ing events. Of potential functional significance is 
the fact that the isoforms found within the rbC and 
rbD classes are often the result of the nearly precise 
substitution of putative transmembrane segments. 
Isoforms for the rbA and rbB subunits have also 
been detected, although most of the divergence, in 
contrast to the rbC and rbD channels, resides in the 
carboxyl segments of the polypeptides. This unantic- 
ipated degree of calcium channel heterogeneity 
may account for variations in the electrophysiologi- 
cal and pharmacological properties of calcium 
channels described in different cell types. 
Differential Expression of Calcium Channels 
The distribution of calcium channels in the mam- 
malian CNS has been mostly defined by studies uti- 
lizing autoradiographic localization of radiolabeled 
antagonists. The availability of gene-specific probes 
allows more-direct analysis of calcium channel gene 
expression. At the regional level, all of the cloned 
calcium channel subunits are expressed through- 
out the CNS. However, the expression of some iso- 
forms generated by alternative splicing is spatially 
regulated. In addition, in situ hybridization studies 
demonstrate that, at the cellular level, the various 
calcium channel isoforms are preferentially ex- 
pressed in subsets of neurons. For example, in the 
cerebellum, rbA-I transcripts are detected at 5- to 
10-fold higher levels in Purkinje cells compared 
with other cell types. Given the complex morphol- 
ogy of neurons, it is likely that both the spatial dis- 
tribution and the density of calcium channel sub- 
types contribute to the integration of neuronal 
excitability. This is being addressed utilizing poly- 
clonal antisera to define the subcellular localization 
of the various subtypes of calcium channels in 
neurons. 
Dr. Snutch is Assistant Professor in the Biotech- 
nology Laboratory and a member of the Depart- 
ment of Zoology and Division of Neuroscience at 
the University of British Columbia, Vancouver. 
STUDIES IN MOLECULAR GENETICS 
Lap-Chee Tsui, Ph.D., International Research Scholar 
Molecular Genetics of Cystic Fibrosis 
To investigate the basic defect in cystic fibrosis 
(CF), Dr. Tsui and his colleagues continue to search 
for mutations in the cystic fibrosis transmembrane 
conductance regulator (CFTR) gene. The large size 
of the gene, with 27 exons spanning 230 kb of DNA, 
has rendered the detection of microscopic muta- 
tions difficult. Furthermore, because of the lack of a 
convenient functional assay for CFTR, it is not easy 
to distinguish a truly disease-causing mutation from 
a benign amino acid substitution. Nevertheless, an 
international consortium has been formed to collect 
the data, and more than 200 putative mutations and 
100 sequence variations have already been identi- 
fied. Dr. Tsui's laboratory has played a central role 
in the consortium, through active compilation and 
dissemination of gene sequences and mutation data. 
The consortium data show that AF508 is still the 
most frequent mutation, accounting for an overall 
67% of the world CF mutant alleles. The remaining 
mutations are mostly rare and highly heterogeneous 
among different populations. After testing more 
than 80 different mutations in a population of more 
than 500 patients attending the CF clinic at the Hos- 
pital for Sick Children, Toronto, it has been possible 
to define the mutations in only 85% of the mutant 
chromosomes. The same level of coverage has been 
observed for most other populations. Therefore 
these results strongly suggest that direct DNA testing 
of mutations should be done in combination with 
closely linked, highly informative DNA markers for 
genetic diagnosis in families with CF. 
The varied degree of severity of the disease among 
different CF patients suggested that the phenotypes 
could be conferred, at least in part, by the genotypes 
at CFTR. Studies in this laboratory demonstrated that 
patients homozygous for the AF508 mutation were 
almost exclusively pancreatic-insufficient (PI) , and 
there was an excess of pancreatic-sufficient (PS) pa- 
tients with non-AF508 mutations. It has been Dr. 
Tsui's hypothesis that patients who are homozygous 
or compound-heterozygous of severe mutations, 
which include AF508, are expected to be PI; in con- 
trast, patients with one or two copies of the other 
class (i.e., mild) of alleles are expected to be PS. 
Based on the hypothesis, ~90% of the mutant al- 
leles are expected to be severe and 10% mild. 
Through further investigation it has been possible to 
classify most of the mutations detected in the pa- 
tient cohort at the Toronto clinic into the two 
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