Characterisation of chromatin and centromere behaviour in Roberts syndrome
Roberts syndrome (RBS) is a rare, recessive genetic condition characterised by a number of developmental defects, which include low birth weight, phocomelia (shortening of limbs) and craniofacial abnormalities. RBS is caused by mutations in the ESCO2 gene which is responsible for establishment of sister-chromatid cohesion by distributing chromosomal cohesins to newly-replicated sister chromatids. Cells from RBS patients show a characteristic pericentric heterochromatin repulsion (HR) in cells arrested at metaphase. There is also anecdotal evidence for premature chromatid separation i.e. separation of sister chromatids before the mitotic checkpoint. RBS cells have also been shown to be slow growing.
At the onset of anaphase, after the mitotic checkpoint, cleavage of centromeric cohesins results in co-ordinated sister chromatid separation. As heterochromatin lies adjacent to human centromeres, the HR defect in RBS has not been fully explained. We have been studying centromeres and pericentric heterochromatin in RBS. We measured centromere and heterochromatin separation using FISH and measured the distribution of a large number of centromere and heterochromatin proteins in a RBS patient. We found that sister chromatids are more separated in the RBS patient compared to controls for all centromeres and regions of heterochromatin studied and linked this separation with an increased mitotic spindle length. Furthermore, we found that the distribution of very few centromere proteins is affected in RBS, indicating that most RBS cells can recover from premature centromere separation, with only a minority passing through to the next cell cycle without cytokinesis resulting in multi-nucleate cells. We will also present evidence that these cells attempt further rounds of DNA replication but do not result in significant levels of apoptosis.