Sørensen Group
We are primarily working on the identification and characterization of novel regulators of the cellular DNA damage response (DDR). This response is critical to avoid gradual accumulation of mutations when cells proliferate, and DDR failure can lead to genomic instability and consequently cancer. It is also of crucial importance in the clinic where cancer cells are treated with DNA damaging agents such as ionizing radiation and cytotoxic drugs. We focus on two partly overlapping areas of research: Large scale functional screens and genome maintenance in the context of chromatin.
1. Large-scale functional siRNA interference screens to identify and characterize the key regulators of DNA damage response pathways.
A central part of the DNA damage response consists of signal transduction cascades that are activated at sites of DNA lesions. These pathways promote DNA repair and delay cell cycle progression until breaks are repaired.
Many key questions are still unresolved:
• What are the rate-limiting components of the DDR machinery?
• What are the key targets downstream of the transduction cascades?
• How is the DDR turned off once lesions are removed?
• Can we target the DDR for therapeutic purposes?
To achieve our goals we are performing large-scale functional siRNA interference screens to find the key regulators of DNA damage response pathways. A robot-automated system and several siRNA libraries from Ambion are currently used. We are using this unique platform to perform rapid and highly efficient semi-genome wide phenotype screens. Current screens deal with identification of genes critical for:
• The radiation induced G2/M checkpoint (see figure 1)
• Genomic stability in the presence and absence of replication inhibitors.
These screens have proven extremely precise and powerful, as the vast majorities of genes identified in the screens have been validated. A large part of the focus in the laboratory now deals with the molecular characterization of these novel DDR regulators.
2. Genome maintenance in the context of chromatin
Cell proliferation depends on the ability to duplicate and maintain the genome. The key DNA replication and DNA repair processes are very much influenced by the chromatin environment that embeds DNA.
Several questions are emerging as very important:
• How does the chromatin environment influence the detection, repair and recovery from DNA lesions?
• What are the modifications at sites of DNA damage?
• How does chromatin environment affect proteins of the DDR proteins?
To answer these questions, we have purified histones from control cells and cells exposed to various types of DNA damage. We then analyzed the histones by quantitative mass spectrometry to compare the status of histone post-translational modifications. This is now being followed up by detailed analysis of the importance of these modifications as well as the enzymes that catalyze them. Several projects are ongoing in this direction, as an example we recently identified the histone methyltransferase SET8 (aka PR-Set7) as a critical regulator of genome stability during DNA replication (see figure 2).
Our research has already lead to the identification and characterization of novel cancer-relevant genes and molecular pathways. We are also unraveling new regulatory roles of well-known genes. We thereby provide new insights to the complex biology of cancer cells, and our work may bring us closer to clinically relevant targets for cancer therapy.
If you would like to be part of our exciting scientific environment, we welcome informal applications from enthusiastic students and post-doctoral fellows. We have a number of exciting but unexplored projects dealing with the molecular characterization of novel and validated DDR regulators. You are also welcome to approach us with ideas for siRNA screens that could form the basis of a solid project If you are interested in coming to work in the lab please email Claus.
Selected recent publications:
Beck H, Nähse V, Larsen MS, Groth P, Clancy T, Lees M, Jørgensen M, Helleday T, Syljuåsen RG, Sørensen CS. Regulators of cyclin-dependent kinases are crucial for maintaining genome integrity in S phase. J Cell Biol. 2010 Mar 1. Epub ahead of print.
Melixetian M, Klein DK, Sørensen CS#, Helin K# (2009). NEK11 regulates CDC25A degradation and the IR-induced G2/M checkpoint. Nat Cell Biol. 11(10):1247-53. # shared corresponding authorship.
Jørgensen S, Elvers I, Trelle MB, Menzel T, Eskildsen M, Jensen ON, Helleday T, Helin K, Sørensen CS. (2007). The histone methyltransferase SET8 is required for S-phase progression. J Cell Biol. 179(7):1337-45
Sørensen CS, Hansen LT, Dziegielewski J, Syljuasen RG, Lundin C, Bartek J and Helleday T (2005). The cell-cycle checkpoint kinase Chk1 is required for mammalian homologous recombination repair. Nat Cell Biol. 7(2):195-201
Syljuåsen RG, Sørensen CS, Hansen LT, Fugger, K, Lundin C, Johansson F, Helleday T, Lukas J, Bartek J (2005): Inhibition of human Chk1 causes increased initiation of DNA replication, phosphorylation of ATR targets, and DNA breakage. Mol Cell Biol 25(9): 3553-62.
Sørensen CS, Syljuåsen RG, Lukas J and Bartek J (2004). ATR, Claspin and the Rad9-Rad1-Hus1 complex regulate Chk1 and Cdc25A in the absence of DNA damage. Cell Cycle. 3(7):941-5.
Sørensen CS, Syljuasen RG, Falck J, Schroeder T, Ronnstrand L, Khanna KK, Zhou BB, Bartek J and Lukas J (2003). Chk1 regulates the S phase checkpoint by coupling the physiological turnover and ionizing radiation-induced accelerated proteolysis of Cdc25A. Cancer Cell. 3(3):247-58.
Sørensen CS, Lukas C, Kramer E, Peters JM, Bartek J and Lukas J (2001). Conserved cyclin binding domain determines phosphorylation of mammalian Cdh1 and its dissociation from the anaphase-promoting complex. Mol Cell Biol. 21(11):3692-703.