9 
durinq the second or third pregnancy. Tumors did not develop in other tissues 
even though the MMTV -myc aenes were expressed in other tissues. 
Dr. Scangos said a high percentage of the animals expressing these genes trans- 
mit the effect to their proqeny. 
Dr. Scangos said insert ional mutaqenesis occurs in regions essential to viability 
in transqenic animals at a fairly high frequency. The effects of insertional 
mutaqenesis are usually not evident in heterozygous animals. H\en transgenic 
animals are inbred, hanozygous offspring display these lethal mutations. The 
available data suggest that 20% of transgenic mice may harbor recessive insertional 
mutations of essential genes. 
Three phenotypes observed with insertional mutagenesis in hanozygous animals 
are: (1) embryonic death; (2) male sterility; and (3) limb deformity. One 
embryonic lethal mutation arose frcm insertion of murine leukemia virus in the 
alpha 1 collaqen gene. 
Dr. Motulsky asked if there are any examples of a heterozygous dominant arising 
fran insertional mutagenesis, rr. Scangos said he had heard of a case in 
which the product of the introduced qene was toxic. 
Dr. Scangos said transqenic animals have provided two disease models: (1) 
transgenic animals obtained throuqh microinjection of the gene coding for the 
hepatitis B surface antigen mimic chronic carriers of hepatitis B; and (2) 
animals expressing the JC virus mimic the pathology of progressive multifocal 
leukoencephalopathy. 
Mr. Capron said the human growth hormone qene has been microinjected into animals; 
he asked Dr. Scanqos to comment on these experiments. Dr. Scangos said the 
human growth hormone gene had been microinjected into sheep, pig, mice, and 
rabbit embryos. The piqs expressed the qene; however, they were not affected 
by expression of the human growth hormone qene and grew to, normal size. The 
mice also expressed the qene and grew to larger than normal size. 
Dr. Scanqos then described the experiments in which the human growth hormone gene 
was microinjected into mice. The human growth hormone gene was engineered to 
be under the control of the requlatory sequences of a metallothionein gene. 
The metallothioneins bind heavy metals such as copper, zinc, cadmium, and 
mercury and the expression of these genes is increased by the presence of 
heavy metals in the diet. The diets of mice microinjected with the human 
qrowth hormone qene were supplemented by zinc. The mice expressed the human 
qrowth hormone qene in tissues that normally synthesize metallothionein and 
qrew to more than normal size; however, since the normal feedback control 
mechanisms were bypassed, the animals expressed many secondary effects such as 
pituitary abnormalities, abnormal liver function, and female infertility. The 
investigators injected subsequent groups of embryos with a fusion gene composed 
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