Conditional deletion of human STN1 leads to telomere dysfunction and telomerase-dependent genome instability and proliferation defects

CTC1-STN1-TEN1 (CST) is a heterotrimeric, RPA-like complex that binds single-stranded DNA, stimulates DNA polymerase α-primase, and functions in several genome maintenance pathways, including telomere maintenance and DNA replication/repair. During telomere replication, CST prevents telomerase from overextending the G-rich single-stranded overhang (G-OH) and promotes fill-in of the C-rich strand by stimulating DNA polymerase α-primase. Previous work characterized the effects of CST loss by deleting CTC1 or TEN1. Interestingly, CTC1 knockout (KO) caused severe proliferation defects and telomeric damage signaling, whereas these phenotypes were absent following TEN1 KO. Molecular analysis revealed that, while loss of CTC1 or TEN1 leads to defective C-strand fill-in, only CTC1 KO exhibited excessive G-OH lengthening. Here, we characterized conditional STN1 KO cells and determined that STN1 KO leads to proliferation defects and telomeric damage signaling. Moreover, STN1 KO caused genome instability in the form of anaphase bridges and micronuclei. Interestingly, these phenotypes and growth inhibition were largely dependent on telomerase activity. Our findings indicate that STN1 KO closely resembles CTC1 versus TEN1 KO and that excessive G-OH extension underlies the genome instability caused by STN1 deletion.