The Role of the Poly(ADP-ribose) Polymerase Tankyrase1 in Telomere Length Control by the TRF1 Component of the Shelterin Complex

Abstract

Tankyrase1 is a multifunctional poly(ADP-ribose) polymerase that can localize to telomeres through its interaction with the shelterin component TRF1. Tankyrase1 poly(ADPribosyl)ates TRF1 in vitro, and its nuclear overexpression leads to loss of TRF1 and telomere elongation, suggesting that tankyrase1 is a positive regulator of telomere length. In agreement with this proposal, we show that tankyrase1 RNA interference results in telomere shortening proportional to the level of knockdown. Furthermore, we show that a tankyrase1-resistant form of TRF1 enforced normal telomere length control, indicating that tankyrase1 is not required downstream of TRF1 in this pathway. Thus, in human cells, tankyrase1 appears to act upstream of TRF1, promoting telomere elongation through the removal of TRF1. This pathway appears absent from mouse cells. We show that murine TRF1, which lacks the canonical tankyrase1-binding site, is not a substrate for tankyrase1 poly(ADP-ribosyl)sylation in vitro. Furthermore, overexpression of tankyrase1 in mouse nuclei did not remove TRF1 from telomeres and had no detectable effect on other components of mouse shelterin. We propose that the tankyrase1-controlled telomere extension is a human-specific elaboration that allows additional control over telomere length in telomerase positive cells.Telomeres can be elongated by the telomere-specific reverse transcriptase telomerase and shortened through the effects of DNA replication and nucleolytic attack. The TTAGGG repeat array of vertebrate telomeres has a speciesspecific length setting, suggesting that these forces are balanced in the germ line. Telomere length control has been primarily studied in human tumor cells that express telomerase (reviewed in Ref. 1). Such cells often maintain the length of their telomeres within a set range. This telomere length homeostasis is achieved through a negative feedback loop involving shelterin, the telomere-specific protein complex (2). Shelterin is comprised of six proteins (TRF1, TRF2, POT1, TPP1, TIN2, and Rap1) whose abundance at chromosome ends is dictated by the length of the duplex telomeric repeat array. All shelterin components behave as negative regulators of telomere elongation by telomerase. Inhibition of TRF1, TPP1, TIN2, and POT1 results in telomere elongation, whereas overexpression of several shelterin components shortens the length of the telomeres. Telomere healing experiments demonstrated that cells have the ability to monitor and regulate telomerase at individual telomeres, and tethering of TRF1 at subtelomeric sites showed that TRF1 can modulate telomere length in cis. These findings have resulted in a model for shelterin-dependent telomere length homeostasis whereby long telomeres contain more shelterin and thus have a diminished chance of being elongated further by telomerase. A key player in this negative feedback loop is POT1, whose binding to the single-stranded telomeric DNA appears to block telomerase in vivo (3-5) and in vitro (6 -9).The length of human telomeres can be reset by mani...

PARP1 is activated at telomeres upon G4 stabilization: possible target for telomere-based therapy

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TRF1 Controls Telomere Length and Mitotic Fidelity in Epithelial Homeostasis

“…Mounting evidence coming from overexpression of wild-type and dominant-negative TRF1 alleles in human cultured cells suggests that TRF1 is a key regulator of telomere length (18). However, the demonstration of a role of TRF1 or its interacting factor tankyrase (12,19,37,49) in telomere maintenance in the context of mammalian organisms has been lacking to date. Mice genetically ablated for TRF1 are embryonically lethal but show a normal telomere length and normal telomere protection, arguing that TRF1 is not essential for telomere length regulation in early embryonic development.…”

Shelterin complex and associated factors at human telomeres

“…61 The expression of a TRF1 mutant allele unable to be modified by Tankyrase led to telomere shortening, concomitant with an increase in TRF1 function at telomeres, arguing for Tankyrase being required upstream of TRF1 in telomere length regulation. 62 A great deal of insights on the significance of the activity on Tankyrase has been discovered through RNAi depletion of the protein. HeLa cells arrest at metaphase with a specific phenotype observed on condensed, metaphase chromosomes: the sister telomeres are joined and prevent chromatid separation.…”