19 
Dr. Anderson said MoMLV is currently the vector of choice for human gene therapy. 
Recombination between the human genome and MoMLV is minimal even if the vector 
is packaged amphotropically. There is same homology between MoMLV sequences 
and the human genome under low stringency conditions; the only region of MoMLV 
somewhat homologous to human endogenous viruses is the pol region which displays 
40% homology. This region is deleted frcm the vectors designed for use in 
human gene therapy. Dr. Anderson said Drs. Malcolm Martin and Steve Goff of 
the NIH had published a letter in Science stating they tried to recombine 
MoMLV and human endogenous sequences and failed. Therefore, a potentially 
"safe" proviral vector construct might be composed of mouse LTRs with their 
enhancer and promoter regions deleted and a human gene controlled by the appro- 
priate human genomic regulatory signals. 
Dr. Mulligan said the issues facing investigators constructing human gene therapy 
vectors is how to obtain high titers of helper-free virus, efficient infection 
of cells and a stable vector capable of expressing the desired gene in appropriate 
cells. As a consequence of retroviral properties, there is strong selection 
in infectious retroviruses for particular types of sequence organization and 
for the absence of certain sequences. In addition, the level of expression of 
genes with internal premotors relative to the level of virus production varies 
with the details of vector construction often in an unpredictable fashion. 
[Rapporteur's Note: Retroviruses normally must modulate the amount of splicing 
of full-length viral RNA so that proper amounts of RNA are available for these 
functions: (1) full length retrcviral FNA for use as mRNA for gag and pol 
genes; (2) subgenomic mRNA for env gene expression; and (3) full length RNA for 
packaging into progeny virions. It is not surprising that insertions and 
deletions would influence the amount of subgenomic RNA produced.] 
Dr. Gottesman asked how HTLV III/IAV controls transcription. Dr. Mulligan 
said HTLV III/LAV has not only acquired additional genetic information but has 
also evolved a very complex splicing mechanism. He said many rules concerning 
the processing of retroviral transcripts are not yet understood. 
Dr. Gottesman said the NIH Guidelines currently treat the association of helper 
virus and defective virus as infectious. Defective retroviruses can persist 
in association with helper virus. Should this situation be specifically addressed 
by the NIH Guidelines? 
Dr. Mulligan said generally the defective retrovirus/helper virus situation 
is not stable. For example, with embryos in tissue culture the infection of 
defective mutant and helper virus spreads through the embryo; however, after 
several replication cycles, defective viruses can no longer be detected. 
Dr. Joklik said one case is known in chickens in which the helper and defective 
virus situation is stable. All sorts of rearrangments occur but follow very 
strict rules as to what is permitted. At the mcment, we do not knew these rules. 
Dr. Hartley said mouse retroviruses cause an ecotropic spreading infection 
with regular generation of a spectrum of reccmbinants between ecotropic and 
endogenous sequences. This activity is host organ dependent in mice occurring 
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