“The heritability of genomic methylation patterns clearly shows that once established, DNA methylation is dominant over chromatin modifications.”
Some neurodevelopmental processes (here) seem to depend on DNA methylation, but, is this the main purpose of all this methylation?
Our genome is a huge junk pile. That’s right … we are built from a genome, of which some 40%, are old retroviruses, transposons and other broken legacies of foreign DNA that inserted themselves into the genomes of our mammalian ancestors. These ancient viruses can be very dangerous and wreak havoc if they are allowed to be transcribed. DNA methylation helps keep this from happening. Its a HUGE job … some 60% of all CpG’s are methylated … likely THE main purpose of DNA methylation.
“The lack of cell-type-specific methylation at either enhancers or promoters indicates that DNA methylation is likely to have a negligible or very small role in development, and that the methylation changes seen at some low-CpG promoters are likely to be a result of transcriptional activation rather than a cause.”
“The data indicate that the bulk of the genome is methylated as the default state, and unmethylated regions are protected from a promiscuous DNA methylating system by a combination of very high CpG densities and histone modifications and variants that repel DNA methyltransferase complexes.”
So, we must keep in mind when reading the epigenetic literature (a methyl group here or there contributes to less anxiety) that there is a much more vital process happening (ie., lack of a methyl group here or there can lead to a lethal viral attack). Occasionally, in the process of keeping us alive, our physiological systems can make life difficult. C’est la vie!
Also, it appears that methylation is like an enormous fire-hose spraying methyl groups everywhere in the genome to dampen the ground and prevent any small fires (viruses) from igniting. How much stock can you put in research findings that hinge on the appearance/disappearance of 1 or 2 errant methyl groups?