HEPATOCELLULAR TRANSPLANTATION AND TARGETING GENETIC MARKERS TO HEPATIC CELLS 
immunosuppressive protocol similar to that used in organ transplantation (Cornetta et 
al, 1990). Other non-human primates used for trials of an adenosine deaminase vector 
(Kantoff et al, 1987), have shovm no pathological effects up to 5 years after 
transduction with recombinant vectors (Cornetta et al., 1991). The most important 1 ; 
assessment of these vectors is the recent report of Rosenberg et al (1990) in which this 
vector was used in five patients without any evidence of complications. Recent follow- 
up on these five patients plus five additional patients revealed no complications from |i 
the vector insertion (Anderson, personal communication) . ii 
Several additional lines of evidence also indicate that these vectors are safe for i 
hiiman use. First, extensive research in the 1960 's and 1970 's as part of the "war on ^ 
cancer" attempted to Identify evidence for murine type retroviruses causing human , 
disease. This attempt was uniformly unsuccessful despite the fact that these viruses ! 
are ubiquitous in rodent populations and human populations are almost certainly exposed i 
to these agents (Weiss, 1984a). Second, it has been demonstrated that human serum will | 
lyse MMLV and related retroviruses by an antibody independent, complement mediated : 
pathway (Welsh, 1975). Thus, viral particles which may remain with infected cells, or ij 
might be formed by recombination, would most likely be unstable in humans (Cornetta et '' 
al, 1990). i 
G. The liver as a target for somatic gene therapy. 
Many somatic sites have been explored as targets for gene therapy including I'' 
hematopoietic cells, epidermal cells, fibroblasts, and endothelial cells, as well as i; 
hepatic cells, though most research has been directed at delivering gene therapy to bone j' 
marrow (reviewed Friedmann 1989, Ledley, 1989). The major limitation of targeting gene 
therapy to the bone marrow has been that the totipotential stem cell has not been j! 
unequivocally isolated or transduced in all studies, and that the complex developmental r 
events associated with differentiation of the totipotential cell into its fully jj 
differentiated progeny is associated with repression of many normal and recombinant || 
genes. I: 
![ 
It has been suggested that some diseases which do not explicitly involve marrow i; 
derived cells could be treated by delivering gene therapy to the bone marrow (Parkman, |, 
1986; Hobbs, 1987). Gene therapy for many diseases, however, will require ortho topic ! 
replacement of gene functions in the liver. This is particularly important in i 
considering gene therapy for inborn errors of metabolism, since provision of an enzyme j| 
protein will not constitute holoenzyme function in the absence of cofactor, substrate, 
or heteromeric subunits (Ledley, 1990). For example: Phenylalanine hydroxylase (PAH) ^ 
requires a tetrahydrobiopterin cofactor which is synthesized in hepatic cells (Ledley | 
et al, 1987b); li') Ornithine transcarbamylase requires carbamyl phosphate as a || 
substrate which is produced by a liver- specific enzyme carbamyl phosphate synthase; 
in') Organic acidemias such as methylmalonic acidemia lead to disruptions in 
ureagenesis and gluconeogenesis which may be mediated by the local effects of abnormal ja 
organic acids within hepatic mitochondria (Ledley, 1990b). Moreover, the metabolites j!j 
which serve as substrates for these enzymes (e.g. propionate, methylmalonate or the u 
corresponding acyl-CoA and acyl -carnitine derivatives) may be present at higher i 
[790] 
Recombinant DNA Research, Volume 14 
