Double Strand Breaks Can Initiate Gene Silencing and SIRT1-Dependent Onset of

DNA Methylation in an Exogenous Promoter CpG Island

Heather M. O’Hagan, Helai P. Mohammad, Stephen B. Baylin

Green et al. EMBO rep. 2002

ARR Model for DNA Repair

Can the DNA Repair Process Lead to Aberrant Gene Silencing?

1) SIRT1 - protein/histone deacetylase that can be part of a PcG complex

2) EZH2 - HMT responsible for repressive histone marks, also in PcG complex

3) DNMT1 - involved in maintaining DNA methylation

4) DNMT3B - involved in de novo DNA methylation

• Tumor suppressor genes are often silenced in cancer cells.

• This silencing often occurs through epigenetic means such and chromatin modification and DNA methylation

Possible candidates for repair-induced silencing:

Treatment with tetracycline induces a double strand break in the inserted E-cad promoter

Treatment with tetracycline induces a double strand break in the inserted E-cad promoter

DSB damage and/or repair induces the transient recruitment of SIRT1, DNMT1, and DNMT3B

DSB damage and/or repair induces the transient recruitment of SIRT1, DNMT1, and DNMT3B

Effects of knockdown of SIRT1 by siRNA

Effects of knockdown of SIRT1 by siRNA

Changes in enrichment of silencing proteins and chromatin marks with knockdown of SIRT1

Changes in enrichment of silencing proteins and chromatin marks with knockdown of SIRT1

Inducing a DSB in a promoter can lead to silencing and the seeding of methylation

Inducing a DSB in a promoter can lead to silencing and the seeding of methylation

Reduction of SIRT1 during DNA damage decreases the number of silent clones that have methylation

Reduction of SIRT1 during DNA damage decreases the number of silent clones that have methylation

Conclusions

• Silencing proteins can be recruited to the site near a double strand break

• Prolonged recruitment may lead to seeding and spreading of DNA methylation

• Supports a role for DNA damage in the epigenetic silencing of genes in tumors.

Final questions or thoughts

• Are silencing proteins recruited during gene expression in order to rapidly turn genes off?– Could the same theory apply for genes being

expressed and not repaired?