Girard, F. | Search bioRxiv

bioRxiv2020

Cohesin regulates homology search during recombinational DNA repair

Homologous recombination (HR) is a ubiquitous DNA double-strand break (DSB) repair mechanism that promotes cell survival. It entails a potentially genome-wide homology search step, carried out along a conserved RecA/Rad51-ssDNA nucleoprotein filament (NPF) assembled on each DSB ends1-3. This search is subdued to NPF-dsDNA collision probability, dictated in part by chromatin conformation2,4. In contrast to the extensive knowledge about chromatin composition and mobility changes elicited by the DNA damage checkpoint (DDC)5-7, whether, how, and to which extent a DSB impacts spatial chromatin organization, and whether this organization in turns influences the homology search process, remains ill-defined8,9. Here we characterize two layers of spatial chromatin reorganization following DSB formation in S. cerevisiae. While cohesin folds chromosomes into cohesive arrays of 10-20 kb long chromatin loops as cells arrest in G2/M10,11, the DSB-flanking regions locally interact in a resection- and 9-1-1 clamp-dependent manner, independently of cohesin and HR proteins. This local structure blocks cohesin progression, constraining the extending NPF at loop base. Functionally this organization promotes side-specific cis DSB-dsDNA interactions that scales with loop expansion span, and provides a kinetic advantage for identification of intra- over inter-chromosomal homologies. We propose that cohesins regulate homology search by promoting cis dsDNA over-sampling, both upon loop expansion-coupled unidimensional dsDNA scanning, NPF trapping, and chromosome individualization, largely independent of their role in sister chromatid cohesion.

genetics↗

bioRxiv2020

The enrichment of breakpoints in late-replicating chromatin provides novel insights into chromoanagenesis mechanisms

The rise of pangenomic molecular assays allowed uncovering complex rearrangements named chromoanagenesis that were hypothesized to result from catastrophic shattering events. Constitutional cases have typically been reported individually preventing identification of common features and uncovering the mechanisms at play. We characterized 20 new chromoanagenesis and discovered yet undescribed features. While literature differentiates chromothripsis and its shattering event repaired through non-homologous end joining from chromoanasynthesis born to aberrant replicative processes, we identified shattered chromosomes repaired through a combination of mechanisms. In particular, three samples present with "rearrangement hubs" comprising a fragmented kilobase-long sequence threaded throughout the rearrangement. To assess the mechanisms at play, we merged our data with those of 20 published constitutional complex chromosomal rearrangement cases. We evaluated if the distribution of their 1032 combined breakpoints was distinctive using bootstrap simulations and found that breakpoints tend to keep away from haplosensitive genes suggesting selective pressure. We then compared their distribution with that of 13,310 and 468 breakpoints of cancer complex chromosomal rearrangements and constitutional simple rearrangement samples, respectively. Both complex rearrangement groups showed breakpoint enrichment in late replicating regions suggesting similar origins for constitutional and cancer cases. Simple rearrangement breakpoints but not complex ones were depleted from lamina-associated domains (LADs), possibly as a consequence of reduced mobility of DNA ends bound to lamina. The enrichment of breakpoints in late-replicating chromatin for both constitutional and cancer chromoanagenesis provides an orthogonal support to the premature chromosome condensation hypothesis that was put forward to explain chromoanagenesis.

genomics↗