Sørensen Group – University of Copenhagen

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Sørensen Group

The goal of our studies is to understand the mechanisms responsible for maintenance of genomic stability in mammalian cells.
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.

Figure 1. Cells escaping the G2/M checkpoint are easily detected in our robotic-assisted siRNA screens. U2OS cells were treated with ionizing radiation (6 Gy) for 6 hours in the absence or presence of the key checkpoint regulator Chk1. 
2. Genome maintenance in the context of chromatinTil toppen

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).

Figure 2. Depletion of SET8 leads to massive genomic instability as evidenced by gamma-H2AX foci and RAD51 foci.

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 publications

Galanos P, Vougas K, Walter D, Polyzos A, Maya-Mendoza A, Haagensen EJ, Kokkalis A, Roumelioti FM, et al. (2016) Chronic p53-independent p21 expression causes genomic instability by deregulating replication licensing.  Nature Cell Biology PMID: 27323328

Nielsen FC, Hansen TvO, Sørensen CS. (2016) Hereditary Breast and Ovarian Cancer: New Genes in Confined Pathways. Nature Reviews Cancer, in press

Ahlskog J, Larsen BD, Kavya A, Sørensen CS. (2016) ATM/ATR‐mediated phosphorylation of PALB2 promotes RAD51 function. EMBO Reports. PMID: 27113759

Walter D, Hoffmann S, Komseli ES, Rappsilber J, Gorgoulis V, Sørensen CS. (2016) SCF(Cyclin F)-dependent degradation of CDC6 suppresses DNA re-replication. Nature Communications. PMID: 26818844

Boyer AS, Walter D, Sørensen CS. (2016) DNA replication and cancer: From dysfunctional replication origin activities to therapeutic opportunities. Seminars in Cancer Biology, Pp 1044-57. PMID: 26805514

Shoaib M & Sørensen CS. (2015) Epigenetic Deficiencies and Replicative Stress: Driving Cancer Cells to an Early Grave. Cancer Cell, PMID: 265551681.

Dobbelstein, M & Sørensen, CS. (2015) Exploiting replicative stress to treat cancer. Nature Reviews Drug Discovery, PMID: 25953507.

Fugger K, Mistrik M, Neelsen KJ, Yao Q, Zellweger R, Kousholt AN, Haahr P, Chu WK, Bartek J, Lopes M, Hickson ID, Sørensen CS. (2015) FBH1 Catalyzes Regression of Stalled Replication Forks. Cell Reports, PMID: 25772361.

Klein DK, Hoffmann S, Ahlskog JK, O'Hanlon K, Quaas M, Larsen BD, Rolland B, Rösner HI, Walter D, Kousholt AN, Menzel T, Lees M, Johansen JV, Rappsilber J, Engeland K, Sørensen CS. (2015)  Cyclin F suppresses B-Myb activity to promote cell cycle checkpoint control. Nature Communications, PMID: 25557911.

Klein DK, Hoffmann S, Ahlskog JK, O’Hanlon K, Quaas M. Larsen BD, Rolland B, Rösner HI, Walter D, Kousholt AN, Menzel T, Lees M, Johansen JV, Rappsilber J, Engeland K, Sørensen CS (2014). Nature Communications.

Hühn D, Kousholt AN, Sørensen CS, Sartori AA. (2014). miR-19, a component of the oncogenic miR-17∼92 cluster, targets the DNA-end resection factor CtIP. Oncogene. doi: 10.1038/onc.2014.329.

Fugger K, Chu WK, Haahr P, Kousholt AN, Beck H, Payne MJ, Hanada K, Hickson ID, Sørensen CS (2013). FBH1 co-operates with MUS81 in inducing DNA double-strand breaks and cell death following replication stress. Nature Communications; 4:1423.

Jørgensen S, Schotta G, Sørensen CS (2013). Histone H4 lysine 20 methylation: key player in epigenetic regulation of genomic integrity. Nucleic Acids Research 41(5):2797-806. Review.

Beck H, Nähse V, Larsen MS, O’Hanlon KA, Patzke S, Holmberg C, Mejlvang J, Groth A, Nielsen O, Syljuåsen RG, Sørensen CS (2012). CDK suppression by WEE1 kinase protects the genome through control of replication initiation and nucleotide consumption. Molecular and Cellular Biology, 32(20):4226-36.

Kousholt AN, Fugger K, Hoffmann S, Larsen BD, Menzel T, Sartori AA, Sørensen CS
(2012). CtIP-dependent DNA resection is required for DNA damage checkpoint maintenance but not initiation. Journal of Cell Biology, 197(7):869-76.

Sørensen CS & Syljuåsen RG (2012). Safeguarding genome integrity: The checkpoint kinases ATR, CHK1 and WEE1 restrain CDK activity during normal DNA replication.
Nucleic Acids Research, 40(2):477-86. Review.

Menzel T, Nähse-Kumpf V, Kousholt AN, Klein DK, Lund-Andersen C, Lees M, Johansen JV, Syljuåsen RG, Sørensen CS. A genetic screen identifies BRCA2 and PALB2 as key regulators of G2 checkpoint maintenance. EMBO Rep. 2011 Jul 1;12(7):705-12.

Jørgensen S, Eskildsen M, Fugger K, Hansen L, Larsen MSY, Nedergaard AK, Syljuåsen RG, Trelle MB, Jensen ON, Helin K, Sørensen CS (2011). SET8 is degraded via PCNA-coupled CRL4(CDT2) ubiquitylation in S phase and after UV irradiation. J Cell Biol, 192(1): 43-54.

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