Overexpression of TET dioxygenases in seminomas associates with low levels of DNA methylation and hydroxymethylation

Abstract

Germ cell tumors and particularly seminomas reflect the epigenomic features of their parental primordial germ cells (PGCs), including genomic DNA hypomethylation and expression of pluripotent cell markers. Because the DNA hypomethylation might be a result of TET dioxygenase activity, we examined expression of TET1-3 enzymes and the level of their product, 5-hydroxymethylcytosine (5hmC), in a panel of histologically characterized seminomas and non-seminomatous germ cell tumors. Expression of TET dioxygenase mRNAs was quantified by real-time PCR. TET1 expression and the level of 5hmC were examined immunohistochemically. Quantitative assessment of 5-methylcytosine (5mC) and 5hmC levels was done by the liquid chromatography-mass spectroscopy technique. We found highly increased expression of TET1 dioxygenase in most seminomas and strong TET1 staining in seminoma cells. Isocitrate dehydrogenase 1 and 2 mutations were not detected, suggesting the enzymatic activity of TET1. The levels of 5mC and 5hmC in seminomas were found decreased in comparison to non-seminomatous germ cell tumors and healthy testicular tissue. We propose that TET1 expression should be studied as a potential marker of seminomas and mixed germ cell tumors and we suggest that elevated expression of TET dioxygenase enzymes is associated with the maintenance of low DNA methylation levels in seminomas. This "anti-methylator" phenotype of seminomas is in contrast to the CpG island methylator phenotype (CIMP) observed in a fraction of tumors of various types.

The Role of DNA/Histone Modifying Enzymes and Chromatin Remodeling Complexes in Testicular Germ Cell Tumors

“…Also, there is room for prognostic impact of these markers, as DNMTs / TET2 are upregulated and downregulated, respectively, in advanced stage disease. These findings are in accordance with most studies published so far, which also report DNMTs’ overexpression in ECs [36,40,42,47,48,50] and of TETs in SEs [45] (Table 1).…”

“…Table 1 displays the result of our query, listing original articles addressing the role of these players in TGCTs pathogenesis and summarizing their major findings [34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55]. Despite the overwhelming evidence that stem cells and germ cells display dynamic epigenetic modifications during differentiation and spermatogenesis, including changes in the expression of these enzymes (e.g., with DNA methyltransferases more expressed in spermatogonia and histone methyltransferases mainly in spermatocytes) [10,56,57,58,59,60,61,62], there is still a lack of studies on the role of these players and related modifications in TGCTs (especially in certain families, with most studies published so far focusing on DNA-modifying enzymes).…”

DNA hypomethylation and aberrant expression of the human endogenous retrovirus ERVWE1/syncytin-1 in seminomas

“…Recently, we have described elevated expression of TET enzymes in GCTs, particularly seminomas [30]. We therefore explored the levels of 5-mC and 5-hmC at the ERVWE1 promoter using bisulfite sequencing and oxidative bisulfite modification.…”

“…The deeply hypomethylated genome of seminoma cells has been recently correlated with elevated expression of the TET1 enzyme in GCTs [ 30 ]. The DNA demethylation activity of TET dioxygenases proceeds through 5-hydroxymethylcytosine (5-hmC) intermediate [ 31 ], which subsequently converts to 5-formylcytosine (5-fC), 5-carboxycytosine and unmodified cytosine (C) [ 32 , 33 ].…”

“…Based on the knowledge of DNA hypomethylation in GCTs and the observation of syncytin-1 expression in testicular tumors [ 13 , 14 ], we explored the ERVWE1 expression systematically within a panel of GCTs with particular respect given to discrimination between seminomas and non-seminoma GCTs. In addition, we included several samples of lymphomas and endometrial carcinomas in our analysis because of (1) the presence of multinuclear giant Reed-Sternberg cells in Hodgkin lymphomas [ 36 ], (2) the detection of full-length ERVWE1 mRNA in endometrial carcinomas [ 17 , 18 , 37 ], and (3) the recently described increased expression of DNA demethylating dioxygenases TET2 and TET3 in seminomas, lymphomas and endometrial carcinomas [ 30 ] that could contribute to ERVWE1 transcriptional derepression. We estimated, for the first time, the levels of both 5-methylcytosine (5-mC) and 5-hmC modifications at the ERVWE1 promoter within the 5′LTR.…”