Measuring relative telomere length: Is tissue an issue?

the specialized structures at chromosome ends, consist of long stretches of protein-bound TTAGGG repeats [1]. In certain cell types, such as germ cells and stem cell populations, the length of telomeric DNA is maintained by the enzyme telo-merase. Most somatic cells, however, lack sufficient telomerase; consequently, telomeric DNA shortens progressively with each cell division. Upon reaching a critically short length, proliferation is halted and cells often enter a state of senescence. Thus, progressive telomere shortening is thought to serve as a molecular clock for cellular replicative aging. Based upon this fundamental tenet of telomere biology, an ever-increasing number of studies spanning a wide spectrum of human conditions have utilized telomere length, typically measured in buccal cells or blood mono-nuclear cells, as a biomarker for cellular replicative age. Individuals with relatively short age-adjusted telomere lengths, due to a combination of inherent factors and environmental stressors, are considered to have accelerated cellular replicative aging, potentially resulting in increased disease susceptibility [2]. Telomere length is also used as a diagnostic tool in diseases characterized by fundamental derangements of telomere biology, such as dyskeratosis congenita (DC). DC is a rare genetic disorder stemming from a defect in telomere maintenance. This defect results in a broad and highly variable clinical phenotype consisting of predisposition to bone marrow failure and malignancy, a triad of mucocutaneous features, and a number of less frequent manifestations such as pulmonary fibrosis and liver disease [3]. In nearly all reported cases, affect-ted individuals have severely shortened telomeres, which can be directly attributed to mutations in genes Commentary encoding components of telomerase or a telo-mere-associated protein in approximately half of cases. Hence, DC is considered the prototype of a heritable disorder of telomere maintenance. In DC, very short telomeres are observed across different classes of peripheral white blood cells, affecting both lymphoid subpopulations and granulocytes [4]. In contrast, in other inherited bone marrow failure syndromes (IBMFS), telomere length shortening is less pronounced and the effect is largely restricted to granulocytes. Rather than being due to an inherent defect in telomere maintenance, the short telomeres in granulocytes in these cases are thought to reflect accelerated progenitor/stem cell turnover secondary to bone marrow stress. The utility of telomere length both as a biomarker for accelerated cellular replicative aging and as a diagnostic marker for a constitutional defect in telomere maintenance solicits a question as to how telomere length varies in different tissues in conditions …