The High-Throughput C-Circle Assay for Identifying Modulators of Alternative Lengthening of Telomeres(ALT) Activity

Telomeres are long, repetitive nucleoprotein structures that protect the ends of linear chromosomes during DNA replication. In differentiated cells, telomeres shorten with each cell division, eventually becoming too short to protect chromosome ends from DNA end fusions or recombination. This critical event is known as the "end replication" problem and results in cell senescence. To achieve replicative immortality, tumor cells must evade senescence by maintaining telomere length using one of two telomere maintenance mechanisms: reactivation of telomerase or the alternative lengthening of telomeres (ALT) pathway. The telomerase pathway is normally turned off in differentiated cells, but is reactivated in approximately 85% of cancers. Telomerase uses reverse transcription to elongate telomere ends during DNA replication. The ALT pathway is activated in approximately 15% of cancers. ALT relies on the homologous recombination DNA repair pathway to lengthen telomeres. Activation of ALT is associated with poor prognosis in several cancers, including osteosarcoma and neuroblastoma. ALT is a tumor-specific pathway, which makes it an attractive therapeutic target; however, there are currently no ALT-specific chemical inhibitors. To advance biological understanding of the ALT pathway and to identify potential ALT therapeutics, the high-throughput C-circle assay was developed.