Telomerase activity is present in most primary human tumours but not in normal somatic tissues except for proliferative cells of renewal tissues (e.g. crypts of the intestine, basal layer of the epidermis, haemopoietic and inflammatory cells). In some instances telomerase activity is detected in preinvasive lesions, whereas in others it is only detected at later stages. Lower telomerase activity levels are detected in some specimens obtained from regions adjacent to primary tumours. The key clinical challenge is to determine if the presence or level of telomerase activity has diagnostic or prognostic utility. Almost any clinical specimen can be used to assay telomerase activity including frozen sections, fine needle aspirates, brushes, washes and sedimented cells in voided urine. In certain cancers increased telomerase activity levels may identify patients that will have either favourable or poor prognostic outcomes, whereas in other instances telomerase activity can distinguish between benign and malignant lesions. New approaches to improve the diagnostic value of telomerase determinations include application of in situ hybridization methods for detecting human telomerase RNA expression on archival paraffin-embedded material. Results show that this assay easily distinguishes cancer from normal cells, and thus may complement the telomerase activity assays.
