Classically, DNA is thought to comprise 4
nucleotide bases: adenine, thymine, cytosine, and guanine (A, T, C, and G).
However, the discovery and identification of variants of the classical 4 bases
including 5-methylcytosine, 5-hydroxymethylcytosine, and 5-formylcytosine in
human and mouse brain tissues has revolutionized the conception of DNA, its
composition, regulation and expression.
While the sequence of the 4 classical bases
determine what genes encode, the additional bases are involved in controlling
how DNA sequences are interpreted, what genes are expressed, and importantly
when genes are expressed. For instance, epigenetic modifications made to
cytosine can affect the DNA structure by exposing regions of the DNA to attract
different proteins and transcription factors, which could either directly or
indirectly influence which genes are expressed or repressed.
* 5-Methylcytosine is formed by the
addition of a methyl group to the 5th carbon of cytosine. This process
typically occurs at cytosines located in CpG dinucleotide sequences, although,
methylation has also been found to occur at non-CpG dinucleotide sites.
5-Methylcytosine has been shown to function as a repressor of gene
transcription. In the promoters of genes, 5-methylcytosine is associated with
stable, long-term transcriptional silencing. By enabling genes to be turned on
and off in specific cell types, the gene regulatory function of
5-methylcytosine is an important mechanism in mediating genomic imprinting,
controlling cellular differentiation, and the expression of specific genes for
normal tissue patterning and development.
* 5-Hydroxymethylcytosine is abundant in the
brain and in embryonic stem cells. It is formed by oxidation of
5-methylcytosine and reduced levels of this cytosine variant in DNA has been
regarded as a hallmark of cancer. Genomic profiling of 5-hydroxymethylcytosine
has revealed that in contrast to 5-methylcytosine, 5-hydroxymethylcytosine is
particularly associated with gene regulatory elements where 5-methylcytosine is
depleted. 5-Hydroxymethylcytosine has been found to be associated with cell
proliferation, having been demonstrated to form immediately during DNA
replication at the stage of synthesis via isotopic labelling studies of DNA in
mouse tissues.
* 5-Formylcytosine is derived from
5-methylcytosine by Tet-mediated oxidation. In mice, 5-formylcytosine has been
found to be present in all tissues, including embryonic tissues, and
preferentially occurs at poised enhancers among other gene regulatory elements.
5-Formylcytosine has been implicated in various roles including demethylation,
chromatin remodeling, and DNA structural changes such as changes to groove
geometry and base pairs associated with 5-formylcytosine-modified bases that
lead to helical underwinding.
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