The Metabolic-Epigenetic Role of IDH1 In The Initiation And Progression Of High-Grade Serous Ovarian Cancer
Open Access
- Author:
- Dahl, Erika
- Graduate Program:
- Biomedical Sciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 05, 2020
- Committee Members:
- Katherine Aird, Dissertation Advisor/Co-Advisor
Katherine Aird, Committee Chair/Co-Chair
Nadine Hempel, Committee Member
Shengyu Yang, Committee Member
George Lucian Moldovan, Outside Member
Edward Joseph Gunther, Committee Chair/Co-Chair
Nathaniel Snyder, Special Member
Ralph Lauren Keil, Program Head/Chair
Edward Joseph Gunther, Dissertation Advisor/Co-Advisor - Keywords:
- Ovarian cancer
Metabolism
Senescence
Epigenetics
DNA repair - Abstract:
- Epithelial ovarian cancer (EOC) is the most lethal gynecological cancer and is composed of several histological subtypes. High-grade serous carcinoma (HGSC) is the most common histosubtype, accounting for approximately 70% of all EOC diagnoses. Almost all HGSC patients have a mutation in TP53, which is an initiating factor in the hypothesized cell-type of origin, the fallopian tube fimbriae. In addition, 20% of HGSC patients have an amplification in the oncogene cyclin E1 (CCNE1), which canonically regulates the G1 to S-phase of the cell cycle. Due to the lack of symptoms and current screening methods, HGSC is typically diagnosed at late stages when it has disseminated from the fallopian tube and ovarian surface epithelium into the peritoneal cavity. Standard-of-care treatments for these patients include debulking surgery, which aims to remove visible tumor, and platinum and taxol-based chemotherapies. Poly(ADP-ribose) polymerase (PARP) inhibitors have recently emerged as a novel therapeutic for women with homologous recombination pathway defects. Unfortunately, 75% of these patients will relapse with chemoresistant disease for which there are limited second-line therapies currently available. As metabolic adaptations are known to occur in cancer and can be specifically targeted, I aimed to identify and target metabolic vulnerabilities in HGSC as novel therapeutic strategies. I found that compared to normal fallopian tube (FT) cells, HGSC cells have increased TCA cycle enzymes and metabolites, suggesting that the TCA cycle may be a novel target for these ovarian cancers. Through further analysis, I identified wildtype isocitrate dehydrogenase I (wtIDH1) to be associated with poor HGSC progression-free survival. Therefore, I hypothesized that targeting wtIDH1 in HGSC may be a novel therapeutic for this patient population. wtIDH1 reversibly converts isocitrate to alpha-ketoglutarate (αKG), a co-substrate for several DNA and histone demethylases, which can effectively alter gene expression. I found that knockdown of wtIDH1 in HGSC cells depleted whole cell αKG pools. Decreased αKG availability increased repressive histone methylation at proliferation-promoting gene loci, which led to a stable cell cycle arrest, termed senescence, in vitro. These data suggest that wtIDH1 is necessary for HGSC proliferation and targeting wtIDH1 may be a pro-senescent therapeutic strategy. Additionally, cyclin E1 amplifications have been identified in FT lesions, suggesting they occur early in ovarian cancer initiation. Cyclin E1 amplifications are mutually exclusive for HR mutations and, therefore, are not sensitive to FDA-approved PARP inhibitors, a currently used therapeutic for HGSC patients. Oncogene activation typically induces senescence; however, it is hypothesized that these cells bypass senescence to progress into HGSC. I found a positive correlation between cyclin E1 and wtIDH1 in FT cells, HGSC cells, and patient data from The Cancer Genome Atlas (TCGA). Defects in HR are known to increase DNA damage in cells and are a hallmark of senescence. Knockdown of wtIDH1 exacerbated DNA damage markers and induced senescence of FT cells, suggesting it is in part necessary for cyclin E1-high FT proliferation. Knockdown of wtIDH1 in cyclin E1-high cells depleted expression of several HR genes, including Breast Cancer Susceptibility Type 1 and 2 (BRCA1/2). To determine whether reduced BRCA1/2 expression is due to epigenetic alterations, I performed histone proteomic screening and found an increase in repressive marks such as H3K9me2 and H4K20me2/3 upon wtIDH1 knockdown. It is well-known that tumors that are HR-deficient are hypersensitive to PARP inhibitors. Interestingly, combination treatment of wtIDH1 and PARP inhibitors increased cell death in vitro, suggesting that changes in HR gene expression due to wtIDH1 inhibition may sensitize these cells to PARP inhibitors. Altogether, these data suggest that wtIDH1 is necessary for proliferation of cyclin E1-high FT cells. Targeting wtIDH1 induces senescence, depletes HR gene expression, and may be a novel combinatorial strategy with PARP inhibitors. Together, these studies identified the critical role of wtIDH1 in the proliferation of HGSC and FT cells with high cyclin E1 expression. wtIDH1 was responsible for regulating key proliferation-promoting and HR genes. Future studies will determine whether inhibition of wtIDH1 alone or in combination with PARP inhibitors may lead to senescence or apoptosis, respectively, of ovarian cancer cells in vivo.