Prospects for Human Gene Therapy 
W. French Anderson 
Gene therapy, the insertion into an 
organism of a normal gene which then 
corrects a genetic defect. has Seen car- 
ried out in fruit dies t/l (Drosophila 
inelanouaster) and mice I ’I. How soon 
gene therapy might he available for the 
treatment of human genetic diseases and 
what criteria should be used in determin- 
son-like elements have not been identi- 
fied in vertebrates. Retroviruses, howev- 
er. arc structurally and functionally simi- 
lar in many ways to the mobile genetic 
elements found in lower organisms, and 
retroviral vectors have now been used to 
transfer functioning genes into mouse 
hone marrow cells. 
Summary Procedures have now been developed lor inserting functional genes into 
the bone marrow ol mice. The most eflective delivery system at present uses 
retroviral-based vectors to transfer a gene into murine bone marrow cells in culture. 
The genetically altered bone marrow is then implanted into recipient animals. These 
somatic cell gene therapy techniques are becoming increasingly efficient Their luture 
application in humans should result in at least partial correction of a number of genetic 
disorders However, the safety ol the procedures must still be established by further 
animal studies before human clinical trials would be ethical 
ing when clinical trial-, should begin are 
issues examined in this article. Several 
investigators are now preparing proto- 
cols for clinical trials of gene therapy in 
seriously ill patients t.f). Since most of 
these protocols will be based on the use 
of retroviral vectors as a delivery sys- 
tem. these structures will be empha- 
sised. It may well be. however, that one 
of the other delivery systems described 
below-, or a new one not yet developed, 
will be the procedure of choice in the 
future. 
Gene Therapy in Lower Species 
The most elegant system thus far dem- 
onstrating successful gene therapy is the 
work in Drosophila (/). The transpo sable 
genetic element, the P factor, has been 
used to transfer a normal gene coding for 
the enzyme that produces the wild-type 
red eye color in Drosophila embryos 
which have a genetically defective gene. 
The result is that (he treated flies acquire 
normal eye color. Similar transfer ex- 
periments under way use other genes. 
Despite considerable searching, transpo- 
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The first genetic "cure" reporled in a 
mammal (.?) was in a strain of mice, 
called little. These have a mutation that 
results in reduced serum levels of grow th 
hormone, and the mice are therefore 
dwarfs. The equivalent human disease is 
pituitary dwarfism Hammer cl al. (2) 
succeeded in inserting a rat growth hor- 
mone gene into the cells of these mice in 
such a way that the gene is expressed at 
a high level. The deficiency in growth 
hormone was corrected, and the animals 
grew rapidly . but the gene was not con- 
trolled appropriately, and gigantism re- 
sulted — namely, a mouse one-and-a-half 
times as large as a normal animal. A 
major research effort is focusing on how 
to correctly regulate transferred genes. 
Gene Therapy in Humans 
/Ionian disease candidates lor nene 
therapy. Pituitary dwarfism in humans is 
not a reasonable initial candidate. Genes 
making hormones that circulate in (he 
bloodstream are probably not appropri- 
ate for early attempts at gene therapy in 
humans. First, the normal feedback con- 
trols m DNA that regulate the expression 
of hormone genes in the body are not 
now known. Therefore, physiologically 
correct levels of hormone production 
would probably not be possible. Second, 
it would be easier and safer to use 
recombinant DNA manufacturing tech- 
niques to produce sufficiently large 
quantities of hormone so that the active 
polypeptide itself could be given to the 
patient. Hormone levels could then be 
titrated precisely. 
At first, clinical investigators thought 
that the human genetic diseases most 
likely to be the initial ones successfully 
treated by gene therapy would be the 
hemoglobin abnormalities (specifically. 
(J-thalasscmia) because these disorders 
are the most obvious ones carried by 
blood cells, and bone marrow is the 
easiest tissue to manipulate in vitro (•?>. 
Regulation of globin synthesis, however, 
is unusually complicated. Not only are 
the embryonic, fetal, and adult globin 
chains carefully regulated during devel- 
opment. but also the u- and (5-globin-like 
chains arc always maintained in a I to I 
ratio despite the fact that the u- and 3 - 
globin loci are on different chromo- 
somes. To understand the regulatory sig- 
nals that control such a complicated sys- 
tem and to develop means for obtaining 
controlled expression of an exogenous 3 - 
glohin gene will take considerably more 
research effort. The recent development 
of a mouse model for 3 -thalassemia 
should aid these investigations (.'I. 
Gene therapy should be beneficial pri- 
marily for the replacement of a defective 
or missing enzyme or protein that must 
function inside the cell that makes it. or 
of a deficient circulating protein whose 
level does not need to be exactly regulat- 
ed ifor example, factor VIII). Early at- 
tempts at gene therapy will almost cer- 
tainly be done with genes for enzymes 
that have a simple "always-on" type of 
regulation. Three genes are the initial 
prime candidates: hypoxanthine-guanine 
phosphoribosyl transferase (HPRT). the 
absence of which results in Lesch-Ny- 
han disease: purine nucleoside phos- 
phorylasc (PNP). the absence of which 
results in a severe immunodeficiency 
disease: and adenosine deaminase (ADA), 
the absence of which results in severe 
combined immunodeficiency disease. 
For all three, the clinical syndrome is 
profoundly debilitating. The defect in 
each is found in the patient's bone mar- 
row (although the severe central nervous 
system manifestations of Lesch-Nyhan 
disease are due to absence of HPRT in 
brain cells and probably cannot be cor- 
rected with current techniques). In all 
three there is no. or minimal, delectable 
enzyme in marrow cells from patients 
homozygous (or hemizygousi for the de- 
fect. and the production of a small frac- 
tion of the normal enzyme level should 
Jiu 
Recombinant DNA Research, Volume 12 
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