Albinism and Tyrosinase 
Paul A. Overbeek, Ph.D. — Assistant Investigator 
Dr. Overbeek is also Associate Professor of Cell Biology at Baylor College of Medicine and has adjunct 
appointments in the Institute for Molecular Genetics, the Division of Neuroscience, and the Department 
of Ophthalmology at Baylor College of Medicine. He received his B.A. degree in chemistry from Kalamazoo 
College, his Ph.D. degree in cellular and molecular biology from the University of Michigan, and an M.B.A. 
degree from the University of Chicago. His postdoctoral research was done in the laboratory of Heiner 
Westphal at NIH. 
THE genetic disorder of albinism, or loss of 
pigmentation, has been identified in many 
species. Albinism typically is caused by the loss 
of production of the black pigment termed mela- 
nin. Specialized cells known as melanocytes are 
responsible for melanin synthesis in the skin, 
hair, and iris. In addition to melanocytes, there 
are cells at the posterior of the retina, referred to 
as retinal pigment epithelial cells, that normally 
synthesize melanin. When these two types of 
cells lose their melanin-synthesizing ability, al- 
binism results. Since melanin helps protect the 
skin from ultraviolet radiation and the visual sys- 
tem from bright light, albinism is often associated 
with secondary health problems, including in- 
creased risk of skin cancer and visual system 
deterioration. 
Melanin is derived from the amino acid tyro- 
sine. An enzyme known as tyrosinase converts ty- 
rosine to dopaquinone, which is then polymer- 
ized to produce melanin. Studies of albino 
individuals, including humans and laboratory 
mice, have revealed that albinism in often asso- 
ciated with a decrease or loss of tyrosinase enzy- 
matic activity. This observation has led to the hy- 
pothesis that albinism may be caused by a 
mutation in the gene that encodes tyrosinase. A 
mutation could result in synthesis of an abnormal 
tyrosinase protein that no longer has enzymatic 
activity. We have recently undertaken a series of 
experiments to test this hypothesis in albino labo- 
ratory mice. 
One important prediction of this hypothesis is 
that the nucleic acid sequence of the tyrosinase 
gene in pigmented mice will be different from 
the sequence in albino mice. A procedure known 
as the polymerase chain reaction was used to am- 
plify specific regions of the tyrosinase gene from 
more than 20 different pigmented and albino 
mouse strains. The nucleic acid sequences were 
then determined, and a computer program was 
used to compare them. Interestingly, every pig- 
mented strain was found to encode a cysteine at 
amino acid 103 of the tyrosinase gene, while the 
albino mice all had a single base pair change that 
would cause amino acid 103 to become a serine. 
These results suggested that conversion of cys- 
teine to serine at amino acid 103 was sufficient to 
inactivate the tyrosinase enzyme and produce 
albinism. 
In order to confirm this prediction, we used 
recombinant DNA techniques to construct two 
miniature versions of the tyrosinase gene: one 
with a cysteine, the other with a serine at amino 
acid 103. The two constructs were tested in an 
albino strain of mice. Embryos of the albino strain 
were harvested at the one-cell stage and injected 
under the microscope with purified DNA. After 
injection the embryos were reimplanted into the 
reproductive tracts of pregnant females and al- 
lowed to develop to birth. 
The newborn mice were then screened to iden- 
tify those mice (termed transgenic) in which the 
injected DNA had become stably incorporated 
into the genome. Mice that had incorporated the 
cysteine 103 version of tyrosinase became pig- 
mented, while all of the mice that had integrated 
the serine 103 version of tyrosinase were still al- 
bino. These experiments have confirmed that al- 
binism can be caused by a mutation in the tyro- 
sinase gene and that a single base pair 
substitution is sufficient to inactivate the gene. 
These studies of albinism have provided an im- 
portant new strategy for the identification of 
transgenic mice. Prior to the design of the minia- 
ture tyrosinase gene, transgenic mice were iden- 
tified by techniques that required genomic DNA 
isolation. The miniature tyrosinase gene allows 
transgenic mice to be recognized by simple vi- 
sual inspection for pigmentation. 
To test the general usefulness of this system, 
experiments were done in which two different 
recombinant DNAs were co-injected into em- 
bryos. One was the miniature tyrosinase gene; the 
other was designed to generate a mouse model 
for predisposition to kidney cancer. The newborn 
mice were first checked for pigmentation, then 
genomic DNA was isolated and tested for integra- 
tion of the co-injected DNA. Altogether five pig- 
mented mice were obtained, and all five had inte- 
grated both DNAs. Breeding studies showed that 
the two different DNAs were always transmitted 
together. 
When the kidneys were checked for cancer in- 
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