Altered splicing leads to reduced activation of CPEB3 in high-grade gliomas.

Cytoplasmic polyadenylation element binding proteins (CPEBs) are auxiliary translational factors that associate with consensus sequences present in 3'UTRs of mRNAs, thereby activating or repressing their translation. Knowing that CPEBs are players in cell cycle regulation and cellular senescence prompted us to investigate their contribution to the molecular pathology of gliomas-most frequent of intracranial tumors found in humans. To this end, we performed methylation analyses in the promoter regions of CPEB1-4 and identified the CPEB1 gene to be hypermethylated in tumor samples. Decreased expression of CPEB1 protein in gliomas correlated with the rising grade of tumor malignancy. Abundant expression of CPEBs2-4 was observed in several glioma specimens. Interestingly, expression of CPEB3 positively correlated with tumor progression and malignancy but negatively correlated with protein phosphorylation in the alternatively spliced region. Our data suggest that loss of CPEB3 activity in high-grade gliomas is caused by expression of alternatively spliced variants lacking the B-region that overlaps with the kinase recognition site. We conclude that deregulation of CPEB proteins may be a frequent phenomenon in gliomas and occurs on the level of transcription involving epigenetic mechanism as well as on the level of mRNA splicing, which generates isoforms with compromised biological properties.

Nuclear exclusion of TET1 is associated with loss of 5-hydroxymethylcytosine in IDH1 wild-type gliomas.

The recent identification of isocitrate dehydrogenase 1 (IDH1) gene mutations in gliomas stimulated various studies to explore the molecular consequences and the clinical implications of such alterations. The Cancer Genome Atlas Research Network showed evidence for a CpG island methylator phenotype in glioblastomas that was associated with IDH1 mutations. These alterations were associated with the production of the oncometabolite, 2-hydroxyglutarate, that inhibits oxygenases [ie, ten-eleven translocation (TET) enzymes involved in the oxidation of 5-methylcytosine to 5-hydroxymethylcytosine (5hmC)]. We investigated 60 gliomas for 5hmC presence, 5-methylcytosine content, TET1 expression, and IDH1 mutation to gain insight into their relationships on a histological level. Of gliomas, 61% revealed no immunoreactivity for 5hmC, and no correlation was observed between IDH1 mutations and loss of 5hmC. Interestingly, expression of TET1 showed remarkable differences regarding overall protein levels and subcellular localization. We found a highly significant (P = 0.0007) correlation between IDH1 mutations and nuclear accumulation of TET1, but not with loss of 5hmC. Of 5hmC-negative gliomas, 70% showed either exclusive or dominant cytoplasmic expression, or no detectable TET1 protein (P = 0.0122). Our data suggest that the loss of 5hmC is a frequent event in gliomas, independent of IDH1 mutation, and may be influenced by the nuclear exclusion of TET1 from the nuclei of glioma cells.