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Nancy C. Walworth, Ph.D.

Professor and Interim Chair
Phone: (732) 235-5661
Fax:(732) 235-4073
E-mail: walworna@rwjms.rutgers.edu

 

Research Interest:

Cell cycle checkpoints, genome stability, fission yeast chromatin

 

Research Description:

When eukaryotic cells are exposed to agents that cause DNA damage, such as UV light, X-rays or drugs used in cancer chemotherapy, progression through the cell cycle is transiently arrested. Research in our laboratory is directed toward understanding the mechanism through which eukaryotic cells arrest the cell cycle in response to DNA damage. We use the fission yeast, Schizosaccharomyces pombe, as a model system for these studies since the molecules that regulate cell cycle progression have been highly conserved through evolution and are well characterized in this organism. Furthermore, yeast is amenable to both classical and molecular genetic analysis.

The signal transduction pathway that couples the detection of DNA damage to control of cell cycle progression has been described as the DNA damage checkpoint. Dissection of the DNA damage checkpoint pathway in fission yeast will foster our understanding of how eukaryotic cells respond to DNA damage and how defects in this response may contribute to the development of cancer. We have focused on the role played by a conserved protein kinase encoded by the chk1 gene of fission yeast in this signal transduction process. Cells that lack chk1 function are unable to arrest the cell cycle when DNA damage takes place, and instead enter mitosis with damaged DNA and subsequently die. To characterize the role of Chk1 in mediating cell cycle arrest in response to DNA damage, we have taken a variety of approaches: biochemical studies to characterize the activity of Chk1 and its response to DNA damage; cell biological studies to understand its localization in the cell and how that changes in response to DNA damage; and genetic screens to identify additional proteins that act with Chk1 to promote cell survival following DNA damage.

The Chk1 protein is phosphorylated in response to DNA damaging agents. Phosphorylation is dependent on the activity of several other gene products that are required for cell cycle arrest in response to DNA damage. Phosphorylation of Chk1 correlates with cell cycle arrest and dephosphorylation accompanies recovery of cell cycle progression following DNA repair. Phosphorylation of Chk1 on a serine residue at position 345 is critical for Chk1 function and correlates with an increase in Chk1 catalytic activity. Following DNA damage, the local concentration of Chk1 in the nucleus increases. The interaction of Chk1 with a 14-3-3 protein called Rad24 is stimulated by DNA damage and we have shown that the interaction of Chk1 with Rad24 is important for governing Chk1 localization within the cell and for checkpoint function.

A genetic screen using the anti-cancer drug camptothecin identified novel checkpoint defective alleles of the chk1 gene. Suppressor analysis of these mutants has identified genes that are able to compensate for the loss of Chk1 function. One of these genes, msc1, is of particular interest as it encodes a protein with motifs that suggest a role for it in regulating chromatin structure. Msc1 co-fractionates with chromatin and associates with a histone deacetylase activity. The phenotype of cells lacking msc1 is consistent with a chromatin-related role for the protein as cells are hypersensitive to a drug that inhibits histone deacetylase activity. Cells lacking msc1 exhibit genomic instability characterized by an increased frequency of chromosome loss. While human homologues of Msc1 exist, very little is known about them. One homologue associates with the Rb tumor suppressor and another is upregulated in breast cancer cells. A careful analysis of Msc1 in fission yeast will shed light on the possible roles of the human homologues in normal and cancer cells. Msc1 has the capacity to facilitate the conjugation of ubiquitin onto target proteins. Msc1 exhibits genetic interactions with proteins that function at the kinetochore, a specialized protein complex that interacts with the centromeric region of chromosomes to facilitate their segregation at mitosis. In addition, Msc1 appears to facilitate the appropriate distribution of specialized proteins within the centromere which may explain its role in maintaining chromosome stability.