Telomeres are the protective caps on chromosome ends found in eukaryotic cells. They consist of DNA and associated proteins that are essential for chromosome integrity and stability. In human chromosomes, but also in other species, the telomeres consist of thousands of copies of 6 base repeats (TTAGGG) or very closely related repeats. Broken chromosomes lacking telomeres undergo fusion, rearrangement and translocation. In somatic cells, telomere length is progressively shortened with each cell division, both in vivo and in vitro due to the inability of the DNA polymerase complex to replicate the very 5` end of the lagging strand of DNA.

In humans, Telomerase, the enzyme that maintains the ends of chromosomes, is absent from the majority of somatic cells but is present and active in most tumors. Telomerase is a ribonucleoprotein that synthesises and directs the telomeric repeats onto the 3` end of existing telomeres using its RNA component as a template. Telomerase activity has been shown to be specifically expressed in immortal cells, cancer cells, and germ cells, where it compensates for telomere shortening during DNA replication and thus stabilises telomere length. These observations have led to a hypothesis that telomere length may function as a mitotic clock to sense cell aging and eventually signal replicative senescence or programmed cell death. Therefore, expression of telomerase activity in cancer cells is seen as a necessary and essential step for tumor development and progression.

Introduction of the telomerase catalytic protein component into normal telomerase-negative human cells may result in restoration of telomerase activity and extension of cellular life span. Human cells with introduced telomerase maintain a normal chromosome complement and continue to grow in a normal manner. Therefore induction of Telomerase activity is nowadays used to immortalize cell lines. Telomerase-induced manipulations of telomere length may thus be important not only for cell and tissue engineering but also for dissecting the
molecular mechanisms underlying inherited genetic diseases, as well as defining the genetic pathways leading to cancer. Because almost all human tumors express telomerase activity, inhibition of telomerase may result in gradual erosion of telomeres and eventual cessation of cell proliferation or induction of apoptosis. Thus telomerase may also be a promising target for cancer therapy.

TRAP assay – The development of a sensitive and efficient PCR-based telomerase activity detection method, TRAP (Telomeric Repeat Amplification Protocol), has made possible large scale analysis of telomerase activity in human cells and tissues. To date, telomerase activity has been detected in almost all tumors.