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Telomerase is a ribonucleoprotein enzyme that elongates telomeres and therefore maintains chromosomal stability in germline, and in the majority of cancer cells, during cell doubling. However, up to 30 % of human tumors of different types do not express telomerase, but instead use an alternative lengthening of telomeres (ALT). Here authors show that human tumor-derived ALT cell lines express a LINE-1 (L1) retrotransposon, which suggests its participation in telomere maintenance, possibly by a «slippage» mechanism of telomeric DNA synthesis. Moreover, suppression of the L1 encoded reverse transcriptase activity using an antisense strategy, or treatment of the ALT cells with the reverse transcriptase inhibitor 3'-azido-2',3'-dideoxythymidine (AZT), induces progressive telomere loss, arrest in G2-phase of the cell cycle, and, eventually, in cancer cell death. This finding suggests an exciting opportunity for the cure of up to 30 % of cancer cases.

The review analyzes telomere lengthening mechanisms in cancer cells of two different tumor types--carcinomas and sarcomas. Basic attention is given to sarcomas which cells lengthen their telomeres through alternative mechanisms for lengthening telomeres (ALT). The features of ALT mechanisms functioning in these cells have been examined in detail. Moreover, the actually existing anti-telomerase drugs and their efficiency in carcinomas treatment were analyzed. The experimental data showing that, contrary to theoretical expectations, activation of ALT was not observed on a background of such anti-telomerase, are presented. This portends a good prognosis in the treatment of the patients with carcinoma. Finally, on the basis of the data analyzed the candidates for a role of targets are proposed for development of anti-ALT antineoplastic therapy. Expected efficiency and toxicity of such drugs are considered in detail.