Different telomere maintenance mechanisms in alveolar and embryonal rhabdomyosarcoma

The activation of a telomere maintenance mechanism (TMM) is crucial for the immortalization of tumor cells. Most human cancers apply telomerase-dependent TMM but some use a mechanism called alternative lengthening of telomeres (ALT). The latter was suggested to be mainly characterizing sarcomas with nonspecific complex karyotypes, whereas telomerase activation is typical of sarcomas generated by specific translocations. In this study, we investigated the TMM and its association with survival in rhabdomyosarcoma (RMS), which is characterized by two major subtypes: one that is harboring a specific translocation (alveolar) and one that has a nonspecific karyotype (embryonal). Telomerase activity (TA), using telomerase repeat amplification protocol (TRAP) assay, and telomere length (TRF), using Southern blotting, were analyzed in tumor samples from 31 patients (16 embryonal and 15 alveolar). Alveolar RMS tumors exhibited no ALT phenotype and the majority presented TA. Some embryonal tumors exhibited an ALT or "ALT-like" phenotype which lacked TA, whereas others expressed telomerase-dependent TMM, and neither TA nor ALT correlated with outcome. The average TRF length of the embryonal tumors was significantly higher than that of the alveolar tumors (10.8 vs. 7.2 kb, P = 0.003). Interestingly, some tumors of both subtypes presented no TMM. These observations suggest that alveolar RMS predominantly use telomerase-dependent TMM, whereas in embryonal tumors both telomerase and ALT may play a role. These findings have important implications for understanding the role of TMM in the development of RMS tumors, and for future designing adapted treatment strategies.