Human Sarcomas Are Mosaic for Telomerase-Dependent and Telomerase-Independent Telomere Maintenance Mechanisms

Abstract

Telomere shortening necessitates that tumor cells activate a telomere maintenance mechanism (TMM) to support immortalization. Although most tumor cells activate expression of the enzyme telomerase, some cells elongate telomeres in a telomerase-independent manner, termed alternative lengthening of telomeres (ALT). Previous studies have evaluated the presence of telomerase or ALT mechanisms or both in a variety of tumor types. Our studies also show that TMMs are not mutually exclusive in some tumors. In contrast, our IHC analyses of human sarcomas identified a subset of tumors with some cells containing ALT-associated PML bodies, a hallmark of ALT, and separate cells expressing telomerase in the same tumor. By using a second set of human osteosarcomas, we merged IHC and biochemical analyses to characterize more fully the tumor TMM. The IHC data reveal the presence of both telomerase- and ALT-positive tumor cells in samples that demonstrate characteristics of both telomerase and ALT in biochemical assays. These assays, which measure telomere length and telomerase activity of tumor extracts, are conventionally used to classify tumor TMM. Our results suggest that TMM is not a single or perhaps static characteristic of some tumors and that TMM heterogeneity should be considered in tumor stratification. Furthermore, clinical interest in telomere-based therapies may necessitate accurate characterization of tumor TMM before treatment to maximize therapeutic efficacy.

Differential nuclear ATRX expression in sarcomas

“…Also, we do not yet know if such tumours were composed both of cells using the telomerase‐dependent mechanism (ATRX‐positive cells) and of cells that use the ALT mechanism (ATRX‐negative cells). Nevertheless, previous studies detected both the ALT phenotype (exceedingly long telomeres) and telomerase reactivation (measured by enzyme activity) in individual sarcoma cases . Gocha et al .…”

ALTernative Telomere Maintenance and Cancer

“…However, there is no universal definition for ALT activity and most studies of patient samples investigate only one or two of the phenotypic markers. Additionally, tumors can have intratumoral heterogeneity in telomere lengths and TMM activity, with ALT and telomerase functioning in different cells within the same tumor [23,27]. Therefore, it is possible that the prevalence of ALT may be over- or under- represented by the markers currently used.…”

Alternative mechanisms of telomere lengthening: Permissive mutations, DNA repair proteins and tumorigenic progression

“…Our recent work demonstrates that some human sarcomas contain two types of tumor cells—those that express telomerase and those that use ALT [56]. Tumors were simultaneously analyzed for expression of the telomerase subunit TERT and APBs through colocalization of TRF2 and PML using the Nuance multispectral imaging system.…”

“…Distinct cells expressing TERT and distinct cells with APBs were observed within the same tumor in 14/27 osteosarcomas, 3/4 osteoclastomas and 9/26 chondrosarcomas. Five fresh frozen osteosarcomas were evaluated using the telomere repeat amplification protocol (TRAP) assay to measure telomerase activity and telomere-specific Southern blotting to characterize telomere characteristics; three of these tumors were characterized by mixed phenotypes [56]. These results demonstrate that some tumors can be mosaic for cells using different telomere maintenance mechanisms.…”