Genetic interactors of the SWI/SNF chromatin remodelling complex in Caenorhabditis elegans
Chromatin remodellers are adenosine triphosphate (ATP)-powered molecular machines that can directly alter the structure of chromatin by reshuffling or evicting nucleosomes. Thereby they control the access to DNA elements like promoters and replication origins that need to be exposed to execute essential cellular processes such as transcription and replication. SWItch/Sucrose Non-Fermenting (SWI/SNF) complexes are a family of chromatin remodellers that are conserved across eukary- otes. Mutations in subunits of SWI/SNF complexes cause a multitude of different developmental disorders including Coffin-Siris syndrome, Nicolaides-Baraitser syn- drome, and autism. However, the molecular mechanism(s) by which mutations in SWI/SNF cause developmental disorders remain unclear and there are currently no treatments that can restore normal development in humans or animal models lacking proper SWI/SNF function.
In this thesis, I used a unique conditional Caenorhabditis elegans swsn-1 (SMARCC1/2 in humans) mutant model that resembles a complete loss of SWI/SNF function only when animals are exposed to a restrictive temperature. By employing chemical mutagenesis, I identified compensating mutations that can restore viability and prevent developmental defects from manifesting in these swsn-1 mutants. Firstly, I found that a specific alanine to valine mutation in the SWI/SNF core subunit snfc-5 (SMARCB1 in humans) can prevent embryonic lethality in animals harbouring the loss-of-function mutation in the SWI/SNF core subunit swsn-1. Secondly, I found that the combination of this specific mutation in snfc-5 and a loss-of-function mutation in either of the E3 ubiquitin ligases ubr-5 (UBR5 in humans) or hecd-1 (HECTD1 in humans) can restore development to adulthood in swsn-1 loss-of-function mutants that otherwise die as embryos. Furthermore, I showed that SWI/SNF protein levels are reduced in swsn-1; snfc-5 double mutants and partly restored to wild-type levels in swsn-1; snfc-5; ubr-5 triple mutants consistent with a model in which UBR-5 regulates SWI/SNF levels by tagging the complex for proteasomal degradation. Finally, using my mutant models, I established a set of approximately 1200 genes that are dysregulated in swsn-1 mutants but exhibit wild-type expression in swsn-1; snfc-5 double and swsn-1; snfc-5; ubr-5 triple mutants, suggesting that the proper regulation of these genes by the SWI/SNF complex promotes animal development. In summary, my findings establish a novel link between two E3 ubiquitin ligases and SWI/SNF function and suggest that UBR5 and HECTD1 might be viable therapeutic targets for the for the many developmental disorders caused by missense mutations in SWI/SNF subunits.
Wellcome Trust (219475/Z/19/Z)