Deciphering the role of heterochromatin in telomere function and maintenance mechanisms

PROJECT SUMMARY
Telomeres are nucleoprotein structures that protect the ends of linear chromosomes and thereby
maintain genome stability. Telomeres solve both the end-protection and the end-replication problems: 1)
They inhibit DNA damage at chromosome ends, which would otherwise resemble broken DNA, 2) Since
chromosome ends shorten during replication, telomeres act as buffer sequences to prevent loss of coding
regions, 3) Once telomeres become too short, they can no longer inhibit DNA damage, leading to
permanent cell cycle arrest (senescence). This “mitotic clock” is a critical tumor-suppressive barrier that
forces aging cells to stop dividing. To become cancerous, cells must acquire unlimited division potential
by activating a telomere maintenance mechanism, either reactivation of telomerase, the enzyme that
elongates telomeres during development, or through the alternative lengthening of telomeres (ALT)
mechanism, which is based on recombination.
Telomeres consist of 5-15kb of (TTAGGG)n repeats organized into tightly packed nucleosomes and
bound by the shelterin, a complex of six non-histone proteins. Telomeres are considered as
heterochromatin and are enriched in the repressive H3K9me3 “histone mark”. Intense focus has been
placed on trying to decipher the exact chromatin status of telomeres, but the much more important
question has been neglected and remains unanswered: What is the role and function of
heterochromatin at telomeres? While the roles of shelterin proteins have been extensively studied, the
function of heterochromatin at telomeres remains largely unexplored.
Using a novel approach to locally and specifically modulate histone methylation at telomeres, we
will thoroughly dissect the function of H3K9me3 in telomere protection and maintenance. By
fusing histone modifying enzymes to the shelterin protein TRF1, we can locally enrich or deplete
H3K9me3 at telomeres.
Our preliminary data revealed that loss of H3K9me3 leads to severe replication defects and de-repression
of telomere transcription. These data suggest that heterochromatin could play unanticipated roles in the
regulation of replicative aging and the onset of senescence. Moreover, while the general consensus is
that ALT is associated with less condensed chromatin at telomeres, we found that H3K9me3 is a driver
of ALT activity.
Using this unique approach to manipulate H3K9 trimethylation at telomeres, we will methodically
determine the function of this heterochromatin mark on the protective properties of telomeres (end-
protection, end-replication, entry into senescence) as well as on the ALT mechanism of telomere
maintenance.

Grant Number: 5R35GM143108-05
NIH Institute/Center: NIH
Principal Investigator: Nausica Arnoult