UHRF1 is an important epigenetic regulator of the human genome, which contains multiple chromatin binding domains. While its role in maintaining methylation patterns through the recruitment of DNMT1 to hemi-methylated DNA has been well established in mouse, less is known about its involvement in human cells. UHRF1 binds to histone modifications and, therefore, facilitates crosstalk between DNA methylation and histone marks. It binds methylated H3K9 through its TTD domain, an interaction that also requires a functional PHD domain. This association of UHRF1 with H3K9 methylation has been implicated in directing the maintenance of DNA methylation. However, evidence for the reliance on this binding for faithful epigenetic inheritance of DNA methylation has been conflicting. Additionally, knockout studies in mice have suggested other possible roles for UHRF1 in cell cycle progression and the DNA damage response. To gain insight into the function of UHRF1 in human cells, stable depletion of UHRF1 using shRNA was carried out in the normal, differentiated human fibroblast hTERT-1604 cell line. This resulted in a transcriptional response consistent with a state of “viral mimicry”, as seen with the use of the DNMT inhibitor 5-AZA-CdR in cancer cells, and the upregulation of ERV and L1 elements. Subsequent restoration of UHRF1 abrogated this response without full remethylation at ERV and L1 elements but not when the PHD or TTD domains were mutated. This implicated a function of UHRF1 in H3K9 mediated silencing of transposable elements. In comparison, depletion of DNMT1 did not induce a notable immune response but exhibited more pronounced effects on the cell cycle and greater DNA damage. 5-AZA-CdR treatment in UHRF1 depleted cells further attenuated the immune response seen with UHRF1 depletion and transient knockdown of UHRF1 in a colorectal cancer cell line induced a similar immune response as seen hTERT-1604 cells. Taken together, these results present a novel concept for the targeting of UHRF1 in cancer therapy in order to enhance the immune response against tumour cells through upregulation of transposable elements.
|Date of Award||Oct 2019|
|Sponsors||Department for Employment and Learning, Northern Ireland.|
|Supervisor||Rachelle Irwin (Supervisor), Colum Walsh (Supervisor) & Declan Mc Kenna (Supervisor)|