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Vectors used in gene therapy: Viral vectors


Viral vector
These are one of the most favourable vectors that are currently being used is harmless viruses. In a pathogenic manner, viruses have progressed a way of delivering and summarizing their genes to human cells. Scientists have tried to control and replace them with human genes which are working and they tried to take benefit of their ability. This transformed virus can be then used with great efficiency to smuggle genes into cells. Some viruses do not enter the cell but in host genomes they insert their genes. Others enter the cell membrane concealed as molecule of protein and enter the cell. Once within the cell of an infected person the transplanted gene is turned on at the right location, it can then produce the instructions which are necessary to make protein for the cell which was formerly altered or missed. (walther W et al., 2000)

Viral vectors Tyoes:
• Adenovirus1
• Retrovirus
• Herpes simplex virus [HSV]
• Adeno-associated viruses (AAVs)


Retrovirus
Retroviruses were the first vectors to be used in experiments of gene therapy. They belong to the class of viruses which has RNA as genetic material. With the enzyme reverse transcriptase these can create double stranded DNA copies. In the host cell, these copies of its genome can be assimilated by another enzyme that carried the virus called as integrase.  If these modified host cells distributes later than their offspring will have the new genes. In most gene therapy experiments, retroviruses have been used they represents many problems.   (Baun C et al., 2003).
Until now, the most successful application of vectors has been seen in gene therapy trials by using retroviral vectors to treat X-linked severe immunodeficiency.  (Gasper Hb, 2005). With relative success this has been also tried to treat ADA and SCID deficiency. Where the corrected genes are needed, researchers began to inject altered retroviruses directly into tissues. e.g. in cystic fibrosis, in the lining of bronchial tube healthy genes are implanted directly. The studies on experimental animals are being conducted to launch the effect in muscular dystrophy. (Gasper Hb, 2005)

Adenovirus
Some researchers use other types of viruses to avoid the problems related with the insertion of genes at the wrong sites. It is a class of virus with the genome of double stranded DNA that may cause intestinal, respiratory and eye infection (especially; common cold).
These viruses introduce their DNA molecule into the host when they infect the host cell. Adenovirus does not incorporate its genetic material into the host cell’s genetic material. In the nucleus of the host cell the DNA molecule is left free and just like any other gene the instructions in this extra DNA molecule is copied (Fig 1). Adenovirus has the ability to infect a wider variety of cells than the retrovirus, including the cells that split more slowly, for an example lungs cells. Though patient’s immune system attacks adenovirus more likely and for the treatment, the high levels of virus required often provoke an adverse inflammatory response. Regardless of these drawbacks, for treating cancer of ovaries and liver this vector system has been encouraged. Certainly, Gendicine p53 based adenoviral product is the first gene therapy product to be licensed to treat the cancer of neck and head (Peng Z, 2005).
In 1999, after the death of Jesse Gelsinger while he was participating in a trial of gene therapy the concerns about the above mentioned vectors relating safety was raised.   (Sibbald , 2001). Later the work using adenovirus as a vector has focused on the version of genetically crippled.

Adeno-associated viruses [AAVs]
A recently discovered virus known as AAV is one of the most promising potential vector which infects a wide range of cells including non-dividing and dividing cells both.  From the Parvovirus family, AAVs are the small viruses with a genome of single stranded DNA. At a specific site on chromosome 19 it can insert genetic material with near 100% certainty. Researchers consider that AAV is carried within most of the people which do not provoke an immune response and do not cause disease. To correct the genetic defects, scientists have established the animal experiments by using AAV.  (Sibbald , 2001).
In preliminary studies, it is now being used to treat muscle, eye and the congenital blood disease haemophilia. Many clinical trials are also initiated to use AAV vectors. The use of these vectors delivers the genes to brain. As the virus can infect neurons which are the non-dividing cells and in which their genome are express for a long time.
The main drawback of AAV is its small size and that in its natural state it may carry only 2 genes. Because the virus inserts its genes directly into DNA of the host cells, it can produce genetic damage accidently.  In the production of large quantities of the altered viruses, the researchers had many complications. By Amsterdam Molecular Therapeutics, this production problem has recently being solved.  (Sanjukta Misra, 2013)

Cis and trans-acting elements
 Replication-defective vectors always have a “transfer construct”. This transfer construct carries the gene to be “transgene”. The necessary sequences for the general functioning of the viral genome are carried by the transfer construct: repetition for replication, packaging sequence and when needed priming of the reverse transcription. These are known as the denominated cis-acting elements because they need to be on the similar piece on DNA as the viral genome and the gene of interest.(Harwood,Adrian J, 1994)

 Herpes simplex viruses-
In nervous system, it is mostly used for gene transfer. It is a neurotropic virus. Due to its large genome as compared to other viruses, the scientists are enabled to insert more than a single therapeutic gene into a single virus that paves the way for the treatment and cure of the disorders caused by more than one gene defect. Herpes Simplex Viruses makes an ideal vector because it can infect a broad range of tissues e.g., liver, muscles, pancreas, lungs and nerve cells. HSV-1 virus of wild type is able to infect the neurons which are not rejected by the immune system. In humans, antibodies related to HSV-1 virus are common. Though, the herpes infections and complications relating these are rare.  (Varghese S,Robkin DS., 2002)

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