The role of the cohesin loader in genome stability : a journey from yeast to human

Abstract

Structural Maintenance of Chromosome (SMC) complexes, as their name suggests, have a central role in maintaining the higher structure of genomes, from bacteria to human, and in doing so protecting their integrity.

Cohesin, one of three SMC complexes, is required to hold sister chromatids together until anaphase, and for homologous recombination-based DNA repair. In these cellular processes, a separate complex, named NIPBL/MAU2 (Scc2/4 in Saccharmomyces cerevisiae) is needed to drive the loading of cohesin onto DNA.

This thesis focuses on the cohesin loader, in different model organisms and in the different cellular functions in which NIPBLScc2 is involved.

Paper I describes the requirements for Scc2 binding at an HO-induced DNA double strand break. ChIP-qPCR profiles show presence of Scc2 after break induction 30 kb around the break with strong binding 5 kb from the HO cut-site. Moreover, these Scc2 levels are found to depend on the MRX complex, the Tel1 kinase and H2A phosphorylation, but unlike cohesin not on Mec1.

Conversely Paper II, performed in human cell lines, shows a dual recruitment model for NIPBL at laser and FokI endonuclease-induced DNA damage. First, NIPBL is recruited to DSB via an HP1 binding motif located in its N-terminal. On the contrary NIPBL truncations containing the HEAT repeat rich C-terminal region, but lacking the HP1 motif, are not recruited at FokI foci but localizes only at laser tracks. The latter pathway depends on the activity of ATR/ATM kinases. Moreover a role for the ubiquitin ligases RNF8/RNF168 in the NIPBL recruitment to DNA damage is also described.

In recent years a new function was discovered, for cohesin and its loader, in gene regulation. Paper III shows that Scc2 affects both general gene expression and DNA damage dependent transcription by microarray analysis. Lastly paper IV focuses on another important process in which cohesin is involved, meiosis, describing NIPBL chromosomal localization in male and female murine germ cells, during meiotic prophase I.