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The Effects of Pol Beta on Homologous Recombination

Jeffrey T. Loh
Department of Biology,Massachusetts Institute of Technology, Cambridge, MA 02139
Work conducted in the laboratory of Prof. Bevin Engelward, Division of Biological Engineering

 
   
Non-Scientific Abstract

Every cell of every organism contains the genetic material DNA. If DNA is damaged, cells are more susceptible to mutation or cell death. Unfortunately, anything from UV light from the sun to the preservatives in the food we eat is a source of DNA-damaging agents. In order to repair damaged DNA, the cell uses a repair mechanism called the base excision repair (BER) pathway. Although the BER pathway consists of many proteins, the enzyme beta-polymerase (δ-pol) is most essential in regulating this pathway.While δ-pol is not necessary to start the repair of DNA through the base excision repair pathway, δ-pol is necessary to finish the processing of damaged nucleotides in DNA. In this study,we seek to investigate the method that δ-pol deficient cells use to repair damaged DNA.We propose that cells unable to fully correct damaged DNA in the BER pathway produce intermediates, which results in the repair of damaged DNA through homologous recombination. To test this hypothesis,we measured the rate of recombination in δ-pol null and δ-pol WT cells exposed to DNA damaging agents. Initial results show that δ-pol null cells appear more susceptible to homologous recombination.

Abstract

δ -pol is a necessary component of the base excision repair pathway (BER). Cells that lack δ -pol are incapable of fully processing damaged nucleotides in DNA,which leads to a higher incidence of apoptosis when cells are exposed to DNA damage (Ochs et al, 2002).While cells often use the BER pathway to repair damaged DNA, other repair mechanisms are also available in cells. It has been shown in Escherichia coli that endonuclease induced single strand breaks from the BER pathway can be repaired by homologous recombination through the use of double strand breaks, which result from replication fork breakdown in cells lacking δ-pol (Michel et al, 2001). By using mouse embryo fibroblast δ- pol WT and δ-pol null cells to study any differences in the rate of recombination,we propose that a similar pathway exists in mammalian cells. Preliminary results show that δ-pol null cells treated with DNA damaging agents appear to have greater increases in recombination frequency than δ-pol WT cells. If δ-pol null cells indeed undergo recombination more frequently than δ-pol WT cells, this would provide evidence that cells lacking δ-pol are more susceptible to homologous recombination due to the presence of BER intermediates.