Shorter telomere length in Europeans than in Africans due to polygenetic adaptation

The telomere world and aging: Analytical challenges and future perspectives

Telomeres, the terminal nucleoprotein structures of eukaryotic chromosomes, play pleiotropic functions in cellular and organismal aging. Telomere length (TL) varies throughout life due to the influence of genetic factors and to a complex balancing between "shortening" and "elongation" signals. Telomerase, the only enzyme that can elongate a telomeric DNA chain, and telomeric repeat-containing RNA (TERRA), a long non-coding RNA involved in looping maintenance, play key roles in TL during life. Despite recent advances in the knowledge of TL, TERRA and telomerase activity (TA) biology and their measurement techniques, the experimental and theoretical issues involved raise a number of problems that should carefully be considered by researchers approaching the "telomere world". The increasing use of such parameters - hailed as promising clinically relevant biomarkers - has failed to be paralleled by the development of automated and standardized measurement technology. Consequently, associating given TL values to specific pathological conditions involves on the one hand technological issues and on the other clinical-biological issues related to the planning of clinically relevant association studies. Addressing these issues would help avoid major biases in association studies involving TL and a number of outcomes, especially those focusing on psychological and bio-behavioral variables. The main challenge in telomere research is the development of accurate and reliable measurement methods to achieve simple and sensitive TL, TERRA, and TA detection. The discovery of the localization of telomeres and TERRA in cellular and extracellular compartments had added an additional layer of complexity to the measurement of these age-related biomarkers. Since combined analysis of TL, TERRA and TA may well provide more exhaustive clinical information than a single parameter, we feel it is important for researchers in the various fields to become familiar with their most common measurement techniques and to be aware of the respective merits and drawbacks of these approaches.

Reflections on telomere dynamics and ageing-related diseases in humans

Epidemiological studies have principally relied on measurements of telomere length (TL) in leucocytes, which reflects TL in other somatic cells. Leucocyte TL (LTL) displays vast variation across individuals—a phenomenon already observed in newborns. It is highly heritable, longer in females than males and in individuals of African ancestry than European ancestry. LTL is also longer in offspring conceived by older men. The traditional view regards LTL as a passive biomarker of human ageing. However, new evidence suggests that a dynamic interplay between selective evolutionary forces and TL might result in trade-offs for specific health outcomes. From a biological perspective, an active role of TL in ageing-related human diseases could occur because short telomeres increase the risk of a category of diseases related to restricted cell proliferation and tissue degeneration, including cardiovascular disease, whereas long telomeres increase the risk of another category of diseases related to increased proliferative growth, including major cancers. To understand the role of telomere biology in ageing-related diseases, it is essential to expand telomere research to newborns and children and seek further insight into the underlying causes of the variation in TL due to ancestry and geographical location.

This article is part of the theme issue ‘Understanding diversity in telomere dynamics’.

Paternal Age and Transgenerational Telomere Length Maintenance: A Simulation Model

Telomere length (TL) in offspring is positively correlated with paternal age at the time of the offspring conception. The paternal-age-at-conception (PAC) effect on TL is puzzling, and its biological implication at the population level is unknown. Using a probabilistic model of transgenerational TL and population dynamics, we simulated the effect of PAC on TL in individuals over the course of 1,000 years. Findings suggest a key role for an isometric PAC midpoint (PACmp) in modulating TL across generations, such that offspring conceived by males younger than the isometric PACmp have comparatively short telomeres, while offspring conceived by males older than the isometric PACmp have comparatively long telomeres. We further show that when cancer incidence escalates, the average PAC drops below the isometric PACmp and transgenerational adaptation to cancer ensues through TL shortening. We propose that PAC serves to maintain an optimal TL across generations.

Leukocyte Telomere Length Reflects Prenatal Stress Exposure, But Does Not Predict Atopic Dermatitis Development at 1 Year

Purpose

Prenatal maternal stress affects offspring's atopic dermatitis (AD) development, which is thought to be mediated by the oxidative stress. We aimed to evaluate the difference in leukocyte telomere length (LTL), a marker for exposure to oxidative stress, according to the prenatal stress exposure and the later AD development.

Methods

From a birth cohort (the COhort for Childhood Origin of Asthma and allergic diseases) that had displayed a good epidemiologic association between the exposure to prenatal stress and AD development in the offspring, we selected 68 pairs of samples from 4 subject groups based on the level of prenatal maternal stress and later AD development. The LTL was measured from both cord blood and 1-year peripheral blood, and their LTLs were compared between subject groups. Finally, the proportion of AD development was examined in the subject groups that are reclassified based on subjects' exposure to prenatal stress and there LTL.

Results

Cord-blood LTL was shorter in prenatally stressed infants than in unstressed ones (P = 0.026), which difference was still significant when subjects became 1 year old (P = 0.008). LTL of cord blood, as well as one of the 1-year peripheral blood, was not different according to later AD development at 1 year (P = 0.915 and 0.174, respectively). Shorter LTL made no increase in the proportion of later AD development in either prenatally high-stressed or low-stressed groups (P = 1.000 and 0.473, respectively).

Conclusions

Cord-blood LTL may reflect subjects' exposure to maternal prenatal stress. However, the LTL shortening is not a risk factor of increasing AD development until the age of 1, and a longer investigation may be necessary for validation. Currently, the results doubt the role of LTL shortening as a marker for risk assessment tool for the prenatal stress associated with AD development in the offspring.

The contribution of parent-to-offspring transmission of telomeres to the heritability of telomere length in humans

Leukocyte telomere length (LTL) is a heritable trait with two potential sources of heritability (h²): inherited variation in non-telomeric regions (e.g., SNPs that influence telomere maintenance) and variability in the lengths of telomeres in gametes that produce offspring zygotes (i.e., "direct" inheritance). Prior studies of LTL h² have not attempted to disentangle these two sources. Here, we use a novel approach for detecting the direct inheritance of telomeres by studying the association between identity-by-descent (IBD) sharing at chromosome ends and phenotypic similarity in LTL. We measured genome-wide SNPs and LTL for a sample of 5069 Bangladeshi adults with substantial relatedness. For each of the 6318 relative pairs identified, we used SNPs near the telomeres to estimate the number of chromosome ends shared IBD, a proxy for the number of telomeres shared IBD (T_shared). We then estimated the association between T_shared and the squared pairwise difference in LTL ((ΔLTL)²) within various classes of relatives (siblings, avuncular, cousins, and distant), adjusting for overall genetic relatedness (ϕ). The association between T_shared and (ΔLTL)² was inverse among all relative pair types. In a meta-analysis including all relative pairs (ϕ > 0.05), the association between T_shared and (ΔLTL)² (P = 0.01) was stronger than the association between ϕ and (ΔLTL)² (P = 0.43). Our results provide strong evidence that telomere length (TL) in parental germ cells impacts TL in offspring cells and contributes to LTL h² despite telomere "reprogramming" during embryonic development. Applying our method to larger studies will enable robust estimation of LTL h² attributable to direct transmission of telomeres.

Shorter telomere length in schizophrenia: Evidence from a real-world population and meta-analysis of most recent literature

Schizophrenia is a severe, chronic mental disorder. Schizophrenia is visualized as an accelerated cellular aging syndrome characterized by early onset of cardiovascular disease causing premature mortality. In human aging involves alterations in telomere length (TL). To investigate the presence of TL shortening in schizophrenia and psychiatric syndromes associated, this condition was studied in leukocytes (LTL) of a sample of patients suffering from schizophrenia and other psychotic disorders, and compared with a group of non-psychiatric controls. We explored the relationship between LTL and age, gender, and smoking habit with the aim to control whether these potential confounding factors may influence the rate of telomeres shortening. We also performed a new comprehensive meta-analysis including studies on LTL in schizophrenia patients compared to healthy subjects published in the last two years and the results of the present study. Our results suggest that a diagnosis of schizophrenia, more than gender, age, cigarette smoking or alcohol drinking, is the most important condition responsible of the LTL shortening. A strong LTL shortening was observed in patients affected by schizophrenia, Schizoaffective disorder, and Psychosis not otherwise specified when they were younger than 50 years, while in the group of older subjects no major differences were observed. Additional evidence supporting the causal link of schizophrenia with accelerated telomeres shortening came from the analysis of the updated meta-analysis. The availability of a personalized profile of mechanistic pathways, risk factors, and clinical features may pose the basis for a rehabilitative treatment addressing individual needs of the psychiatric patients.

Early Senescence and Leukocyte Telomere Shortening in SCHIZOPHRENIA: A Role for Cytomegalovirus Infection?

Schizophrenia is a severe, chronic mental disorder characterized by delusions and hallucinations. Several evidences support the link of schizophrenia with accelerated telomeres shortening and accelerated aging. Thus, schizophrenia patients show higher mortality compared to age-matched healthy donors. The etiology of schizophrenia is multifactorial, involving genetic and environmental factors. Telomere erosion has been shown to be accelerated by different factors including environmental factors such as cigarette smoking and chronic alcohol consumption or by psychosocial stress such as childhood maltreatment. In humans, telomere studies have mainly relied on measurements of leukocyte telomere length and it is generally accepted that individuals with short leukocyte telomere length are considered biologically older than those with longer ones. A dysregulation of both innate and adaptive immune systems has been described in schizophrenia patients and other mental diseases supporting the contribution of the immune system to disease symptoms. Thus, it has been suggested that abnormal immune activation with high pro-inflammatory cytokine production in response to still undefined environmental agents such as herpesviruses infections can be involved in the pathogenesis and pathophysiology of schizophrenia. It has been proposed that chronic inflammation and oxidative stress are involved in the course of schizophrenia illness, early onset of cardiovascular disease, accelerated aging, and premature mortality in schizophrenia. Prenatal or neonatal exposures to neurotropic pathogens such as Cytomegalovirus or Toxoplasma gondii have been proposed as environmental risk factors for schizophrenia in individuals with a risk genetic background. Thus, pro-inflammatory cytokines and microglia activation, together with genetic vulnerability, are considered etiological factors for schizophrenia, and support that inflammation status is involved in the course of illness in schizophrenia.

Short Telomere Length as a Biomarker Risk of Lung Cancer Development Induced by High Radon Levels: A Pilot Study

Long-term exposure to radon has been determined to be the second leading cause of lung cancer after tobacco smoking. However, an in-depth study of this topic has not been explicitly carried out in Chiang Mai (Thailand). This paper presents the results of an indoor radon level measurement campaign in dwellings of Chiang Mai using total of 110 detectors (CR-39) during one year. The results show that the average radon levels varied from 35 to 219 Bq/m³, with an overall average of 57 Bq/m³. The finding also shows that the average value is higher than the global average value of 39 Bq/m³. In addition, to examine the cause of lung cancer development among people with risk of chronic exposure to radon during their lifetime, 35 non-smoker lung cancer patients and 33 healthy nonsmokers were analyzed for telomere length. As expected, telomere length was significantly shorter in lung cancer patients than in healthy nonsmokers. Among healthy nonsmokers, the telomere length was significantly shorter in a high radon group than in an unaffected low radon group. To the best of our knowledge, our research provides the first attempt in describing the shortened telomeres in areas with high levels of environmental radon that might be related to lung cancer development.

Genetic Determinants of Telomere Length in African American Youth

Telomere length (TL) is associated with numerous disease states and is affected by genetic and environmental factors. However, TL has been mostly studied in adult populations of European or Asian ancestry. These studies have identified 34 TL-associated genetic variants recently used as genetic proxies for TL. The generalizability of these associations to pediatric populations and racially diverse populations, specifically of African ancestry, remains unclear. Furthermore, six novel variants associated with TL in a population of European children have been identified but not validated. We measured TL from whole blood samples of 492 healthy African American youth (children and adolescents between 8 and 20 years old) and performed the first genome-wide association study of TL in this population. We were unable to replicate neither the 34 reported genetic associations found in adults nor the six genetic associations found in European children. However, we discovered a novel genome-wide significant association between TL and rs1483898 on chromosome 14. Our results underscore the importance of examining genetic associations with TL in diverse pediatric populations such as African Americans.

Global Inequities in Precision Medicine and Molecular Cancer Research

Precision medicine based upon molecular testing is heralded as a revolution in how cancer is prevented, diagnosed, and treated. Large efforts across the world aim to conduct comprehensive molecular profiling of disease to inform preclinical models, translational research studies and clinical trials. However, most studies have only been performed in patients from high-income countries. As the burden on non-communicable diseases increases, cancer will become a pressing burden across the world, disproportionately affecting low-middle income settings. There is emerging evidence that the molecular landscape of disease differs geographically and by genetic ancestry, which cannot be explained by environmental factors alone. There is a lack of good quality evidence that characterises the molecular landscape of cancers found in low-middle income countries. As cancer medicine becomes increasingly driven by molecular alterations in high-income settings, low-income settings may become left behind. Further efforts on an international scale must be made by researchers, funders, and policymakers to ensure cancer research addresses disease across the world, so models are not limited to subtypes of disease found in high-income countries. In this review, we discuss differences found in the molecular profiles of tumours worldwide and the implication this has for the future of global cancer care. Finally, we identify several barriers currently limiting progress in this field and innovative solutions, which may address these shortcomings.

Maternal human telomerase reverse transcriptase variants are associated with preterm labor and preterm premature rupture of membranes

Objective

Premature aging and short telomere lengths of fetal tissues are associated with spontaneous preterm labor (PTL) and preterm premature rupture of membranes (pPROM). Maintenance of telomere length is performed by the enzyme telomerase. Human telomerase reverse transcriptase (hTERT) is a subunit of telomerase, and its dysfunction affects telomere shortening. This study assessed whether maternal or fetal genetic variations in the hTERT gene are associated with PTL or pPROM.

Methods

A case (PTL or pPROM) control (term birth) genetic association study was conducted in 654 non-Hispanic white mothers (438 term, 162 PTL, 54 pPROM) and 502 non-Hispanic white newborns (346 term, 116 PTB, 40 pPROM). Maternal and fetal DNA samples were genotyped for 23 single nucleotide polymorphisms (SNPs) within the hTERT gene. Allele frequencies were compared between cases and controls, stratified by PTL and pPROM. Maternal and fetal data were analyzed separately.

Results

Allelic differences in one SNP of hTERT (rs2853690) were significantly associated with both PTL (adjusted OR 2.24, 95%CI 1.64–3.06, p = 2.32e-05) and with pPROM (adjusted OR 7.54, 95%CI 3.96–14.33, p = 2.39e-07) in maternal DNA. There was no significant association between the hTERT SNPs analyzed and PTL or pPROM in the fetal samples.

Conclusion

hTERT polymorphisms in fetal DNA do not associate with PTL or pPROM risk; however, maternal genetic variations in hTERT may play a contributory role in risk of PTL and PPROM.

Beginning at the ends: telomeres and human disease

Studies of rare and common illnesses have led to remarkable progress in the understanding of the role of telomeres (nucleoprotein complexes at chromosome ends essential for chromosomal integrity) in human disease. Telomere biology disorders encompass a growing spectrum of conditions caused by rare pathogenic germline variants in genes encoding essential aspects of telomere function. Dyskeratosis congenita, a disorder at the severe end of this spectrum, typically presents in childhood with the classic triad of abnormal skin pigmentation, nail dystrophy, and oral leukoplakia, accompanied by a very high risk of bone marrow failure, cancer, pulmonary fibrosis, and other medical problems. In contrast, the less severe end of the telomere biology disorder spectrum consists of middle-age or older adults with just one feature typically seen in dyskeratosis congenita, such as pulmonary fibrosis or bone marrow failure. In the common disease realm, large-scale molecular epidemiology studies have discovered novel associations between illnesses, such as cancer, heart disease, and mental health, and both telomere length and common genetic variants in telomere biology genes. This review highlights recent findings of telomere biology in human disease from both the rare and common disease perspectives. Multi-disciplinary collaborations between clinicians, basic scientists, and epidemiologist are essential as we seek to incorporate new telomere biology discoveries to improve health outcomes.

Neighborhood Disadvantage and Telomere Length: Results from the Fragile Families Study

Telomeres are repetitive nucleotide sequences located at the ends of chromosomes that protect genetic material. We use data from the Fragile Families and Child Wellbeing Study to analyze the relationship between exposure to spatially concentrated disadvantage and telomere length for white and black mothers. We find that neighborhood disadvantage is associated with shorter telomere length for mothers of both races. This finding highlights a potential mechanism through which the unique spatially concentrated disadvantage faced by African Americans contributes to racial health disparities. We conclude that equalizing the health and socioeconomic status of black and white Americans will be very difficult without reducing levels of residential segregation in the United States.

Local adaptation in European populations affected the genetics of psychiatric disorders and behavioral traits

Background

Recent studies have used genome-wide data to investigate evolutionary mechanisms related to behavioral phenotypes, identifying widespread signals of positive selection. Here, we conducted a genome-wide investigation to study whether the molecular mechanisms involved in these traits were affected by local adaptation.

Methods

We performed a polygenic risk score analysis in a sample of 2455 individuals from 23 European populations with respect to variables related to geo-climate diversity, pathogen diversity, and language phonological complexity. The analysis was adjusted for the genetic diversity of European populations to ensure that the differences detected would reflect differences in environmental exposures.

Results

The top finding was related to the association between winter minimum temperature and schizophrenia. Additional significant geo-climate results were also observed with respect to bipolar disorder (sunny daylight), depressive symptoms (precipitation rate), major depressive disorder (precipitation rate), and subjective well-being (relative humidity). Beyond geo-climate variables, we also observed findings related to pathogen diversity and language phonological complexity: openness to experience was associated with protozoan diversity; conscientiousness and extraversion were associated with language consonants.

Conclusions

We report that common variation associated with psychiatric disorders and behavioral traits was affected by processes related to local adaptation in European populations.

Electronic supplementary material

The online version of this article (10.1186/s13073-018-0532-7) contains supplementary material, which is available to authorized users.

A Simple Test Identifies Selection on Complex Traits

Important traits in agricultural, natural, and human populations are increasingly being shown to be under the control of many genes that individually contribute only a small proportion of genetic variation. However, the majority of modern tools in quantitative and population genetics, including genome-wide association studies and selection-mapping protocols, are designed to identify individual genes with large effects. We have developed an approach to identify traits that have been under selection and are controlled by large numbers of loci. In contrast to existing methods, our technique uses additive-effects estimates from all available markers, and relates these estimates to allele-frequency change over time. Using this information, we generate a composite statistic, denoted [Formula: see text] which can be used to test for significant evidence of selection on a trait. Our test requires pre- and postselection genotypic data but only a single time point with phenotypic information. Simulations demonstrate that [Formula: see text] is powerful for identifying selection, particularly in situations where the trait being tested is controlled by many genes, which is precisely the scenario where classical approaches for selection mapping are least powerful. We apply this test to breeding populations of maize and chickens, where we demonstrate the successful identification of selection on traits that are documented to have been under selection.

Sperm telomere length in donor samples is not related to ICSI outcome

PURPOSE

Variations in sperm telomere length (STL) have been associated with altered sperm parameters, poor embryo quality, and lower pregnancy rates, but for normozoospermic men, STL relevance in IVF/ICSI is still uncertain. Moreover, in all studies reported so far, each man's STL was linked to the corresponding female partner characteristics. Here, we study STL in sperm donor samples, each used for up to 12 women, in order to isolate and determine the relationship between STL and reproductive outcomes.

METHODS

Relative STL was determined by qPCR in 60 samples used in a total of 676 ICSI cycles. Univariable and multivariable statistical analyses were used to study the STL effect on fertilization rate; embryo morphology; biochemical, clinical, and ongoing pregnancy rates; and live birth (LB) rates.

RESULTS

The average STL value was 4.5 (relative units; SD 1.9; range 2.4-14.2). Locally weighted scatterplot smoothing regression and the rho-Spearman test did not reveal significant correlations between STL and the outcomes analyzed. STL was not different between cycles resulting or not in pregnancy and LB (Mann-Whitney U test, p > 0.05). No significant effect of STL on reproductive outcomes was found, with the OR for each unit increase in STL (95% CI) of 0.94 (0.86-1-04), 0.99 (0.9-1.09), 0.98 (0.89-1.09), and 0.93 (0.8-1.06) for biochemical, clinical, and ongoing pregnancy and LB, respectively. The multilevel analysis confirmed that the effect of STL on fertilization; biochemical, clinical, and ongoing pregnancy; and LB was not significant (p > 0.05).

CONCLUSION

After addressing STL independently from female variables, results show that STL measurement is not useful to predict reproductive outcomes in ICSI cycles using donor semen.

Telomere Homeostasis: Interplay with Magnesium

Telomere biology, a key component of the hallmarks of ageing, offers insight into dysregulation of normative ageing processes that accompany age-related diseases such as cancer. Telomere homeostasis is tightly linked to cellular metabolism, and in particular with mitochondrial physiology, which is also diminished during cellular senescence and normative physiological ageing. Inherent in the biochemistry of these processes is the role of magnesium, one of the main cellular ions and an essential cofactor in all reactions that use ATP. Magnesium plays an important role in many of the processes involved in regulating telomere structure, integrity and function. This review explores the mechanisms that maintain telomere structure and function, their influence on circadian rhythms and their impact on health and age-related disease. The pervasive role of magnesium in telomere homeostasis is also highlighted.

Telomeres, Aging and Exercise: Guilty by Association?

Telomeres are repetitive tandem DNA sequences that cap chromosomal ends protecting genomic DNA from enzymatic degradation. Telomeres progressively shorten with cellular replication and are therefore assumed to correlate with biological and chronological age. An expanding body of evidence suggests (i) a predictable inverse association between telomere length, aging and age-related diseases and (ii) a positive association between physical activity and telomere length. Both hypotheses have garnered tremendous research attention and broad consensus; however, the evidence for each proposition is inconsistent and equivocal at best. Telomere length does not meet the basic criteria for an aging biomarker and at least 50% of key studies fail to find associations with physical activity. In this review, we address the evidence in support and refutation of the putative associations between telomere length, aging and physical activity. We finish with a brief review of plausible mechanisms and potential future research directions.

Genome-wide association study of telomere length among South Asians identifies a second RTEL1 association signal

Background

Leucocyte telomere length (TL) is a potential biomarker of ageing and risk for age-related disease. Leucocyte TL is heritable and shows substantial differences by race/ethnicity. Recent genome-wide association studies (GWAS) report ~10 loci harbouring SNPs associated with leucocyte TL, but these studies focus primarily on populations of European ancestry.

Objective

This study aims to enhance our understanding of genetic determinants of TL across populations.

Methods

We performed a GWAS of TL using data on 5075 Bangladeshi adults. We measured TL using one of two technologies (qPCR or a Luminex-based method) and used standardised variables as TL phenotypes.

Results

Our results replicate previously reported associations in the TERC and TERT regions (P=2.2×10⁻⁸ and P=6.4×10⁻⁶, respectively). We observed a novel association signal in the RTEL1 gene (intronic SNP rs2297439; P=2.82×10⁻⁷) that is independent of previously reported TL-associated SNPs in this region. The minor allele for rs2297439 is common in South Asian populations (≥0.25) but at lower frequencies in other populations (eg, 0.07 in Northern Europeans). Among the eight other previously reported association signals, all were directionally consistent with our study, but only rs8105767 (ZNF208) was nominally significant (P=0.003). SNP-based heritability estimates were as high as 44% when analysing close relatives but much lower when analysing distant relatives only.

Conclusions

In this first GWAS of TL in a South Asian population, we replicate some, but not all, of the loci reported in prior GWAS of individuals of European ancestry, and we identify a novel second association signal at the RTEL1 locus.

Accumulative effects of indoor air pollution exposure on leukocyte telomere length among non-smokers

Indoor air pollution is an important environmental factor that contributes to the burden of various diseases. Long-term exposure to ambient air pollution is associated with telomere shortening. However, the association between chronic indoor air pollution from household fuel combustion and leukocyte telomere length has not been studied. In our study, 137 cancer-free non-smokers were recruited. Their exposure levels to indoor air pollution from 1985 to 2014 were assessed using a face-to-face interview questionnaire, and leukocyte telomere length (LTL) was measured using a monochrome multiplex quantitative PCR method. Accumulative exposure to solid fuel usage for cooking was negatively correlated with LTL. The LTL of residents who were exposed to solid fuel combustion for three decades (LTL = 0.70 ± 0.17) was significantly shorter than that of other populations. In addition, education and occupation were related to both exposure to solid fuel and LTL. Sociodemographic factors may play a mediating role in the correlation between leukocyte telomere length and environmental exposure to indoor air pollution. In conclusion, long-term exposure to indoor air pollution may cause LTL dysfunction.

Cardiac telomere length in heart development, function, and disease

Telomeres are repetitive nucleoprotein structures at chromosome ends, and a decrease in the number of these repeats, known as a reduction in telomere length (TL), triggers cellular senescence and apoptosis. Heart disease, the worldwide leading cause of death, often results from the loss of cardiac cells, which could be explained by decreases in TL. Due to the cell-specific regulation of TL, this review focuses on studies that have measured telomeres in heart cells and critically assesses the relationship between cardiac TL and heart function. There are several lines of evidence that have identified rapid changes in cardiac TL during the onset and progression of heart disease as well as at critical stages of development. There are also many factors, such as the loss of telomeric proteins, oxidative stress, and hypoxia, that decrease cardiac TL and heart function. In contrast, antioxidants, calorie restriction, and exercise can prevent both cardiac telomere attrition and the progression of heart disease. TL in the heart is also indicative of proliferative potential and could facilitate the identification of cells suitable for cardiac rejuvenation. Although these findings highlight the involvement of TL in heart function, there are important questions regarding the validity of animal models, as well as several confounding factors, that need to be considered when interpreting results and planning future research. With these in mind, elucidating the telomeric mechanisms involved in heart development and the transition to disease holds promise to prevent cardiac dysfunction and potentiate regeneration after injury.

Telomeres and the natural lifespan limit in humans

An ongoing debate in demography has focused on whether the human lifespan has a maximal natural limit. Taking a mechanistic perspective, and knowing that short telomeres are associated with diminished longevity, we examined whether telomere length dynamics during adult life could set a maximal natural lifespan limit. We define leukocyte telomere length of 5 kb as the 'telomeric brink', which denotes a high risk of imminent death. We show that a subset of adults may reach the telomeric brink within the current life expectancy and more so for a 100-year life expectancy. Thus, secular trends in life expectancy should confront a biological limit due to crossing the telomeric brink.

Mutations, Cancer and the Telomere Length Paradox

Individuals with short telomeres should be at increased risk for cancer, since short telomeres lead to genomic instability - a hallmark of cancer. However, individuals with long telomeres also display an increased risk for major cancers, thus creating a cancer-telomere length (TL) paradox. The two-stage clonal expansion model we propose is based on the thesis that a series of mutational hits (1st Hit) at the stem-cell level generates a clone with replicative advantage. A series of additional mutational hits (2nd Hit) transforms the expanding clone into cancer. By proposing that the 1st Hit is largely telomere length-independent, while the 2nd Hit is largely TL-dependent, we resolve the paradox, highlighting a regulatory role of telomeres in cancer.

Widespread signatures of positive selection in common risk alleles associated to autism spectrum disorder

The human brain is the outcome of innumerable evolutionary processes; the systems genetics of psychiatric disorders could bear their signatures. On this basis, we analyzed five psychiatric disorders, attention deficit hyperactivity disorder, autism spectrum disorder (ASD), bipolar disorder, major depressive disorder, and schizophrenia (SCZ), using GWAS summary statistics from the Psychiatric Genomics Consortium. Machine learning-derived scores were used to investigate two natural-selection scenarios: complete selection (loci where a selected allele reached fixation) and incomplete selection (loci where a selected allele has not yet reached fixation). ASD GWAS results positively correlated with incomplete-selection (p = 3.53*10⁻⁴). Variants with ASD GWAS p<0.1 were shown to have a 19%-increased probability to be in the top-5% for incomplete-selection score (OR = 1.19, 95%CI = 1.11–1.8, p = 9.56*10⁻⁷). Investigating the effect directions of minor alleles, we observed an enrichment for positive associations in SNPs with ASD GWAS p<0.1 and top-5% incomplete-selection score (permutation p<10⁻⁴). Considering the set of these ASD-positive-associated variants, we observed gene-expression enrichments for brain and pituitary tissues (p = 2.3*10⁻⁵ and p = 3*10⁻⁵, respectively) and 53 gene ontology (GO) enrichments, such as nervous system development (GO:0007399, p = 7.57*10⁻¹²), synapse organization (GO:0050808, p = 8.29*10⁻⁷), and axon guidance (GO:0007411, p = 1.81*10⁻⁷). Previous genetic studies demonstrated that ASD positively correlates with childhood intelligence, college completion, and years of schooling. Accordingly, we hypothesize that certain ASD risk alleles were under positive selection during human evolution due to their involvement in neurogenesis and cognitive ability.

Predisposition to psychiatric disorders is due to the contribution of many genes involved in numerous molecular mechanisms. Since brain evolution has played a pivotal role in determining the success of the human species, the molecular pathways involved with the onset of mental illnesses are likely to be informative as we seek an understanding of the mechanisms involved in the evolution of human brain. Accordingly, we tested whether the genetics of psychiatric disorders is enriched for signatures of positive selection. We observed a strong finding related to the genetics of autism spectrum disorders (ASD): common risk alleles are enriched for genomic signatures of incomplete selection (loci where a selected allele has not yet reached fixation). The genes where these alleles map tend to be expressed in brain and pituitary tissues, to be involved in molecular mechanisms related to nervous system development, and surprisingly, to be associated with increased cognitive ability. Previous studies identified signatures of purifying selection in genes affected by ASD rare alleles. Accordingly, at least two different evolutionary mechanisms appear to be present in relation to ASD genetics: 1) rare disruptive alleles eliminated by purifying selection; 2) common alleles selected for their beneficial effects on cognitive skills. This scenario would explain ASD prevalence, which is higher than that expected for a trait under purifying selection, as the evolutionary cost of polygenic adaptation related to cognitive ability.

Less Than Ideal Cardiovascular Health Is Associated With Shorter Leukocyte Telomere Length: The National Health and Nutrition Examination Surveys, 1999-2002

Background

The associations between individual cardiovascular disease risk factors and leukocyte telomere length (LTL) have been inconclusive. We investigated the association between LTL and overall cardiovascular health (CVH) as defined by the American Heart Association and whether the association is modified by sex and race/ethnicity.

Methods and Results

We included 5194 adults (aged ≥20) from the National Health and Nutrition Examination Survey 1999–2002. CVH was defined as a composite score of the 7 metrics (smoking, physical activity, diet, body mass index, blood pressure, total cholesterol, and fasting blood glucose) and categorized as “poor,” “intermediate,” and “ideal.” LTL was assayed from whole blood using the quantitative polymerase chain reaction method relative to standard reference DNA. Multivariable linear regression models were used to estimate the association between CVH and log‐transformed LTL. We found strong graded association between CVH and LTL in the overall sample, with evidence of dose‐response relationship (P for trend=0.013). Individuals with poor and intermediate CVH had significantly shorter LTL than individuals with ideal CVH (−3.4% [95% CI=−6.0%, −0.8%] and −2.4% [−4.4%, −0.3%], respectively), after adjustment for demographic variables, socioeconomic status, and C‐reactive protein. The association was stronger in women (−6.6% [−10.2%, −2.9%] for poor vs ideal CVH) and non‐Hispanic whites (−4.3% [−7.1%, −1.4%] for poor vs ideal CVH).

Conclusions

The findings suggest that less‐than‐ideal CVH is associated with shorter LTL, but this association varies by sex and race/ethnicity. Future longitudinal research is needed to elucidate the mechanisms that underlie the association between CVH and LTL.

Maternal Social Disadvantage and Newborn Telomere Length in Archived Dried Blood Spots from the Michigan Neonatal Biobank

Telomeres are the protective caps at the ends of eukaryotic chromosomes. Short telomere length is associated with morbidity and mortality among adults and may mark the biological impact of social experiences. Using archived dried blood spots from the Michigan Neonatal Biobank, this study examined markers of maternal social disadvantage (educational attainment, receipt of public assistance, marital status, and race/ethnicity) from linked birth certificates as predictors of telomere length at birth in a sample of 192 singleton neonates born to non-Hispanic black, non-Hispanic white, and Latina mothers aged 20-35 years. Consistent with two recent studies in newborns, but counter to the idea that maternal social disadvantage is associated with shorter offspring telomere length, we found that infants born to black mothers had longer telomeres than those born to white mothers (b = 0.12, SE = 0.06, p = .05). However, black/white differences in newborn telomere length varied by receipt of public assistance. Among newborns whose mothers received WIC and/or Medicaid, there were no significant black/white differences in telomere length (b = 0.09, SE = 0.08, p = .25). In contrast, among those whose mothers did not receive public assistance-just 6 out of 69 infants born to black mothers versus 41 out of 69 infants born to white mothers-we found that babies born to black mothers had longer telomere length than babies born to white mothers (b = 0.37, SE = 0.16, p = .03). The interaction between black race/ethnicity and receipt of public assistance did not reach the conventional threshold for statistical significance (b = -0.22, SE = 0.15, p = .13), suggesting that this finding may be due to chance. No other markers of maternal social disadvantage were related to infant telomere length. Although replication of these results in a larger sample with more infants born to black mothers with relatively high socioeconomic status is needed, this study offers preliminary support for the hypothesis that race/ethnic differences in newborn telomere length depend on social context.

Telomere Length and the Cancer-Atherosclerosis Trade-Off

Modern humans, the longest-living terrestrial mammals, display short telomeres and repressed telomerase activity in somatic tissues compared with most short-living small mammals. The dual trait of short telomeres and repressed telomerase might render humans relatively resistant to cancer compared with short-living small mammals. However, the trade-off for cancer resistance is ostensibly increased age-related degenerative diseases, principally in the form of atherosclerosis. In this communication, we discuss (a) the genetics of human telomere length, a highly heritable complex trait that is influenced by genetic ancestry, sex, and paternal age at conception, (b) how cancer might have played a role in the evolution of telomere biology across mammals, (c) evidence that in modern humans telomere length is a determinant (rather than only a biomarker) of cancer and atherosclerosis, and (d) the potential influence of relatively recent evolutionary forces in fashioning the variation in telomere length across and within populations, and their likely lasting impact on major diseases in humans. Finally, we propose venues for future research on human telomere genetics in the context of its potential role in shaping the modern human lifespan.