Cold Spring Harbor, NY - The enzyme telomerase has received a great deal of attention since 1998 when researchers showed that expressing this enzyme in human tissue culture cells significantly extended the life-span of the cells. Telomerase expression was immediately recognized as a useful strategy for growing the large number of cells required for cell-based therapeutic procedures. Now, however, scientists report that using telomerase to extend the life-span of human tissue culture cells is associated with activation of the c-myc oncogene and thus may present some level of cancer risk if the cells are intended for therapeutic use in humans.
David Beach, of the Wolfson Institute for Biomedical Research (University College London) and his colleagues reported these findings in the June 15 issue of Nature. Joining Beach in the study were Jing Wang of Genetica, Inc. (Cambridge, Massachusetts) and Gregory Hannon of Cold Spring Harbor Laboratory.
The ends of chromosomes, called telomeres, consist of specialized repeated sequences of DNA (TTAGGG in humans) that serve to maintain the integrity of the chromosome. In the absence of telomerase, telomeres shorten with each cell division. Eventually, cells stop dividing when they sense that their telomeres are too short to maintain chromosomal integrity. In contrast, telomerase maintains telomere length by adding nucleotides one at a time to existing chromosomal ends in a regulated fashion.
Experiments in other laboratories had indicated that the use of telomerase expression to extend the life-span of cultured cells did not appear to transform these cells into a hyperproliferative, cancerous state. Now, Beach and his colleagues have shown that at least one hallmark of cancer cells is observed in such cells, namely, activation of the c-myc oncogene.
Frequently, cells grown in culture-such as the human mammary epithelial cells (HMEC) used in this study-undergo 50 to 60 population doublings before they stop growing or "senesce." To determine whether permanent telomerase expression was necessary to enable HMEC cells to continue growing beyond their usual senescence point, Beach and his colleagues employed a retrovirus that carried a gene encoding human telomerase. After 40 doublings, HMEC cells were infected with the retrovirus, and the resulting telomerase expression enabled the cells to divide beyond their usual senescence point. Then the scientists used a genetic trick to eliminate the retroviral copy of the telomerase gene, thus preventing any further telomerase gene expression. Or so they thought.
To their surprise, telomerase activity remained high in the cells for at least 20 population doublings after the retrovirus-borne telomerase gene was eliminated. This meant that expression of the cells' own telomerase genes had been unexpectedly switched on.
Beach, Wang, and Hannon (and their associates) had previously shown that the potent transcription factor encoded by the c-myc oncogene stimulates telomerase gene expression. Thus, they reasoned that the elevated telomerase expression they observed in the recent study might be a consequence of c-myc oncogene activation.
Consistent with this idea, the researchers observed that the expression level of c-myc was two- to three-fold higher in cells that had been immortalized (enabled to grow beyond their usual senescence point) by introduction of the retrovirus-borne telomerase gene. This degree of c-myc overexpression was similar to that in a breast cancer cell line the researchers included as a control. Although the immortalized HMEC cells were not fully transformed into a cancerous state, elevated c-myc expression is a major step in the multistep process leading to the malignant transformation of cells.
Altered expression of the c-myc oncogene is observed in approximately 70,000 fatal cancers per year in the United States. Therefore, the new study indicates that the use of telomerase expression to extend the life-span of cultured cells for therapeutic purposes must be approached with caution.