Cohesin mutations and chromatin changes in cancer

The identification of recurrent mutations in genes encoding the cohesin complex in cancer was among the most unexpected findings from cancer exome sequencing studies. Cohesin is a multi-subunit protein complex that is essential for sister chromatid cohesion, three-dimensional chromosome organization, DNA damage repair, and gene regulation. It forms a ring around DNA, with four structural subunits, SMC1A, SMC3, RAD21, and either STAG1 or STAG2. In particular, the cohesin subunit STAG2 is one of only 12 human genes to be significantly mutated in four or more distinct types of cancer. Cohesin mutations are typically heterozygous and result in haploinsufficiency and/or loss-of-function, and although they might be expected to cause defects in chromosome segregation, the sister chromatid cohesion function of the complex is unlikely the driving mechanism of tumorigenesis, given the lack of aneuploidy in cohesin-mutant cancers. In this review, we will focus on the prevalence of somatic mutations in cohesin subunits across different cancer types, the influence of these mutations on chromatin organization and gene regulation, the resulting cellular and disease phenotypes, and the therapeutic potential of targeting the mutant cohesin complex.