MOLECULAR GENETICS OF X-LINKED DISEASE 
Robert L. Nussbaum, M.D., Associate Investigator 
Dr. Nussbaum's laboratory has extended its re- 
search efforts in mapping and characterizing a 
number of human X-Unked diseases. 
Mental retardation with Xq27 fragile site has re- 
mained a major focus in the laboratory. Under- 
standing the nature of the fragile site requires isola- 
tion of DNA from the region of the mutation. 
Although new yeast vectors for yeast artificial chro- 
mosomes were originally used, an apparently high 
rate of recombination of artificial chromosomes 
made with these vectors convinced this laboratory 
to return to the original vectors of Burke and 
Olson. A new library is now being constructed from 
a hybrid containing a translocation between a host 
rodent chromosome and a human X, with the 
human breakpoint at what is felt to be the Xq27 
fragile site. Dr. Stephen Warren, a collaborator, 
made this hybrid by inducing the fragile site in a hy- 
brid containing an intact human X and then screen- 
ing for breakage at the fragile site and loss of the 
distal Xq28 markers. The immediate goal is to iden- 
tify an artificial chromosome containing the translo- 
cation breakpoint, in order to obtain human se- 
quences from in and around the fragile site. 
Continued progress has been made in the study 
of the X-linked retinal dystrophy choroideremia. A 
female with choroideremia due to an X;13 translo- 
cation, with nonrandom inactivation of the normal 
X, allows more precise localization of the disease 
locus. Using the marker DXS165 (identified by Dr. 
Ropers in Holland as being within deletions in pa- 
tients with choroideremia) the laboratory has initi- 
ated jumping and walking experiments that have 
allowed isolation of sequences within 40 kb of 
the translocation breakpoint. These sequences are 
being used for continued walking experiments and 
are being screened for exons of genes expressed in 
the retina and retinal pigmented epithelium in a 
search for a candidate gene for this disease. 
The Lowe oculocerebrorenal syndrome is an 
X-linked disorder characterized by mental retarda- 
tion, renal tubular abnormalities, and congenital 
cataracts. In collaboration with Richard Lewis of the 
CuUen Eye Institute at Baylor College of Medicine, 
X-linked restriction fragment length polymorphisms 
(RFLPs) have been used to study six families in 
which the Lowe syndrome is segregating. Two 
markers mapping to the region Xq24-26 were 
identified that are tightly linked to the disease 
locus. One locus, DXS42, shows no recombinants 
with lod score of 8.67; the other, DXSIO, is linked 
at 2% recombination with lod score 8.89. However, 
DXSIO and another RFLP, DXS86, share large re- 
striction fragments on field-inversion gel electro- 
phoresis analysis, and DXS86 is linked at 10% with 
lod 4.74. This discrepant result occurs because one 
family in which DXS86 is informative and shows 
crossovers is not informative for DXSIO. 
Fibroblasts from a female affected with Lowe syn- 
drome due to an X;3 translocation were used to 
construct somatic cell hybrids in which the deriva- 
tive 3 containing only Xq25-qter and the derivative 
X containing Xpter-q25 have been isolated from the 
normal X. With such hybrids, DXSIO and DXS86 
are shown to be distal to the breakpoint, while 
DXS42 and DXS37 are proximal. 
Dr. Nussbaum is also Associate Professor of 
Human Genetics and Pediatrics at the University of 
Pennsylvania School of Medicine and Attending 
Physician in Genetics and Metabolism at Children's 
Hospital of Philadelphia. 
PUBLICATIONS 
Books and Chapters of Books 
Nussbaum, R.L., and Ledbetter, D.H. 1989. Fragile X syndrome. In The Metabolic Basis of Inherited Disease 
(Scriver, C.R., Beaudet, A.L. , Sly, WS., and Valle, D. , Eds.). New York: McGraw-Hill, pp 327-339. 
Articles 
Cremers, F.RM., van de Pol, D.J.R., Diergaarde, RJ., Wieringa, B., Nussbaum, R.L. , Schwartz, M., and Ropers, 
H.-H. 1989. Physical fine mapping of the choroideremia locus using Xq21 deletions associated with com- 
plex syndromes. Genomics 4:41-46. 
Maddalena, A. , Sosnoski, D.M., Berry, G.T, and Nussbaum, R.L. 1988. Mosaicism for an intragenic deletion in 
a boy with mild ornithine transcarbamylase deficiency. N Engl J Med 319:999-1003. 
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