Human Gene Therapy Subcommittee - July 29-30, 1991 
vein. The investigators propose to use a new vector system. The parent vector is 
derived from a series called DOl. The packaging cell line is T-CRIP. There are safety 
concerns in this new system: (1) the likelihood of generating replication competent 
virus that might be introduced directly into the patient or generated at a later time, 
and (2) the likelihood of a tumorigenic response developing as a result of the gene 
transfer protocol. The investigators should also address any overlap that may exist 
between the vector sequence and the sequences that are used to generate the retroviral 
proteins in the packaging cell lines. Dr. D. Miller has shown that the extent of overlap 
correlates with the likelihood of generating replication competent virus. In this new 
system, the packaging cell line splits the protein coding capacity into two different 
sequences; therefore, this cell line should be safer than those which have previously 
been used for generating virus. In terms of gene transfer and its associated risk, the 
only concern revolves around the possibility of an immune response to the receptor. 
This protocol does not present any new risks over previous protocols in terms of 
insertional mutagenesis because the vector 3' LTR has been disabled. 
Dr. Mclvor said the substantial preclinical data indicates that the gene transfer 
procedure is effective in both the introduction and expression of the gene. Prior to 
infusion, the hepatocyte population will be evaluated for gene expression. This would 
provide verification that the gene transfer procedure had been successful prior to 
infusion. He asked the investigators what proportion of the cells in the liver are 
actually derived from the donor cells when the hepatocytes are infused into a recipient 
animal. Dr. Mclvor said the follow-up studies performed on the patients should 
include the determination of their lipid levels post-infusion and molecular studies. 
Also, there needs to be an evaluation to determine whether the gene transfer and 
repopulation in the liver have been effective. He asked for a list of post-treatment 
studies. 
Dr. Mclvor said there are three factors that affect the anticipated efficacy of the 
procedure: (1) the efficiency of gene transfer into the isolated hepatocyte population, 
(2) the representation of the infused cell population in the recipient liver, and (3) the 
level of expression obtained in the recipient liver. As a related question, he wanted to 
know if cells could over express the LDL receptor, and if such cells could handle the 
additional load of cholesterol. 
Mr. Capron asked if the LDL receptor gene can be transferred to adult human 
hepatocytes in vitro efficiently, reliably, and without complications. Can these 
transduced cells be reintroduced into human subjects and produce permanent or long- 
term expression of the gene. Will the level of the LDL receptor activity be sufficient 
to produce a clinically significant benefit, and will the change in the LDL receptor level 
affect patient morbidity and mortality? 
Mr. Capron said that the number of adults with FH justifies beginning the trial with 
Recombinant DNA Research, Volume 14 
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