Transcription and alternative splicing of telomerase reverse transcriptase in benign and malignant breast tumours and in adjacent mammary glandular tissues: implications for telomerase activity

Abstract

Telomerase activity was determined in 15 breast cancers, 24 benign breast lesions, and 36 breast tissues adjacent to benign or malignant tumours. A positive TRAP (telomeric repeat amplification protocol) signal was detected in 67% of carcinomas and 29% of benign tumours. In five of ten cases, non-invaded breast tissues adjacent to telomerase-positive carcinomas also displayed telomerase activity. Conversely, in peritumoural specimens adjacent to benign lesions, telomerase activity was never detected. To investigate the regulatory mechanisms of telomerase activity in breast tissues, the expression of telomerase subunits was assessed, as well as the presence of alternatively spliced variants of human telomerase reverse transcriptase (hTERT). The presence of the hTERT full-length transcript appeared necessary for telomerase activity in breast carcinomas. Specifically, all telomerase-positive carcinomas expressed the hTERT full-length message, together with different combinations of alternatively spliced variants, whereas in telomerase-negative cancers, the hTERT full-length transcript was not detectable, or its abundance was markedly lower than that of alternatively spliced variants. Results obtained in benign tumours and normal tissues surrounding carcinomas instead showed that the presence of hTERT full-length transcript was not sufficient to determine telomerase activity. These findings suggest that in non-neoplastic tissues there are other mechanisms that suppress telomerase activity downstream from hTERT transcription and mRNA splicing and that such mechanisms have been lost during neoplastic transformation.