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Cytosine is a molecule that has left a big mark in the field of epigenetics; with research into it revealing a complex role in the regulation of gene expression.
5-Hydroxymethylcytosine (5-hmC) has recently emerged as an epigenetic mark. Despite its initial discovery in bacteriophages in 1952 (Wyatt & Cohen, 1952), it was only reported in mammals in the 1970s (Penn et al., 1972). However, researchers failed to replicate these findings in later studies (Kothari & Shankar, 1976) and it took more than 30 years to confirm the existence of 5-hmC and gain insights into its molecular function.
Progress was made in 2009 when, while assaying for 5-mC, researchers identified 5-hmC in human and mouse brains (Kriaucionis & Heintz, 2009). At the same time, another research group independently observed 5-hmC to be abundant in embryonic stem cells (Tahiliani et al., 2009).
These two high-impact Science publications, led to a revolution in epigenetics, with 5-hmC successfully staging a coup for the top methylation mark of interest.
Just as most researchers were beginning to realize that molecular inheritance was a process much more complex than simply four base pairs, a sixth base pair had emerged.
Although initially observed in neurons and embryonic stem cells, 5-hmC has since been extensively characterized for tissue specificity.
High levels are often found in the central nervous system and the spinal cord (Globisch et al., 2010), while significantly lower levels are observed elsewhere. 5-hmC truly does appear to be a molecular mark of the nervous system.
5-hmC was initially believed to be a by-product of oxidative stress, rather than a functional epigenetic mark. However, researchers observed an absence of other markers for oxidative stress in the presence of high 5-hmC levels.
It was then shown that 5-hmC is the product of DNA methylation, an active process involving the TET family of enzymes. Now, recent research suggests that not only do the intermediates of this reaction (5-hmC, 5-fC and 5-caC) have a role in the demethylation of DNA, but they may also act as epigenetic signals on their own (Iurlaro, et al., 2013).
However, it appears that 5-hmC's true functional potential is only beginning to be realized. This is exemplified in the case of active demethylation, which occurs in post-mitotic adult neurons (Gavin et al., 2013).
Furthermore, recent research has also shown that there is a global loss of 5-hmC in cancerous cells of a number of non-brain tissues (Pfeifer et al., 2013), suggesting a potential role in cellular growth regulation.
5-hmC is a key intermediate in cytosine demethylation as it can either be passively depleted through DNA replication or actively reverted to cytosine through oxidation reactions and base excision repair (Kohli & Zhang, 2013). Successive oxidation of 5-hmC by the TET enzymes produces 5-formylcytosine (5-fC) and 5-carboxycytosine (5-caC), which are found at a lower abundance in the genome (He et al., 2011).
Ultimately, methylation, oxidation and repair provide a model for a cycle of dynamic cytosine modification, with mounting evidence for its significance in the biological processes known to involve active demethylation (Tan & Shi, 2012).
The most common approaches for studying DNA methylation are based on sodium bisulfite conversion. Sodium bisulfite conversion deaminates cytosine into uracil, but does not affect 5-mC. When the bisulfite-treated DNA is subjected to PCR, the uracil pairs to an adenine and is then amplified as a thymine, whereas the methylated cytosines remain unchanged.
Downstream techniques (e.g. next generation sequencing and microarray) can be used to analyze the bisulfite converted DNA, where cytosines that are read as cytosine represent methylated cytosine, while those that are read as thymine represent unmethylated cytosine.
However, upon the popularization of 5-hmC, it was realized that conventional assays, including bisulfite and sequencing (BS-seq), were not able to distinguish between 5-mC and 5-hmC (Huang et al., 2010 and Jin et al., 2010). Hence, the distinction has become a new frontier in the understanding of the regulation of gene expression.
Two novel, bisulfite-based technologies have recently emerged that address the challenge of mapping 5-hmC:
Although these approaches address the core issue of distinguishing 5-mC from 5-hmC, the additional oxidative step combined with bisulfite can introduce another layer of potentially variability and sample loss.
Researchers are working towards both improving the above methods and developing new ones for 5-hmC detection. Some of the more recent advancements include:
Although these methods are very promising, they still have their limitations. For example, the nanopore technology is still in very early stages of development, while other techniques suffer from selection biases (AbaSI for certain CpGs and 5-hmC for antibody specificity).
Investigations into 5-hmC have begun to yield great insight into our understanding of development and complex disorders.
Recent investigation has generated single-base resolution 5-hmC maps that show that 5-hmC marks regulatory regions in the developing fetal brain genome (Lister et al., 2013). These regions then go on to be CpG demethylated by TET2 and activated in the adult brain.
Ultimately, this suggests that there are 5-hmC signatures for developmentally important regions (Tan & Shi, 2012). Furthermore, 5-hmC may have great implications for our understanding of disease, given its environmentally responsive nature (Blaschke et al., 2013), as evidenced by its dynamic response to cell media conditions and life experience.