Epithelial ovarian cancer (EOC) is a highly lethal malignancy due to a lack of early detection approaches coupled with poor outcomes for patients with clinically advanced disease. Cancer-testis (CT) or cancer-germline genes encode antigens known to generate spontaneous anti-tumor immunity in cancer patients. CT45 genes are a recently discovered 6-member family of X-linked CT genes with oncogenic function. Here, we determined CT45 expression in EOC and fully defined its epigenetic regulation by DNA methylation. CT45 was silent and hypermethylated in normal control tissues, but a large subset of EOC samples showed increased CT45 expression in conjunction with promoter DNA hypomethylation. In contrast, copy number status did not correlate with CT45 expression in the TCGA database for EOC. CT45 promoter methylation inversely correlated with both CT45 mRNA and protein expression, the latter determined using IHC staining of an EOC TMA. CT45 expression was increased and CT45 promoter methylation was decreased in late-stage and high-grade EOC, and both measures were associated with poor survival. CT45 hypomethylation was directly associated with LINE-1 hypomethylation, and CT45 was frequently co-expressed with other CT antigen genes in EOC. Decitabine treatment induced CT45 mRNA and protein expression in EOC cells, and promoter transgene analyses indicated that DNA methylation directly represses CT45 promoter activity. These data verify CT45 expression and promoter hypomethylation as possible prognostic biomarkers, and suggest CT45 as an immunological or therapeutic target in EOC. Treatment with decitabine or other epigenetic modulators could provide a means for more effective immunological targeting of CT45.
Keywords: CNA, copy number alteration; CT antigen genes, cancer-testis or cancer-germline antigen genes; CT45; DAC, decitabine, 5-Aza-2′-deoxycytidine; DFS, disease-free survival; DNA methylation; DNMT, DNA methyltransferase; EOC, epithelial ovarian cancer; FTE, normal fallopian tube epithelia; HGSOC, high-grade serous ovarian cancer; IHC, immunohistochemistry; NO, bulk normal ovary; OS, overall survival; OSE, normal ovary surface epithelia; RLM-RACE, 5′ RNA ligase-mediated rapid amplification of cDNA ends; RNA-seq, RNA sequencing; TCGA, The Cancer Genome Atlas; TMA, tissue microarray; TSS, transcription start site; cancer germline genes; cancer testis antigen genes; decitabine; epithelial ovarian cancer; tumor antigens.