Institute of Development, Aging and Cancer, Tohoku University


Seminars and Symposia

IDAC Seminar, 21 November 2012

Secretariat, Alumni Association, IDAC
Date Wednesday, 21 November 2012, 17:00~
Room Smart Aging Research Building International Conference Room
Title A Chemicogenetic screen reveals a role for TORC2 in the maintenance of genome stability
Speaker Susan M Gasser
Affiliation Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland 
Person-in-charge Kozo Tanaka Dept.Molecular Oncology (ex8491)
Abstract The maintenance of genome stability is a major challenge to all living organisms, because cells are continuously threatened by endogenous and exogenous sources of DNA damage. Failed DNA repair can lead to genetic aberrations, such as chromosome loss, translocations, deletions or point mutations. The risk of damage is accentuated in S phase, when two enzymes contribute significantly and synergistically to genome stability: the ATR kinase (Mec1 in budding yeast), a members of the phosphoinositide 3-kinase-related protein kinase family, and the Bloom’s Syndrome RecQ helicase (Sgs1 in budding yeast). To understand how these pathways are regulated and to identify more players in their control, we carried out a high-throughput screen in yeast, using either an S-phase specific ATR mutant (mec1-100) or a null allele of BLM (sgs1Δ). We scored for synthetic lethality between low levels of hydroxyurea and compounds in the Novartis chemical compound library. We identified a compound that inhibits both TOR (Target Of Rapamycin) kinase complexes in yeast, Torc1 and Torc2. Both of these PIKK kinases play central roles in cell growth and response to nutritional conditions. Further genetic analysis has shown that this novel inhibitor renders cells very sensitive to DSB inducing agents like Zeocin?, and that the Torc2C?Ypk1/Ypk2 cascade of phosphorylation pathway is crucial for the maintenance of genomic stability in the presence of low levels of DSBs. Phosphoproteomics identified numerous proteins downstream of TORC2 and the YPK1/2 kinase that are specifically phosphorylated in response to DNA damage. Most striking among these are proteins involved in clatrin-coated endocytosis and actin polymerization. Actin depolymerization induced by latrunculin A resembles TORC2 inhibition with respect to Zeocin-sensitivity, as does attenuating actin polymerization by depleting ARP2/3 activators, Pan1 or Las17. These results suggest that the depolymerization of the actin cytoskeleton (G-actin level) impairs the repair of lesions or enhances generation of breaks. It is unclear if high nuclear actin triggers chromosomal breakdown or if it is an indirect effect.
Chromatin remodelers are important regulators of transcription and DNA repair, and generally they contain both actin and actin related proteins (Arp proteins). We have examined the role of various chromatin remodelers in the subnuclear movement of DNA upon damage. Long-range chromatin movement seems to be driven by the large ATPases that mediate chromatin remodeling. Indeed, targeting subunits of the ATP-dependent chromatin remodeling complex INO80 to chromatin increases the volume that a tagged locus can explore, in a manner entirely dependent on the ATPase activity of the complex. Furthermore, we found that increased chromatin movement increases the rates of homologous recombination between distant sequences. This led to the notion that double strand breaks might specifically make use of movement within the nucleus to facilitate the homology search for repair by homologous recombination.