Structure
The TET enzymes have a cysteine-rich region closely followed by a double stranded beta-helix (DSBH) domain near their C-terminus.[2] The DSBH domain contains three Fe2+ binding sites and an α-ketoglutarate binding site.[2] This DSBH domain, along with the preceding cysteine-rich region, performs the main catalytic activity of these enzymes and more generally, for all α-ketoglutarate oxygenases.
In addition, TET1 has a CXXC-type zinc finger domain near the N-terminus. However, the TET1 CXXC domain lacks the conserved lysine-phenylalanine-glycine-glycine (KFGG) motif commonly seen within the CXXC domains of other DNA binding proteins, such as DNA methyltransferase-1 (DNMT1). A study conducted by Frauer et al. in 2011 showed that the isolated CXXC domain of TET1 has no DNA binding activity, which agrees with the evidence suggesting that the KFGG motif increases affinity for unmethylated DNA.[3] Frauer et al. also speculated that the CXXC domain of TET1 may be involved with protein-protein interactions instead of DNA binding.[3]
Function
Common Function
All three TET enzymes and their isoforms are involved in the biochemical pathway that converts 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC). They also regulate the further conversions of 5hmC to 5-formylcytosine (5fC) and then 5fC to 5-carboxylcytosine (5caC).[4] Although experimental data shows that TET3 does so to a lesser extent than TET1 and TET2.[4]
While the oxidation performed by TET enzymes was originally thought to be a source of DNA damage, new research has implied that this catalytic activity may actually be the initial steps of a process of DNA demethylation. This hypothesized DNA demethylation pathway starts with the conversion of 5mC to 5caC after several rounds of oxidation by TET enzymes. The next step is the removal of the modified cytosine base by thymine DNA glycosylase (TDG) which leaves an abasic site on the DNA. The last step is then the process of base excision repair in which a new unmodified cytosine is regenerated at the site, thus completing the process of DNA demethylation.[4][5]
Specific Functions
Experimental data shows that the TET genes have different expression patterns and at different levels, which indicates that each of the TET enzymes do not fully overlap in their functionality.
TET1 is usually expressed in fetal heart, lung, and brain tissue and in adult skeletal muscle, thymus, and ovary. It is not generally expressed in adult heart, lung, and brain tissue. Moreover, studies have shown that TET1 expression in adult brain tissue is correlated with brain cancer. This occurs through TET1’s indirect activation of brain cancer-related genes such as EGFR, AKT3, CDK6, CCND2, and BRAF through creation of 5hmC which recruits the CHTOP-methylosome complex that activates these genes.[6]
TET2 is broadly expressed, but it is especially highly expressed in hematopoietic cells, which develop into blood cells. Regarding this, it is suggested that TET2 plays a role in hematopoiesis due to the presence of TET2 mutations in many myelodysplastic syndromes.[7][8][9]
TET3 is highly expressed in zygotes and is involved with epigenetic chromatin reprogramming in the zygote after fertilization. Specifically, it plays a role in DNA demethylation of the paternal pronucleus before implantation.[5]
Disease
Structural highlights
