Eye Development, Pigmentation, and Insertional Mutagenesis 
\ 
ment synthesis results in albinism. Albinism is of- 
ten accompanied by a loss of activity of the en- 
2yme tyrosinase, suggesting that albinism might 
be due to a mutation in the tyrosinase gene. To 
test this hypothesis, we constructed a nonmutant 
tyrosinase minigene and introduced it into the 
genome of albino mice. The resultant transgenic 
mice were pigmented and could be identified at 
birth by simple visual inspection. The tyrosinase 
minigene was expressed properly in the different 
pigmented cell types within the eye. These re- 
sults confirm the hypothesis that albinism is due 
to loss of endogenous tyrosinase activity. 
Through genomic sequencing studies, the albino 
mutation in laboratory mice was subsequently 
identified as a single-base pair change that 
causes a conversion from cysteine to serine at 
amino acid 103 of the tyrosinase protein. The ty- 
rosinase minigene is a useful new research tool, 
since it allows transgenic mice to be recognized 
by simple visual inspection. 
The other major focus of our research is the 
characterization of recessive mutations that are 
caused by integration of transgenic DNA. Such 
mutations are valuable, because the transgenic 
DNA can be used to isolate the inactivated genes. 
Once the transgenic DNA has integrated into the 
genome, it becomes linked to adjacent genomic 
sequences. Using recombinant DNA technology, 
we can fragment the genome into a large number 
of segments, and the segments can be individu- 
ally cloned to give a genomic "library." The li- 
brary can be screened by hybridization tech- 
niques to identify the clones that contain the 
transgenic DNA along with the adjacent genomic 
sequences. In this fashion, the transgenic DNA 
provides a molecular tag that can be used to iso- 
late genes that play essential roles in normal 
mammalian development. 
In one of our mouse families, the transgenic 
insert has inactivated the downless gene, a gene 
essential for normal induction of hair follicle for- 
mation. In another transgenic family, there is a 
mutation that blocks sperm development. For 
both of these mutants, the genomic sequences 
that flank the transgenic DNA have been isolated. 
Putative coding regions have been identified and 
sequenced, and efforts are in progress to confirm 
that the proteins encoded by these regions play 
essential roles in specific stages of development. 
Such studies can provide important insights 
into the mechanisms that specify differentiation 
and morphogenesis during mammalian embry- 
onic development, and may provide clues for 
strategies to help prevent birth defects in 
humans. 
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