Deleting a Single Protein Restores Critical DNA Repair Process in Mice with Brca1 Gene Mutations

The 53BP1 protein interferes with homologous recombination in BRCA-deficient cells by blocking a nuclease from generating the single-stranded DNA necessary for recombination.  As a result, error-prone DNA repair is mediated by ligase IV.  When BRCA1 and 53BP1 are both defective, the break is not “clogged,” the nuclease can make single-stranded DNA, and recombination can proceed.

The 53BP1 protein interferes with homologous recombination in BRCA-deficient cells by blocking a nuclease from generating the single-stranded DNA necessary for recombination. As a result, error-prone DNA repair is mediated by ligase IV. When BRCA1 and 53BP1 are both defective, the break is not “clogged,” the nuclease can make single-stranded DNA, and recombination can proceed.

Women who carry a harmful mutation in the BRCA1 gene have up to an 85 percent greater lifetime risk of developing breast cancer than other women, and up to a 40 percent greater chance of developing ovarian cancer. Thus far, no effective therapies have been developed that overcome the susceptibility to cancer caused by mutations in BRCA1.

The protein produced by BRCA1 functions in a DNA repair process called homologous recombination (HR). Cells use HR to repair DNA damage that occurs spontaneously when cells duplicate their DNA in preparation for cell division. When HR is impaired, as it is when the BRCA1 gene is defective, a different DNA repair process kicks in. But that process is more prone to mistakes than HR, and these errors can lead to the formation of abnormal and unstable chromosome structures that set the stage for tumor development.

Mice with genetically engineered mutations in the BRCA1 gene frequently develop mammary tumors that are similar to human BRCA1-associated breast cancers. In previous studies, researchers had found that, when these BRCA1 mutant mice lack a protein called 53BP1, the formation of mammary tumors was dramatically reduced. To investigate how the loss of 53BP1 function could suppress mammary tumor formation in BRCA1-deficient mice, researchers from the National Cancer Institute and their colleagues did a series of experiments using cells grown in the laboratory. The researchers found that the loss of 53BP1 function restores HR in BRCA1-deficient mouse cells.

Further analysis led the investigators to propose that the BRCA1 and 53BP1 proteins are involved in a kind of tug-of-war to decide whether damaged DNA is fixed using the HR pathway or the alternative DNA repair pathway that promotes chromosome instability and cancer development. When cells have functional BRCA1 and 53BP1, the BRCA1 protein wins the tug-of-war and damaged DNA is repaired through the HR process. When cells are deficient in BRCA1, 53BP1 shifts the balance toward the more mistake-prone repair process. And, when both BRCA1 and 53BP1 are deficient, HR once again takes place normally.

“Our results show that the choice of pathway used to repair DNA damage determines whether the repair is error free or error prone,” said the study’s senior author, André Nussenzweig, Ph.D., who heads the Molecular Recombination Section of NCI’s Experimental Immunology Branch. “This opens the possibility of using drugs to inhibit mutagenic DNA repair pathways and promote error-free repair.”

The results also shed light on how some BRCA1 deficient cells become resistant to drugs that inhibit another protein called PARP, which functions in the repair of a specific type of DNA damage. Normally, HR can compensate for the loss of PARP activity. However, inhibition of PARP in cells that have defects in HR, such as those with BRCA1 mutations, is highly toxic to the cells. This discovery led to the development of PARP inhibitors, which are being tested in clinical trials to treat women with breast and ovarian cancer who have BRCA1 mutations. The results of the mouse study suggest that BRCA1-deficient tumors could become resistant to PARP inhibitors if they acquire additional mutations that allow HR to be restored, such as mutations in 53BP1.

By eliminating a single protein, researchers restored a critical DNA repair process that is deficient in mice with mutations in the BRCA1 breast cancer susceptibility gene, thereby dramatically reducing the risk of cancer in these mice. These results suggest that it may one day be possible to restore the same DNA repair process and reduce cancer risk in women with BRCA1 mutations.

Summary Posted: 04/2010

Reference

Cell. 2010 Apr 16;141(2):243-54 PubMed Link