Human Molecular Genetics in Two X-linked Diseases 
We are focusing on three major efforts related 
to the FMR-1 gene. The first is to determine the 
mechanism of expansion of the CGG repeat by 
both the direct sequence analysis of the fragile X 
mutation and the introduction of long synthetic 
CGG repeats into mammalian cells and, subse- 
quently, into mice. Second, we are attempting to 
understand the normal function of the FMR-1 
gene product. Toward this goal, we have recently 
cloned and sequenced the homologous gene 
from the mouse and are in the process of cloning 
the yeast and the nematode genes. Finally, we are 
excited about finding a number of other human 
genes, distinct from FMR-1, that similarly contain 
long CGG repeats. These genes, whose functions 
are presently not understood, may share a similar 
functional utilization of the CGG repeat; may un- 
dergo similar mutational changes, perhaps lead- 
ing to diseases exhibiting unusual genetic pat- 
terns; and may represent other, autosomal fragile 
sites. 
Emery-Dreifuss Muscular Dystrophy 
Emery-Dreifuss muscular dystrophy (EDMD) is 
the most frequent X-linked muscular dystrophy 
following the Duchenne and Becker types. It is a 
slowly progressive disease that usually leaves af- 
fected males ambulatory until middle age. Heart 
muscle involvement is frequent, sometimes re- 
sulting in early sudden death due to heart block. 
If the disease is identified early, such a death can 
be prevented by pacemaker implantation. 
We have performed genetic linkage studies in 
two large families and have localized the EDMD 
gene to the terminal band of the X chromosome 
(band Xq28) just distal from the fragile X site. 
We can now place the gene within an approxi- 
mate 2,000 kb of DNA. Using selective cDNA li- 
braries and DNA of this region cloned into cos- 
mids or YACs, we are mapping muscle-expressed 
genes within Xq28. This entails use of a somatic 
cell hybrid mapping panel containing six distinct 
Xq28 fragments. Any genes that map within the 
region believed to contain the EDMD gene will be 
used to search for mutations in patients. 
Another outcome of our genetic mapping stud- 
ies has been the identification of young, asymp- 
tomatic males who will eventually suffer from 
EDMD. Cardiac function of these males is being 
carefully followed, and pacemaker implantation 
is performed when warranted prior to full heart 
block. Thus we have virtually eliminated sudden 
death due to EDMD in these two families. 
In situ hybridization detecting human DNA 
within the metaphase chromosomes of a so- 
matic cell hybrid ( micro21D), which contains 
a human-rodent translocation between the 
centric fragile X chromosome (yellow) and a 
rodent chromosome arm ( red). The hybrid was 
constructed under conditions favoring re- 
arrangements specific for the fragile X site. In- 
deed, the human translocation breakpoint in 
this hybrid is within the CGG repeat of the frag- 
ile X mutation. Somatic cell hybrids with trans- 
locations of this nature proved instrumental in 
cloning the fragile X site and its associated gene 
(FMR-1). 
Research of Stephen Warren. 
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