3' TERC tail sequencing of WT and TOE1 KO HeLa cells

In most human cancers, telomere is elongated by telomerase which contains reverse transcriptase TERT and the RNA template TERC. The mature TERC is non-polyadenylated while the oligo(A)-containing precursors are unstable. Poly(A) specific PARN mediated TERC 3' trimming completes with the precursor decay. However, the precise mechanism of TERC 3' maturation remained largely unknown. We identified a protein TOE1 accumulated specifically in Cajal bodies, where telomerase RNP assemblies and TERC accumulates. TOE1 immunoprecipitation complex harbors telomerase activity in a DEDD dependent way. Using siRNA and CRISPR-Cas9 knockout technology, we found that deficiency of TOE1 resulted in accumulation of polyadenylated TERC precursors, while did not decrease the levels of total TERC. Accordingly, TOE1 deficiency impaired telomerase activity and finally shortened telomere. Diminished telomerase activity and telomere length are rescued by restoring TOE1-FL but not TOE1-DE mutation which is disable in deadenylase activity. 3' RACE sequencing showed TOE1 deficiency causes an accumulation of polyadenylated TERC precursors, as well as 3'-extended TERC precursors. In vitro assay supports TOE1 as a DEDD dependent 3' exonuclease. We concluded TOE1 is a 3' exonuclease participating in TERC 3'-ends precise processing to mature 451 nt after deadenylation. Our results provide a mechanism linking TOE1 to telomere related diseases.