The epigenetics of testicular germ cell tumors: looking for novel disease biomarkers

DNA methylation has a profound impact on genome stability, transcription and development. Although enzymes that catalyse DNA methylation have been well characterized, those that are involved in methyl group removal have remained elusive, until recently. The transformative discovery that ten-eleven translocation (TET) family enzymes can oxidize 5-methylcytosine has greatly advanced our understanding of DNA demethylation. 5-Hydroxymethylcytosine is a key nexus in demethylation that can either be passively depleted through DNA replication or actively reverted to cytosine through iterative oxidation and thymine DNA glycosylase (TDG)-mediated base excision repair. Methylation, oxidation and repair now offer a model for a complete cycle of dynamic cytosine modification, with mounting evidence for its significance in the biological processes known to involve active demethylation.

The germ-cell tumours are a fascinating group of neoplasms because of their unusual biology and the spectacular therapeutic results that have been obtained in these tumours. Traditionally, this group of neoplasms is presented in an organ-oriented approach. However, recent clinical and experimental data convincingly demonstrate that these neoplasms are one disease with separate entities that can manifest themselves in different anatomical sites. We propose five entities, in which the developmental potential is determined by the maturation stage and imprinting status of the originating germ cell. Recent progress begins to explain the apparent unpredictable development of germ-cell tumours and offers a basis for understanding their exquisite sensitivity to therapy.

The base 5-hydroxymethylcytosine (5hmC) was recently identified as an oxidation product of 5-methylcytosine in mammalian DNA. Here, using sensitive and quantitative methods to assess levels of 5-hydroxymethyl-2'-deoxycytidine (5hmdC) and 5-methyl-2'-deoxycytidine (5mdC) in genomic DNA, we investigated whether levels of 5hmC can distinguish normal tissue from tumor tissue. In squamous cell lung cancers, levels of 5hmdC were depleted substantially with up to 5-fold reduction compared with normal lung tissue. In brain tumors, 5hmdC showed an even more drastic reduction with levels up to more than 30-fold lower than in normal brain, but 5hmdC levels were independent of mutations in isocitrate dehydrogenase-1. Furthermore, immunohistochemical analysis indicated that 5hmC is remarkably depleted in many types of human cancer. Importantly, an inverse relationship between 5hmC levels and cell proliferation was observed with lack of 5hmC in proliferating cells. The data therefore suggest that 5hmdC is strongly depleted in human malignant tumors, a finding that adds another layer of complexity to the aberrant epigenome found in cancer tissue. In addition, a lack of 5hmC may become a useful biomarker for cancer diagnosis.