Final Report Summary - GENOMIC STABILITY (Genomic stability -chromosome segregation and repair)
This project has elucidated molecular mechanisms behind genome stability and dynamics. These two are intimately connected since genome stability depends on correct chromosome replication, segregation and repair. Failure of any of these processes is either lethal or can trigger genome instability, developmental diseases and cancer. To increase the knowledge in this area, the project has determined the genome wide of chromosome binding pattern of proteins involved in replication, segregation and repair in the budding yeast model system.
One of the most important findings has been that the so called Smc5/6 complex (Smc5/6) binds to chromosomes during DNA replication in a chromosome length-dependent manner. It was also shown to be required for the resolution of replication-induced topological tension in the DNA molecule. Topological tension arises when the replication machinery pries apart the parental DNA double-helix into the two single strands which are to be duplicated by the polymerases (Fig. 1). This strand-separation leads to DNA over-winding, known as positive supercoiling, ahead of the replication fork. If this supercoiling is not removed, it will lead to replication fork blockage which increases the risk of genomic rearrangements. So far, topoisomerases which work by creating transient breaks in the DNA molecule have been the only enzymes known to be involved in this removal. It has also been proposed that positive supercoils ahead of the fork can be avoided if the advancing replication machinery rotates with the turn of the DNA helix. This way of supercoil-avoidance will create sister chromatid intertwinings behind the replication fork (Fig. 1 (see attached PDF, Project summary).
The results obtained have allowed us to propose a model where Smc5/6 removes topological tension by sequestration of sister chromatid intertwinings. They also indicate that topological tension increases with the size of budding yeast chromosomes, and that this should be the reason behind the length-dependent binding pattern of Smc5/6. Furthermore, the association of Smc5/6 has proven to be under the control of the so called cohesin complex, which tethers sister chromatids until cell division takes place. Taken together, the research has open for the possibility that the interplay between topological tension, Smc5/6 and a related complex called cohesin, sets the structure of chromosomes and allow their correct segregation during cell division. This indicates that changes DNA supercoiling not is an obstacle which has to be overcome in order to complete replication, but instead is essential for chromosome segregation. This is an important and surprising finding in itself, and taken the central position of topoisomerases as cancer drug targets, could also have impact on future cancer treatment regimes.
One of the most important findings has been that the so called Smc5/6 complex (Smc5/6) binds to chromosomes during DNA replication in a chromosome length-dependent manner. It was also shown to be required for the resolution of replication-induced topological tension in the DNA molecule. Topological tension arises when the replication machinery pries apart the parental DNA double-helix into the two single strands which are to be duplicated by the polymerases (Fig. 1). This strand-separation leads to DNA over-winding, known as positive supercoiling, ahead of the replication fork. If this supercoiling is not removed, it will lead to replication fork blockage which increases the risk of genomic rearrangements. So far, topoisomerases which work by creating transient breaks in the DNA molecule have been the only enzymes known to be involved in this removal. It has also been proposed that positive supercoils ahead of the fork can be avoided if the advancing replication machinery rotates with the turn of the DNA helix. This way of supercoil-avoidance will create sister chromatid intertwinings behind the replication fork (Fig. 1 (see attached PDF, Project summary).
The results obtained have allowed us to propose a model where Smc5/6 removes topological tension by sequestration of sister chromatid intertwinings. They also indicate that topological tension increases with the size of budding yeast chromosomes, and that this should be the reason behind the length-dependent binding pattern of Smc5/6. Furthermore, the association of Smc5/6 has proven to be under the control of the so called cohesin complex, which tethers sister chromatids until cell division takes place. Taken together, the research has open for the possibility that the interplay between topological tension, Smc5/6 and a related complex called cohesin, sets the structure of chromosomes and allow their correct segregation during cell division. This indicates that changes DNA supercoiling not is an obstacle which has to be overcome in order to complete replication, but instead is essential for chromosome segregation. This is an important and surprising finding in itself, and taken the central position of topoisomerases as cancer drug targets, could also have impact on future cancer treatment regimes.