Telomeres are repetitive nucleoprotein structures at the ends of linear chromosomes. In humans, telomeric DNA repeats consist of the tandem repeated hexanucleotides TTAGGG. Telomeres do not contain protein-coding genes but are involved in the stabilization and protection of chromosomal ends from events such as illegitimate recombination, the determination of chromosomal localization within the nucleus, and the regulation of cellular replicative capacity.
Telomeres shorten with each round of normal somatic cell division at a rate of 50–100 base pairs per cell division, as inferred from in vitro analysis of cultured human fibroblasts or lymphocytes. After several cell divisions, telomeres reach a critical length, forcing the cells to undergo replicative senescence, which is correlated to physiological cell death. This system thus provides a “counting” mechanism to limit the number of times a normal cell can divide. Consensus suggests telomere shortening may thus be involved in the ageing of normal somatic cells, and the restoration of telomeric repeats to the ends of the chromosome can overcome this limitation.
The most widely studied telomere lengthening mechanism is mediated by telomerase, a ribonucleoprotein enzyme that mediates RNA-dependent synthesis of telomeric repeats in species ranging from yeast to human. Telomerase activity is present in cells of the germ line and appears to be critical in maintaining stable telomere length, thereby conserving the capacity for essentially limitless replication in highly proliferative male and female germ cells.
|Figure 1:||The reverse transcriptase activity of telomerase on Telomeric DNA. not shown are other protein components of the telomerase complex.|
Initial analysis of human cell populations detected telomerase activity in germ line cells and in most malignant transformed cells, but not in normal somatic cells. However, more recently it has been established that modest levels of telomerase activity exist in proliferative tissues with high renewal potential, e.g. bone marrow, tissue from the gastrointestinal tract and uterine endometrium, and activated lymphocytes.
The ribonucleoprotein complex of human telomerase was first isolated in 2007 as two molecules each of human telomerase reverse transcriptase (hTERT), telomerase RNA (hTR or TERC), and dyskerin. The minimum components necessary for telomerase activity, at least in vitro, are the hTERT protein, a 1132-amino acid protein containing a conserved catalytic reverse transcriptase motif and hTR RNA, a non-coding RNA of 451 nucleotides (in human). The reverse transcriptase activity of hTERT adds deoxynucleotide triphosphates against the hTR template contained within the telomerase complex (Figure 1).
The 'telomere hypothesis' proposes that activation of telomerase is necessary for cells to become immortal or capable of extended proliferation. Cells that can overcome this limitation have the potential for prolonged survival and indefinite proliferation. Telomere maintenance is regarded as an important mechanism by which tumour cells evade senescence, and in most cases, it is achieved by reactivating or up-regulating telomerase activity. In fact, telomerase is activated in ~90% of human carcinomas and also appears to be present in circulating cancer cells. The actual point at which telomerase reactivation occurs during the process of carcinogenesis is still not fully understood. In some instances, telomerase can be activated gradually throughout the progression of the cancer, whereas in other cases, telomerase might already be ubiquitously expressed at the in situ or precancerous stage.
Telomerase is up-regulated in 90% of human epithelial cancers