Abstract 83: Hypomethylation of TGF-beta target gene, ABCA1 in ovarian cancer and cancer initialing cell and is associated with poor prognosis in cancer patients

Abstract

Abstract Dysregulation of TGF-β signaling plays a crucial role in ovarian carcinogenesis and maintaining cancer stem cell properties. We have previously identified the TGF-β responsive targets in immortalized ovarian surface epithelial cells (IOSE) by expression microarray and found that ABCA1, a family members of ATP-binding cassette is up-regulated following TGF-β treatment (Qin et al, BMC Syst Biol 2009). Thus, we hypothesize that ABCA1 may be involved in ovarian cancer and its initiation. First, we compared the expression level of ABCA1 in IOSE and a panel of ovarian cancer cell lines and found that ABCA1 was up-regulated in HeyC2, SKOV3, MCP3, and MCP2 ovarian cancer cell lines but down-regulated in A2780 and CP70 ovarian cancer cells. The down-regulation in A2780 and CP70 cells was associated with promoter hypermethylation as demonstrated by bisulphite sequencing and demethylation treatment. To investigate if ABCA1 is involved in ovarian cancer initiating cells (OCIC), we compared the methylation level of ABCA1 in ovarian tumor samples and their corresponding spheroids which is previously found to be enriched with OCIC. Lower methylation of ABCA1 could be detected in 2/6 OCIC samples as compared to their original tumors. We further analyzed the expression and methylation level of ABCA1 in CP70 spheroid (CP70sp) derived from suspension culture of CP70 ovarian cancer cells. Comparing to CP70, up-regulation of stem cell markers Nanog (3.8 fold) and Oct4 (12 fold) was detected in CP70sp cells suggesting that CP70sp may contain OCIC. Interestingly, up-regulation of ABCA1 (5.2 fold) concomitant with promoter hypomethylation was detected in CP70sp cells. Additionally, active demethylation may be involved in the hypomethylation of ABCA1 as expression of the methylcytosine dioxygenase, TET1 was up-regulated in the CP70sp cells (5.3 fold). Furthermore, ABCA1 was involved in drug resistance of ovarian cancer which is a feature of cancer initiating cells, as lenti-viral knockdown of ABCA1 in a platinum resistant MCP2 ovarian cancer cells resensitized the cells to cisplatin (IC50: shABCA1 vs shGFP: 0.267 ug/ml vs 0.402 ug/ml). We further analyzed the methylation level of ABCA1 in 97 ovarian cancer, 46 benign, and 4 normal samples using real-time quantitative MSP assay and found that higher methylation level of ABCA1 was detected in cancer than benign (P=0.051) and normal tissues (P<0.05). Importantly, cancer patients showing lower or no methylation of ABCA1 has significantly shorter progression free survival than patients with higher methylation (log rank, P=0.0463). In conclusion, ABCA1 is hypomethylated in a sub-set of ovarian cancer and its cancer initiating cells and is associated with poor prognosis in cancer patients. The role of ABCA1 in OCIC and the involvement of 5-hydroxymethylcytosine (5hmC) in the active demethylation of ABCA1 deserves further investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 83. doi:10.1158/1538-7445.AM2011-83