successful recombination between the vector and packaging cell genome that 
might result in replication-competent virus. The modifications include deletion 
of 5’ and 3’ sequences to minimize homology and substitution of a stop codon 
at the gag start codon. 
4.2 The LNL-6 Producer Cell Line 
The LNL-6 producer cell line was created by transfection of the LNL-6 vector 
into the PA317 producer cell line. The viral containing supernate obtained 
from the LNL-6 producer cell line was extensively tested for exogenous 
pathogens and replication-competent virus. 
The supernatant (provided by Genetic Therapy, Inc.) to be used for human 
clinical use has passed all studies for contaminating microorganisms, and no 
pathogens were detected. The supernatant is approved for clinical use by the 
U.S. Food and Drug Administration. 
No side effects have been seen in animal studies nor in human clinical use to 
date. However, there are theoretical risks. First the LNL-6 vector preparation 
used could be contaminated with replication-competent murine retrovirus (see 
4.3 below). Secondly, even though the virus used to insert the gene into the 
bone marrow cells cannot grow and is considered harmless, it is possible that 
oncogenic events could occur within a transduced cell. Finally, the inserted 
gene may produce a protein that inactivates some aminoglycoside antibiotics. 
Satisfactory alternative antibiotics are available. 
4.3 Replication Competent Virus 
The LNL-6 vector has been modified so that it entirely lacks viral genes. The 
only coding genetic material that will be transferred to the patient’s cells will 
be the marker gene (neo^). Since LNL-6 has no remaining viral genes, it is 
incapable of producing the virion proteins necessary to package its RNA into 
an intact infectious virus. 
Since retroviral vectors are replication-detective, their genome must be 
"packaged" into a virion so that they may transfer their genetic material to the 
target cell. This is accomplished by use of a "packaging cell line" which 
supplies the necessary proteins for virion formation (Figure 2). The cell to be 
used, PA317, contains a modified murine leukemia virus (MLV) genome with 
intact packaging genes, but with deletions which make the virus incapable of 
inserting its own RNA genome into a virion. Thus, the patient’s cells are 
never exposed to a replication-competent virus, but only to a viral "coat" within 
which is contained the neomycin vector. The LNL-6/PA317 combination has 
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