Uncovering the Molecular Basis 
of X-linked Disorders 
Jane M. Gitschier, Ph.D. — Assistant Investigator 
Dr. Gitschier is also Associate Professor of Medicine ( Genetics) at the University of California, San 
Francisco. She received a B.S. degree in engineering science from Pennsylvania State University, an M.S. 
in applied physics from Harvard University, and a Ph.D. in biology from the Massachusetts Institute 
of Technology. She did postdoctoral research with Richard lawn at Genentech, before joining the faculty 
at the University of California. 
X-LINKED disorders result from mutations in 
genes on the X cliromosome, one of the two 
sex chromosomes in humans. These common in- 
herited conditions almost exclusively affect 
males because they lack the protection of a sec- 
ond, normal X chromosome. Females, though 
they have such protection, can be silent carriers 
of X-linked diseases and genetically transmit 
them to their sons. Consequently, X-linked dis- 
eases are often referred to as "sex-linked." 
Our laboratory is interested in uncovering the 
molecular biology of some of these disorders. We 
are seeking to identify both the genes responsible 
for several diseases of unknown biochemical 
basis and the underlying mutations. 
Much of our effort is concentrated on a particu- 
lar region of the X chromosome called Xq28, the 
terminal portion of the chromosome's long arm. 
To date, 24 inherited disorders have been 
mapped to this region, which consists of approxi- 
mately 9 million base pairs of DNA. As this density 
of disease loci is unusually high compared with 
that found in other regions of the chromosome, it 
suggests that Xq28 may be very rich in genes. 
The incidence of Xq28-linked disorders is vari- 
able. Color blindness is extremely common, af- 
fecting about 1 of every 10 males. However, the 
majority of diseases occur infrequently, making 
accurate genetic mapping difficult. These rare 
diseases are clinically diverse and of unknown 
biochemical basis. They include Emery-Dreifuss 
muscular dystrophy, nephrogenic diabetes insi- 
pidus, adrenoleukodystrophy, incontinentia pig- 
menti, and dyskeratosis congenita. 
We are attempting to isolate genes in Xq28 and 
determine whether they are mutated in individ- 
uals affected by Xq28-linked disorders. To date 
we have isolated six candidate genes and have 
collected 5 1 patient samples, including at least 
one example of most of the rare diseases. In some 
cases the sequence of the cloned gene has pro- 
vided a clue to its function and suggested possi- 
ble involvement in a particular disease. On the 
other hand, one of the isolated genes does not 
appear to produce a protein. Until a match is 
made between candidate gene and inherited dis- 
ease, we are screening all patient samples for mu- 
tations in all genes, searching for gross rearrange- 
ments in genes as well as single-base alterations. 
One Xq28-linked disorder we have studied in 
depth is classic hemophilia, or hemophilia A. It 
results from mutations in the gene coding for a 
blood coagulation protein called factor VIII. The 
gene was identified previously because part of 
the amino acid sequence was known. Our labora- 
tory is interested in understanding what types of 
mutations lead to hemophilia A and how these are 
generated. 
Hemophilia is well suited to studies on muta- 
genesis because it is clinically heterogeneous, 
implicating a wide variety of mutations in the fac- 
tor VIII gene. Correlating the types of mutations 
with particular clinical findings may be very 
helpful in understanding the role of factor VIII in 
coagulation. To that end we recently completed a 
study of mutations that lead to amino acid 
changes in the protein. These mutations, which 
affect a single base pair in DNA, were revealed by 
a sensitive technique called denaturing gradient 
gel electrophoresis. By comparing the amino acid 
sequence of factor VIII with sequences from evo- 
lutionarily related proteins, we were able to infer 
that some mutations are likely to destroy activity 
by disrupting the protein's structure, while 
others alter amino acids that play a role unique to 
factor VIII function. 
Hemophilia can occur in families lacking a ge- 
netic history of hemophilia, reflecting newly aris- 
ing mutations. It is possible to uncover the origin 
of factor VIII gene mutations in these cases. In the 
past, mutations were assumed to occur as isolated 
events in either eggs or sperm. However, we have 
found evidence in several sporadic cases for "mo- 
saicism" — i.e., a mixture of cells with and with- 
out mutation within the individual. These results 
suggest that mutations can occur during embryo- 
logical development. In addition, these findings 
demonstrate that the unaffected mother of a child 
with sporadic disease may be at substantial risk of 
having a second child with the disease. 
Another aspect of the hemophilia-related re- 
search has been a continued investigation of the 
factor VIII gene. We discovered that it may en- 
