How do DNA molecules undergo methylation? Is it a selective process or do they undergoes methylation irregularly? How can we know why it's expressing more in particular CpG island cells in cases of cancer?
This is a complex and well studied process that cannot be explained in a simple and short answer. CpG are methylated by DNA methyltransferases (DNMTs), some of this happens in development during differentiation. Genome patterns of CpG methylation are described in various cancer types and the effects on gene expression are correlated. Methylation can be the result of Polycomb mediated repression, but it is still not clear how specific genes are recognized. Cancer cells can amplify DNMTs and PcG genes to increase expression and to deregulate the genome. There are many review articles on DNA methylation in development and in cancer that you should read for more information.
It varies a lot. Theoretrically as well a practically, there are many instances where we have found that tumor tissues have a way-ward expression pattern compared to the normal tissue. Even a drastic change can be observed between lymphocytes and tissue of the same individual. You can follow the following paer. Hope this shall be useful:
http://toxsci.oxfordjournals.org/content/135/2/300.short
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