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Functional characterization of the tumour suppressor gene Hypermethylated in cancer-1 (HIC1)
thesisposted on 22.02.2017, 01:08 authored by McKenzie, Lisa
Epigenetic gene silencing, associated with promoter CpG island hypermethylation, is now recognized as one of the main mechanisms by which cancers inactivate tumour suppressor genes. HYPERMETHYLATED IN CANCER-1 (HIC1) is a tumour suppressor commonly silenced by epigenetic mechanisms in cancer. Although HIC1 is a sequence-specific transcriptional repressor, its role as a tumour suppressor is poorly understood. The most well characterized transcriptional target of HIC1 is SIRT1, a class III histone deactylase known to promote mammalian cell survival under oxidative stress and DNA damage. Importantly, SIRT1 actively deactylates, and thus inactivates, P53. I therefore hypothesized that loss of Hic1 function could substitute for p53 mutations in a cancer initiation model. To address this question, I utilized mouse embryonic fibroblasts (MEFs) as a model for neoplastic transformation. In addition, I aimed to determine whether the loss of Hic1 could substitute for p53 loss in the background of Kras constitutive activation (KrasG12D). Surprisingly, my research found that Hic1 loss did not result in the deregulation of the Hic1-Sirt1-p53 loop in MEFs, either alone or in a KrasG12D background. In addition, no known Hic1 target genes were deregulated in this model, and its loss did not result in major gene expression changes. Despite this, the loss of Hic1 in MEFs resulted in the immortalization and full transformation into cancer cells. In addition, tumours arising from Hic1 KO MEFs resembled pleomorphic rhabdomyosarcoma (RMS), a highly aggressive soft-tissue sarcoma. My research found that the loss of Hic1 caused these cells to have a delayed ability to repair DNA double strand breaks (DSBs), along with massive widespread genomic and chromosomal instability, and a decrease in nucleosome remodelling and deacetylase (NuRD) complex activity. The findings from this thesis demonstrate that Hic1 has a previously unidentified non-transcriptional role in maintaining stability of the genome, and suggests that HIC1 loss by epigenetic silencing is a potentially important event in cancer initiation.