Supervisors info:
Χ. Πιπέρη, Αναπληρώτρια Καθηγήτρια, Ιατρική, ΕΚΠΑ
Μ. Καραμούζης, Αναπληρωτής Καθηγητής, Ιατρική, ΕΚΠΑ
Κ. Τρούγκος, Αναπληρωτής Καθηγητής, Ιατρική, ΕΚΠΑ
Summary:
This study investigates the expression frequency of epigenetic enzymes in pediatric gliomas, the most common type of primary brain tumors. The degree of malignancy of primary gliomas varies from well-differentiated non-invasive tumors to a high degree of malignant invasive tumors that are difficult to respond to existing therapeutic methods. Their pathogenesis includes environmental and genetic factors, as well as epigenetic modifications. Epigenetics refer to the study of reversible hereditary changes in the function of genes that occur without any change in the sequence of nuclear DNA. The main epigenetic mechanisms are DNA methylation, changes in chromatin structure and the presence of miRNAs. The present study investigates the expression of enzymes that cause chemical modifications of histones that make up the nucleosome, the basic constituent of chromatin. Histones (H2A, H2B, H3, and H4) are responsible for the proper structure of chromatin and are involved in DNA replication and repair mechanisms, controlling the expression of important genes. They are subject to epigenetic chemical modifications, which cause post-transcriptional changes of genes and are involved in the development of diseases, including brain tumors.
Histone methylation at arginine and lysine residues of their N-terminus has been associated with oncogenenic mechanisms. Methylation of histones is carried out by specific enzymes called methyltransferases. The present study investigated the expression of methyltransferases SETD2, SUV39H1 and MLL2 that catalyze trimethylation of histone 3 on lysine 36 (H3K36me3), trimethylation of histone 3 on lysine 9 (H3K9me3) and trimethylation of histone 3 on lysine 4 (H3K4me3) respectively, in pediatric brain tumors compared to normal brain.
The expression levels of methyltransferases and their corresponding modifications were then investigated at protein level by Western blotting and immunohistochemistry techniques and were correlated with clinicopathological features. In particular, the expression of SETD2 was found to be significantly increased in abnormal brain tissue relative to normal tissue, whereas the highest expression was observed in low-grade malignant pilocytic astrocytes in relation to high-grade glioblastomas, while the expression of H3K36me3 modification, which is exclusively performed by methyltransferase SETD2, is also very high in all pediatric astrocyte samples of varying degrees of malignancy. MLL2 expression was found at moderate levels, with increased expression of lysine 4 trimethylation in histone 3 in abnormal brain tissue relative to normal. In contrast, the expression of SUV39H1 shows no significant differentiation between brain tumors and normal brain. However, the expression levels of H3K9me3 modification in human normal brain and glioma tissues were differentiated, and more specifically it was found that in grade II and IV malignant astrocytes the expression was increased relative to normal brain tissue.
In conclusion, methyltransferase SETD2 and methylation of histone 3 in lysine 4 and 9 were shown to be involved in glioma pathogenesis and need further investigation. Elucidation of the mechanisms that participate to the epigenetic changes associated with the development of pediatric brain tumors could lead to the identification of new therapeutic targets and the design of new therapeutic approaches for patients.
Keywords:
Epigenetics, Pediatric gloiomas, Histone methylation, Astrocytomas, Methyltransferase
