Complete sequence of DNA repair mechanism identified

The entire repair cycle of DNA replication has been identified for the first time.

DNA replication, the process in which DNA makes an exact copy of itself during cell division, occurs for growth and repair. This natural process is vital for life, but sometimes errors occur. In humans, break induced replication (BIR) efficiently addresses the faults, and repairs the damaged DNA using undamaged, intact DNA as a template. BIR can however induce changes in DNA during the repair resulting in genome instability, a feature in the development of cancer.

'It's kind of a double-edged sword,' said Professor Anna Malkova, who led the study. 'The basic ability to repair is a good thing, and some DNA breaks can't be repaired by other methods. So, the idea is very good. But the outcomes can be bad.'

Publishing their findings in Nature, Professor Malkova and her team from the University of Iowa, provided a complete insight into the mechanism of BIR and how it contributes to genome instability. The research explains, for the first time, the complete sequence of the repair mechanism from start to end.

Liping Liu, a biology graduate student and first author, developed a novel DNA purification technique, which was used with Droplet Digital PCR to precisely analyse the entire BIR process.

The team intentionally introduced roadblocks to observe how BIR responds to obstacles. Professor Malkova explained, 'If you imagine this as a train, Liping installed a bunch of stations, and she watched how the train proceeded at each station, tracking the increase in DNA at each station, how much increase is occurring at each station, and thus, in aggregate, how the entire process unfolds.'

Obstacles introduced included transcription. The scientists uncovered that when transcription occurs at the beginning of the BIR process, repairs are suppressed. Furthermore, the scientists proposed that BIR is able to affect the repair cycle and as such may be an important factor in the promotion of genomic instability and thus promoting cancer.

Professor Malkova clarified: 'When BIR meets transcription, it can introduce even more instability, which can lead to even higher mutations. As a result, we think that instabilities that mainly were found at collisions between transcription and replication that have been suggested to lead to cancer might be caused by BIR that came to the rescue. It comes, it rescues, but it's kind of questionable how helpful it really is.'

This research provides fundamental insight into the repair mechanism of DNA, and its role in cancer development. 'Scientists already know there's a lot of instability in places where high transcription meets normal replication,' Professor Malkova concluded. 'What we did not know until now is where is it coming from and why is it happening.'