Cell unable to complete the recombinational repair process show abnormalities at cell division.

Introduction

Mammalian cells possess a large repertoire of DNA repair processes that maintain the integrity of our genetic material. Some individuals, however, carry mutations in genes required for DNA repair, and this often leads to inheritable disease.

Three-dimensional image reconstructions of the BRCA2-RAD51 complex.

Figure 1: Three-dimensional image reconstructions of the BRCA2-RAD51 complex (shown in side and top views). The BRCA2 subunits are shown in cyan and yellow, and four RAD51 monomers arranged in a semi-helical arrangement are indicated in red.

An important repair process involves recombination, and defects in this process are linked with cancer predisposition, in particular breast cancers caused by mutation of the BRCA2 gene, acute leukaemias associated with Fanconi anemia, and a wide range of cancers found in individuals with the chromosome instability disorder known as Bloom's syndrome. The primary focus of our research is to determine the molecular mechanisms of recombinational repair, and to define why defects in these processes cause cancer.

For several years we have been interested in the mechnisms of homologous recombination, how they contribute to the repair of DNA double-strand breaks, and how they promote genome stability. Many of the proteins required for recombination have been purified in this laboratory, and we use biochemical, structural and molecular and cell biological approaches to understand how they bring about the repair of DNA breaks.

RAD51 protein, the human ortholog of the bacterial RecA protein, is responsible for the initiation of DNA strand break repair by catalysing homologous pairing and strand exchange, two reactions that are essential for recombinational repair. The RAD51 protein is targeted to DNA break sites by BRCA2, a well-known tumour suppressor.

The key role that BRCA2 plays in DNA repair is indicated by the fact that women carrying BRCA2 mutations have a 70 per cent chance of developing breast or ovarian cancers. An important aim of the laboratory is therefore to define the precise role that BRCA2 plays in recombination-directed DNA repair.

Using biochemical and structural approaches we recently determined the three-dimensional structure of BRCA2 protein, both alone and in complex with RAD51. The structure revealed that BRCA2 is a dimeric protein that interacts directly with RAD51. Two sets of RAD51 monomers are arranged on the BRCA2 dimer in readiness for the establishment of RAD51-ssDNA nucleoprotein filaments.

We find that BRCA2 overcomes the rate-limiting step of nucleoprotein filament formation by RAD51, providing a molecular basis for the role of BRCA2 in the maintenance of genome stability and suggesting why mutations in this protein lead to tumourigenesis.