Telomere length measurement in UK Biobank: advancing understanding of biological ageing and age-related diseases

Why do some individuals develop age-associated diseases such as coronary artery disease (CAD) early while others go through their entire life without being affected? The answer to this question is profoundly important for both understanding these diseases as well as healthy ageing. Because of their age-association, we have proposed that these diseases, at least to some extent, are manifestations of accelerated biological (as distinct from chronological) ageing.

In this project, we will investigate this hypothesis by measuring a marker of biological ageing called telomere length (TL). Telomeres are structures that cap our chromosomes at each end. People are born with telomeres of different lengths and as cells divide a small amount of telomere is lost each time. Furthermore, the amount lost is known to be increased by some of the same factors that influence the risk of some age-related diseases. Importantly, cells "sense" their TL and become senescent and subsequently die when TL reaches a critically short value. Thus, TL acts as a biological clock that determines cell behaviour and lifespan. Translating this to a whole person, those born with shorter telomeres or whose telomeres attrition faster may "biologically" age more quickly than others. In turn, this could explain why some people develop specific age-associated diseases at an earlier age while others can remain healthy to an advanced age despite similar life exposures.

In support of our hypothesis, in previous work we and others have shown an association between shorter mean TL in DNA from blood cells (leucocytes) (LTL) and risk of several chronic diseases, including CAD. Other studies have shown correlations between LTL and diet, life-style characteristics and disease risk factors such as obesity and smoking. However, many of the findings require further definitive evidence especially of a prospective nature i.e. whether variation in LTL predicts future occurrence of age-related disease.

UK Biobank provides an ideal cohort to undertake such research. It comprises 500,000 participants, aged 45-69 years at enrolment, recruited UK-wide. A vast amount of information has been collected on each participant including data on diet, lifestyle, behaviour and risk factors and on existing diseases. By linkage to a variety of health-related records, information is also being collected on new diseases as they occur. Each participant is having their DNA typed using a genome-wide array.

Using a method that we have optimised to achieve high throughput, we propose to measure LTL of all 500,000 UK Biobank participants. Standardised measurement of LTL in all participants in one go will enable a wide-range of hypotheses to be explored and provide a long-term enhancement of UK Biobank whose value will only increase with time as more disease outcomes accrue. Furthermore, we will be taking advantage of a one-off opportunity that has presented itself from an ongoing genotyping project in UK Biobank. By using the residual DNA samples left over from this project to measure LTL, we will avoid the substantial costs of further DNA extraction, quantification and arraying that would otherwise be required.

Using the data we will:
(i) Identify personal, environmental and life-style factors that correlate with LTL, which may be determinants of LTL.
(ii) Characterise associations of LTL with cardiovascular traits/risk factors as well as with selected cardiovascular disease outcomes that occur after enrolment into the study.
(iii) Test whether LTL is likely to be a causative factor in cardiovascular diseases using genetic methods, which, if positive, would support the biological ageing hypothesis and potentially open up new strategies for prevention and treatment.
(iv) Make the data widely available to enable other researchers to test the association of LTL with other age-associated diseases, such as dementia and cancers, and with the likelihood of healthy ageing and longevity.

Mean leucocyte telomere length (LTL) will be measured using a well-established quantitative PCR assay which compares the amplification of a telomere length PCR product (T) against a PCR product (S) of a reference single copy gene to produce a T/S ratio. We have extensively validated the assay and confirmed its reliability. To enable a faster and more cost-effective throughput we have developed a multiplex assay which allows amplification and quantification of both T and S in the same reaction to achieve an average weekly throughput of ~5000 samples per week (ie, completion of LTL assays for 500,000 participants within 3 years).

We will characterise any independent cross-sectional associations of LTL with the following characteristics:

- personal (age, gender, ethnicity, birth weight) and lifestyle and environmental factors (e.g. diet, smoking, physical activity) available in UK Biobank.
- cardio-metabolic traits and risk factors (e.g. blood pressure, body-mass index, diabetes, pulse wave velocity) and soluble biomarkers (e.g. lipids, glycaemia measures, sex hormones, inflammatory factors).

We will also characterise any independent prospective associations of LTL with the following characteristics:

- novel cardiovascular traits obtained through MRI imaging such as left ventricular mass and function.
- incident fatal and nonfatal cardiovascular diseases such as CAD, stroke, heart failure, atrial fibrillation.

To evaluate whether inter-individual LTL variation is causally relevant to the cardiovascular traits and diseases mentioned above, we will use the genome-wide SNP data currently being obtained on UK Biobank participants to identify genetic variants associated with LTL and use these as instruments in mendelian randomisation analyses.

The LTL measurements together with metadata from the project will be deposited with UK Biobank so that other investigators can access them for their own studies.

The project addresses the grand societal challenge of an increasing ageing population and the consequent rise in age-associated chronic diseases. Therefore in addition to the academic beneficiaries, the main beneficiary will be society itself. Prevention of these diseases and increase in the health (as distinct from life) span will only be possible through a better understanding of the ageing process and the fundamental mechanisms underlying variation in risk of age-associated diseases. This project will enable a robust testing of the role of biological ageing in age-associated diseases and an exploration of factors (nutritional, life-style and others) that in turn may impact on this process. The benefits and impact of the project will only become greater with time as more data and outcomes accrue in UK Biobank.

The potential impact of the project is underscored by the fact that it addresses multiple priorities in the MRC's Research Strategy:

- Research Priority Theme One (Resilience, repair and replacement): Tissue disease and degeneration: To advance knowledge in the biology of ageing and degeneration of human tissue and to progress research tackling dementia;

- Research Priority Theme Two (Living a long and healthy life): Molecular datasets and disease: To use genetics, imaging and biological indicators to understand predispositions to disease.

Furthermore it specifically addresses the objectives of the cross-council initiative on Lifelong Health and Well Being (LHWB) and specifically its objective of "Understanding the determinants of life expectancy and the relationship between natural ageing and the development of age-related conditions."

More broadly, the results from the proposal could inform the efforts of biotechnology companies to develop diagnostic tests for LTL and new therapies that target TL. Already a couple of nascent companies offering TL measurements have come into this space but require more robust evidence. Similarly, there are early small molecule activators of telomerase (e.g. TA-65) that have shown to elongate short telomeres and increase health span of adult/old mice without increasing cancer incidence.

Finally, the project will provide another high-quality example of how the MRC and its partner funders optimise the use and benefits that can be gained from the large prospective cohorts created through willing voluntary participation of the UK population. (Pell J, Valentine J, Inskip H. One in 30 people in the UK take part in cohort studies. Lancet, 2014; 383: 1015 - 1016.).