Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 235

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Genus Lentivirus; type species: human immunodeficiency virus 1; others include simian,
feline immunodeficiency viruses.

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Genus Spumavirus; type species: simian foamy virus.

5.4.1.1.2 Endogenous
Endogenous retroviruses are broadly classified into three classes, on the basis of
relatedness to exogenous genera:

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Class I are most similar to the gammaretroviruses.

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Class II are most similar to the betaretroviruses and alpharetroviruses.

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Class III are most similar to the spumaviruses.

5.4.1.1.3 Group VI
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Family Metaviridae

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Family Pseudoviridae

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Family Retroviridae—retroviruses, e.g., HIV

All the members of Group VI use virally encoded RT, an RNA-dependent DNA
polymerase, to produce DNA from the initial virion RNA genome. This DNA is
often integrated into the host genome, as in the case of retroviruses and pseudovi-
ruses, where it is replicated and transcribed by the host.

5.4.1.1.4 Group VII
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Family Hepadnaviridae—e.g., hepatitis B virus

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Family Caulimoviridae—e.g., cauliflower mosaic virus

All members of Group VII have DNA genomes contained within the invading
virus particles. The DNA genome is transcribed into both mRNA, for use as a tran-
script in protein synthesis, and pregenomic RNA (pgRNA), for use as the template
during genome replication. Virally encoded RT uses the pgRNA as a template for the
creation of genomic DNA.

5.4.1.2 Life Cycle of the Retrovirus
The detailed process of the retrovirus life cycle is shown in Fig. 5.8 . Retroviruses
enter cells via an interaction between the viral envelope protein and a cellular recep-
tor that controls the host tropism and target cell specificity of retroviral vectors. The
whole process is followed by loss of the envelope and viral capsid. After entry into
the cytoplasm, the viral RNA/DNA and then DNA/DNA double helix are synthe-
sized by RT. Double-strand DNA integrates into the host cell genome. The integrated
provirus generates new RNA genomes, which are condensed into core particles and
further encased in the lipoprotein envelope by budding from the cell surface [148] .
Retrovirus genomes commonly contain these three ORFs that encode for proteins
that can be found in the mature virus:

1. Group-specific antigen (gag) codes for core and structural proteins of the virus
2. Polymerase (pol) codes for RT, protease, and integrase
3. Envelope (env) codes for the retroviral coat proteins