartile with the longest telomeres (P(trend) = 0.006). We observed a statistically significant difference in telomere length among men and women (P < 0.001); however, the interaction between gender, telomere length, and bladder cancer risk was not significant. We also observed a significant difference in telomere length across categories of pack-years of smoking (P = 0.01).” However, a conclusion was drawn by the authors that does not appear to be justified by the data: “These findings suggest that truncated telomeres are associated with an increased risk of bladder cancer.” Since the shorter telomeres were observed in patients that actually had bladder cancer it is not clear whether the patients tended to have shorter telomeres to start with, or whether telomere shortening was a consequence of the cancer process.
The 2007 paper Short telomeres in aggressive non-Hodgkin’s lymphoma as a risk factor in lymphomagenesis characterizes how telomeres are shorter in lymphoma cancer patients but again may incorrectly conclude that such shorter telomeres may serve as disease-predictive biomarkers. “OBJECTIVE: Telomeres cap chromosomal ends and help to maintain chromosomal integrity. Telomere shortening may result in chromosomal instability and, ultimately, malignant transformation of cells. It has not been systematically studied whether patients with malignancy have shortened telomeres in their normal, nontransformed cells, which might point to a preexisting disposition for chromosomal instability. — METHODS: We designed an (age-) matched pair analysis that compared telomere length in nonmalignant peripheral leukocytes from previously untreated patients who recently developed an aggressive non-Hodgkin’s lymphoma, with leukocytes from healthy individuals. — RESULTS: Telomere lengths in B and T lymphocytes as well as granulocytes from the patients’ group were significantly shorter than those from age-matched healthy controls. We were able to rule out increased proliferation, telomerase defects, or increased oxidative stress in patients as confounding factors of shortened telomeres.” CONCLUSION: Short telomeres in nontransformed leukocytes may constitute a risk factor for lymphomagenesis.” The conclusion may not be valid since the observation of telomeres in nontransformed leukocytes took place in patients who had already developed an aggressive non-Hodgkin’s lymphoma.
Telomere lengths as predictors of stroke consequences
The 2006 publication Telomere length predicts poststroke mortality, dementia, and cognitive decline looked at telomere lengths in stroke survivors. “Objective:Long-term cognitive development is variable among stroke survivors, with a high proportion developing dementia. Early identification of those at risk is highly desirable to target interventions for secondary prevention. Telomere length in peripheral blood mononuclear cells was tested as prognostic risk marker. – Methods: A cohort of 195 nondemented stroke survivors was followed prospectively from 3 months after stroke for 2 years for cognitive assessment and diagnosis of dementia and for 5 years for survival. Telomere lengths in peripheral blood mononuclear cells were measured at 3 months after stroke by in-gel hybridization. Hazard ratios for survival in relation to telomere length and odds ratios for dementia were estimated using multivariate techniques, and changes in Mini-Mental State Examination scores between baseline and 2 years were related to telomere length using multivariate linear regression. – Results: Longer telomeres at baseline were associated with reduced risk for death (hazard ratio for linear trend per 1,000bp = 0.52; 95% confidence interval, 0.28–0.98; p = 0.04, adjusted for age) and dementia (odds ratio for linear trend per 1,000bp = 0.19; 95% confidence interval, 0.07–0.54; p = 0.002) and less reduction in Mini-Mental State Examination score (p = 0.04, adjusted for baseline score). Interpretation: Telomere length is a prognostic marker for poststroke cognitive decline, dementia, and death.”
Telomere shortening and cardiovascular disease
The 2008 paper Telomere biology in heart failure offers a thoughtful discussion: “Conclusions and perspectives Telomere and telomerase have recently been shown to be associated with cardiovascular disease and its risk factors. Critically short telomeres, changes in telomere-binding proteins, and decreased telomerase activity have all been implicated in the activation of cellular damage pathways, and eventually cellular dysfunction, senescence and apoptosis. It remains to be elucidated whether WBC telomere shortening, which is frequently observed in CHD and CHF is a cause or a consequence of the disease. Future experimental and epidemiological studies to determine telomere length in relation to cardiac function will contribute to our understanding of the role of telomeres in cardiovascular disease and might open up new avenues for risk stratification and interventions.”
The 2010 publication Telomere length trajectory and its determinants in persons with coronary artery disease: longitudinal findings from the heart and soul study illustrates the complexity of telomere biology and why simplistic correlations of telomere lengths with disease states may not work. “Leukocyte telomere length, an emerging marker of biological age, has been shown to predict cardiovascular morbidity and mortality. However, the natural history of telomere length in patients with coronary artery disease has not been studied. We sought to investigate the longitudinal trajectory of telomere length, and to identify the independent predictors of telomere shortening, in persons with coronary artery disease. — METHODOLOGY/PRINCIPAL FINDINGS: In a prospective cohort study of 608 individuals with stable coronary artery disease, we measured leukocyte telomere length at baseline, and again after five years of follow-up. We used multivariable linear and logistic regression models to identify the independent predictors of leukocyte telomere trajectory. Baseline and follow-up telomere lengths were normally distributed. Mean telomere length decreased by 42 base pairs per year (p<0.001). Three distinct telomere trajectories were observed: shortening in 45%, maintenance in 32%, and lengthening in 23% of participants. The most powerful predictor of telomere shortening was baseline telomere length (OR per SD increase = 7.6; 95% CI 5.5, 10.6). Other independent predictors of telomere shortening were age (OR per 10 years = 1.6; 95% CI 1.3, 2.1), male sex (OR = 2.4; 95% CI 1.3, 4.7), and waist-to-hip ratio (OR per 0.1 increase = 1.4; 95% CI 1.0, 2.0). — CONCLUSIONS/SIGNIFICANCE: Leukocyte telomere length may increase as well as decrease in persons with coronary artery disease. Telomere length trajectory is powerfully influenced by baseline telomere length, possibly suggesting negative feedback regulation. Age, male sex, and abdominal obesity independently predict telomere shortening. The mechanisms and reversibility of telomeric aging in cardiovascular disease deserve further study.”
Telomere lengths, insulin resistance and obesity
Two interesting 2010 studies look at Arab populations. The very-recent October 2010 study Adiposity and insulin resistance correlate with telomere length in middle-aged Arabs: the influence of circulating adiponectin reports: “OBJECTIVE: Studies in obesity have implicated adipocytokines in the development of insulin resistance, which in turn may lead to accelerated aging. In this study, we determined associations of chromosomal telomere length (TL) to markers of obesity and insulin resistance in middle-aged adult male and female Arabs with and without diabetes mellitus type 2 (DMT2). — DESIGN AND METHODS: One hundred and ninety-three non-diabetic and DMT2 subjects without complications (97 males and 96 females) participated in this cross-sectional study. Clinical data, as well as fasting blood samples, were collected. — RESULTS: Circulating chromosomal leukocyte TL had significant inverse associations with body mass index (BMI), systolic blood pressure, fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR), low-density lipoprotein (LDL)- and total cholesterol, ANG II and hsCRP levels. Adiponectin, BMI, systolic blood pressure, and LDL cholesterol predicted 47% of the variance in TL (P<0.0001). HOMA-IR was the most significant predictor for TL in males, explaining 35% of the variance (P=0.01). In females, adiponectin accounted for 28% of the variance in TL (P=0.01). — CONCLUSION: Obesity and insulin resistance are associated with chromosomal TL among adult Arabs. Evidence of causal relations needs further investigation. The positive association of adiponectin to TL has clinical implications as to the possible protective effects of this hormone from accelerated aging.” Yet again, what is cause and what is effect is unclear.
The July 2010 study Telomere length in relation to insulin resistance, inflammation and obesity among Arab youth reports “ AIM: The aim of this study was to determine the associations of telomere length to markers of obesity, insulin resistance and inflammation in Saudi children. — METHODS: A total of 69 boys and 79 girls, aged 5-12 years, participated in this cross-sectional study. Anthropometrics were measured. Serum glucose and lipid profile were measured using routine laboratory methods. Serum insulin, leptin, adiponectin, resistin, tumour necrosis factor-alpha and active plasminogen activator inhibitor 1 were quantified using customized multiplex assay kits. C-reactive protein and angiotensin II were quantified using ELISA. Leucocyte telomere length was examined by quantitative real time PCR utilizing IQ cycler. —RESULTS: Mean telomere length was significantly shorter in obese boys compared with their lean counterparts (p = 0.049), not in girls. It was not associated to insulin resistance, adipocytokines and markers of inflammation. In girls, the significant predictor of telomere length was waist circumference, explaining 24% of variance (p = 0.041) while in boys, systolic blood pressure explained 84% of the variance (p = 0.01). — CONCLUSION: Childhood obesity in boys corresponds to shorter leucocyte telomere length which is not evident in girls. The association of leucocyte telomere length to blood pressure and waist circumference in children suggests clinical implications as to the contribution of these parameters in premature ageing.”
If you have a subscription to the Annals of the New York Academy of Sciences and want to explore more on this subject, you could read the September 2010 article Telomeres and life histories: the long and the short of it.
My take on these sometimes-contradictory studies is that:
· The extent to which telomere shortening is cause or consequence of a disease process is an open question not only for several cancers but also for other conditions like coronary heart disease and obesity. Much further clarification is needed.
· In general, shorter telomeres appear to be associated with the presence of diseases and conditions of poor health or health risk like smoking, poor diet or obesity and with advancing age.
· Most likely, initial lengths of blood cell telomeres are predictive of both cancer risk, cancer mortality and even overall mortality in the most general sense.
· The value of short telomeres as a biomarker of cancer risk may depend critically on the type of cancer or precancerous condition. E.g. The indicator could be valuable in the case of Barrett’s esophagus but not be useful for predicting susceptibility for breast and colorectal cancers.
· Although several cancers are associated with shorter telomeres in healthy tissues, this does not appear to be the case for all cancers, e.g. prostate cancer.
· For some special conditions such as prediction of stroke consequences, telomere length appears to be a useful biomarker.
· Simplistic correlations of telomere lengths with disease states or progression for prognostic or diagnostic purposes may be misleading or not work. And even in some people with serious diseases, telomeres may grow longer over a period of years as well as get shorter. Many causes operate to determine telomere lengths. Part 3 of this mini-series of blog posts will be concerned with some of the molecular biological mechanisms for telomere length homeostasis.
· There appears to be a significant amount of research going on related to telomere lengths, and new results can be expected to appear frequently. Hopefully, this research will clarify some of the existing confusion as to the value of telomere lengths as a predictive biomarker.
· My current opinion is that there is so much variability associated with telomere lengths that for most disease processes a telomere length biomarker will be useful only in a patterned combination with other predictive biomarkers, probably several of them.