Neotelomere formation by human telomerase

The maintenance of genome integrity requires that telomerase action be limited to telomeres and not convert DSBs into neotelomeres. Using the breakpoint sequence from an apparent germline neotelomere formation event, we developed an assay to detect and quantify telomeric repeat addition at Cas9-programmed DSBs in human cells. The data show that telomerase can add telomeric repeats to DSBs and that this process can generate functional neotelomeres. Neotelomere formation is increased when telomerase is overexpressed, suggesting that in most human cells, low (or absent) telomerase activity limits the deleterious effects of de novo telomere addition. We show that neotelomere formation at DSBs is inhibited by long-range resection and the accompanying activation of ATR signaling. Our findings reveal that telomerase can cause genome instability by generating neotelomeres at DSBs. We propose that neotelomere formation can promote tumorigenesis by ending detrimental breakage-fusion-bridge cycles in cancer cells whose genome alterations engender dicentric chromosomes.