Adenovirus Vector
Adenoviruses are double-stranded DNA viruses that can cause a number of disorders, such as respiration infections. The virion is non-enveloped, spherical and about seventy to ninety nm in size. The adenovirus DNA is linear, double stranded of approximately 36,000 bp wrapped in a histone-like protein and has inverted terminal repeats (ITRs) of 50-200 bp. Both strands of adenovirus DNA encode genes and the genome encodes about thirty proteins.
Soon after its isolation in 1953 adenovirus was recognized as a preferred
vehicle for gene delivery because of its many distinguishing features. The
first generation recombinant adenoviruses are E1 deleted to insure replication
deficiency of the virus and to prevent cell lysis. Once packaged into a
complementing cell line (a cell line that provides the E1 products in trans,
e.g. HEK 293A cells), viral replication will be enabled. In addition to being
E1 deleted, the first generation adenoviruses are also often E3 deleted (
E1/E3).
Lentiviruses are a subclass of retroviruses. The viral genome in the form of RNA is reverse-transcribed when the virus enters the cell to produce DNA, which is then inserted into the genome at a random position by the viral integrase enzyme. Lentiviruses have been adapted as vectors thanks to their ability integrate into the genome of non-dividing as well as dividing cells. The vector can be used to provide highly effective gene therapy as lentiviruses can change the expression of their target cell's gene for up to six months. They can be used for nondividing or terminally differentiated cells such as neurons, macrophages, hematopoietic stem cells, retinal photoreceptors, and muscle and liver cells, cell types for which previous gene therapy methods could not be used. The only cells lentiviruses cannot gain access to are quiescent cells (in the G0 state) because this blocks the reverse transcription step.
For safety reasons lentiviral vectors never carry the genes required for their replication. Lentiviral vectors are usually created in a transient transfection system in which a cell line is transfected with three separate plasmid expression systems. These include the transfer vector plasmid ( portions of the HIV provirus), the packaging plasmid or construct, and a plasmid with the heterologous envelop gene (ENV) of a different virus. The three-plasmid components of the vector are put into a packaging cell which is then inserted into the HIV shell. The virus portions of the vector contain insert sequences so that the virus cannot replicate inside the cell system.
Small interfering RNA (siRNA), sometimes known as short interfering RNA, are a class of 20-25 nucleotide-long double-stranded RNA molecules that play a variety of roles in biology. Most notably, it is involved in the RNA interference(RNAi) pathway where the siRNA interferes with the expression of a specific gene.
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