Poster Presentation 31st Lorne Cancer Conference 2019

Dysregulated transcriptional response to differentiation signals in cohesin-mutant leukemia cells (#119)

Jisha Antony 1 2 , Gregory Gimenez 1 , Umaima Khatoon 1 , Robert Day 3 , Ian Morison 1 2 , Julia Horsfield 1 2
  1. Department of Pathology, University of Otago, Dunedin, Otago, New Zealand
  2. Maurice Wilkins Centre for Molecular Biodiscovery, Auckland, New Zealand
  3. Department of Biochemistry, University of Otago, Dunedin, Otago, New Zealand

Cohesin is a multiprotein complex with essential roles in cell division and chromatin architecture, in which it contributes to the control of transcriptional programs. Mutations in the subunits of cohesin complex occur in significant proportion of myeloid malignancies1,2. In particular, the frequency of cohesin subunit mutation is as high as 53% in Down Syndrome-associated megakaryoblastic leukemia subtype3.

 

We used CRISPR-Cas9 to engineer chronic myelogenous leukemia cell line, K562 to contain leukemia-associated mutation, R614* in the cohesin subunit, STAG2. Homozygous mutant cells showed complete loss of STAG2 expression and displayed altered morphology with gain of adherent characteristics. RNA-sequencing revealed dysregulation of more than 5,000 genes in STAG2 mutants, which was accompanied by changes in chromatin accessibility (profiled by ATAC-sequencing). Upregulated genes showed enrichment for stem cell and extracellular matrix genes, which is consistent with the adherent phenotype.

 

We hypothesised that loss of STAG2 would impact signalling in response to differentiation cues. To test this, we stimulated our isogenic cell lines with phorbol myristate acetate (PMA) to induce megakaryocyte differentiation. PMA stimulation of STAG2 homozygous mutant cells caused a precocious spike in RUNX1 expression that was associated with enhanced transcription from its proximal P2 promoter. A similar spike was observed in transcription of ERG. Interestingly, the spike in transcription of these genes was not observed at 48 hours, indicating that the ectopic transcription of RUNX1 and ERG is confined to early response to stimulation. The BET inhibitor JQ1, was previously shown to block enhancer activity4. Treatment of STAG2 mutant cells with JQ1 dampened ectopic RUNX1 P2 expression and led to a complete loss of RUNX1 P1 and ERG activation. Furthermore, JQ1 treatment caused a 2-fold decrease in the stem cell marker c-KIT in STAG2- deficient cells, indicating a reversal of immature properties in these cells.

 

In summary, our results suggest that STAG2 mutation results in unconstrained enhancer-driven transcription which could underlie the aberrant gene expression in STAG2-deficient cells exposed to differentiation cues.

  1. Kon A, Shih LY, Minamino M, et al. Recurrent mutations in multiple components of the cohesin complex in myeloid neoplasms. Nat Genet. 2013;45(10):1232-1237.
  2. Thota S, Viny AD. Genetic alterations of the cohesin complex genes in myeloid malignancies. 2014;124(11):1790-1798.
  3. Yoshida K, Toki T, Okuno Y, et al. The landscape of somatic mutations in Down syndrome-related myeloid disorders. Nature genetics. 2013.
  4. Bhagwat AS, Roe J-S, Mok BA, Hohmann AF, Shi J, Vakoc CR. BET bromodomain inhibition releases the Mediator complex from select cis-regulatory elements. Cell reports. 2016;15(3):519-530.