A sensitive direct human telomerase activity assay

Abstract

Expression of telomerase, the specialized reverse transcriptase that adds 5′-TTAGGG-3′ repeats to the ends of human chromosomes, is upregulated in ≥85% of human cancers and tumor cell lines. We describe a direct primer-extension activity assay^1,2 for human telomerase that displays sensitivity to ∼10⁶ telomerase-positive cells, making the method suitable for use with standard cell culture–based research (Fig. 1). Telomerase is first immunoaffinity purified from cell lysate using an antibody to telomerase and captured using protein G–agarose beads (Steps 14–17). Then telomerase is dissociated from the beads using excess peptide antigen (Step 19). A second affinity purification exploits the stable binding interaction between human telomerase and the telomeric DNA substrate 5′-(TTAGGG)₃-3′ (dissociation half-life ≥ 10 h at 23 °C)³. Modifying neutravidin beads with 5′-biotin-CTAGACCTGTCATCA(TTAGGG)₃-3′ (Step 5) generates an affinity reagent that captures >90% of immunopurified telomerase⁴ (Step 22), providing highly enriched telomerase bound to its DNA substrate in a volume of 20 μl of beads. Addition of assay buffer results in extension of the bead-immobilized DNA substrate (Step 24). Telomerase extension products are released by heating in denaturing formamide buffer to disrupt the avidin-biotin interaction, and then separated and visualized by standard techniques.