Cystic Fibrosis, Gene Expression in the Mammalian 
Lens, and Mapping of Chromosome 7 
Lap-Chee Tsui, Ph.D. — International Research Scholar 
Dr. Tsui is Senior Scientist and Sellers Chair of Cystic Fibrosis Research in the Department of Genetics at 
the Research Institute of the Hospital for Sick Children, Toronto, and Professor of Molecular and Medical 
Genetics at the University of Toronto. He was born in Shanghai, raised and educated in Hong Kong, and 
there awarded degrees from the Chinese University. His Ph.D. degree is from the University of Pittsburgh, 
where his thesis was on the structure and assembly of bacteriophage X (with Roger Hendrix). After 
training briefly in the Biology Division of Oak Ridge National Laboratory, he joined the laboratories of 
Manuel Buchwald and Jack Riordan at the Hospital for Sick Children to work on cystic fibrosis. Dr. Tsui's 
honors include the titles of Scientist of the Medical Research Council of Canada, Fellow of the Royal Society 
of Canada, and Fellow of the Royal Society of London. 
THE research interests of my laboratory consist 
of three general topics in the molecular biol- 
ogy of mammalian gene regulation and function. 
Molecular Genetics of Cystic Fibrosis 
Through classical genetic linkage analysis and 
various molecular cloning strategies, the gene re- 
sponsible for cystic fibrosis (CF) , the commonest 
severe autosomal recessive disorder among Cau- 
casians, has been localized and identified. We 
have shown that the major CF mutation, account- 
ing for approximately 70 percent of all mutant 
alleles, is a deletion of the phenylalanine residue 
at amino acid position 508 of the predicted poly- 
peptide, which is named cystic fibrosis trans- 
membrane conductance regulator (CFTR) . 
To investigate the basic defect in CF, we are 
continuing our search for the other CFTR muta- 
tions. Since the CFTR gene contains 27 exons and 
spans 230 kb of DNA, detecting a microscopic 
mutation is not straightforward. Furthermore, the 
lack of a convenient functional assay for CFTR 
makes it difficult to distinguish a truly disease- 
causing mutation from a benign amino acid sub- 
stitution. To coordinate the detection eff'ort, our 
laboratory has taken a central role in the forma- 
tion of an international consortium of 90 groups 
of researchers from 26 countries. Through active 
exchange of gene sequences and mutation data 
before submission for publication, the consor- 
tium has already identified more than 150 appar- 
ent CF mutations and 30 sequence variations. The 
collective data from two years of operation also 
show that most of the remaining 30 percent of 
mutant alleles are individually rare and highly 
heterogeneous among different populations. 
Despite our intensive DNA sequencing effort, 
covering each of the exons plus their flanking 
regions, we have not been able to identify the 
mutations for 6 of the 94 CF chromosomes exam- 
ined. These alleles probably harbor different mu- 
tations located in regions that affect transcription 
or RNA processing. Nevertheless, we have devel- 
oped a highly informative marker for use in ge- 
netic counseling of CF families. The observed he- 
terozygosity for this marker, a dinucleotide 
repeat polymorphism in one of the introns of the 
CFTR gene, is over 95 percent. In a case of prena- 
tal diagnosis, we applied the marker successfully 
after all other available tests failed. 
The common CF symptoms include chronic 
obstructive lung disease, pancreatic enzyme in- 
sufficiency, and elevated sweat electrolytes. 
Other organs and tissues, such as the hepatobili- 
ary tree, intestines, and vas deferens, may also be 
involved. The varied degree of severity of the 
symptoms among CF patients suggests that the 
phenotypes are at least partly conferred by the 
genotypes at CFTR. Furthermore, information 
about genotypes and phenotypes should also pro- 
vide important clues to the function of CFTR and 
the basic defect in the various affected organs and 
tissues. Based on this assumption and the use of 
the extensive clinical data collected at the CF 
clinic in our hospital, we have demonstrated a 
good correlation between pancreatic involve- 
ment and genotypes at the CFTR locus. 
To facilitate direct biochemical and physiologi- 
cal analysis, we have constructed a full-length 
cDNA for CFTR in several expression vectors. 
Site-directed mutagenesis has been used to re- 
move sequences that are toxic to the host bacte- 
rium as well as to generate mutant constructs for 
functional evaluation. Using DNA transfection 
into heterologous cell types, we showed that this 
cDNA could confer a cAMP-regulated chloride 
channel activity de novo, suggesting that CFTR is 
a chloride channel itself. Preliminary data from 
the analysis of constructs reproducing some of 
the naturally occurring mutations appear to be in 
good agreement with those predicted from the 
severity of pancreatic involvement. 
In order to understand the regulation of the 
CFTR gene, we have performed a series of dele- 
tion and transfection studies to determine the se- 
quence elements responsible for basal promoter 
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