Loss of Function Cohesin Complex Gene Mutations Create Neomorphic Cell States Advantageous to Oncogenesis

Multiple large scale sequencing projects have identified a significant number of mutations in Acute Myeloid Leukaemia (AML) and high-risk Myelodysplastic syndromes (MDS) patients. Emerging from this are those affecting members of the cohesin complex, predominantly STAG2, and it has been highlighted that the poorer overall survival of this chromatin structure group is second only to patients carrying mutations affecting TP53. The Cohesin complex is an evolutionary conserved, multimeric protein complex consisting of SMC1, SMC3, RAD21 and STAG1/ STAG2. The complex plays a pivotal role during mitosis, ensuring sister chromatid cohesion however, substantial data exists highlighting roles in DNA damage repair, homologous recombination and most significantly, contribution to long-range interactions between cis-regulatory elements of the genome.

Here, we have used the CRISPR/Cas9 genome editing system to create an isogenic model featuring a premature stop codon within STAG2 at a region which presents as highly mutated in patient data sets. This stop codon generates a loss of function phenotype due to the haplo-insufficient X-linked nature of the STAG2 gene. Following model validation, the impact on genome wide distribution of the cohesin complex was assessed using ChIP-Seq for STAG1, STAG2, SMC3 & the transcriptional insulator CTCF.

A modest increase in overall CTCF binding from ~67,000 to ~72,000 high confidence peaks was observed, despite a ~7-fold decrease in enrichment, indicating decreased global insulator activity as a result of the inactivating STAG2 mutation. As expected, compensation between the STAG members was observed at a number of sites within transcriptionally active topologically associated domains, with an increase in STAG1 binding peaks (~17,000 to >25,000 sites) within the mutant model, despite no changes in average global enrichment. This data was integrated with RNA-seq of the isogeneic parental and mutant cells to determine the transcriptional impact of changes in cohesin complex binding on gene expression within active and repressive domains.

Assessment of the active and repressive super-enhancer associated marks H3K27ac and H3K27me3 showed increased global levels of both marks without increase in the polycomb repressive complex 2 member, EZH2 total protein levels. These findings indicate that increased H3K27ac and H3K27me3 marks, in this model, are due to altered domain boundaries, associated with local activation of both H3K9ac and H3K9me3, leading to aberrant gene expression and consequently a neomorphic state within these mutant cells.

With recent further stratification of de-novo AML, secondary AML and high-risk MDS emerging, this study on the aberrant and non-canonical nature of the cohesin complex provides avenues for an attractive therapeutic target and personalised treatment approach for patients within the chromatin subgroup.