To attain the immortal phenotype, cancer cells must overcome the mitotic clock. Telomerase activity has been identified to be activated in malignant tumors including breast cancer. Telomerase activity was evaluated in 71 breast cancer tissues and paired normal tissues with the TRAP (telomerase repeat amplification protocol) assay. Telomerase activity was calculated and translated into arbitrary units by computer-assisted densitometry with the control of telomerase activity in the 293 control cell line. In 59 paired breast tissues with telomerase activity, terminal restriction fragment (TRF) lengths were measured using Southern blotting. Relative inhibition (RI), the ratio of inhibited telomerase activity in each tumor tissue compared to that of the 293 control cell line after pre-treatment with 150 microg/ml of RNAse A, was measured. Sixty-three of 71 cancer tissues showed telomerase activity (88.7%) with 75.3+/-17.9 units in densitometry, while no telomerase activity was detected in their paired normal tissues. Telomerase activity was correlated to node metastasis (p=0.02) and stage (p=0.005), but not to tumor size or the hormonal receptor status. TRF lengths were 11. 0+/-4.7 kb in 59 tumor tissues and 11.7+/-2.2 kb in paired normal tissues. TRF lengths did not correlate to any of the clinical parameters. However changes of TRF lengths in tumor tissues compared to those of normal tissues correlated to telomerase activity. RI in the tumor tissues was proportional to telomerase activity without RNAse A pre-treatment. In breast cancer, telomerase activity was specific to tumor tissues and increased with tumor progression. Telomerase activity and changes in TRF lengths can be used as guidelines in detecting candidates for the telomerase inhibitor.

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