TIN2 is an important regulator of telomere length, and mutations in TINF2, the gene encoding TIN2, cause short telomere syndromes. While genetics underscore the importance of TIN2, the mechanism through which TIN2 regulates telomere length remains unclear. Here, we tested the effects of human TIN2 on telomerase activity. We identified a new isoform in human cells, TIN2M, that is expressed at similar levels to previously studied TIN2 isoforms. All three TIN2 isoforms localized to and maintained telomere integrity in vivo, and localization was not disrupted by telomere syndrome mutations. Using direct telomerase activity assays, we discovered that TIN2 stimulated telomerase processivity in in vitro Each TIN2 isoform stimulated telomerase to a similar extent. Mutations in the TPP1 TEL-patch abrogated this stimulation, suggesting that TIN2 functions with TPP1/POT1 to stimulate telomerase processivity. We conclude from our data and previously published work that TIN2/TPP1/POT1 is a functional shelterin subcomplex.ImportanceTelomere length regulation maintains the fine balance between cancer and short telomere syndromes, which manifest as complex degenerative diseases including bone marrow failure and pulmonary fibrosis. The enzyme telomerase maintains telomere equilibrium through highly regulated addition of telomere sequence to chromosome ends. Here, we uncover a previously unknown biochemical role for human shelterin component TIN2 in regulating telomerase enzyme processivity, suggesting that TIN2 functions with TPP1/POT1 as a specialized telomeric single-stranded DNA-binding complex. These findings suggest that previous descriptions of TIN2 as a tethering or bridging protein is incomplete and imply that TIN2 mutants could affect telomere length through regulating telomerase processivity. This new perspective on shelterin components regulating telomere length at the molecular level will help advance understanding of clinical manifestations of short telomere syndromes.Copyright © 2019 Pike et al.
Journal: Molecular and cellular biology