MOLECULAR CYTOGENETICS 
Laboratory of the late Samuel A. Latt, M.D., Ph.D., Investigator 
BY Marc Lalande, Ph.D., Senior Associate 
I. Molecular Analysis of the Human 15qllql3 Sub- 
region. 
Dr. Lalande is continuing the pioneering work of 
Dr. Latt on molecular studies of two distinct human 
genetic disorders, Prader-Willi syndrome (PWS) and 
Angelman syndrome (AS), both of which are asso- 
ciated with abnormalities of the 15qllql3 chromo- 
somal subregion. Ten cloned DNA markers specific 
for this region have been isolated from a flow- 
sorted recombinant DNA library. Five of these have 
been mapped to a subregion of 15qllql3 that is 
deleted in 60% of patients suffering from PWS. In 
collaboration with Dr. Joan Knoll, it has been dem- 
onstrated that the similar region of 15qllql3 is 
also deleted in a number of AS patients. Although 
the deletions are indistinguishable at the molecular 
and cytological levels, the clinical phenotypes of 
the two disorders are different. The clinical features 
of PWS include obesity, small hands and feet, 
hypogonadism, hypotonia, growth delay in infancy, 
and mild mental retardation; AS is characterized 
by severe mental retardation, microcephaly, sei- 
zures, prognathism with tongue protrusion, and 
puppet-like ataxic gait with jerky arm movements. 
Using restriction fragment length polymorphisms 
characterized by Dr. Robert D. NichoUs, Dr. Knoll 
has determined that in each of the seven AS cases 
studied, the deleted chromosome is inherited from 
the mother. This is in contrast to PWS, where 
the deleted chromosome is paternally inherited. 
These results suggest that the sex of the parent 
who transmits the abnormal chromosome plays a 
role in determining the clinical phenotype, i.e., AS 
or PWS. 
These studies have now been extended to the 
analysis of PWS patients who display an apparently 
normal karyotype, in that no deletion of 15qllql3 
can be detected at either the molecular or cytoge- 
netic level. Drs. NichoUs, Knoll, and Lalande have 
shown in the first two families studied that the PWS 
patient inherits both of the mother's normal and 
intact (at least for the 15qllql3 critical region) 
chromosome 15s. This is the first time that the in- 
heritance of both the normal chromosomes from 
one parent in an individual has been demonstrated 
in humans and the first time that such an anomaly 
has been associated with a genetic disease (PWS). It 
appears that a gene or genes within 15qllql3 are 
required for normal human development. The ab- 
sence of a paternal contribution of genes to this 
region, whether by deletion of the paternal 
15qllql3 or by inheritance of two maternal chro- 
mosome 15s will result in PWS. This implies that 
there are functional differences in alleles of a gene 
or genes from 15qllql3 that depend upon the sex 
of the transmitting parent. This phenomenon is 
termed genetic imprinting. Future studies will con- 
centrate on the possible imprinting mechanism and 
the role of imprinting in PWS, AS, and, perhaps, 
other human genetic disorders. 
II. Chromosome Mapping Studies. 
Dr. Latt's laboratory continues to generate DNA 
fragments specific for several important chromo- 
somal regions and to analyze these subregions 
using various molecular techniques. In collabora- 
tion with Dr. Matthew L. Warman, a physical map of 
~1 million base pairs (bp) of DNA surrounding the 
oncogene N-myc was constructed in normal cell 
lines. This map was compared with that of neu- 
roblastoma tumor cell lines, where the N-myc 
oncogene is specifically amplified. Complex rear- 
rangements in the amplified N-myc locus of tumor 
cells were observed that may be important in un- 
derstanding the process of neoplastic change and 
tumor progression. In collaboration with Dr. Ulrich 
Miiller, a physical map of almost the entire short 
arm of the human Y chromosome has been con- 
structed. The map encompasses —13 million bp 
and was generated using a number of Y chro- 
mosome-specific DNA fragments isolated from 
flow-sorted recombinant DNA libraries and a one- 
dimensional pulsed-field gel electrophoresis sys- 
tem. Similar techniques have been employed to ex- 
tend the physical map of human chromosome 13. 
This map now spans —12 million bp of the 13ql4 
subregion, which contains the retinoblastoma sus- 
ceptibility (RB) gene locus. Rearrangements, such 
as deletions and translocations, that involve the RB 
gene can be detected using conventional as well as 
long-range mapping techniques and DNA fragments 
cloned in Dr. Latt's laboratory. The RB gene that 
was originally isolated using such fragments has 
now been shown to be involved in the develop- 
ment of several different human tumors, including 
breast and colon cancer. 
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