Cell Reports Report - Cycloastragenol

1 Cell ReportsReportTelomere Length Correlateswith Life Span of Dog BreedsLaura J. Fick,1 Gordon H. Fick,2 Zichen Li,1 Eric Cao,1Bo Bao,1 Doug Heffelfinger,3 Heidi G. Parker,4 Elaine A. Ostrander,4and Karl Riabowol1,*1 Department of Biochemistry and Molecular Biology2 Department of Community Health Sciences, Faculty of MedicineUniversity of Calgary, Calgary, AB T2N 4N1, Canada3 Crestwood Veterinary Clinic, Edmonton, AB T5P 1J9, Canada4 National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892, DNA repeats are lost as normal somaticcells replicate.

2 When telomeres reach a criticallyshort length, a DNA damage signal is initiated,inducing cell senescence. Some studies have indi-cated that telomere length correlates with mortality,suggesting that telomere length contributes tohuman life span; however, other studies Report nocorrelation, and thus the issue remains dogs show parallels in telomere biology tohumans, with similar telomere length, telomere attri-tion, and absence of somatic cell telomerase this model, we find that peripheral bloodmononuclear cell (PBMC) telomere length is a strongpredictor of average life span among 15 differentbreeds (p < ), consistent with telomeres play-ing a role in life span determination.

3 Dogs lose telo-meric DNA 10-fold faster than humans, which issimilar to the ratio of average life spans betweenthese species. Breeds with shorter mean telomerelengths show an increased probability of deathfrom cardiovascular disease, which was previouslycorrelated with short telomere length in T2AG3 DNA repeats are lost in most mammalian celltypes that replicate (Harley et al., 1990), and telomere lengthpredicts the capacity of normal diploid cells in culture to replicate(Allsopp et al., 1992). Oxidative stress (Parrinello et al., 2003) andthe inability of DNA polymerase to replicate the ends of linearDNA molecules (Olovnikov, 1971) play major roles in telomereloss in different species (Richter and von Zglinicki, 2007).

4 When sufficiently short, telomeres initiate a stress responsethat includes activation of ATM (Vaziri et al., 1997) and p53(Atadja et al., 1995), resulting in cell senescence (Kipling et al.,1999; Hemann et al., 2000). This can be reversed by expressionof the enzyme telomerase, a reverse transcriptase that elongatestelomeres (Bodnar et al., 1998; Vaziri and Benchimol, 1998).A previous analysis of banked human blood samples uncov-ered a correlation between short telomere length and increasedprobabilities of mortality from age-associated heart disease(>3-fold) and infectious diseases (>8-fold) (Cawthon et al.)

5 ,2003). Subsequent studies of both monozygotic and dizygotictwins and very elderly populations have yielded conflictingresults (Bakaysa et al., 2007; Bischoff et al., 2006; Cawthonet al., 2003; Chiang et al., 2010; Kimura et al., 2008; Martin-Ruiz et al., 2005), leaving the question of whether telomere lengthaffects life span per se, in the absence of premature mortalityfrom age-associated diseases, unresolved. Use of the best-defined mammalian genetic model, mice, has not helped inves-tigators to address this issue directly, because the dynamics ofrodent telomeres differ significantly from that of human telo-meres (for example, rodents have much longer telomeres andexceedingly short life spans).

6 Furthermore, rodent studies havenot found correlations between telomere length and senescenceor life span in mice and rats that contain wild-type telomerase(Blasco et al., 1997; Hemann and Greider, 2000; Melk et al.,2003), most likely due to the predominant role played by stressessuch as oxidative stress in inducing senescence in rodent cells (Parrinello et al., 2003; von Zglinicki et al., 2003), in contrast totelomeric signals, which are only seen in mice after severalgenerations of telomere loss (Strong et al., 2011).The domestic dog (Canis lupis familiaris) is a well-definedmodel that theoretically can reduce the effects of the intrinsicgenetic diversity of the outbred human population.

7 Dogs have78 acrocentric autosomes, with males having XY and femaleshaving XX sex chromosomes (Parker et al., 2004, 2010). Withintheir genomes, dog breeds on average show an up to 100-foldgreater linkage disequilibrium than humans (Sutter et al., 2004),indicating significant interbreed genetic divergence. This pres-ents an ideal model for genetic analysis because the breedsare genetically isolated and inbred, especially compared withthe relatively outbred human population. Relevant to this study,the average life span of dog breeds also ranges widely, from years for French mastiffs to >14 years for miniature poodles( ).

8 In both dogsand humans, cultured fibroblasts lose telomeric sequence(McKevitt et al., 2002), and telomere length decreases withincreasingdonor age. In addition, telomere length and telome-rase activity in different tissues are comparable between the1530 Cell Reports2, 1530 1536, December 27, 2012 2012 The Authorstwo species (Nasir et al., 2001). These factors indicate that if telo-mere length does influence mortality and life span, as suggestedby some studies of humans, long-lived dog breeds should havesignificantly longer this study, we first confirmed that, as in humans, canineleukocyte telomere length decreases with age (Figure 1A).

9 However, this occurred at a rate of 360 bp/year (p < ) versus20 40 bp/year in humans (Unryn et al., 2005). Also as inhumans, we found that male dogs lose telomeric sequenceslightly faster than females (Unryn et al., 2005; Figure 1A). Themethod we used to measure telomeres allowed us to representthe mean telomere length from quantitative PCR (qPCR) inkilobases for each dog sample, because there is an absolutequantification step inherent in the protocol (O Callaghan and Fe-nech, 2011). The average telomere length determined from 175dogs of 26 breeds varied from kbp in Great Danes to beagles (Figure 1B), with variability noted between and withinbreeds.

10 Some of this variability may be due to differential ratesof telomere loss with age, if dog telomere attrition followspatterns similar to those observed in humans (Frenck et al.,1998; Unryn et al., 2005). Figure 1B represents the box plotdistribution of the data within the 15 breeds for which we hadtelomere measurements for three or more animals. Determina-tion of telomere length by the qPCR method (O Callaghan andFenech, 2011) was followed by age adjustment based on thetelomere attrition rate determined by linear regression of telo-mere length versus age for all dogs inFigure 1A.