DNA METHYLATION AND CANCER STEM CELLS
A second crucial epigenetic event is DNA methylation. It consists of the covalent addition of a methyl group (-CH3) on the cytosine pyrimidine ring in DNA, and occurs almost exclusively at cytosines located 5’ to a guanine in CpG dinucleotides. It is generally repressive to transcription. Many tumor suppressor genes and other cancer-related genes are transcriptionally silenced by DNA hypermethylation on the CpG islands of their promoters, while tumor cells present a global hypomethylation (decrease of total methyl-cytosine content) as compared to normal tissue. An ever-growing list of studies underlines the role of DNA methylation in cancer development.
This led Zdenko Herceg and his Group to explore the role of DNA methylation in breast cancer stem cells: “Breast cancer stem cells may be defined by their epigenetic signature. Because epigenetic changes are in principle reversible, by targeting the epigenetic mechanisms, which are underlining their development and maintenance, we may be able to target these cells more efficiently. Using a so-called “mammosphere model”, we looked at the epigenome of putative cancer stem cells, more specifically the methylome, and we identified interesting methylation changes or profiles that may define these cells.”
Mammospheres allow an enrichment of presumed cancer stem cell populations using breast cancer cell lines. They are used in combination with bead array , for methylation patterns to be determined at specific promoters. The Illumina bead-array technology allows the analysis of CpG methylation in genome-wide and high-throughput settings.
Using these tools, Dr Herceg identified aberrant DNA methylation in a number of genes belonging to signaling pathways such as the Jak-STAT pathway. However BRCA1, a well known breast cancer susceptibility gene, was not targeted by aberrant methylation profile. Moreover this methylation pattern is correlated to the expression level of the concerned genes . This implies, on one hand, that the expression of specific cancer-stem like pathways is dependent on epigenetic mechanisms; and on the other hand, that BRCA1 expression, a phenomenon often observed in sporadic breast cancer, is not the result of aberrant methylation, although this does not rule out the possibility that other epigenetic mechanisms may be involved.
"We did more work with profiling microarray. The vast majority of breast cancers are not familiar cases, where BRCA1 is mutated, but sporadic cases that are not associated with BRCA1 mutations. There is a kind of downregulation of the expression of BRCA1in sporadic cases that might be epigenetic and contribute to these sporadic cases. The BRCA1 downregulation is not linked to DNA methylation, which does not exclude indirect regulation. It will be important to answer this question in the future."