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The
focus of our research is to understand the molecular basis
of telomere function in human cells. Telomeres, act as
protective caps and serve as templates for replication by the enzyme telomerase. Telomere integrity is essential for
chromosome stability and maintenance, and telomeres play a key role in aging and cancer.
Mammalian telomeres are comprised of TTAGGG repeats and shelterin, a six-subunit complex.
Tankyrase 1 is a poly(ADP-ribose) polymerase that localizes to telomeres via association with the shelterin subunit
TRF1 and regulates telomere function. Our studies have shown that overexpression of tankyrase 1 evicts TRF1 and its
binding partner TIN2 from telomeres, leading to access of telomerase and telomere elongation.
Thus, tankyrase 1 functions as a positive regulator of telomere length.
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| Tankyrase
1 and sister telomer cohesion |
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Tankyrase 1 is also required after DNA replication for sister telomere separation prior
to mitosis. In the absence of tankyrase 1 sister chromatids resolve normally at centromeres and arms, but remain
associated at telomeres.
Sister chromatids are held together along their length by cohesin, a multisubunit complex comprised of a ring
containing Smc1, Smc3, and Scc1, and a peripheral component SA1 or SA2. We found that cohesion between sister telomeres is mediated by
a complex comprised of the cohesin ring,
the peripheral component SA1 (but not SA2) and the shelterin subunits TRF1 and TIN2. Thus sister telomere cohesion is mediated by
a unique complex that requires tankyrase 1 for its resolution. We are currently investigating the consequences of aberrant cohesion
at telomeres.
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| Persistent cohesion leads to fusion |
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In tankyrase 1 deficient cells sister telomere resolution is blocked.
Unexpectedly, cohered sister telomeres become deprotected and are inappropriately fused.
By contrast to telomeres rendered dysfunctional by removal of shelterin, which
engage in chromatid fusions predominantly between chromatids from different chromosomes, telomeres
rendered dysfunctional by tankyrase 1 engage in chromatid fusions almost exclusively between sister
chromatids. We show that cohered sister telomeres are fused by DNA Ligase IV mediated NHEJ.
Our work shows that timely removal of sister telomere cohesion is essential for formation of a
protective structure at chromosome ends following DNA replication in S/G2 phase of the cell cycle.
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| Premature loss of telomere cohesion |
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We are also investigating
the effects of premature loss of cohesion. We found that cells deficient in SA1 or TIN2 were unable to establish
or maintain cohesion between sister telomeres following DNA replication in S phase.
By contrast, in SA2 depleted cells telomere cohesion was normal, but centromere cohesion was prematurely lost.
Thus, cohesinSA1 and cohesinSA2 are differentially required for telomere and centromere cohesion.
We further found that loss of telomere cohesion had dramatic consequences on chromosome structure and function.
Loss of telomere cohesion led to loss of cohesion all along chromosome arms. Moreover, in the absence of sister
telomere cohesion cells were unable to repair chromatid breaks and suffered sister telomere loss, revealing
an essential role for sister telomere cohesion in genomic integrity.
Currently we are continuing to investigate the role of telomere cohesion in chromosome integrity by studying the role of cohesion
in ALT (alternative lengthening of telomeres) cells. ALT cells are cancer cells that maintain their telomeres in the absence of
telomerase by homologous recombination. We predict that loss of telomere cohesion will have dramatic effects on telomere maintenance
and cell growth in ALT cells. In addition, we are investigating the mechanism by which tankyrase 1 resolves sister telomere cohesion.
We are using biochemical and cell biological approaches to determine precisely when and how tankyrase 1 goes to telomeres and how it resolves cohesion.
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