The Role of the Epigenetic Regulator UHRF1 in Colon Tumorigenesis

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PROJECT SUMMARY Alterations to the normal DNA methylation patterns that regulate all chromatin-templated processes is a hallmark of cancer. Global DNA hypomethylation and local hypermethylation contribute to malignant transformation and the silencing of tumor suppressor genes (TSGs). Specific patterns of DNA hypermethylation are particularly present in a subclass of proximal colorectal cancers (CRC). These right-sided colorectal tumors often present with the CpG Island Methylator Phenotype (CIMP) in which many normally unmethylated promoter CpG islands are hypermethylated with high consistency and frequency. How these abnormal DNA methylation patterns are established and propagated remains unclear, while therapeutic agents intended to reverse DNA hypermethylation do not currently meet the needs of CRC patients. One protein with appealing attributes that may explain how DNA hypermethylation occurs in CRC is the E3 ligase and epigenetic regulator UHRF1. UHRF1 directs DNA methylation through recruitment of the maintenance DNA methyltransferase DNMT1 to newly replicated DNA. Evidence suggests UHRF1 is overexpressed and its function in DNA methylation is misappropriated in many cancers, including CRC. The objective of this proposal is to define the subclass of CRCs influenced by high UHRF1 levels and determine the mechanism through which UHRF1 contributes to CRC by altering DNA methylation levels and patterns. Aim 1 will identify the subclass of CRC most associated with high UHRF1 expression. This is critical to determining the CRC context for which inhibition of UHRF1 would have therapeutic benefit. This Aim will use primary human colon tumor tissue to segregate UHRF1-high tumors with epigenetic phenotypes. Aim 2 addresses the role of UHRF1 in colon tumor formation and maintenance. This aim will utilize an ex vivo model system of intestinal organoids derived from mouse intestinal epithelia. This system allows for precise control of the onset of an intestinal tumor phenotype to facilitate understanding of the contribution of UHRF1 to either tumor initiation or maintenance and changes in DNA methylation during this course. Finally, Aim 3 queries other gene targets identified to regulate the epigenetic-mediated silencing of TSGs. These targets were determined through a genome-wide screen and include both known epigenetic regulators and proteins previously unknown to regulate TSG silencing. As reversal of DNA hypermethylation is necessary for reactivation of TSGs, a long-term goal of this project is to gain a better understanding of the mechanisms necessary for reactivating TSGs that have been epigenetically silenced and using that information to refine CRC treatments. Thus, this proposal will address in which genetic/epigenetic subclass and at what point in tumor formation UHRF1 is important, in addition to better understanding TSG silencing through epigenetic modifications. This has high relevance to public health given the prevalence of CRC and the critical need for identifying potential therapeutic targets to reverse abnormal DNA methylation and TSG silencing in CRC.