Tumors are caused by the demethylation of DNA sections. But what causes the demethylation?
I can't quite explain this in any way but I thought that, for instance in breast cancer, it is possible that the cancer is caused by the methlyation of DNA, not the DNA demethylation. The methylation has an effect on inhibiting transcription machinery, and in this case on tumor suppressor genes.
If I am wrong, is there someone who can clarify this.
Methylation in the promotor region plays a role in transcription level. CpG island are often associated with methylation.
Regarding cancer, the gene with the inappropriate transcription level will influence the result. For example, demethylation of a tumor promoting gene will lead in cancer due to increase level of the tumor-promoting protein. The methylation of tumor suppressor genes will also lead to cancer but due the repression of a tumor suppressor gene.
Demethylation is a normal process of gene transcription; however, defective DNA repair pathway or error made the DNA transcritption machinery will lead to prolonged demethylation.
Demethylation is the symptom, not the cause. Faulty receptors do not recognize the signal. Active receptors get overstimulated and assumes the signal means they should do something - even if its wrong. And it usually is wrong. A cascading response typically follows.
There are enzymes activated that regulate methylation, DNA methyltransferases, and their absence or suppression can lead to reduced methylation. Depending on the region of the genome, hypo- or hypermethylation can affect tumor formation. As iansmith pointed out, methylation is important for transcriptional regulation, so alterations in either direction can lead to errors in important regulatory steps and promote tumorigenesis.
Here are two abstracts, selected somewhat randomly from the literature addressing this, that serve to illustrate the point (I didn't choose these because they are any better or more relevant than any number of papers out there, but because the abstract contains enough information to illustrate the point if you can't get to the full text of the article).
Separate names with a comma.