Thymidine kinase from herpesvirus is a sub-family of thymidine kinases that catalyses the transfer of phospho group of ATP to thymidine to generate thymidine monophosphate, which serves as a substrate during viral DNA replication. Its presence in herpesvirus-infected cells is used to activate a range of antivirals against herpes infection, and thus specifically target the therapy towards infected cells only. Such antivirals include:
Purine analogues of guanine: Aciclovir, Famciclovir, Ganciclovir, Penciclovir, Valaciclovir, Valganciclovir Vidarabine Pyrimidine analogues of uridine: Idoxuridine, Trifluridine Brivudine Mutations in the gene coding thymidine kinase in herpes viruses can endow the virus with resistance to aciclovir. In these situations, alternative medications that are of use include other guanine analogues such as famciclovir, valaciclovir and penciclovir.
Function and role Overall, herpesviruses are large DNA viruses varying from 120 to 260 nm long. Each virus particle contains a core with linear DNA, a capsid with 162 capsomeres, a loose protein integument, and an outer fatty membrane. Some genes within the herpesvirus can be removed which makes the virus weaker, allowing it to train the immune system without causing major disease which is useful for vaccines. Along with this, deleted sections can be replaced with foreign genes, which the virus can deliver to target cells. Thymidine kinase is part of the nucleoside salvage pathway which recycles broken-down DNA to be used again. TK helps the virus make DNA. With HSV-1 TK (herpes virus type 1 thymidine kinase) specifically, it has the ability to phosphorylate multiple different nucleotides and nucleoside analogues. Herpesvirus genes are turned on in a specific order – starting with immediate early (IE) genes, then early (E) genes, and finally late (L) genes. The groups are reliant on each other for when the next group is activated. The thymidine kinase gene is part of the early (E) gene group. It can be expressed at low levels in few non-virus systems due to the fact that its promoter contains cellular control elements that can be used by regular cell transcription factors. In an HSV (herpes simplex virus) infection, thymidine kinase expression requires activation by immediate early (IE) proteins and is dependent on the host cell’s RNA polymerase II apparatus to transcribe efficiently. Thymidine kinase is seen in a wide variety of bacterial, eukaryotic, and prokaryotic cells. Cells have specific TK enzymes including TK1 which is systolic and mainly active when cells prepare to divide, and TK2 which is mitochondrial and helps maintain the mitochondrial DNA. They both have the role of adding a phosphate to thymidine, creating dTMP. In order for this to happen, ATP acts as the phosphate donor, and magnesium ions (Mg2+) ions are present to help the reaction occur. A majority of herpesviruses, along with some DNA viruses, including vaccinia virus, African swine fever virus, and Rana grylio virus contain a gene encoding for thymidine kinase. Specifically in HSV-1, the expression of TK is important and it relies heavily on it during its lifecycle. HSV-1 TK is able to recognize nucleosides (thymidine and deoxycytidine) and nucleoside analogue drugs (acyclovir). When thymidine kinase phosphorylates these, they become active and interfere with viral DNA replication. When regarding antiherpesvirus treatment and therapeutic targets in cancer gene therapy, thymidine kinase is a key component.
Pyrimidine salvage pathway Thymidine kinase is also a core enzyme with the pyrimidine salvage pathway which is the cell’s recycling system for DNA building blocks. It takes a thymidine (DNA base) and adds a phosphate group with the use of ATP, which produces dTMP which is needed for the cell to make DNA. HSV-1 TK can recognize and phosphorylate many different molecules including pyrimidines and pyrimidine analogs, and purine analogs. Pyrimidine analogs contain thymidine, deoxycytidine, and AZT. Purine analogs contain acyclovir, ganciclovir, buciclovir, and penciclovir. This broad activity is important as the thymidine kinase can activate not just nucleosides, but their analogs as well. When HSV-1 TK phosphorylates a nucleoside analog, it adds the first phosphate, and then cellular kinases add two more phosphates which converts it to a nucleoside triphosphate, which is then inserted into the DNA causing synthesis to stall and ultimately leads to cell death. These cells that express HSV-1 TK contain a vulnerability of being able to be killed by drugs that normal cells do not have the ability to activate efficiently. A guanosine analog known as acyclovir (ACV) is an antiviral drug that is, even in high dosage, mostly nontoxic. HSV-1 TK contains a high Km for ACV, meaning that the enzyme binds poorly and activates inefficiently. ACV is an option used for herpes infections, but it is not potent enough for cancer suicide gene therapy. Concerning the ability of thymidine kinase being used to cause cell death, not all cells need to contain the TK gene to be susceptible. This is called the bystander effect where nearby cells can die due to gap junctions where activated drug molecules can pass through its channels, apoptotic vesicles, and through immune response where the immune system clears nearby tumor cells due to inflammation.
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