W. l : rench Anderson 
By attaching a rat growth hormone gene to an active regulatory 
sequence (specifically, the promoter that normally directs the 
synthesis of metallothionein messenger RNA in mice), researchers 
obtained a recombinant DNA construct that actively produces 
growth hormone in the genetically defective mouse and in a number 
of its offspring. Although the level of growth hormone production 
was inappropriately controlled (that is, influence by signals that 
normally regulate metallothionein synthesis), these experiments did 
show that microinjection can be used as a delivery system that can 
put a gene into every cell of an animal’s body, that a genetic disorder 
can, as a result, be corrected, and that the correction can be passed 
on to the next generation of animals. 
Why is the technique of microinjecting a fertilized egg not accept- 
able for use for human gene therapy at the present time? First, the 
procedure has a high failure rate; second, it can produce a deleterious 
result; and third, it would have limited usefulness. Microinjection has 
a high failure rate because the majority of eggs are so damaged by the 
microinjection and transfer procedures that they do not develop into 
live offspring. In one recent experiment (Brinster et al., 1983) involv- 
ing microinjection of an immunoglobulin gene into mouse eggs, 300 
eggs were injected, 192 (64 percent) were judged sufficiently healthy 
to be transferred to surrogate mothers, only 11 (3.7 percent) pro- 
ceeded to live birth, and just 6 (2 percent) carried the gene. These 
results are from a highly experienced laboratory in which thousands 
of identical eggs from the same hybrid cross of inbred mice have 
been injected over several years. The mice were chosen precisely 
because they gave the best results for gene transfer by microinjection. 
Attempts to microinject functional growth hormone genes into live- 
stock eggs met with several major biological and technical problems 
before being accomplished. Successful gene transfer by microinjec- 
tion of human eggs, without a long period of trial and error experi- 
mentation, is extremely unlikely. 
Second, microinjection of eggs can produce deleterious results 
because there is no control over where the injected DNA will inte- 
grate in the genome. For example, the integration of an exogenous 
rabbit beta-globin gene in transgenic mice can sometimes occur at a 
chromosomal location that results in expression of the beta-globin 
gene in an inappropriate tissue, viz., muscle or testis (Lacy et al., 
1983). There have also been several cases reported where integration 
of microinjected DNA has resulted in a pathological condition. Al- 
though there is no control over where exogenous DNA will integrate 
Recombinant DNA Research, Volume 12 
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