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Cohesin proteins arrange chromatin within the nucleus, and defects in this organizational role may underlie two human diseases, suggest Gard et al.
The cohesin complex and its accessory factors are best known for holding sister chromatids together before their separation in mitosis, but the cohesin pathway may have other functions, too. Cornelia de Lange syndrome (CdLS) and Roberts syndrome (RBS) patients have mutations in these proteins and suffer developmental problems such as growth and mental retardation. Yet their cells have no significant defects in chromosome segregation.
Gard et al. recreated CdLS and RBS mutations in the cohesin pathway proteins of budding yeast. Sure enough, none of the mutants had problems with chromosome cohesion, but two mutants in particular were unable to order their genomes correctly. Yeast with point mutations in either Eco1 or Scc2—two cohesin complex regulatory proteins—couldn't condense their chromosomes as much as wild-type cells. The mutants also had abnormally shaped nucleoli and showed defects in the subnuclear localization, clustering, and silencing of several genes.
Gene expression may be similarly affected during the development of CdLS and RBS patients. Senior author Jennifer Gerton now wants to explore the connection between the cohesin network, chromosome organization, and transcription in more detail. Her laboratory has already performed a screen for factors that genetically interact with Eco1 and Scc2 mutants, identifying a number of chromatin-modifying proteins.