Molecular Genetics ofX-linked Disease 
Robert L. Nussbaum, M.D. — Associate Investigator 
Dr. Nussbaum is also Associate Professor of Human Genetics, Pediatrics, and Medicine at the University of 
Pennsylvania School of Medicine and Consultant in Clinical Genetics at the Children's Hospital of Phila- 
delphia. He received his undergraduate training in applied mathematics at Harvard College and his M.D. 
degree at Harvard Medical School in the Harvard-MIT Joint Program in Health Sciences and Technology. 
After his residency in internal medicine at Barnes Hospital, Washington University School of Medicine, he 
moved to Baylor College of Medicine, first for a genetics fellowship with Thomas Caskey and Arthur 
Beaudet and later as a faculty member. He then moved to the University of Pennsylvania, where he devel- 
oped his research program in molecular genetics and its application to the diagnosis and elucidation of 
human genetic disease. 
THE research in my laboratory is directed to- 
ward elucidating the molecular bases for a 
number of human genetic diseases. Each disease 
under investigation is known to be caused by a 
gene on the X chromosome, but the molecular 
mechanism, the gene involved, and the nature of 
the underlying mutations have been generally un- 
known. Recombinant DNA techniques are being 
employed to isolate the responsible genes, with 
the aim of furthering our understanding of the 
normal processes that result in each of these dis- 
eases when disrupted. 
Fragile X Syndrome 
One of the most common genetic forms of 
mental retardation, the fragile X syndrome, af- 
fects nearly 1 in every 2,000 males worldwide. 
The chromosomes of affected males are normal in 
appearance on routine examination. If, however, 
supplies of the molecular building blocks for 
DNA synthesis are stringently limited during cell 
division (m vitro), an abnormal gap or fragile 
site is induced to appear at the tip of the human X 
chromosome. 
The disease is generally inherited in an X- 
linked manner, but there are important excep- 
tions. These are found in "transmitting males," 
the approximately 20 percent of male carriers 
who are not retarded and do not show the chro- 
mosomal fragile site under the usual induction 
conditions. The grandsons of transmitting males, 
however, may inherit the gene through daughters 
and manifest the disease. 
Understanding the fragile X syndrome will ulti- 
mately require isolation of DNA sequences from 
the region involved so that the molecular basis of 
both the chromosomal abnormality (the fragile 
site) and the clinical disease (fragile X mental 
retardation) can be elucidated. My colleagues 
and I are creating yeast artificial chromosomes 
(YACs) containing DNA from the region of the 
fragile site for study. This involves attaching 
functional parts of yeast chromosomes to large 
fragments of DNA from a patient and reintroduc- 
ing these fragments into yeast. The large frag- 
ments are then propagated in the yeast, just like a 
yeast chromosome, and can be grown in large 
amounts for study. Nearly 600,000 bp of contigu- 
ous DNA from the region around the fragile site 
has been isolated to date. Our aim is to under- 
stand how the chromosome abnormality comes 
about and what gene or genes are responsible for 
the retardation. 
Choroideremia 
Choroideremia is a rare X-linked disease of the 
retina that produces blindness in affected males. 
The gene responsible and the mechanism of reti- 
nal damage have until recently been unknown. 
Our laboratory is using information about where 
the choroideremia gene is to identify it and ex- 
plain why mutations in this gene cause the 
disease. 
We have been studying a female patient in 
whom choroideremia has occurred because of a 
disruption of the choroideremia gene caused by a 
chromosome break in the X chromosome in this 
region. A transcribed gene that is disrupted by 
this chromosome translocation has been identi- 
fied and found to be very similar, although not 
identical, to one isolated in the laboratory of 
Frans Cremers by his study of males with choroi- 
deremia and submicroscopic deletions. The gene 
identified in both laboratories bears no sequence 
homology to any previously identified gene, and 
there is no information at present as to what the 
protein encoded by this gene does and why muta- 
tions in the gene cause choroideremia. 
The goal is to learn more about the function of 
the normal retina as well as to find new methods 
of diagnosis and treatment for this and other re- 
lated retinal disorders. 
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