The telomeres at the end of human chromosomes are responsible for safeguarding DNA termini while chromosome replication is taking place. They represent unique structures characterized by a pattern of short tandem repeats rich in guanine that shorten with each cell division.
Shortening of telomeres can also occur as a part of the general stress response that can take place in different pathological states; therefore, measuring telomere length represents an important approach to study the stressed and aging cell, as well as to gain insights into the different mechanisms.
Methods of measurement
Southern blotting and terminal restriction fragment analysis have traditionally been regarded as the gold standard for measuring telomere length. In this technique, a cocktail of common cutting restriction enzymes without recognition sites in the telomeric and subtelomeric regions is used to digest genomic DNA.
Nevertheless, due to its labor-intensive and time-consuming nature, reproducibility problems, as well as the requirement for large amounts of genomic DNA, newer analysis methods based on polymerase chain reaction have been developed. Real-time PCR (qPCR) is the most widely used among them, and the ensuing results show a high degree of correlation with results from Southern blot analyses.
Originally developed by Cawton, qPCR technology entails separate polymerase chain reactions to measure telomeres, which are normalized to a single copy gene, yielding a telomere/gene ration as a measure of telomere length. Still, even when performed by experts, there can be a significant variability across laboratories, reflecting measurement variation between aliquots and differential amplification efficiency.
Other measurement methods use cytometry hybridization techniques, which are designed to measure the shortest telomeres, as well as telomeres from specific chromosomes. The length of the shortest telomere represents a much better indicator of cellular aging than the average telomere length, since cells either die or become senescent once the shortest telomeres are depleted.
Approaches and challenges in studies of telomere length
When a research study wants to include telomere length in the analysis, the most important steps are thorough evaluation of the research question, sample types, population, timing of analysis, as well as resources that are available in order to choose the most suitable measurement method available.
One of the major drawbacks in using telomere length as a clinical measure is its high variability between different individuals, which is determined at birth. Furthermore, shortening with age is more rapid in males than females, and the rates can also differ between various ethnic groups.
All these factor limits the usefulness of telomere measurement in cross-sectional studies; thus longitudinal studies measuring actual telomere erosion rates in individuals over times represent more powerful study designs for demonstrating causal effects.
In any case, telomere measurement can be used as a tool that, either alone or together with other biomedical and biobehavioral attributes, may have significant implications for preventative efforts, disease monitoring, development of different interventions and, of course, further improvement of our knowledge.
- Ridout SJ, Ridout KK, Kao HT, Carpenter LL, Philip NS, Tyrka AR, Price LH. Telomeres, Early-Life Stress and Mental Illness. In: Balon R, Wise TN, editors. Clinical Challenges in the Biopsychosocial Interface: Update on Psychosomatics for the 21st Century. Karger Medical and Scientific Publishers, 2015; pp. 92-108.