Publication highlights

The human RAD52 protein plays an important role in several cellular processes, including the repair of chromosome breaks and the maintenance of telomere length (structures at the end of chromosomes) to avoid cellular aging. During DNA repair, it provides an alternative to the BRCA2 protein, which is mutated in many inheritable breast, ovarian and prostate cancers. Consequently, targeting RAD52 could be used to kill tumours with BRCA2 mutations, where growth is uncontrolled. To elucidate the mechanism of repair by RAD52, we determined the atomic structure of the protein using cryo-electron microscopy, and found that the protein forms a ring in which the broken DNA wraps around the outside of the ring. Having the atomic structure gives us new insights into ways to identify small molecules that can be used to inhibit repair by RAD52 and kill BRCA2-defective tumours.

Individuals with inheritable mutations in the BRCA1 or BRCA2 tumour suppressor genes are unable to carry out a DNA repair process known as homologous recombination, and are predisposed to breast, ovarian and prostate cancers. In the clinic, these cancers are treated with inhibitors of poly [ADP-ribose] polymerase (PARPi) which knocks out a second repair process, and makes the tumour cells die. While effective at initial cancer maintenance, after a period of time the tumours unfortunately develop resistance to PARP inhibition leading to further growth. However, researchers recently discovered that loss or inhibition of a nucleotide pool sanitiser called DNPH1 sensitises BRCA-deficient cells to PARPi, offering a promising strategy for improved therapy for these individuals. The DNPH1 normally removes faulty nucleotides from the cell to stop their incorporation into DNA, so DNPH1 loss leads to an overload in the second repair pathway that is sensitive to PARPi, causing tumour cell death. There is now significant pharmaceutical interest in the development of small molecules that will target and inhibit DNPH1. Towards this goal researchers at the Crick have determined the X-ray crystal structure of DNPH1 bound to the molecule that it acts upon, which will now allow rational drug design.

BRCA2 is a tumour suppressor frequently mutated in breast and ovarian cancer. BRCA2 is a large, 3418 amino acid-long protein that plays a central role in homologous recombination, a pathway to repair broken DNA. How exactly BRCA2 acts during the repair process was unclear. To address this, researchers from the Crick and Imperial College London isolated BRCA2 and its partner, Rad51 recombinase, from cells and labelled them with fluorescent chemicals. They then used optical tweezers, powerful lasers capable of holding and moving small objects, to stretch individual single DNA molecules bearing a gap in one of the strands – mimicking DNA damage that BRCA2 and Rad51 repair in cells.

Finally, after adding labelled BRCA2 and Rad51, the researchers used fluorescence microscopy to make a movie of BRCA2 recognising and delivering Rad51 to gapped DNA in real time. The researchers analysed these movies and observed BRCA2 can recognise the DNA damage directly, bind to a border of damaged and undamaged DNA and surprisingly, also slide along DNA to reach the damaged site. When researchers engineered mutations in BRCA2, it was less able to search for DNA damage. This work shows in unprecedented detail how BRCA2 delivers Rad51 to repair DNA damage to prevent tumour formation.