M.J. Welsh and A.E. Smith, RAC Application 
defective retrovirus, albeit at low titer. The adenovirus genome is much more complex and 
present vectors retain virtually all of the viral genes. The introduced genes replace either a) 
dispensable viral gene functions (such as early region 3) to produce a live recombinant viral 
vector, or b) gene functions that can be provided in trans. In the latter case, a transformed 
human cell line called 293 that expresses the early region 1 genes is available and this can 
be used to grow the otherwise defective El-deleted adenovirus vectors (81). 
The ends of the retrovirus genome contain so called Long Terminal Repeats (LTR). The 
LTRs include elements that promote integration and activation of adjacent genes 
(enhancers). Integration of retrovirus DNA introduces two copies of the LTR. The LTRs 
activate viral gene expression, but they can also activate cellular genes adjacent to the 
integration site. This is the basis for insertional mutagenesis involving gene activation by 
retroviruses (82). Insertional mutagenesis can also occur by gene disruption. The 
adenovirus genome contains an inverted terminal repeat (ITR) involved in DNA 
replication, but the viral promoter enhancer elements map outside the repeated sequences. 
Although integration of adenovirus DNA is not required for virus replication, it can occur 
at low frequency in non-permissive cells or when the virus is disabled (83-86). However, 
integration does not obligatorily occur at the terminal repeats. Mutagensis by gene 
activation therefore appears less likely, although mutagenesis by gene disruption as with 
retroviruses remains a possibility. However, we are unaware of any reports of insertional 
mutagenesis associated with adenovirus integration. Finally, because retrovirus vectors 
integrate their nucleic acid, gene expression from such a vector potentially persists for the 
life time of the treated cell. For adenoviruses, the DNA is predominantly 
extrachromosomal and continued expression may require multiple administrations. 
4.2 Advantages of Adenovirus as a Vector System 
Adenoviruses are widespread in the human population and almost 50 different serotypes 
are known. Viral infection, especially with the Ad2 and Ad5 serotypes of subgenus C, is 
not generally associated with serious disease (75,76); infection usually results in mild 
respiratory tract symptoms. Exposure to the virus is widespread with the majority of adults 
being seropositive for type C viruses. Engineered adenovirus offer an attractive means by 
which gene therapy for CF might be achieved (10,11,80). 
a) The virus can be readily manipulated to encode and express the desired gene product, 
CFTR. 
b) Deletion of viral genes can impair the activity of adenovirus in terms of its ability to 
replicate in a normal lytic viral life cycle. 
c) Adenovirus has a natural tropism for the target cells of the airway epithelium. 
d) The viruses are able to infect quiescent cells, as are found in the airways. 
e) Adenovirus expression can be achieved without integration of the viral DNA into the 
host cell chromosome, thereby reducing concerns about insertional mutagenesis. 
f) In vivo delivery of al antitrypsin and CFTR using adenovirus vectors has been reported 
in experiments using cotton rats (11,87). 
g) The molecular biology of human adenoviruses are relatively well understood. 
h) Recombinant adenovirus can be grown to high titer and the virus purified. 
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Recombinant DNA Research, Volume 16 
