Shelterin: the protein complex that shapes and safeguards human telomeres

May combine two claims.

A complex of six telomere-specific proteins, called shelterin, binds TTAGGG repeats at human chromosome ends and protects those ends.

“A complex formed by six telomere-specific proteins associates with this sequence and protects chromosome ends.”

May combine two claims.

TIN2, TPP1, and Rap1 link TRF1, TRF2, and POT1 together, creating a complex that enables cells to tell telomeres apart from DNA damage sites.

“They are interconnected by three additional shelterin proteins, TIN2, TPP1, and Rap1, forming a complex that allows cells to distinguish telomeres from sites of DNA damage.”

May combine two claims.

Shelterin is not a fixed structural part of the telomere; rather, it remodels telomeric DNA and cooperates with DNA repair factors to alter that DNA's structure, which protects chromosome ends.

“The current data argue that shelterin is not a static structural component of the telomere.”

• Also stated in: § Shelterin: the protein complex that shapes and safeguards human telomeres · hedged

  “Instead, shelterin is emerging as a protein complex with DNA remodeling activity that acts together with several associated DNA repair factors to change the structure of the telomeric DNA, thereby protecting chromosome ends.”

May combine two claims.

TIN2 links TPP1/POT1 to TRF1 and TRF2, and also joins TRF1 to TRF2, a connection that keeps TRF2 stable on telomeres.

“The linchpin of shelterin is TIN2, which tethers TPP1/POT1 to TRF1 and TRF2.”

• Also stated in: § Six shelterin subunits: TRF1, TRF2, TIN2, Rap1, TPP1, and POT1 · plain

  “TIN2 also connects TRF1 to TRF2 and this link contributes to the stabilization of TRF2 on telomeres (Liu et al. 2004a;Ye et al. 2004a).”

• cites Liu et al. 2004aNot checked yet

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• cites Ye et al. 2004aNot checked yet

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May combine two claims.

TRF1 and TRF2 each use SANT/Myb DNA-binding domains to recognize the duplex sequence 5'-YTAGGGTTR-3', and they tolerate single-base changes very poorly.

“The SANT/ Myb-type DNA-binding domains of TRF1 and TRF2 each bind the sequence 5Ј-YTAGGGTTR-3Ј in duplex DNA, showing very low tolerance for single-base changes (Fig. 1; Bianchi et al. 1999;Court et al. 2005;Hanaoka et al. 2005).”

• cites Bianchi et al. 1999Not checked yet

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• cites Court et al. 2005Not checked yet

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• cites Hanaoka et al. 2005Not checked yet

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May combine two claims.

If shelterin's protection is lost, telomeres become visible to DNA damage surveillance and chromosome ends are incorrectly processed by DNA repair pathways.

“Without the protective activity of shelterin, telomeres are no longer hidden from the DNA damage surveillance and chromosome ends are inappropriately processed by DNA repair pathways.”

May combine two claims.

When TRF2 is inhibited or telomeres become critically short, factors such as 53BP1, γ-H2AX, the Mre11 complex, Rif1, and phosphorylated ATM gather at chromosome ends to form TIFs; inhibiting TIN2 or POT1 also produces TIFs.

“After inhibition of TRF2 or when telomeres become critically short, 53BP1, ␥-H2AX, the Mre11 complex, Rif1, and the phosphorylated form of ATM, ATM S1981-P, accumulate at chromosome ends.”

• Also stated in: § Shelterin inhibition activates the canonical DNA damage response · plain

  “TIFs are also formed when other components of shelterin are inhibited (e.g., TIN2 or POT1) (Kim et al. 2004;Hockemeyer et al. 2005).”

• cites Takai et al. 2003Not checked yet

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• cites Kim et al. 2004Not checked yet

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• cites Hockemeyer et al. 2005Not checked yet

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May combine two claims. The quote only partly states this claim.

TRF2 binds weakly to ATM, contacts the region around Ser1981, and when overexpressed in the nucleoplasm can reduce S1981 phosphorylation and dampen ATM signaling, which suggests shelterin might include an ATM inhibitor.

“Unexpectedly, it was found that the shelterin subunit TRF2 has a weak interaction with the ATM kinase (Karlseder et al. 2004), although ATM is not detectable at telomeres.”

• Also stated in: § An ATM inhibitor in shelterin? · plain

  “In overexpression studies, TRF2 has the ability to inhibit S1981 phosphorylation and when TRF2 is abundant in the nucleoplasm, it has a dampening effect on the ATM signaling pathway (Karlseder et al. 2004).”

• cites Karlseder et al. 2004Not checked yet

  Not checked in the free run.

• AssumptionRests on an assumption: Shelterin is plentiful at chromosome ends and does not build up at other sites.

May combine two claims.

The circular product of t-loop HR is observed in ALT cells and at very low levels in unperturbed normal human cells.

“The circular product of t-loop HR is also observed in ALT cells and at very low levels in unperturbed normal human cells.”

• cites Wang et al. 2004Not checked yet

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May combine two claims.

TRF2ΔB protects telomeres from NHEJ but induces sudden telomere truncations.

“T-loop HR was discovered through a separation of function mutant of TRF2, TRF2 ⌬B , which protects telomeres from NHEJ but induces sudden telomere truncations.”

• cites Wang et al. 2004Not checked yet

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May combine two claims.

Resolution of a double Holliday junction with crossover deletes the loop segment and leaves a drastically shortened telomere and a circular telomeric DNA molecule.

“resolution of this structure with crossover would delete the whole loop segment, leaving a drastically shortened telomere at the chromosome end.”

• Also stated in: § Repression of HR · plain

  “Resolution of the double HJ in the direction shown will generate a shortened telomere and a circular telomeric DNA.”

May combine two claims.

Telomerase control depends on POT1's ability to bind single-stranded DNA; a POT1 mutant unable to bind ssDNA completely loses telomere length control.

“This control of telomerase requires the ssDNA-binding activity of the shelterin component POT1 (Loayza and de Lange 2003;Liu et al. 2004b).”

• Also stated in: § Shelterin shapes telomeres · plain

  “A POT1 mutant form that does not bind ssDNA results in complete loss of telomere length control.”

• cites Loayza and de Lange 2003Not checked yet

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• cites Liu et al. 2004bNot checked yet

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May combine two claims.

Purified TRF2 can convert an artificial telomeric substrate into loops in vitro; TRF1 can loop, bend, and pair telomeric repeat arrays; and TIN2 can boost some of TRF1's architectural effects.

“TRF2 can remodel an artificial telomeric substrate into loops (Griffith et al. 1999;Stansel et al. 2001).”

• Also stated in: § Shelterin shapes telomeres · hedged

  “In vitro, TRF1 can loop, bend, and pair telomeric repeat arrays, activities that might stimulate the folding of telomeres in vivo, (Bianchi et al. 1997(Bianchi et al. , 1999;;Griffith et al. 1998) and TIN2 can enhance some of TRF1's architectural effects (Kim et al. 2003).”

• cites Griffith et al. 1999Not checked yet

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• cites Stansel et al. 2001Not checked yet

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• cites Kim et al. 2003Not checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

DNA-processing factors associated with shelterin could facilitate shelterin tasks.

“On the one hand, the factors on its”

May combine two claims.

T-SCE and t-loop HR are similar reactions, one occurring in cis and the other in trans.

“As T-SCE and t-loop HR are similar reactions (one taking place in cis, the other in trans)”

May combine two claims.

Shelterin acts as the cis-acting inhibitor in the negative feedback loop that controls telomere length; the amount of shelterin at a telomere sets the probability that POT1 can occupy the 3' terminus and block telomerase.

“Shelterin is a key component of this pathway, representing the cis-acting inhibitor.”

• Also stated in: § Shelterin shapes telomeres · hedged

  “Based on these data, the current model is that the amount of shelterin on a telomere determines the likelihood that its POT1 component can position itself on the 3Ј terminus and block telomerase (Fig. 2D).”

• AssumptionThe authors state this as an assumption: Shelterin loading at a telomere is approximately proportional to the number of TTAGGG repeats.
• AssumptionThe authors state this as an assumption: Shelterin's inhibition of telomerase is stochastic and depends on how much shelterin is present at the telomere.
• AssumptionThe authors state this as an assumption: More shelterin on longer telomeres is assumed to raise the amount of POT1 loaded onto the overhang.
• AssumptionThe authors state this as an assumption: When POT1 occupies the 3' end, it is assumed to prevent telomerase from extending the telomere.

May combine two claims.

Shelterin controls nucleolytic processing of the 5' strand; without POT1, 5' ends become heterogeneous, ending in AA, AT, TC, CC, CA, or AT.

“Although the nuclease involved has not yet been identified, recent data shows that the nucleolytic processing of the 5Ј strand is controlled by shelterin.”

• Also stated in: § Shelterin shapes telomeres · plain

  “When POT1 is inhibited, 5Ј ends lose their homogeneity and now end with AA, AT, TC, CC, CA, or AT (Hockemeyer et al. 2005).”

• cites Hockemeyer et al. 2005Not checked yet

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May combine two claims.

Other eukaryotes have shelterin-related telomere complexes: POT1-like proteins occur in nearly all eukaryotes, a TRF1/2-like protein occurs in fission yeast and trypanosomes, and Rap1 occurs in fungi; by contrast, TIN2 and TPP1 have been found only in vertebrates so far.

“POT1-like proteins are present in nearly all eukaryotes (de Lange 2001), a TRF1/2 like protein is found in fission yeast and in trypanosomes (Cooper et al. 1998;Sfeir et al. 2005), and Rap1 is present in fungi (Shore 1994;Chikashige and Hiraoka 2001;Kanoh and Ishikawa 2001).”

• Also stated in: § Six shelterin subunits: TRF1, TRF2, TIN2, Rap1, TPP1, and POT1 · hedged

  “In contrast, the shelterin subunits TIN2 and TPP1 have so far only been found in vertebrates; their emergence may have coincided with the gain of a second TRF-like gene.”

• cites de Lange 2001Not checked yet

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• cites Cooper et al. 1998Not checked yet

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• cites Sfeir et al. 2005Not checked yet

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May combine two claims. The quote only partly states this claim.

Because TRF1 and TRF2 each contribute two DNA-binding domains and POT1 contributes one, shelterin can engage telomeric DNA through at least five such domains, which makes it uniquely able to tell telomeres apart from other DNA ends.

“Since these three shelterin subunits are connected through protein-protein interactions, shelterin has the capacity to recognize telomeric DNA with at least five DNA-binding domains (two each in TRF1 and TRF2 and one in POT1).”

• Also stated in: § Six shelterin subunits: TRF1, TRF2, TIN2, Rap1, TPP1, and POT1 · plain

  “As a consequence, shelterin is uniquely qualified to distinguish telomeres from all other DNA ends.”

May combine two claims.

NHEJ between two telomeres yields a circular or dicentric chromosome; such fusions happen when telomeres are too short or TRF2 is inhibited, join the C-strand of one telomere to the G-strand of another, and need the normal NHEJ machinery, since cells lacking DNA ligase IV or Ku70 fuse telomeres 10- to 50-fold less often after shelterin inhibition.

“Nonhomologous end-joining (NHEJ) of two telomeres creates a circular or a dicentric chromosome.”

• Also stated in: § Preventing NHEJ and overhang processing · plain

  “They occur when telomeres have become too short and when TRF2 is inhibited (for review, see de Lange 2002).”

• Also stated in: § Preventing NHEJ and overhang processing · plain

  “Cells lacking DNA ligase IV or Ku70 have a 10-to 50-fold reduced ability to fuse telomeres after shelterin inhibition.”

• cites de Lange 2002Not checked yet

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• cites Smogorzewska et al. 2002Not checked yet

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• cites Celli and de Lange 2005Not checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

Telomeric DNA-containing Double Minute chromosomes could be formed by recombination between a telomere and interstitial telomeric DNA on the same chromosome.

“These elements, referred to as Telomeric DNA-containing Double Minute chromosomes (TDMs) could be formed by recombination between a telomere and interstitial telomeric DNA on the same chromosome.”

May combine two claims.

T-loops can be resolved by Holliday junction resolvases because pairing of the 5' telomere end with the D loop can form a Holliday junction.

“T-loops are at risk for resolution by Holliday junction (HJ) resolvases because an HJ could be formed if the 5Ј end of the telomere base pairs with the displacement loop (D loop).”

Probably not a substantial claim: may not pass S4 (based). May combine two claims.

Protecting the overhang alone cannot prevent the telomere damage response, so shelterin must use at least one additional mechanism to hide telomeres from DNA damage surveillance.

“If overhang protection is not sufficient to prevent the telomere damage response, shelterin must have at least one other mechanism to prevent detection of telomeres by the DNA damage surveillance.”

May combine two claims.

Normal mouse and human cells show little T-SCE, whereas ALT cells show very frequent T-SCE, which suggests that T-SCE is normally suppressed and that ALT cells have relaxed their control of telomeric HR.

“In normal mouse and human cells, T-SCE is not frequent, but ALT cells, which maintain their telomeres by a recombination pathway, have very frequent T-SCE (Bailey et al. 2004;Bechter et al. 2004;Londono-Vallejo et al. 2004).”

• Also stated in: § Repression of HR · hedged

  “The difference may be that T-SCEs are normally repressed and that ALT cells have loosened their control of HR at telomeres, allowing them to maintain their telomeres and therefore to survive (for review, see Neumann and Reddel 2002).”

• cites Bailey et al. 2004Not checked yet

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• cites Bechter et al. 2004Not checked yet

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• cites Londono-Vallejo et al. 2004Not checked yet

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May combine two claims.

Human telomeres that lose shelterin function trigger the canonical DNA damage response; specifically, TRF2 loss turns on ATM signaling, raising p53 and causing a p21-dependent G1/S arrest.

“that dysfunctional human telomeres indeed activate the DNA damage response pathway first emerged from experiments in which the shelterin subunit TRF2 was inhibited with a dominant-negative allele (TRF2 ⌬B⌬M ), that heterodimerizes with the endogenous TRF2, blocking its binding to DNA (van Steensel et al. 1998; for review, see de Lange 2002).”

• Also stated in: § Shelterin inhibition activates the canonical DNA damage response · plain

  “The loss of TRF2 activates the ATM kinase pathway, leading to p53 up-regulation and a p21-mediated G1/S arrest (Karlseder et al. 1999).”

• cites van Steensel et al. 1998Not checked yet

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• cites Karlseder et al. 1999Not checked yet

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May combine two claims.

Short telomeres with less shelterin reduce POT1 loading on the overhang, increasing the chance that telomerase elongates the telomere.

“Due to the diminished amount of shelterin, the chance that POT1 loads on the overhang is reduced, leading to a higher chance that telomerase can elongate the telomere.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

NHEJ can occur prior to DNA replication, giving rise to chromosome-type fusions in metaphase.

“NHEJ can also occur prior to DNA replication, giving rise to chromosome-type fusions in metaphase (not shown).”

May combine two claims.

Forced increase of POT1 on telomeres through shelterin overexpression increases the chance that telomerase is blocked, resulting in telomere shortening.

“Forced increase of POT1 on telomeres (through shelterin overexpression) increases the chance that telomerase will be blocked, resulting in telomere shortening.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

The size of the t-loop is variable.

“The size of the loop is variable.”

Probably not a substantial claim: may not pass S4 (based).

More shelterin on longer telomeres increases loading of POT1 on the telomeric overhang.

“The presence of more shelterin on longer telomeres is proposed to increase the loading of POT1 on the telomeric overhang.”

• AssumptionThe authors state this as an assumption: The t-loop is unlikely to be a substrate for telomerase.

May combine two claims.

Telomeres lose protection after inhibition of shelterin subunits such as TRF2, TIN2, or POT1, or after telomere attrition.

“Telomeres lose protection after inhibition of shelterin subunits (e.g., TRF2, TIN2, or POT1) or telomere attrition.”

Probably not a substantial claim: may not pass S4 (based).

POT1 bound to the 3' end blocks telomerase from acting.

“POT1 bound to the 3Ј end is proposed to block telomerase from acting.”

May combine two claims.

In mouse cells, p16 is induced but its induction does not affect entry into S phase.

“In mouse cells, p16 is induced, but its induction does not affect entry into S phase(Smogorzewska and de Lange 2002).”

• cites Smogorzewska and de Lange 2002Not checked yet

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Probably not a substantial claim: may not pass S4 (based).

After replication, chromosome ends require processing to acquire a long 3' overhang.

“After replication, chromosome ends require processing in order to acquire a long 3Ј overhang.”

May combine two claims.

Lagging-strand telomeric DNA synthesis copies the G-rich strand and is expected to be inhibited by G-quadruplexes.

“which copies the G-rich telomeric strand and is therefore expected to be inhibited by G-quadruplexes”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

WRN is a RecQ helicase with 3' exonuclease activity.

“Another of shelterin's fair-weather friends is WRN, a RecQ helicase with 3Ј exonuclease activity.”

• cites Opresko et al. 2002Not checked yet

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May combine two claims.

Shelterin must control how these DNA repair factors act because some have the ability to destroy chromosome ends.

“At the same time, shelterin must control how these DNA repair factors act because some have the ability to destroy chromosome ends.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

The resulting terminal structure is a preferred binding site for POT1 in vitro.

“the resulting terminal structure is a preferred binding site for POT1 in vitro.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

The Mre11/Rad50/Nbs1 complex is associated with shelterin.

“Mre11/Rad50/Nbs1 | recombinational repair | t-loop formation/resolution? | associated with shelterin”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

The Mre11 complex is required for t-loop HR.

“required for t-loop HR”

May combine two claims.

Telomere damage activates ATM kinase, leading to p53-dependent G1/S arrest and can induce apoptosis or senescence.

“Telomere damage activates the ATM kinase, which leads to a p53-dependent G1/S arrest and can induce either apoptosis or senescence.”

Probably not a substantial claim: may not pass S4 (based).

Other aspects of shelterin, largely unknown, allow telomeres to evade detection by DNA damage surveillance.

“Yet other aspects of shelterin, so far largely unknown, allow telomeres to evade detection by the DNA damage surveillance.”

May combine two claims.

Inhibition of shelterin, or a POT1 mutant that does not bind ssDNA, reduces the chance that telomerase is blocked, resulting in telomere elongation.

“Inhibition of shelterin, or a mutant of POT1 that does not bind ssDNA, reduces the chance that telomerase will be blocked, resulting in telomere elongation.”

Probably not a substantial claim: may not pass S4 (based).

The resulting 5' end always has the sequence ATC-5'.

“The resulting 5Ј end always has the sequence ATC-5Ј.”

May combine two claims.

In vitro, WRN can resolve the t-loop structure and degrade the 3' overhang.

“In vitro, WRN can resolve the t-loop structure and degrade the 3Ј overhang”

• cites Opresko et al. 2002Not checked yet

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• cites Opresko et al. 2004Not checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

The general function of WRN is to allow branch migration of Holliday junctions.

“The general function of WRN is to allow branch migration of HJs.”

May combine two claims.

Upon loss of shelterin, t-loops are destabilized or not formed, allowing engagement of the NHEJ pathway.

“Upon loss of shelterin, t-loops are proposed to be destabilized (or not formed), allowing engagement of the NHEJ pathway.”

Probably not a substantial claim: may not pass S4 (based).

The t-loop conceals the chromosome end from the NHEJ machinery.

“presumably so that the chromosome end is concealed from the NHEJ machinery.”

Probably not a substantial claim: may not pass S3 (load-bearing).

G-quadruplex removal activity is proposed to be necessary for telomere replication.

“This activity was recently proposed to be necessary for telomere replication”

May combine two claims.

WRN interacts with TRF2 and can be detected at telomeres by ChIP and IF in S phase.

“WRN interacts with TRF2 and can be detected at telomeres by ChIP and IF in S phase (Opresko et al. 2002”

• cites Opresko et al. 2002Not checked yet

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• cites Crabbe et al. 2004Not checked yet

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• cites Machwe et al. 2004Not checked yet

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May combine two claims.

Telomere dysfunction induces p16, which can contribute to inhibition of proliferation in p53-deficient human cells.

“Telomere dysfunction also induces p16(Jacobs and de Lange 2004), which can contribute to the inhibition of proliferation in p53-deficient human cells.”

• cites Jacobs and de Lange 2004Not checked yet

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May combine two claims.

Shelterin recognizes telomeric DNA and remodels it into a t-loop.

“Shelterin recognizes telomeric DNA and remodels it into a t-loop”