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A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. [2] After invading a host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backward).
Because retroviruses are able to recombine with each other and with other endogenous DNA sequences, it would be beneficial for gene therapy to explore the potential risks HERVs can cause, if any. Also, this ability of HERVs to recombine can be manipulated for site-directed integration by including HERV sequences in retroviral vectors. [1]
How vectors work to transfer genetic material. Gene therapy utilizes the delivery of DNA into cells, which can be accomplished by several methods, summarized below. The two major classes of methods are those that use recombinant viruses (sometimes called biological nanoparticles or viral vectors) and those that use naked DNA or DNA complexes (non-viral methods).
In gene therapy a gene that is intended for delivery is packaged into a replication-deficient viral particle to form a viral vector. [29] Viruses used for gene therapy to date include retrovirus, adenovirus, adeno-associated virus and herpes simplex virus. However, there are drawbacks to using viruses to deliver genes into cells.
A number of viruses have been used for human gene therapy, including viruses such as lentivirus, adenoviruses, herpes simplex, vaccinia, and adeno-associated virus. [5] Adenovirus viral vectors (Ad) temporarily modify a cell's genetic expression with genetic material that is not integrated into the host cell's DNA.
XMRV is a recombinant virus observed incidentally as a result of recombination between two endogenous mouse retroviruses by prostate cancer researchers in the mid-1990s. Although it can infect human tissue, no known disease is associated with the infection [10] [11] [12] and it is unlikely to exist outside laboratories. [13]
There are two types of oncogenic retroviruses: acute transforming viruses and non-acute transforming viruses. Acute transforming viruses induce a rapid tumor growth since they carry viral oncogenes in their DNA/RNA to induce such growth. An example of an acute transforming virus is the Rous Sarcoma Virus (RSV) that carry the v-src oncogene.
Virotherapy is a treatment using biotechnology to convert viruses into therapeutic agents by reprogramming viruses to treat diseases. There are three main branches of virotherapy: anti-cancer oncolytic viruses, viral vectors for gene therapy and viral immunotherapy.