Lentiviruses are a subclass of retroviruses. They have recently been adapted as gene delivery vehicles because of their ability to integrate into the genome of non-dividing cells, which makes them very unique from other retroviruses, thus lentivirus engineered to be devoid of any pathogenic elements can become efficient gene transfer vectors.
Lentiviruses are characterized by a complex genome that encodes a number of accessory proteins besides the canonical retroviral genes; gag-pol and env. They share all the common characteristic of retroviral replication including receptor-mediate entry, capsid uncoating, reverse transcription of the viral RNA and integration into the host cell genome (Dzierzak et al., 1988).
Advantages of using lentivirus as a gene transfer vector
- They are able to transduce non-replicating/dividing cells which confer to these viruses a special value for the development of clinically functional gene vectors.
- Compared to the oncoretroviral vectors, the stabilization of the proviral mRNA genome by the interaction of the accessory protein Rev with its cognate motif Rev-responsive element (RRE), increases their range of application.
- They allow the integration of larger genomic elements into their genome with limited or no sequence rearrangement (Kalberer et al., 2000 ). Therefore lentiviral vectors are thus likely to be selected as vectors of choice for the stable delivery of regulated transgenes in stem cell-based gene therapy. They allow the introduction of Beta-globin locus, different promoters, enhancers and chromatin structure determinants that led to lineage-specific and elevated level of β- -and -globin expression invivo. This resulted in the amelioration or correction of anemia and secondary organ damage in several murine models of hemoglobinopathies, making the recombinant lentiviruses the most effective vector system to date for gene therapy of these disorders (Laura et al., 2009).
Studies have shown that lentivirus vectors have a lower tendency to integrate in places that potentially cause cancer than gamma-retroviral vectors and also does not cause either an increase in tumor incidence or an earlier onset of tumors in a mouse strain with a much higher incidence of tumors (Montini el al., 2006).
Lentiviral vectors containing complex globin expression cassettes have been successful used to cure mouse models of sickle cell disease and β-thalassaemia (May et al, 2000,). Finally, fairly radical modification of the lentiviral genome can be made to enhance safety, including the deletion of the U3 promoter region of the long terminal repeats (CTR) to produce self-inactivating vectors (SIN) which renders the LTR transcriptionally inactive.
3. Adenoviral Vectors
Adenoviruses are viruses that carry their genetic material in the form of double-stranded DNA. When these viruses infect a host cell, they introduce their DNA molecule into the host, but the genetic material of the adenoviruses is not incorporated (transient) into host cell’s genetic material. The DNA molecule is left free in the nucleus of the host and the instruction in this extra DNA are transcribed just like any other gene. The only difference is that these extra genes are not replicated when the cell is about to undergo cell division so the descendants of that cell will not have the extra gene.
- Absence of integration into the host cell’s genome prevents insertional mutagenesis which may lead to cancer. Adeno and recombinant adeno-associated viral (rAAV) cancer vector can efficiently transduce HSC in G0 phase of the cell cycle.
- This vector system has been promoted for treating cancer and indeed the first gene therapy product to be licensed to treat cancer. For example;
Gendicine:- Is an adenoviral P53-based gene therapy that was approved by the Chinese food and drug regulator in 2003 for treatment of cancer of the head and neck.
Advexin:- A similar gene therapy approach from introgen, was turned down by the US food and Drug Administration (FDA) in 2008 (en.wikipedia.com).
Shortcoming of Adenovirus
It can be replicated during cell division only and thus will always require re-administration in a growing cell population. 4. Envelope Protein Pseudtyping of Viral Vector
For a virus to enter a particular cell, the cell must posses a receptor to which the virus will bind, thus virus exhibits tropisms for cells. For the purposes of gene therapy, one might either want to limit or expand the range of cells susceptible to transduction by a gene therapy vector. Due to this, many vectors have been developed on which the endogenous viral envelope proteins have been replaced by either envelop protein from other viruses, or by chimeric proteins. Such chimera would consist of those parts of viral protein necessary for incorporation into the virion as well as sequences meant to interact with specific host cell proteins.
Viruses in which the envelop proteins have been replaced as described are referred to as pseudo-typed viruses. For example; the most popular retroviral vector for use in gene therapy trials has been the lentivirus simian immunodeficiency virus coated with the envelop proteins, G-protein from vesicular stomatitis virus. This vector is referred to as VSV-G-Pseudotyped lentivirus and infects an almost universal set of cells. This tropism is characterstic of the VSV-G-Protein with which this vector is coated. (wikipedia, free encyclopedia).