Telomere structure. (A) Telomeres are located at the ends of linear chromosomes; they are composed of hundreds to thousands of tandem DNA repeat sequences: hexameric TTAGGG in the leading strand and CCCTAA in the lagging strand in humans. Protective proteins associated with telomere DNA are collectively termed shelterin (TRF1, TRF2, TIN2, POT1, TPP1, and RAP1). The 3′ end of the telomeric leading strand terminates as a single-stranded overhang, which folds back and invades the double-stranded telomeric helix, forming the T loop. (B) Telomeres can be directly visualized under the microscope at the ends of metaphase chromosomes (four telomere signals per chromosome) by fluorescence in situ hybridization (FISH). (Image provided by Peter Lansdorp, MD, PhD.) Average telomere length can be measured by several methods: a technique that combines flow cytometry and FISH (flow-FISH), Southern blotting, and a quantitative polymerase-chain-reaction (qPCR) assay. Flow-FISH can measure the telomere length in different cell subgroups, such as granulocytes or CD4+ T lymphocytes; Southern blotting reveals length and length heterogeneity; and qPCR is a rapid assay that requires very small amounts of DNA. (C) The average length of telomeres in human leukocytes varies, ranging from approximately 11 kb at birth (in umbilical-cord blood) to 6 kb at 90 years of age. Telomere loss is most rapid early in life, and over a life span it is not linear but follows a third-order polynomial. Data are from Yamaguchi et al.14 Reprinted with permission from Calado RT, Young NS. Telomere diseases. N Engl J Med. 2009;361:2353–2365.
