Multiple mechanisms of telomere maintenance exist in liposarcomas.

PURPOSE: Telomeres are specialized nucleoprotein complexes that protect and confer stability upon chromosome ends. Loss of telomere function as a consequence of proliferation-associated sequence attrition results in genome instability, which may facilitate carcinogenesis by generating growth-promoting mutations. However, unlimited cellular proliferation requires the maintenance of telomeric DNA; thus, the majority of tumor cells maintain their telomeres either through the activity of telomerase or via a mechanism known as alternative lengthening of telomeres (ALT). Recent data suggest that constitutive telomere maintenance may not be required in all tumor types. Here we assess the role and requirement of telomere maintenance in liposarcoma. EXPERIMENTAL DESIGN: Tumor samples were analyzed with respect to telomerase activity, telomere length, and the presence of ALT-specific subcellular structures, ALT-associated promyelocytic leukemia nuclear bodies. This multi-assay assessment improved the accuracy of categorization. RESULTS: Our data reveal a significant incidence (24%) of ALT-positive liposarcomas, whereas telomerase is used at a similar frequency (27%). A large number of tumors (49%) do not show characteristics of telomerase or ALT. In addition, telomere length was always shorter in recurrent disease, regardless of the telomere maintenance mechanism. CONCLUSIONS: These results suggest that approximately one half of liposarcomas either employ a novel constitutively active telomere maintenance mechanism or lack such a mechanism. Analysis of recurrent tumors suggests that liposarcomas can develop despite limiting or undetectable activity of a constitutively active telomere maintenance mechanism.

p53 differentially inhibits cell growth depending on the mechanism of telomere maintenance.

Telomere stabilization is critical for tumorigenesis. A number of tumors and cell lines use a recombination-based mechanism, alternative lengthening of telomeres (ALT), to maintain telomere repeat arrays. Current data suggest that the mutation of p53 facilitates the activation of this pathway. In addition to its functions in response to DNA damage, p53 also acts to suppress recombination, independent of transactivation activity, raising the possibility that p53 might regulate the ALT mechanism via its role as a regulator of recombination. To test the role of p53 in ALT we utilized inducible alleles of human p53. We show that expression of transactivation-incompetent p53 inhibits DNA synthesis in ALT cell lines but does not affect telomerase-positive cell lines. The expression of temperature-sensitive p53 in clonal cell lines results in ALT-specific, transactivation-independent growth inhibition, due in part to the perturbation of S phase. Utilizing chromatin immunoprecipitation assays, we demonstrate that p53 is associated with the telomeric complex in ALT cells. Furthermore, the inhibition of DNA synthesis in ALT cells by p53 requires intact specific DNA binding and suppression of recombination functions. We propose that p53 causes transactivation-independent growth inhibition of ALT cells by perturbing telomeric recombination.

Telomerase is frequently activated in tumors with microsatellite instability.

Telomeres are specialized structures at the ends of eukaryotic chromosomes that are required for the complete replication and stability of naturally occurring chromosome ends. Telomere stabilization is critical for the unlimited cellular proliferation that is necessary for tumor formation. While most tumors achieve telomere stabilization through activation of telomerase, a subset of tumors utilize a recombination-based mechanism termed Alternative Lengthening of Telomeres (ALT) to maintain chromosome termini. Tumors utilizing ALT for telomere preservation will likely be refractory to treatment with telomerase inhibitors. Furthermore, tumors carrying mutations that predispose a cell to utilize ALT may activate this pathway when challenged by telomerase inhibition. Mutation of the mismatch repair (MMR) pathway enhances telomerase independent survival in yeast, with the survivors using recombination-based pathways for telomere maintenance. One possibility is that mutation of the MMR pathways alleviates suppression of recombination, thereby abrogating the need for telomerase activation. If true, one might predict an increased frequency of tumors harboring MMR mutation to use ALT for telomere maintenance. Here we characterized tumors with and without MMR mutation for the presence of telomerase activity versus ALT. We found similarly frequent activation of telomerase in tumors with and without MMR mutation, suggesting that human tumors with MMR mutation may respond favorably to treatment with telomerase inhibitors.