Viruses that belong to the genus Lentivirus of the family Retroviridae are exogenous, have specific morphology and contain a relatively complex retroviral genome. Vectors derived from lentiviruses can be used for treatment strategies dependent on both transgene expression and gene correction. Gene vectors are also used extensively in basic biomedical research.
Characteristics of the genome
The specificity of the Lentivirus genus is that it contains a diploid single stranded positive sense RNA-genome, i.e. has two strands of RNA. It is spherical in shape (80-120 nm), and its genome is found in the complex with the nucleocapsid protein, and also bound to the reverse transcriptase, protease and integrase.
The genome is organized from the 5′ to the 3′ end, and major protein components are contained in gag, pol and env genes. Whereas gag is responsible for encoding the structural proteins, pol encodes the reverse transcriptase, protease and integrase, and env encodes the virus envelope glycoprotein. These proteins are synthesized as fusion proteins, which are then post-translationally cleaved by the virus-encoded protease.
The complex lentiviruses contain other accessory genes as well, responsible for precise orchestration of viral replication, biosynthesis and pathogenesis. For example, human immunodeficiency virus type 1 (HIV-1) has 6 regulatory or auxiliary genes: vif, vpr, vpu, tat, rev and nef.
Some of the other additional genes that are important are psi-sequence near the 5’ end of the RNA-genome (essential for packaging viral RNA into virus capsid), the polypurine tract required for reverse transcription, and the long-terminal repeats (LTRs) which are part of the promoter for transcription of the viral genes.
Genetic conversion of a lentivirus into a vector
Lentiviral vectors are created when the genome of the retrovirus (for example, HIV) is stripped down, with a consequence of breaking the provirus into pieces and leaving the replication of the vector defective. What remains are different plasmids required for viral particle formation and infectivity (the packaging and the envelope constructs) and sequences for mobilization of viral genome.
The packaging plasmid represents the backbone of the virus system; in HIV it is known as pCMVAR9. It contains the elements required for vector packaging such as structural proteins, HIV genes (except the gene env because its presence would enable the virus to infect T-cells) and the enzymes that generate vector particles.
The transfer vector plasmid contains cis-acting genetic sequences crucial for vector’s ability to infect the target cell and to transfer selected therapeutic (or reporter) genes. It also contains restriction sites for appropriate insertion of desired genes. The 3’ and 5’ LTRs, the original envelop proteins and gag sequence promoter are removed.
In the case of transfer vectors, a myriad of adjustments can improve the performance of gene transfer. Some of them are the use of post-transcriptional regulatory elements that boost the transgene transcriptional expression, the use of heterologous polyadenylation enhancer elements, or the use of diverse internal promoters to express a specific gene of interest.
Lentiviral vectors are developed with an aim to optimize the efficacy of gene transfer, eliminating at the same time the impending dangers that can arise due to the use of such systems. In the foresight, combinations between specific envelope proteins, viral proteins that target certain cell types and the use of specific promoters could aid in the selectivity of the whole gene transfer process.
Further Reading