Molecular Genetics of Hemophilia A 
of the three genes, or the directions in which 
their messenger RNAs are transcribed. 
A number of much rarer genetic disorders are 
also tightly linked to the factor VIII gene. These 
rare diseases are clinically diverse and of 
unknown biochemical basis. They include 
Emery-Dreifuss muscular dystrophy, nephro- 
genic diabetes insipidus, adrenoleukodystrophy, 
incontinentia pigmenti, and a manic-depressive 
disorder. By isolating genes near the factor VIII 
gene, we may be able to uncover the genetic basis 
for one of these disorders. 
The physical map has formed the framework 
for identifying candidate disease genes in the fac- 
tor VIII neighborhood. These are found by virtue 
of their association with "CpG islands," regions 
of human DNA that are rich in the bases cytosine 
and guanine. Since islands are often found near 
the origin of messenger RNA transcription, it is 
sometimes possible to identify and isolate a tran- 
scribed gene by isolating a CpG island. We have 
cloned five islands near the human factor VIII 
gene. Messenger RNAs have been found for three 
of the islands, and we have isolated cDNAs corre- 
sponding to two. These are being sequenced. 
Eventually the corresponding genes from patients 
will be examined for DNA sequence aberrations 
to determine whether the genes are responsible 
for disease. 
A surprising result of the CpG island investiga- 
tion was the discovery of an island inside a non- 
coding portion of the factor VIII gene. We found 
associated with the island two messenger RNAs, 
transcribed in opposite directions. The function 
of these "nested" genes is unknown, but their 
relationship to factor VIII is interesting to con- 
sider. Gene A is contained entirely within the 
noncoding region and is transcribed in the direc- 
tion opposite to the factor VIII gene. Two other 
transcribed copies of gene A are located near the 
factor VIII gene. Their existence may explain 
why hemophiliacs deleted for the factor VIII 
gene have no disorders other than hemophilia. 
Gene B includes some of the factor VIII coding 
regions, suggesting that it may participate in the 
coagulation pathway. Currently we are trying to 
determine whether either of these genes pro- 
duces a protein. 
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