Tier 3 — preclinical

SIRT6 is a histone H3 lysine 9 deacetylase that modulates telomeric chromatin

Michishita E, McCord RA, Berber E, Kioi M, Padilla-Nash H, Damian M, Cheung P, Kusumoto R, Kawahara TLA, Barrett JC, Chang HY, Bohr VA, Ried T, Gozani O, Chua KF

Nature 2008 Volume 452, issue 7186, pages 492–496

Bibliography

DOI: 10.1038/nature06736
PubMed: PMID 18337721
Funding: US National Institutes of Health grants (to K.F.C., O.G., H.Y.C., R.A.M. and T.L.A.K.), the American Federation for Aging Research / Paul Beeson Scholar Award and the Department of Veterans Affairs Merit Review (to K.F.C.), the Burroughs Wellcome Fund and Searle Scholar Award (to O.G.), and funds from the Intramural Research Program of the NIH, the National Cancer Institute, the Center for Cancer Research and the National Institutes on Aging.
Competing interests: Not explicitly declared in the manuscript text available.

Study snapshot

Design	Interventional, cell-based mechanistic study using RNAi knockdown, in vitro deacetylation assays, and telomere chromatin immunoprecipitation.
Model	WI-38 human diploid fibroblasts, IMR90 primary fibroblasts, U2OS osteosarcoma cells, HeLa cells, HEK293T cells, and Sirt6-knockout mouse embryonic fibroblasts.
Sample	Multiple independent replicates per experiment. Telomere dysfunction-induced foci quantification n=158 control cells and n=110 SIRT6-knockdown cells. Telomere-ChIP experiments n=4 independent replicates.
Intervention	Retroviral shRNA-mediated SIRT6 knockdown (three independent hairpins), Flag-SIRT6 or catalytic-inactive Flag-SIRT6-H133Y overexpression, ectopic hTERT or DNA polymerase-β dRP lyase domain expression, and in vitro incubation of recombinant SIRT6 with 13 acetylated histone tail peptides plus full-length histone H3.
Duration	Serial passaging through approximately 50 population doublings for replicative senescence assays. In vitro deacetylation reactions run for 1 hour at 30 °C.
Endpoints	Population doubling until replicative senescence; senescence-associated β-galactosidase staining; telomere dysfunction-induced foci (γ-H2AX co-localised with telomere signal); chromosomal end-to-end fusions by spectral karyotyping; SIRT6 occupancy at telomeric chromatin (T-ChIP); H3K9 acetylation at telomeric chromatin (S-phase T-ChIP); WRN occupancy at telomeric chromatin; sister telomere loss and telomere doublets per metaphase

What the study showed, in plain terms

Before this paper, SIRT6 was known to be a chromatin-associated protein whose loss in mice caused a premature-ageing-like syndrome, but no one had shown what it actually does at the molecular level. This study is where SIRT6 first gets a defined biochemical job.

The researchers show that human SIRT6 is an NAD⁺-dependent deacetylase that removes an acetyl group from a specific residue on histone H3, called lysine 9 (H3K9), and that it does so preferentially at the ends of chromosomes — the telomeres. When they screened SIRT6 against a panel of acetylated histone tail peptides, only H3K9 was efficiently deacetylated.

When SIRT6 was knocked down in human fibroblasts, several things happened at once. The cells hit replicative senescence about ten population doublings early. Telomeres became dysfunctional and were recognised as sites of DNA damage. Chromosomes fused end-to-end. And the Werner syndrome protein WRN, which normally helps process telomeres during DNA replication, could no longer associate stably with the telomeric chromatin.

The paper's proposed model is that SIRT6 keeps H3K9 acetylation low at telomeres, and that this specific chromatin state is what allows WRN and related factors to work properly at replicating telomeres. Lose SIRT6, and the telomere-replication machinery becomes unstable, causing the same kind of cellular damage seen in Werner syndrome — a rare human disorder that causes premature ageing.

Key findings

Human SIRT6 is an NAD⁺-dependent, highly specific H3K9 deacetylase; a screen against 13 acetylated histone tail peptides showed activity only against H3K9-Ac.
SIRT6-knockdown WI-38 fibroblasts underwent premature replicative senescence approximately 10 population doublings ahead of control cells, with a 3-fold increase in senescence-associated β-galactosidase-positive cells.
Telomere dysfunction-induced foci (γ-H2AX co-localising with telomere signal) rose from ~10% of control cells to ~35% of SIRT6-knockdown cells (p = 1.22 × 10⁻⁶).
Ectopic hTERT expression rescued the premature senescence of SIRT6-knockdown cells, whereas supplementing base excision repair (via DNA polymerase-β dRP lyase domain) did not — placing the senescence phenotype downstream of telomere dysfunction, not base excision repair.
Telomeric H3K9 hyperacetylation was confirmed in SIRT6-knockdown human cells and in Sirt6-knockout mouse cells, giving in vivo evidence that SIRT6 maintains low physiological H3K9 acetylation at telomeric chromatin.
SIRT6 depletion reduced WRN occupancy at telomeric chromatin in S phase without disrupting the SIRT6-telomere association itself, consistent with a model where SIRT6's chromatin modification is upstream of stable WRN recruitment.
SIRT6-knockdown cells showed elevated sister telomere loss and telomere doublets per metaphase, plus non-recurrent chromosomal end-to-end fusions with weak or absent telomere signal at fusion sites — the same pattern seen in Werner-syndrome cells and distinct from TRF2-deficient fusions.

What this study can and cannot tell us

This is a mechanistic cell-based study. The findings describe a molecular pathway inside cultured human and mouse cells, not an intervention in a whole organism, and the connection to human ageing is inferential rather than clinical.

The link to Werner syndrome is drawn from shared cellular phenotypes — telomere dysfunction, chromosomal fusions with weak telomere signal, delayed S-phase completion — rather than from a demonstration that SIRT6 dysfunction is present in Werner syndrome patients. Werner syndrome itself is caused by mutations in the WRN helicase; this paper positions SIRT6 upstream of WRN function, but it does not show that SIRT6 modulation would alter the human disease course.

H3K9 hyperacetylation was demonstrated at telomeric chromatin in both human and mouse cells, but the downstream cellular defects (premature senescence, chromosomal instability) were only seen in the human cells. Mouse cells appear buffered by their much larger telomere reserve, and the authors flag this as evidence that species-level telomere biology matters when translating findings.

The paper does not address whether SIRT6 activity can be pharmacologically increased in humans, and no dosing, safety, or supplement data are relevant to this work.