Chromosome segregation: learning to let go

The zebrafish cohesin protein Sgo1 is required for cardiac function and eye development

BACKGROUND

Cohesinopathies is a term that refers to/covers rare genetic diseases caused by mutations in the cohesin complex proteins. The cohesin complex is a multiprotein complex that facilitates different aspects of cell division, gene transcription, DNA damage repair, and chromosome architecture. Shugoshin proteins prevent the cohesin complex from premature dissociation from chromatids during cell division. Patients with a homozygous missense mutation in SGO1, which encodes for Shugoshin1, have problems with normal pacing of the heart and gut.

RESULTS

To study the role of shugoshin during embryo development, we mutated the zebrafish sgo1 gene. Homozygous sgo1 mutant embryos display various phenotypes related to different organs, including a reduced heart rate accompanied by reduced cardiac function. In addition, sgo1 mutants are vision-impaired as a consequence of structurally defective and partially non-functional photoreceptor cells. Furthermore, the sgo1 mutants display reduced food intake and early lethality.

CONCLUSION

We have generated a zebrafish model of Sgo1 that showed its importance during organ development and function.

A positive feedback mechanism ensures proper assembly of the functional inner centromere during mitosis in human cells

The inner centromere region of a mitotic chromosome critically regulates sister chromatid cohesion and kinetochore-microtubule attachments. However, the molecular mechanism underlying inner centromere assembly remains elusive. Here, using CRISPR/Cas9-based gene editing in HeLa cells, we disrupted the interaction of Shugoshin 1 (Sgo1) with histone H2A phosphorylated on Thr-120 (H2ApT120) to selectively release Sgo1 from mitotic centromeres. Interestingly, cells expressing the H2ApT120-binding defective mutant of Sgo1 have an elevated rate of chromosome missegregation accompanied by weakened centromeric cohesion and decreased centromere accumulation of the chromosomal passenger complex (CPC), an integral part of the inner centromere and a key player in the correction of erroneous kinetochore-microtubule attachments. When artificially tethered to centromeres, a Sgo1 mutant defective in binding protein phosphatase 2A (PP2A) is not able to support proper centromeric cohesion and CPC accumulation, indicating that the Sgo1-PP2A interaction is essential for the integrity of mitotic centromeres. We further provide evidence indicating that Sgo1 protects centromeric cohesin to create a binding site for the histone H3-associated protein kinase Haspin, which not only inhibits the cohesin release factor Wapl and thereby strengthens centromeric cohesion but also phosphorylates histone H3 at Thr-3 to position CPC at inner centromeres. Taken together, our findings reveal a positive feedback-based mechanism that ensures proper assembly of the functional inner centromere during mitosis. They further suggest a causal link between centromeric cohesion defects and chromosomal instability in cancer cells.