Diagnostics, prognostic and therapeutic exploitation of telomeres and telomerase in leukemias

Telomeres are specialized structures at the end of human chromosomes. Telomere length decreases with each cell division, thus, reflecting the mitotic history of somatic cells. Telomerase, the ribonucleoprotein enzyme which maintains telomeres of eukaryotic chromosomes, is up-regulated in the vast majority of human neoplasia but not in normal somatic tissues. In contrast to other somatic cells, normal primitive human hematopoietic cells and some peripheral blood cells expressed low levels of telomerase activity. This activity is thought to play an important role in self-renewal of hematopoietic stem cells. In malignant disorders, telomere lengths are generally shortened and telomerase expression and activity enhanced with high differences in the levels. Although it is necessary to be cautious in interpreting these data, there are indications that telomere length and telomerase expression and activity can serve as a molecular marker of the clinical progression and prognosis of most leukemias. Approaches that directly target telomerase, telomeres or telomerase regulatory mechanisms have been developed. Some of these anti-telomerase strategies in combination with conventional drugs proved to be promising in some types of leukemias.