The interplay of cohesin and DNA replication in sister chromatid cohesion

In order to duplicate, cells need to faithfully replicate their DNA followed by the coordinated transfer of duplicated chromosomes to the future daughter cells. To ensure correct segregation, sister chromatids are paired by the cohesin complex from the moment of their synthesis until their separation in mitosis. Defects in cohesin function and mutations in cohesin genes are associated with cancer and are the cause of developmental disorders called cohesinopathies. In this thesis, we aimed to enhance the understanding of the molecular mechanisms underlying sister chromatid cohesion biology to gain insights relevant for cohesinopathies and cancer. By studying Warsaw Breakage Syndrome patient derived mutant DDX11, we reveal that sister chromatid cohesion establishment and DNA replication stress response depend on the helicase function of DDX11. Interestingly, the all studied patients have decreased, but residual, functional DDX11. Together with the finding that DDX11 loss leads to P53 dependent proliferation defects and mice with helicase defective DDX11 are inviable, this suggests complete loss of functional DDX11 is not compatible with life. In addition, we show that cancer cells of various origins often exhibit cohesion defects. In untransformed cells induction of DNA replication stress is sufficient to trigger sister chromatid cohesion loss. Furthermore, we unveil that the cohesin antagonist WAPL is necessary to deal with replication stress by promoting the repair of broken replication forks. Our data suggests WAPL-dependent cohesin removal at stalled replication forks may contribute to sister chromatid cohesion defects in cancer cells. Both the requirement for WAPL and the cohesion loss phenotype may provide specific vulnerabilities of cancer cells that may be targeted. To expose vulnerabilities of cells with cohesion defects and identify novel genes involved in sister chromatid cohesion, we performed genome-wide CRISPR screens in cohesion defective cells. This revealed multiple synthetic lethal interactions, enriched in genes involved in DNA replication and mitosis. Among these, we identify a role for MMS22L-TONSL in sister chromatid cohesion establishment. Furthermore, we identify a novel regulator of cohesin occupancy on chromatin, the PAXIP1-PAGR1 heterodimer. PAXIP1 co-localizes with cohesin on multiple genomic loci, including at active promoters and enhancers, thereby likely contributing to multiple cohesin related functions. Together, our work increases the understanding of the molecular pathways contributing to sister chromatid cohesion establishment. These findings may have implications for the development of therapeutic strategies for cancer and provide insight in the etiology of cohesinopathies.