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Amplification of Telomerase Reverse Transcriptase Gene in Human Mammary Epithelial Cells with Limiting Telomerase RNA Expression Levels

Children's Medical Research Institute, Westmead, and University of Sydney, Sydney, New South Wales, Australia

Requests for reprints: Roger R. Reddel, Children's Medical Research Institute, 214 Hawkesbury Road, Westmead, Sydney, New South Wales 2145 Australia. Phone: 61-2-9687-2800; Fax: 61-2-9687-2120; E-mail: rreddel{at}cmri.usyd.edu.au.

Key Words: telomerase • gene amplification • hTERT • hTR • telomere

Activation of telomerase is a crucial step during cellular immortalization, and in some tumors this results from amplification of the human telomerase reverse transcriptase (hTERT) gene. Immortalization of normal human cells has been achieved by transduction with hTERT cDNA under the control of a strong heterologous enhancer/promoter, but this is sometimes an inefficient process, with periods of poor growth or even crisis occurring before immortalization. Here, we showed that normal human mammary epithelial cells expressing exogenous hTERT amplified the transgene extensively and expressed high levels of hTERT mRNA and protein. Paradoxically, the cells had low levels of telomerase activity and very short telomeres, indicating that telomerase activity did not correlate with hTERT expression. These cells contained only 20 human telomerase RNA (hTR) molecules/cell (compared with 120 hTR molecules per 293 cell). Expression of exogenous hTR caused increased telomerase activity and telomere lengthening. These data indicate that some hTERT-transduced normal cells may express high levels of the transgene but fail to up-regulate endogenous hTR expression sufficiently to enable expression of robust levels of telomerase activity. [Cancer Res 2008;68(9):3115–23]