Recombinant DNA Advisory Committee - 10/7-8/91 
this tissue. The hepatocytes are then transduced with the retroviral vector, which is 
designed for expression of the low-density lipoprotein (LDL) receptor. These patients 
are defective or deficient in expression of the LDL receptor; this is the molecular basis 
for the disease. After transduction, the hepatocytes are reinfused into the patient 
through the portal vein. These hepatocytes will seed the liver and may function to lower 
the level of cholesterol in these patients, and reduce their risk of coronary disease. The 
proposed therapy uses autologous cells and therefore, is less risky than an orthotopic 
liver transplant, the currently available therapy. 
Dr. Mclvor noted that the investigators have done extensive experiments in an animal 
model, the Watanabe hyperlipidemic rabbit, in which they have demonstrated a long- 
term reduction in the level of serum cholesterol. They have essentially performed the 
same experiment that is being proposed for humans. The vectors to be used are safer 
than the ones that have been proposed in the previous protocols, because the enhancer 
elements in the long-terminal repeats have been inactivated. The packaging cell line is 
one that splits the protein coding sequences into two separate pieces resulting in less 
chance of generating replication competent virus. The PA317 cell line, used by others, 
expresses both of the retroviral protein coding genes from the same segment of DNA. 
The only possible risk would be associated with the expression of the LDL receptor. 
There could be a possible immune response in patients that previously have not 
expressed any LDL receptor. The investigators have indicated that they will follow these 
patients closely for any possible immune response. 
Dr. Mclvor said the efficiency of repopulating the liver with these infused cells was 
determined by doing RNA protection studies on animals that had received transduced 
hepatocytes. The investigators were able to estimate the frequency with which 
hepatocytes were repopulating in the liver. In terms of the molecular and metabolic 
evaluation, the investigators will be doing molecular tests such as PCR and in situ 
hybridization. They may do RNase protection if there is sufficient sample available from 
biopsies. In terms of the anticipated efficacy, there can be a five-fold variability of LDL 
receptor expression from one transduced hepatocyte to the next. The gene transfer 
frequency in the hepatocyte population is at most 5%. Finally, the frequency with which 
the cells repopulate the liver is as much as 2%. All three of these issues factor into the 
efficiency that one can express this gene in a patient and, therefore, affect the disease. 
Dr. Mclvor noted that patient selection was discussed at the subcommittee meeting. The 
treatment may involve children. The question of whether to include individuals who 
could not give their consent needs to be addressed. There was also some discussion on 
limiting the study to only the receptor negative population. The subcommittee decided 
to leave it open to all of these patients, although it was the receptor negative patients 
who were most likely to benefit from the procedure. The protocol was approved by the 
subcommittee with additional information to be provided about quality control data on 
Recombinant DNA Research, Volume 15 
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