TIN2 Mediates Functions of TRF2 at Human Telomeres

Abstract

Telomeres are protective structures at chromosome ends and are crucial for genomic stability. Mammalian TRF1 and TRF2 bind the double-stranded telomeric repeat sequence and in turn are bound by TIN2, TANK1, TANK2, and hRAP1. TRF1 is a negative regulator of telomere length in telomerase-positive cells, whereas TRF2 is important for telomere capping. TIN2 was identified as a TRF1-interacting protein that mediates TRF1 function. We show here that TIN2 also interacts with TRF2 in vitro and in yeast and mammalian cells. TIN2 mutants defective in binding of TRF1 or TRF2 induce a DNA damage response and destabilize TRF1 and TRF2 at telomeres in human cells. Our findings suggest that the functions of TRF1 and TRF2 are linked by TIN2.Telomeres are the DNA protein structures that cap the ends of linear chromosomes thereby protecting them from degradation and fusion by cellular DNA repair systems (1, 2). In mammals, telomeres consist of several kilobase pairs of doublestranded DNA that have the repeat sequence TTAGGG followed by 100 -150 nucleotides of a single-stranded TTAGGG 3Ј-overhang. The telomeric DNA tract is thought to loop back on itself, thereby protecting the overhang, in a structure termed the t-loop (3). This protected or capped telomeric structure can be lost or disrupted by DNA damage, malfunctions in telomere-associated proteins, or as a consequence of the gradual attrition of telomeric DNA that occurs when cells proliferate in the absence of telomerase. Dysfunctional telomeres activate a DNA damage response, resulting in the binding of DNA damage response proteins to the chromosome end and induction of apoptosis or cellular senescence (4 -7).Telomere-associated proteins are crucial for forming and maintaining the protective telomeric structure in vivo. Some telomere-associated proteins bind directly to telomeric DNA, whereas others associate with telomeres via protein-protein interactions. Among the direct telomere-binding factors is a family of Myb domain-containing proteins that specifically bind double-stranded telomeric DNA. These proteins include Rap1p in Saccharomyces cerevisiae (8), Taz1 in Schizosaccharomyces pombe (9), and TRF1 and TRF2 in mammals (10 -12). TRF1 reduces telomere length, whereas a dominant-negative TRF1 mutant increases telomere length when overexpressed in telomerase-positive human cells (13). These results suggest that TRF1 controls the extent to which telomerase can access and hence elongate the telomeres, presumably by modulating the telomeric structure. TRF2 is essential for t-loop formation in vitro (14) and telomere integrity in vivo (3). It has been proposed that t-loop formation by TRF2 provides a mechanism to sequester the telomeric 3Ј-overhang, thereby preventing its degradation and/or fusion to another telomere by DNA repair processes. Disruption of TRF2 function induces (ataxia-telangiectasia-mutated) (ATM)/p53-dependent apoptosis in human tumor cells (4) and premature senescence in normal human and mouse cells (5).Several TRF1-and TRF2-interacting factors have been id...

Cancer and aging: the importance of telomeres in genome maintenance

“…Recently, TIN2 was found to bind TRF2 [60], in addition to directly interacting with TRF1 [51,52] ( Fig. 2).…”

“…The TIN2-TRF2 interaction is very likely crucial for both TRF2 and TRF1 functions in vivo. Depending on the level of expression and type of mutation, dominant negative forms of TIN2, like dominant negative TRF1, can extend telomere length [51], or, like dominant negative TRF2, can induce pervasive telomere uncapping [60]. Thus, TIN2 may link the functions of TRF1 and TRF2.…”

“…Nonetheless, perturbations to either TRF1 or TRF2, or their associated proteins POT1, RAP1 or TIN2, influence both telomere length and capping [50,51,60,61,[64][65][66][67] A major gap in our knowledge of telomere composition is how the myriad proteins that associate with telomeres ( Fig. 3) organize.…”

“…Therefore, dysfunctional telomeres elicit the cellular tumor suppressor mechanisms of apoptosis or senescence [58][59][60]. Whether cells undergo apoptosis or senescence in response to telomere dysfunction depends on the cell type and genetic background, notably the p53 status of the cell.…”

Identification of candidate alternative lengthening of telomeres genes by methionine restriction and RNA interference

“…TRF1 functions as a negative regulator of telomere length (van Steensel and de Lange, 1997), whereas TRF2 is important for maintaining telomere capping (van Steensel et al, 1998; Karlseder et al, 1999). The functions of TRF1 and TRF2 are interconnected and integrated by TIN2 (Houghtaling et al, 2004;Kim et al, 2004;. Also, TRF2 binds RAP1 (Li et al, 2000), which recruits MRN complexes (Zhu et al, 2000;O'Connor et al, 2004).…”