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1 From the Department of Ophthalmology, Sapir Medical Center, Kfar Saba; and the 2 Institute of Human Genetics, Sheba Medical Center, Tel Hashomer, Israel. Both institutions are affiliated with the Sackler Faculty of Medicine, Tel Aviv University, Israel.
| Abstract |
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METHODS. Linkage analysis was performed with polymorphic markers close to 14 loci previously shown to be involved in autosomal dominant congenital cataract. In one of the families a gene segregating with the disease was analyzed by single-strand conformation polymorphism (SSCP) and eventually sequenced.
RESULTS. Three polymorphic markers close to the CRYAA gene located on chromosome 21q segregated with the disease phenotype in one of the families, but not in the other. Sequencing of the CRYAA in this Jewish Persian family revealed a G-to-A substitution, resulting in the formation of a premature stop codon (W9X).
CONCLUSIONS. A nonsense mutation in the CRYAA gene causes autosomal recessive cataract in one family. This constitutes the first description of the molecular defect underlying nonsyndromic autosomal recessive congenital cataract. That there was no linkage to this locus in another family provides evidence for genetic heterogeneity.
| Introduction |
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, ß, and
.2
The crystallins constitute a major
component of the cellular cytoplasm needed for the transparency of the
lens. This requires their refractive index to be relatively constant
over distances approximating the wave length of the transmitted light,
and therefore, maintenance of a high degree of short-range order among
the crystallins is needed.3
Recently, some of the
crystallins were also shown to have enzymatic activity.4
Because fiber cells in the central lens nucleus lose their nuclei
during development, the crystallins in these cells do not turn over and
thus must be extremely stable proteins.2
The transparency
of the lens also depends on a variety of noncrystallin proteins, which
include various enzymes such as those involved in the glutathione redox
cycle and the mercapturic acid pathway,5
and
components of the lens cytoskeleton and membrane proteins such as
MP-26, aquaporins, connexins, and N-cadherins.6
Each
component of these systems could cause hereditary cataract.
Mutations in five different crystallin genes (or clusters) have been
identified as the cause of dominant disease. These include:
CRYAA,7
CRYAB,8
9
CRYBA,10
CRYBB,11
and mutations within the
-crystallin gene cluster at 2q34.12
Two gap junction
gene mutations were identified, GJA8 at
1q21.113
and GJA3 at 13q11-12.14
In addition, in seven families the disease locus has been mapped by
linkage studies, but the disease-causing gene has not yet been
cloned.15
16
17
18
19
20
21
All together, 14 disease loci have been
identified in autosomal dominant congenital cataract.
Despite the progress that has been made in understanding the molecular
basis of autosomal dominant congenital cataract, no loci or genes in
humans has been identified in the autosomal recessive form of the
disease. An autosomal recessive cataract has been previously linked to
the Ii blood group, but the chromosomal location of Ii is not
known.22
In this report, we present data showing that a
nonsense mutation in the human
-crystallin gene (CRYAA),
is responsible for an autosomal recessive form of the disease in an
inbred Jewish Persian family. In an additional Arab Muslim family we
have ruled out this gene as the cause of the disease, thus providing
evidence for genetic heterogeneity in the autosomal recessive form of
the disease.
| Methods |
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Genotyping and SSCP Analysis
Markers used in this study included: D11S1986, D11S1998 (for
CRYAB); D21S2055, D21S226, and D21S1446 (for
CRYAA); and D1S1597, D1S3669, D1S1653, D1S1679, D2S1384,
D2S434, D3S2460, D3S1764, D3S1311, D13S787, D14S587, D14S592, D16S2624,
D16S539, D17S1308, D17S2196, D17S1294, D17S2193, D17S784, and D22S420
for the additional 12 loci described in autosomal dominant
cataract.10
11
12
13
14
15
16
17
18
19
20
21
Amplification was performed in a 10-µl
reaction volume containing 50 ng of DNA, 13.4 ng of each unlabeled
primer, 1.5 mM each dNTP, and 0.08 µg
32P-labeled primer in 1.5 mM
MgCl2 polymerase chain reaction (PCR) buffer,
with 1.2 U Taq polymerase (Bio-Line, London, UK). After an
initial denaturation of 5 minutes at 95°C, 31 cycles were performed
at 94°C for 2 minutes, 52°C for 3 minutes, and 72°C for 1 minute,
followed by a final extension time of 7 minutes at 72°C. Samples were
mixed with 10 µl of loading buffer, denatured at 95°C for 5 minutes
and electrophoresed on a 6% denaturing polyacrylamide gel.
Single-strand conformational polymorphism (SSCP) analysis of the
CRYAA gene was performed by amplifying exons 1, 2, and 3
using the primers 5'-CTCCAGGTCCCCGTGGTACCA-3' and
5'-GCGAGGAGAGGCCAGCACCAC-3', 5'-CTGTCTCTGCCAACCCCAGCAG-3' and
5'-CCCCTGTCCCACCTCTCAGTGCC-3', 5'-GGGGAGCCAGCCGAGGCAATG-3'
and 5'-GGCAGCTTCTCTGGCATGGGG-3', respectively. The reaction was
performed as described earlier. Samples were amplified using the
conditions described for genotyping. Samples were mixed with 10 µl of
loading buffer, denatured at 94°C for 5 minutes, and electrophoresed
on a 6% polyacrylamide-10% glycerol nondenaturing gel at 8 W for 16
hours.
Sequencing and Restriction Analysis
Exons 1, 2, and 3 were amplified as described and sequenced using
an automated sequencing system (ABI Prism-310 Genetic Analyzer; Perkin
ElmerApplied Biosystems, Foster City, CA). The exon 1 mutation was
confirmed by testing for the presence of a restriction site in the
mutated allele. Exon 1 was amplified as described above and digested
with HinfI, to produce three fragments of 146, 51, and 56 bp
in the mutated allele in contrast with the two fragments of 202 and 51
bp in the normal allele.
Linkage Analysis
Linkage was calculated with the LINKAGE (ver 5.1) package of
computer programs, assuming an autosomal recessive model of
inheritance, 100% penetrance in both sexes, and a gene frequency of
0.001. The marker order and distance, taken from published sources
(http://cedar.genetics.soton.ac.uk/pub/), were as follows: 21
qter-D21S2055-2.62 cM-D21S212-0.55 cM-CRYAA-0.24
cM-D21S1446-pter; 11qter-D11S1986-0.06-cM-CRYAB-6.81
cM-D11S1998-pter. Allele frequencies were also taken from published
sources (http://www.gdb.org).
| Results |
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= 0.00 for the marker D21S212 (data not shown). SCCP
analysis performed on the amplification product of the three exons that
compose this gene revealed an abnormal migration pattern in exon 1
(Fig. 2) . Sequencing of this exon in one of the affected siblings showed a
G-to-A substitution at position 27, resulting in a change of a
tryptophan to a stop codon (W9X; Fig. 3
). Subsequent sequencing of the other two exons did not reveal any other
changes.
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-crystallin protein is composed of the products
of CRYAA and CRYAB, after excluding
CRYAA as the gene responsible for the disease in family 2,
we checked for linkage between CRYAB and two chromosome 11
markers located close to this gene in this family. The results,
presented in Table 1
, ruled out CRYAB as the disease-causing
gene in family 2. We also excluded in this family the 12 additional
loci involved in autosomal dominant cataract10
11
12
13
14
15
16
17
18
19
20
21
(data
not shown).
|
| Discussion |
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The
-crystallins that compose up to 50% of the total protein mass
of the lens belong to the small heat shock protein family and function
as chaperons.23
Molecular chaperons facilitate the correct
folding of proteins in vivo and are of extreme importance in keeping
these proteins properly folded and in a functional
state.23
The small heat shock family protein is a group of
closely related proteins that are induced by heat or hypertonic stress,
bind to aggregation-prone proteins, and act as a reservoir of nonnative
folding intermediates that are subsequently refolded by other
chaperons.24
The
-crystallin is a multimeric protein
composed from two gene products,
A and
B. CRYAA
encodes the
A subunit, whereas
B is encoded by the
CRYAB gene located on chromosome 11q. Thus, this multimeric
protein is important in maintaining the transparency of the lens,
possibly by ensuring that the complexes formed by them and other
proteins of the lens remain soluble.25
Brady et al.26 have produced a knockout mouse model for the CRYAA gene. In this animal model, the main disease was detected in the homozygote mice, who at the age of 10 weeks showed development of dense opacity of the lens, whereas no such opacities were detected in the heterozygote mice. In both the knockout mice and family 1, the mutation in the CRYAA gene resulted in a complete or an almost complete absence of the CRYAA protein in the homozygotes and in a recessive mode of inheritance. It seems as though the amount of protein produced by the normal allele in the family 1 heterozygotes and in the knockout mice heterozygotes is sufficient for the normal development and maintenance of the lens, whereas complete absence of the CRYAA protein results in cataract formation. In contrast, a missense mutation (R116C) in the CRYAA gene described by Litt et al.,7 was inherited in an autosomal dominant manner. Heterozygosity for this mutation is sufficient to cause cataract, even in the presence of a normal second allele. The abnormal protein caused by the missense mutation may precipitate in the lens, induce the precipitation of other proteins, or interfere with the function of the normal allele (negative dominant effect).
In family 2 we excluded the CRYAA gene as the cause of the
disease, thus providing proof for genetic heterogeneity in autosomal
recessive congenital cataract. We also ruled out the CRYAB
gene that encodes the
B subunit of the
-crystallin multimer and
obviously constitutes an attractive candidate gene for recessive
cataract, as well as the 12 other loci involved in autosomal dominant
cataract. Ehling27
estimated that approximately 30 loci
are involved in autosomal dominant human congenital cataract. In mice,
40 loci have already been mapped
(http://www.ncbi.nlm.nih.gov/Homology/). The estimations about the
number of loci involved in recessive congenital cataract are much
smaller.27
As shown for the CRYAA gene in this
study, mutations in other loci involved in dominant cataract could very
well also account for the autosomal recessive form of the disease. It
is also evident that not all the loci involved in human cataract have
been mapped.
| Acknowledgements |
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| Footnotes |
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Submitted for publication March 24, 2000; revised May 30, 2000; accepted June 20, 2000.
Commercial relationships policy: N.
Corresponding author: Elon Pras, Institute of Human Genetics, Sheba Medical Center, Tel Hashomer 52621, Israel. epras{at}cc.tau.ac.il
| References |
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