DNA demethylases, are usually taken to be members of the 10-11 translocation protein (TET) family (TET1, TET2 and TET3), which is the key driver of DNA methylation and demethylation. These are the best-suited enzymes to watch DNA demethylation: TET enzymes. Modification of DNA demethylation using TET enzymes. In three stepwise reactions, tet proteins oxidise 5mC to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). Such bases might be new epigenetic states of genomic DNA or demethylation temporary stops in DNA.
Recent biochemical, genetic and functional findings point to Tet proteins being essential for all forms of biological process, from zygotic epigenetic reprogramming to pluripotent stem-cell differentiation to hematopoiesis to leukemia. TETS code for an epigenome, and through their enzymatic activity regulate gene expression and cell growth.
Related Products
DNA Methylation and Demethylation
DNA methylation and demethylation are two of the most common epigenetic regulatory changes that govern many physiological and pathological effects. DNA methylation and demethylation change genes, and their perpetual relative balancing can maintain gene expression. Among the DNA methyltransferases, most often there are three, named DNMT1 and DNMT3A, DNMT3B and DNMT3L.
TET-mediated DNA Demethylation
The epigenetic state of cells is set by the hydroxymethylation and demethylation of DNA by TET enzymes. They control not just gene expression but cellular plasticity, elasticity and environmental adaptation, an active controller of TET enzymes. The TET proteins are iron(II)/-ketoglutarate (Fe(II)/-KG) dioxygenases. Carboxyl on the catalytic core domain is a double-stranded -helix (DSBH) domain and a cysteine rich domain 29. The DSBH domain picks up Fe(II), -KG and 5mC to oxidise; the cysteine domain surrounds the DSBH core to stick the entire system and TET-DNA interaction together. Those TET-DNA contacts aren't methylated, and TETs can be wired up to carry all manner of methylated cytosines.
Mechanisms of TET-mediated DNA demethylation
The primary activity of TET enzymes is to break 5-methylcytosine (5mC) down into 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC), which slowly rewrite the DNA with methyl residues. TET enzymes are diazepam hydroxylase enzymes that catalyze 5mC hydroxylation reaction to convert 5hmC.
Catalytic Activity
TET enzymes are diazepam hydroxylases and they can hydroxylate 5mC to 5hM. TET consists of 3 members, TET1, TET2, and TET3, and they are all expressed at different tissues and in different developmental stages.
Further Modifications
TET enzymes then convert 5hmC to 5fC and 5caC. These derivatives could be used in a variety of ways for gene expression regulation and DNA repair.
Removal of Methyl Groups
DNA repair (e.g., nucleotide excision repair or BER) can be eliminated from 5fC and 5caC. Typically, that takes certain DNA repair enzymes to detect such damage and heal DNA, leading to demethylation.
Gene Expression Regulation
TET enzymes can influence gene expression through demethylation, particularly during embryonic growth and differentiation. Changes in DNA methylation can lead to transcriptional shifts that affect the binding of transcription factors and other epigenetic modifications, ultimately influencing gene transcription.
Fig. 1 Active DNA demethylation with TET active DNA demethylation (Wu, X.; et al. 2017).
Regulation of TET-mediated DNA Demethylation
Demethylation of active DNA is also subject to regulation. The rate of the enzymatic reaction is dependent on the availability of the substrate. Additionally, the reaction can be modulated by cofactors during the enzymatic process. Furthermore, the expression of these genes can be regulated at the transcriptional, post-transcriptional, and post-translational levels. Lastly, perturbations targeting the demethylation machinery in specific genomic regions can also be effective. Many isoforms of TET are generated through various processes that regulate TET expression.
A complex of signalling mechanisms and epigenetic alterations tightly constrain TET enzymes' expression, which gives rise to different isoforms that possess distinct functions. Differential TET isoform expression regulation further complexity epigenetic control, as cells can tailor their responses to internal and external signals. Three main isoforms of TET enzymes, differential expression and/or function revealed to date. A lot of TET enzyme isoforms result from variation in promoter usage and/or alternative splicing, and they are all protein of different sizes. TET activity is not only determined by the amount and type of transcripts made, but also (to a degree) by post-translational modifications.
Biological Functions of TET Enzymes
The biological roles of TET enzymes extend into all cell types, from embryonic development and stem-cell maintenance to immune system regulation. TET enzymes tune the DNA methylation program, fine-tuning genes that are crucial to homeostasis and function in cells. At the heart of the activity of TET enzymes are catalytic reactions that initiate DNA demethylation in a series of oxidative reactions. They combine iron with -ketoglutarate as cofactors to activate the incremental reduction of 5mC to 5hmC, 5fC and 5caC to activated DNA demethylation. The functional value of TET enzymes extends beyond demethylation as well. These enzymes control the regulation of gene expression and the cell's differentiation, growth and adaption to environmental conditions. They make cells epigenetic and functionally specific, by their complex interaction with DNA methylation marks.
Conclusion
TET enzymes are tightly regulated via a series of signalling circuitry and epigenetic changes, so they have many isoforms with different roles and properties. Differential TET isoform regulation makes epigenetic regulation more intricate, since cells can adapt to different types of stimulation (both from inside and outside).
The TET enzymes do the most hydroxymethylating of DNA. There are three enzymes called TET – TET1, TET2, and TET3, which oxidise 5mC to 5hmC, 5caC and then uracil(U). This kind of response is especially important for epigenetic control.
Important regulators of epigenetic regulation, TET enzymes regulate gene expression, cell function, development, and disease progression through their modulation of DNA methylation and its derivatives. As we learn more about how TET enzymes work, new targeted therapies could be delivered for related diseases in the future.
References
- Rasmussen, K.; et al. Role of TET enzymes in DNA methylation, development, and cancer. Genes & development. 2016, 30(7): 733-750.
- Wu, X.; et al. TET-mediated active DNA demethylation: mechanism, function and beyond. Nature Reviews Genetics. 2017, 18(9): 517-534.
.
