CDK1 differentially regulates G-overhang generation at leading- and lagging-strand telomeres in telomerase-negative cells in G2 phase.

Human telomeres contain single-stranded 3' G-overhangs that function in telomere end protection and telomerase action. Previously we have demonstrated that multiple steps involving C-strand end resection, telomerase elongation and C-strand fill-in contribute to G-overhang generation in telomerase-positive cancer cells. However, how G-overhangs are generated in telomerase-negative human somatic cells is unknown. Here, we report that C-strand fill-in is present at lagging-strand telomeres in telomerase-negative human cells but not at leading-strand telomeres, suggesting that C-strand fill-in is independent of telomerase extension of G-strand. We further show that while cyclin-dependent kinase 1 (CDK1) positively regulates C-strand fill-in, CDK1 unlikely regulates G-overhang generation at leading-strand telomeres. In addition, DNA polymerase α (Polα) association with telomeres is not altered upon CDK1 inhibition, suggesting that CDK1 does not control the loading of Polα to telomeres during fill-in. In summary, our results reveal that G-overhang generation at leading- and lagging-strand telomeres are regulated by distinct mechanisms in human cells.