Rad21 is the core subunit of the cohesin complex involved in directing genome organization

The ring-shaped cohesin complex is an important factor regulating genome structure. It is thought to mediate the formation of chromatin loops and topologically associating domains (TADs) by loop extrusion. However, the regulation of association between cohesin and chromatin is poorly understood. In this study, we directly visualized cohesin loading after up-regulation of cohesin subunit Rad21 by identifying the formation of vermicelli-like structures via live cell super-resolution imaging. We also reveal that cohesin loading can be promoted by Rad21-loader interactions and accumulated contacts were shown at TAD corners while inter-TAD interactions increased after vermicelli formation, indicating that Rad21 is an important determinant of chromatin structure. Moreover, we find that cohesin saddle on topologically associating domains by FISH assay, which is consistent with the CTCF/cohesin-anchored chromatin loop model. Importantly, expression of Rad21 is strictly controlled, and aberrant expression of Rad21 leads to the formation of Rad21 “beads” in the nucleus. In summary, our observations provided important new biological insights into the mechanism of cohesin loading and its functions.

The loader complex Scc2/4 forms co-condensates with DNA as loading sites for cohesin

The genome is organized by diverse packaging mechanisms like nucleosome formation, loop extrusion and phase separation, which all compact DNA in a dynamic manner. Phase separation additionally drives protein recruitment to condensed DNA sites and thus regulates gene transcription. The cohesin complex is a key player in chromosomal organization that extrudes loops to connect distant regions of the genome and ensures sister chromatid cohesion after S-phase. For stable loading onto the DNA and for activation, cohesin requires the loading complex Scc2/4. As the precise loading mechanism remains unclear, we investigated whether phase separation might be the initializer of the cohesin recruitment process. We found that, in absence of cohesin, budding yeast Scc2/4 forms phase separated co-condensates with DNA, which comprise liquid-like properties shown by droplet shape, fusion ability and reversibility. We reveal in DNA curtain and optical tweezer experiments that these condensates are built by DNA bridging and bending through Scc2/4. Importantly, Scc2/4-mediated condensates recruit cohesin efficiently and increase the stability of the cohesin complex. We conclude that phase separation properties of Scc2/4 enhance cohesin loading by molecular crowding, which might then provide a starting point for the recruitment of additional factors and proteins.

Recurrent Germline Variant in the Cohesin Complex Gene RAD21 Predisposes Children to Lymphoblastic Leukemia and Lymphoma

Introduction: Cohesin complex genes are commonly mutated in cancer particularly in myeloid malignancies. Yet patients with germline mutations in cohesin genes, leading to cohesinopathies like Cornelia-de-Lange syndrome (CdLS) are generally not known to be tumor-prone. The complex plays a major role in chromosome alignment and segregation (Uhlmann, Nature Reviews Molecular Cell Biology, 2016), homologous recombination-driven DNA repair (Ström et al., Molecular Cell, 2004) and regulation of gene expression (Busslinger et al., Nature, 2017). To deepen the understanding of cohesin variants in cancer predisposition, we performed TRIO Sequencing in two independent pediatric cancer cohorts. Thereby, we identified a novel recurrent heterozygous germline variant in the cohesin gene RAD21 not described in CdLS patients , located in the binding domain of the cofactors WAPL and PDS5B . Methods: Whole exome sequencing (WES) in a TRIO (child-parent datasets) setting was carried out in two independent, unselected cancer cohorts (TRIO-D, n=158 (Wagener et al., European Journal of Human Genetics, 2021) and TRIO-DD, n=60). To investigate the oncogenic potential of the novel RAD21 variant molecular and functional assessment was performed focusing on potential implications on the complex. Results: The newly identified RAD21 variant at amino acid position 298 resulting in a Proline to Serine (p.P298S) and a Proline to Alanine exchange, respectively, (p.P298A) is only rarely mutated in the general population (gnomAD database n=118,479; RAD21 p.P298S MAF <10 -6 and RAD21 p.P298A MAF <10 -5). While both patients did not show any signs of CdLS, they both have a remarkable family history of cancer. Patient 1 (13y) was diagnosed with T-cell acute lymphoblastic leukemia (T-ALL) whose father had died from breast cancer (41y), while patient 2 (2y) presented with precursor B-cell lymphoblastic lymphoma (pB-LBL) whose uncle had died from pediatric cancer of unknown subtype (8y). To assess the influence of RAD21 p.P298S/A on the binding capacity of the complex, RAD21 variants and the wildtype (WT) were cloned and transfected into HEK293T cells, respectively. Immunoprecipitation analysis of RAD21 with the cofactors WAPL and PDS5B showed no differential binding between the WT and the variants, suggesting that RAD21 p.P298S/A does not impact the formation of the complex. Nevertheless, on a transcriptional level 83 genes were significantly differentially expressed in RAD21 p.P298S and p.P298A compared to the wildtype (fc>1.5, adj. p-value <0.05) with enrichment of genes in p53 signaling pathways. We further observed an increased number of γH2AX and 53BP1 co-localized foci compared to the WT (p≤0.01; Student's t-test). In line, following ionizing radiation, primary patients' samples showed increased cell cycle arrest at G2/M cell-cycle stage compared to a healthy control (p.P298S: p=0.0049 [6Gy]; p=0.0026 [10Gy]; p.P298A: p=0.0054 [6Gy]; p=0.0006 [10Gy]; Student's t-test). For cross-validation of the germline variant RAD21 p.P298S/A and its potential role in pediatric lymphoblastic malignancies, we analysed a third cohort of 150 children with relapsed ALL (IntReALL) for RAD21 p.P298S/A. We again identified RAD21 p.P298A in a boy (12y) with B-cell precursor acute lymphoblastic leukemia. To compare our data to a non-pediatric cancer setting, a cohort of 2300 young adults (<51 years) with cancer was mined (MASTER program). Here, one patient carrying RAD21 p.P298A with a solid tumor was identified. Therefore, amongst all cohorts, RAD21 p.P298S/A was found to be enriched in pediatric vs. adult cancers (3/479 vs. 1/2299; Fisher's exact test; p=0.018). Conclusion: Taken together, we present for the first time the potential role of RAD21 germline variants in pediatric lymphoblastic malignancies. This may shed new light on the many roles of the cohesin complex and its implication outside the typical syndromal presentation. Disclosures No relevant conflicts of interest to declare.

The Cohesin Complex and Its Interplay with Non-Coding RNAs

The cohesin complex is a multi-subunit protein complex initially discovered for its role in sister chromatid cohesion. However, cohesin also has several other functions and plays important roles in transcriptional regulation, DNA double strand break repair, and chromosome architecture thereby influencing gene expression and development in organisms from yeast to man. While most of these functions rely on protein–protein interactions, post-translational protein, as well as DNA modifications, non-coding RNAs are emerging as additional players that facilitate and modulate the function or expression of cohesin and its individual components. This review provides a condensed overview about the architecture as well as the function of the cohesin complex and highlights its multifaceted interplay with both short and long non-coding RNAs.

Paralogous synthetic lethality underlies genetic dependencies of the cancer-mutated gene STAG2

STAG2, a component of the mitotically essential cohesin complex, is highly mutated in several different tumour types, including glioblastoma and bladder cancer. Whereas cohesin has roles in many cancer-related pathways, such as chromosome instability, DNA repair and gene expression, the complex nature of cohesin function has made it difficult to determine how STAG2 loss might either promote tumorigenesis or be leveraged therapeutically across divergent cancer types. Here, we have performed whole-genome CRISPR-Cas9 screens for STAG2-dependent genetic interactions in three distinct cellular backgrounds. Surprisingly, STAG1, the paralog of STAG2, was the only negative genetic interaction that was shared across all three backgrounds. We also uncovered a paralogous synthetic lethal mechanism behind a genetic interaction between STAG2 and the iron regulatory gene IREB2. Finally, investigation of an unusually strong context-dependent genetic interaction in HAP1 cells revealed factors that could be important for alleviating cohesin loading stress. Together, our results reveal new facets of STAG2 and cohesin function across a variety of genetic contexts.

Biochemically distinct cohesin complexes mediate positioned loops between CTCF sites and dynamic loops within chromatin domains

The ring-like cohesin complex mediates sister chromatid cohesion by encircling pairs of sister chromatids. Cohesin also extrudes loops along chromatids. Whether the two activities involve similar mechanisms of DNA engagement is not known. We implemented an experimental approach based on isolated nuclei carrying engineered cleavable RAD21 proteins to precisely control cohesin ring integrity so that its role in chromatin looping could be studied under defined experimental conditions. This approach allowed us to identify cohesin complexes with distinct biochemical, and possibly structural properties, that mediate different sets of chromatin loops. When RAD21 is cleaved and the cohesin ring is opened, cohesin complexes at CTCF sites are released from DNA and loops at these elements are lost. In contrast, cohesin-dependent loops within chromatin domains and that are not anchored at CTCF sites are more resistant to RAD21 cleavage. The results show that the cohesin complex mediates loops in different ways depending on genomic context and suggests that it undergoes structural changes as it dynamically extrudes and encounters CTCF sites.

The Zebrafish Meiotic Cohesin Complex Protein Smc1b Is Required for Key Events in Meiotic Prophase I

The eukaryotic structural maintenance of chromosomes (SMC) proteins are involved in key processes of chromosome structure and dynamics. SMC1β was identified as a component of the meiotic cohesin complex in vertebrates, which aids in keeping sister chromatids together prior to segregation in meiosis II and is involved in association of homologous chromosomes in meiosis I. The role of SMC1β in meiosis has primarily been studied in mice, where mutant male and female mice are infertile due to germ cell arrest at pachytene and metaphase II stages, respectively. Here, we investigate the function of zebrafish Smc1b to understand the role of this protein more broadly in vertebrates. We found that zebrafish smc1b is necessary for fertility and has important roles in meiosis, yet has no other apparent roles in development. Therefore, smc1b functions primarily in meiosis in both fish and mammals. In zebrafish, we showed that smc1b mutant spermatocytes initiated telomere clustering in leptotene, but failed to complete this process and progress into zygotene. Furthermore, mutant spermatocytes displayed a complete failure of synapsis between homologous chromosomes and homolog pairing only occurred at chromosome ends. Interestingly, meiotic DNA double strand breaks occurred in the absence of Smc1b despite failed pairing and synapsis. Overall, our findings point to an essential role of Smc1b in the leptotene to zygotene transition during zebrafish spermatogenesis. In addition, ovarian follicles failed to form in smc1b mutants, suggesting an essential role in female meiosis as well. Our results indicate that there are some key differences in Smc1b requirement in meiosis among vertebrates: while Smc1b is not required for homolog pairing and synapsis in mice, it is essential for these processes in zebrafish.

BETting on a Transcriptional Deficit as the Main Cause for Cornelia de Lange Syndrome

Cornelia de Lange Syndrome (CdLS) is a human developmental syndrome with complex multisystem phenotypic features. It has been traditionally considered a cohesinopathy together with other phenotypically related diseases because of their association with mutations in subunits of the cohesin complex. Despite some overlap, the clinical manifestations of cohesinopathies vary considerably and, although their precise molecular mechanisms are not well defined yet, the potential pathomechanisms underlying these diverse developmental defects have been theoretically linked to alterations of the cohesin complex function. The cohesin complex plays a critical role in sister chromatid cohesion, but this function is not affected in CdLS. In the last decades, a non-cohesion-related function of this complex on transcriptional regulation has been well established and CdLS pathoetiology has been recently associated to gene expression deregulation. Up to 70% of CdLS cases are linked to mutations in the cohesin-loading factor NIPBL, which has been shown to play a prominent function on chromatin architecture and transcriptional regulation. Therefore, it has been suggested that CdLS can be considered a transcriptomopathy. Actually, CdLS-like phenotypes have been associated to mutations in chromatin-associated proteins, as KMT2A, AFF4, EP300, TAF6, SETD5, SMARCB1, MAU2, ZMYND11, MED13L, PHIP, ARID1B, NAA10, BRD4 or ANKRD11, most of which have no known direct association with cohesin. In the case of BRD4, a critical highly investigated transcriptional coregulator, an interaction with NIPBL has been recently revealed, providing evidence on their cooperation in transcriptional regulation of developmentally important genes. This new finding reinforces the notion of an altered gene expression program during development as the major etiological basis for CdLS. In this review, we intend to integrate the recent available evidence on the molecular mechanisms underlying the clinical manifestations of CdLS, highlighting data that favors a transcription-centered framework, which support the idea that CdLS could be conceptualized as a transcriptomopathy.

The loopy world of cohesin

DNA loops can be formed by a mechanism in which the cohesin complex pulls DNA strands through its ring structure using biased Brownian motion.