Gene Delivery Using Viral Vectors - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 234

Table 5.5 Advantages and Limitations of Retroviral Vectors

Advantages of Retroviral Vector Limitations of Retroviral Vectors

They can integrate with high efficiency
They contain no viral genes, so they can
mediate long-term expression and avoid
host cellular immune responses

They cannot transduce nondividing cells
It is difficult to obtain high-titer production
The instability of these lipid-enveloped viruses
renders purification and concentration difficult
The viral long terminal repeat (LTR) sequence
may interfere with gene expression of various
gene cassettes

DNA is then incorporated into the host's genome by an integrase enzyme. The virus
thereafter replicates as part of the host cell's DNA. Once in the host cell, reverse tran-
scription of the RNA strands occurs in the cytosol, and they are integrated into the host
genome, at which point the retroviral DNA is referred to as a provirus. Retroviruses
are enveloped viruses that belong to the viral family Retroviridae . Inside the particle,
the retroviral genome is made of two identical, partially dimerized, capped mRNAs
surrounding the proviral sequence, with the exception of two short stretches unique in
the RNA form. Thus, the retroviral genome is single stranded and of positive polarity.
The retrovirus itself stores its nucleic acid in the form of an mRNA genome (including
the 5 cap and 3 PolyA inside the virion) and serves as a means of delivery of that
genome into cells it targets as an obligate parasite; this constitutes the infection. It is
difficult to detect the virus until it has infected the host. Virions of retroviruses con-
sist of enveloped particles about 100 nm in diameter. The virions contain two identical
single-stranded RNA molecules 7-10 kb in length. Virions do not share the same mor-
phology or biology, but all virion components are very similar [147] .
The major advantages and limitations of retroviral vectors are shown in Table 5.5 .

5.4.1 Biology and Structure of the Retrovirus Life Cycle
Retroviruses are found in a wide variety of animal species, and some are causally
associated with particular diseases. Previously retroviruses were divided into three
subfamilies: Oncovirinae , Lentivirinae , and Spumavirinae .

5.4.1.1 Classification
5.4.1.1.1 Exogenous
l

Genus Alpharetrovirus; type species: avian leukosis virus; others include rous sarcoma virus.

l

Genus Betaretrovirus; type species: mouse mammary tumor virus.

l

Genus Gammaretrovirus; type species: murine leukemia virus (MLV); others include feline
leukemia virus.

l

Genus Deltaretrovirus; type species: bovine leukemia virus; others include the cancer-caus-
ing human T-lymphotropic virus.

l

Genus Epsilonretrovirus; type species: walleye dermal sarcoma virus.