The American population is aging. The fastest growing segment of the population is persons over 85 years of age, who are expected to number over 30 million by the year 2040. This demographic surge is creating significant needs for drugs for the treatment of age-related diseases and has led to increased interest in the aging process and diseases associated with it, including cancer.
Organisms age, in part, because their cells have a finite capacity to continue dividing. As they reach that limit, cells become senescent. Cell senescence has been traced to the ends of a cell's chromosomes, the telomeres. With each cell division, the telomeres lose some DNA and become shorter. At some point this shortening becomes critical. Cells sense this and arrest chromosome replication to avoid further loss. Hence, the cell is no longer able to divide.
Not all cells become senescent. Single-cell organisms and certain mammalian cells have no fixed cell division limit. Investigators have discovered that many of these cells contain a ribonucleoprotein enzyme called telomerase. Telomerase replaces the DNA that is usually lost from the telomeres during cell division. (E. H. Blackburn, (1992) Annu. Rev. Biochem. 61:113-29.) Consequently, the telomeres never shorten past the critical length and the cells never reach senescence.
Particularly interesting, investigators have found that the cells of many human cancers have telomerase. (C. B. Harley, Mutation Research 1991 256:271-282.) This helps explain why cancer cells continue dividing without becoming senescent. It also suggests a potent weapon in the battle against cancer: If telomerase activity in cancer cells can be inhibited, the cancer cells are expected to reach senescence and cease dividing.
Developing methods to regulate telomerase activity requires sources of purified telomerase and, in particular, purified human telomerase. Purified telomerase would be useful in developing and testing assays for measuring telomerase activity, for example, to evaluate the assay and for use as a standard in the assay. Assays for telomerase are useful in characterizing cancer cells or pre-cancer cells, because most cancer cells express telomerase. Purified telomerase would be more useful than crude telomerase preparations to identify and test regulators, inhibitors or activators of telomerase activity in in vitro assays. Moreover, purified telomerase would facilitate a thorough biochemical analysis of the enzyme's mechanism, which may provide insight for development of mechanism-based regulators. Purified telomerase also would be useful in the preparation of antibodies against telomerase. Such antibodies would in turn be especially useful as reagents to purify human telomerase and may be useful in cancer diagnosis or prognosis. Purified telomerase also will help provide amino acid sequence information useful in cloning the various components of the ribonucleoprotein.
While there is a need for purified telomerase, the purification of the human enzyme has posed technical challenges. Telomerase is a rare ribonucleoprotein expressed in human cells only in very low abundance. It has been estimated that human cells known to express the highest levels of telomerase activity may have only about one hundred molecules of the enzyme per cell. The fact that telomerase is a complex, multi-component structure further impedes its purification. Human cells also possess comparatively very high levels of non-telomerase ribonucleoproteins. These other ribonucleoproteins might have chromatographic purification properties similar to the telomerase ribonucleoprotein, which makes purification of telomerase from human cells difficult. Thus, there is a need for purified telomerase and purified human telomerase, in particular.