Surprising Discovery in the Repair of DNA Double-Strand Breaks

Zhiyuan Shen
University of New Mexico, Albuquerque
R29ES8353

Background: DNA can suffer a wide variety of damage. A particularly bad problem caused by ionizing radiation and oxidative stress is double-strand breaks (DSBs). DSBs form when both strands of the DNA double helix are broken either through the direct action of the damaging particle or through the action of repair enzymes. In mammalian cells these, DSBs can be rejoined by homologous recombination or non-homologous end joining. This latter form of repair has been suggested to be the major pathway for repair in mammalian cells. The repair of DSBs is mediated by a large protein machine made up of many proteins including: RAD51, RAD52, RAD54, XRCC2, and XRCC3. Gene products that have been shown to be defective in certain cancers such as, p53, BRCA1 or BRCA2 are also believed to be involved in DSB repair. Furthermore, increased RAD51 has been associated with sporadic ductal breast cancer, whereas a decrease in this protein has been associated with 30% of breast carcinomas. An increase in RAD51 has also been associated with resistance to ionizing radiation. RAD51 is believed to be the key protein that promotes two homologous strands of DNA to pair together.

Advance: Dr. Shen and colleagues have engineered special mammalian and human cell lines that target an endonuclease, an enzyme that makes DSBs, to a particular chromosome location. This group can follow the repair events following the DSB. In contrast to earlier results, Dr. Shen found that over expression of either RAD51 or RAD52, led to a decrease in targeted DSB repair. Interestingly over expression of both proteins actual depressed repair of DSBs by homologous recombination more than over expressing just one.

Implication: RAD51 and RAD52 are absolutely required for the exchange of genetic information though homologous recombination. It is therefore surprising that over expression of either or both these proteins leads to a decrease in DSB repair through homologous recombination. These results indicate that the repair of DSBs through homologous recombination is mediated by a complex process in which each component must be precisely regulated in order for the protein machine to function efficiently. Gene therapy strategies often use the cell's host recombination machinery to splice in the corrected gene. Results presented in this study suggest that over expression of either RAD51 or RAD52 will not increase and would probably decrease gene targeting.

Citation: Kim PM, Allen C, Wagener BM, Shen Z, Nickoloff JA. Overexpression of human RAD51 and RAD52 reduces double-strand break-induced homologous recombination in mammalian cells. Nucleic Acids Res. 2001 Nov 1;29(21):4352-60.