Roles of the sister chromatid cohesion apparatus in gene expression, development, and human syndromes

Abstract

The sister chromatid cohesion apparatus mediates physical pairing of duplicated chromosomes. This pairing is essential for appropriate distribution of chromosomes into the daughter cells upon cell division. Recent evidence shows that the cohesion apparatus, which is a significant structural component of chromosomes during interphase, also affects gene expression and development. The Cornelia de Lange (CdLS) and Roberts/SC phocomelia (RBS/SC) genetic syndromes in humans are caused by mutations affecting components of the cohesion apparatus. Studies in Drosophila suggest that effects on gene expression are most likely responsible for developmental alterations in CdLS. Effects on chromatid cohesion are apparent in RBS/SC syndrome, but data from yeast and Drosophila point to the likelihood that changes in expression of genes located in heterochromatin could contribute to the developmental deficits.

Fig. 1

Proposed functions for sister chromatid cohesion proteins. a Structure of cohesin; b topological linkage of sister chromatids by a cohesin ring. Other models for how cohesin mediates cohesion are discussed in the text. c Speculative model for how the Nipped-B/Mau-2 (Scc1/Scc4) adherin complex facilitates binding of cohesin to chromosomes. It is proposed that adherin stimulates opening of the cohesin ring, which requires ATP hydrolysis by the SMC dimer, allowing the chromosome to enter the ring. d Speculative models for the functions of Pds5 and Ctf7/Eco1 (Eso1, Esco2, Deco) in establishing cohesion during DNA replication. Ctf7/Eco1 interacts with certain DNA replication factors and with Pds5. Pds5 also binds to cohesin and inhibits establishment of cohesion in the absence of Ctf7/ Eco1. Based on structural similarities between Pds5 and Nipped-B, and the ability of a mutant Drosophila Pds5 protein to reduce cohesin binding, it is proposed that Pds5 opens cohesin after DNA replication to allow both sisters to enter the ring.

Fig. 2

Cohesin functions at the boundaries surrounding the yeast HMR silent mating-type locus. HMR silencing is mediated by binding of SIR (silent information regulator) protein complexes (red circles) to the a2 and a1 genes. The E and I sequence elements are needed to establish silencing. Cohesin binds to the boundary elements that block the spread of SIR protein binding beyond the HMR locus, and boundary function is compromised in smc1 mutants, indicating that cohesin blocks the spread of SIR complexes

Fig. 3

Model for the effects of Nipped-B and cohesin on long-range activation of the Drosophila cut gene. A wing margin-specific transcriptional enhancer is located more than 80 kbp upstream of the promoter. Cohesin binds to four regions between the enhancer and the promoter and inhibits gene expression. Nipped-B facilitates gene activation, and it is proposed that Nipped-B facilitates a dynamic equilibrium between bound and unbound cohesin. Removal of cohesin facilitates enhancer–promoter contact and gene activation. Reduction of Nipped-B activity slows the rate of cohesin removal and the efficiency of gene activation