Transcript Double Strand Breaks Can Initiate Gene Silencing

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
ARR Model for DNA Repair
Green et al. EMBO rep. 2002
Can the DNA Repair Process Lead to Aberrant Gene Silencing?
• 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:
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
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?