Ana Losada

Recurrent inactivation of STAG2 in bladder cancer is not associated with aneuploidy

Urothelial bladder cancer (UBC) is heterogeneous at the clinical, pathological and genetic levels. Tumor invasiveness (T) and grade (G) are the main factors associated with outcome and determine patient management. A discovery exome sequencing screen (n = 17), followed by a prevalence screen (n = 60), identified new genes mutated in this tumor coding for proteins involved in chromatin modification (MLL2, ASXL2 and BPTF), cell division (STAG2, SMC1A and SMC1B) and DNA repair (ATM, ERCC2 and FANCA). STAG2, a subunit of cohesin, was significantly and commonly mutated or lost in UBC, mainly in tumors of low stage or grade, and its loss was associated with improved outcome. Loss of expression was often observed in chromosomally stable tumors, and STAG2 knockdown in bladder cancer cells did not increase aneuploidy. STAG2 reintroduction in non-expressing cells led to reduced colony formation. Our findings indicate that STAG2 is a new UBC tumor suppressor acting through mechanisms that are different from its role in preventing aneuploidy.

Cohesin-SA1 deficiency drives aneuploidy and tumourigenesis in mice due to impaired replication of telomeres

Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1‐null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin‐SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin‐SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin‐SA1, with clear implications in tumourigenesis.

A unique role of cohesin‐SA1 in gene regulation and development

Vertebrates have two cohesin complexes that consist of Smc1, Smc3, Rad21/Scc1 and either SA1 or SA2, but their functional specificity is unclear. Mouse embryos lacking SA1 show developmental delay and die before birth. Comparison of the genome‐wide distribution of cohesin in wild‐type and SA1‐null cells reveals that SA1 is largely responsible for cohesin accumulation at promoters and at sites bound by the insulator protein CTCF. As a consequence, ablation of SA1 alters transcription of genes involved in biological processes related to Cornelia de Lange syndrome (CdLS), a genetic disorder linked to dysfunction of cohesin. We show that the presence of cohesin‐SA1 at the promoter of myc and of protocadherin genes positively regulates their expression, a task that cannot be assumed by cohesin‐SA2. Lack of SA1 also alters cohesin‐binding pattern along some gene clusters and leads to dysregulation of genes within. We hypothesize that impaired cohesin‐SA1 function in gene expression underlies the molecular aetiology of CdLS.

Cohesin, a chromatin engagement ring.

Cohesin is a four subunit complex, conserved from yeast to man, with the ability to hold together two DNA segments within its ring-shaped structure. When the two segments belong to sister chromatids, cohesin is mediating cohesion, which is essential for chromosome segregation in mitosis and meiosis and for homologous DNA repair. When the two DNA segments are in the same chromatid, a loop is formed. These chromatin loops are emerging as a mechanism for controlling the communication between enhancers and promoters and thereby regulate gene expression. They also facilitate DNA replication and recombination. Given all its essential functions, it is not surprising that mutations in cohesin and its interacting factors have been associated to cancer and developmental syndromes known as cohesinopathies.

Cohesin in development and disease

Cohesin is a ring-shaped complex, conserved from yeast to human, that was named for its ability to mediate sister chromatid cohesion. This function is essential for chromosome segregation in both mitosis and meiosis, and also for DNA repair. In addition, more recent studies have shown that cohesin influences gene expression during development through mechanisms that likely involve DNA looping and interactions with several transcriptional regulators. Here, we provide an overview of how cohesin functions, highlighting its role both in development and in disease.

Heterochromatin Protein 1 (HP1) Proteins Do Not Drive Pericentromeric Cohesin Enrichment in Human Cells

Sister chromatid cohesion mediated by cohesin is essential for accurate chromosome segregation. Classical studies suggest that heterochromatin promotes cohesion, but whether this happens through regulation of cohesin remains to be determined. Heterochromatin protein 1 (HP1) is a major component of heterochromatin. In fission yeast, the HP1 homologue Swi6 interacts with cohesin and is required for proper targeting and/or stabilization of cohesin at the centromeric region. To test whether this pathway is conserved in human cells, we have examined the behavior of cohesin in cells in which the levels of HP1 alpha, beta or gamma (the three HP1 proteins present in mammalian organisms) have been reduced by siRNA. We have also studied the consequences of treating human cells with drugs that change the histone modification profile of heterochromatin and thereby affect HP1 localization. Our results show no evidence for a requirement of HP1 proteins for either loading of bulk cohesin onto chromatin in interphase or retention of cohesin at pericentric heterochromatin in mitosis. However, depletion of HP1gamma leads to defects in mitotic progression.

Cohesin ties up the genome

Cohesin was originally identified as a mediator of sister chromatid cohesion both in mitosis and meiosis. Emerging evidences suggest that it also participates in the organization of interphase chromatin. The ring-shaped complex regulates gene expression by constraining chromatin topology in concert with factors such as the insulator CTCF, at least in certain loci. The global relevance of this function of cohesin remains to be assessed, but its contribution to the pathology of the Cornelia de Lange syndrome seems evident. Our current knowledge of the molecular mechanisms underlying cohesin behavior should now be considered from the perspective of its novel functions, which promise to be as relevant for cell viability as cohesion.

The regulation of sister chromatid cohesion

Sister chromatid cohesion is a major feature of the eukaryotic chromosome. It entails the formation of a physical linkage between the two copies of a chromosome that result from the duplication process. This linkage must be maintained until chromosome segregation takes place in order to ensure the accurate distribution of the genomic information. Cohesin, a multiprotein complex conserved from yeast to humans, is largely responsible for sister chromatid cohesion. Other cohesion factors regulate the interaction of cohesin with chromatin as well as the establishment and dissolution of cohesion. In addition, the presence of cohesin throughout the genome appears to influence processes other than chromosome segregation, such as transcription and DNA repair. In this review I summarize recent advances in our understanding of cohesin function and regulation in mitosis, and discuss the consequences of impairing the cohesion process at the level of the whole organism.

Reduction of Nipbl impairs cohesin loading locally and affects transcription but not cohesion-dependent functions in a mouse model of Cornelia de Lange Syndrome

Cornelia de Lange Syndrome (CdLS) is a genetic disorder linked to mutations in cohesin and its regulators. To date, it is unclear which function of cohesin is more relevant to the pathology of the syndrome. A mouse heterozygous for the gene encoding the cohesin loader Nipbl recapitulates many features of CdLS. We have carefully examined Nipbl deficient cells and here report that they have robust cohesion all along the chromosome. DNA replication, DNA repair and chromosome segregation are carried out efficiently in these cells. While bulk cohesin loading is unperturbed, binding to certain promoters such as the Protocadherin genes in brain is notably affected and alters gene expression. These results provide further support for the idea that developmental defects in CdLS are caused by deregulated transcription and not by malfunction of cohesion-related processes.

The specific contributions of cohesin-SA1 to cohesion and gene expression: implications for cancer and development.

Besides its well-established role in sister chromatid cohesion, cohesin has recently emerged as major player in the organization of interphase chromatin. Such important function is related to its ability to entrap two DNA segments also in cis, thereby facilitating long-range DNA looping which is crucial for transcriptional regulation, organization of replication factories and V(D)J recombination. Vertebrate somatic cells have two different versions of cohesin, containing Smc1, Smc3, Rad21/Scc1 and either SA1 or SA2, but their functional specificity has been largely ignored. We recently generated a knockout mouse model for the gene encoding SA1, and found that this protein is essential to complete embryonic development. Cohesin-SA1 mediates cohesion at telomeres, which is required for their replication. Telomere defects in SA1- deficient cells provoke chromosome segregation errors resulting in aneuploidy despite robust centromere cohesion. This aneuploidy could explain why heterozygous animals have an earlier onset of tumorigenesis. In addition, the genome-wide distribution of cohesin changes dramatically in the absence of SA1, and the complex shows reduced accumulation at promoters and CTCF sites. As a consequence, gene expression is altered, leading to downregulation of biological processes related to a developmental disorder linked to cohesin function, the Cornelia de Lange Syndrome (CdLS). These results point out a prominent role of cohesin-SA1 in transcriptional regulation, with clear implications in the etiology of CdLS.