In Saccharomyces cerevisiae
, the NAD+
-dependent histone deacetylase Sir2 (silent information regulator 2) plays a key role in generating and maintaining silent chromatin near telomeres. This epigenetic silencing of telomere-proximal genes—a phenomenon termed telomere position effect (TPE)1–4
—is lost with replicative yeast aging, concomitant with aberrant hyperacetylation of sub-telomeric histones5,6
. One important feature of TPE is that the strength of silencing increases with telomere length7,8
, a property thought to reflect an increased repressive chromatin environment that results from longer telomeres9,10
. Like their yeast counterparts, mammalian telomeres can exert silencing effects on adjacent genes11–13
. However, aside from downstream changes such as altered histone methylation patterns and suggestions that HP1 is involved12,14
, the upstream factors required for telomeric gene silencing in human cells remain largely unknown.
The Sir2 family member SIRT6 is a highly substrate-specific histone deacetylase that promotes proper chromatin function in several physiologic contexts, including genome stabilization, DNA repair, and gene expression. Early work implicated SIRT6 in regulating aging-associated pathologies and lifespan in mice15
. Subsequently, SIRT6 was shown to possess NAD+
-dependent histone deacetylase activity, with specificity for lysines 9 and 56 of histone H3 (H3K9 and H3K56, respectively)16–18
Since the discovery of its enzymatic activity, SIRT6 has been shown to function in several important genomic contexts. First, SIRT6 modulates telomeric chromatin in mammalian cells16,17
. By deacetylating H3K9 and H3K56 at telomeres during S-phase, SIRT6 is required for proper replication of telomeres. Depletion of SIRT6 from human cells results in abnormal telomere structures and stochastic replication-associated telomere sequence loss, ultimately leading to genomic instability, chromosomal end-to-end-fusions, and premature cellular senescence11
In addition to its role at telomeres, SIRT6 also plays a critical role in regulating DNA repair. SIRT6 associates dynamically with chromatin flanking DNA double-strand breaks (DSBs) and is required for stabilization of the non-homologous end-joining (NHEJ) protein DNA-PKcs at DSBs and for efficient repair of these breaks19
. Recently, SIRT6 was shown to promote DNA DSB repair through several additional mechanisms, including stimulating DNA end-resection in homology-directed repair (HDR; ref20
) and activating the poly-ADP ribosyltransferase PARP-1 to promote NHEJ and HDR21
Finally, histone deacetylation by SIRT6 is important for active transcriptional repression of gene expression networks linked to aging and metabolism22,23
. For example, SIRT6 functions at specific NF-κB target gene promoters to attenuate gene expression, providing an important link between SIRT6 and active gene repression22
. More recently, SIRT6 was also shown to repress expression of genes involved in glucose and fat metabolism23–26
. However, no role for SIRT6 in the generation or maintenance of silent chromatin domains—at telomeres or elsewhere—has yet been described.
Here we uncover a novel role for SIRT6 in maintaining TPE in human cells. We show that SIRT6 dynamically regulates the silencing of a telomere-proximal transgene and that depletion of SIRT6 leads to loss of TPE-associated heterochromatic marks. In addition, we show that SIRT6 is required for the repression of an endogenous telomere-proximal gene. These findings indicate that SIRT6 is important for maintaining a proper telomeric chromatin structure that is required for silencing of nearby genes.