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A-CrystallinEncoding Gene
1 From the National Research Center for Environment and Health (GSF), Institute of Mammalian Genetics and the 2 Institute of Experimental Genetics, Neuherberg, Germany; the 3 Institute of Molecular Genetics, Max-Delbrück-Center for Molecular Medicine, Berlin, Germany; and 5 Lehrstuhl für Molekulare Tierzucht und Biotechnologie, Ludwig-Maximilians-Universität, Munich, Germany.
| Abstract |
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METHODS. Isolated lenses were photographed and histologic sections of the eye were analyzed according to standard procedures. Linkage analysis was performed with a set of microsatellite markers covering all autosomal chromosomes. cDNA was amplified after reverse transcription of lens mRNA. For PCR, cDNA or genomic DNA was used as a template.
RESULTS. Nuclear opacity and posterior suture anomaly were visible at eye
opening and progressed to a nuclear and zonular cataract at 2 months of
age. The opacity as well as the microphthalmia was more pronounced in
the homozygotes than in the heterozygotes. The mutation was mapped to
chromosome 17 between the markers D17Mit133 and
D17Mit180. This position made the
A-crystallinencoding gene (Cryaa) an excellent
candidate gene. Sequence analysis revealed a mutation of a T to an A at
position 371 in the Cryaa cDNA. The mutation was
confirmed by an additional MnlI restriction site in the
genomic DNA of homozygous mutants leading to replacement of Val with
Glu at codon 124 affecting the C-terminal region of the
A-crystallin.
CONCLUSIONS. The Aey7 mutant represents the first dominant mouse
cataract mutation affecting the Cryaa gene. The mutation
leads to progressive opacification of the lens. Compared with the ß-
and
-crystallinencoding genes, mutations in the
-crystallinencoding genes are rare.
| Introduction |
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-crystallins represent the major class of
water-soluble proteins in the lens, forming large complexes of
approximately 800 to 1000 kDa composed of two subunits,
A- and
B-crystallin. The isolated subunits have molecular masses of 20 and
22 kDa (
A- and
B-crystallin, respectively), and the isoelectric
points of the native proteins have been reported to range from pH 4.5
to 5.0.1
2
The
-crystallins are considered to be
structural proteins and the subject of a variety of posttranslational
modifications (truncation, glycosylation, glycation, carbamylation, and
acetylation; for review see Groenen et al.3
).
The two related proteins
A- and
B-crystallin are encoded by two
genes, Cryaa and Cryab, which are located on
mouse chromosomes 17 and 9, respectively. Both genes have been cloned
in a variety of species, including chicken, hamster, human, mouse,
rabbit, and rat. Both genes contain three exons of similar
size.4
In rodents (mouse, rat, hamster, rabbit) an
alternative splice product can be observed in 10% to 20% of the
Cryaa transcripts. From intron A an additional 69 base pairs
("insert exon") are included in the mature mRNA leading to a
protein 23 amino acids longer than the usual
A-crystallin, which is
referred to as aAins-crystallin.5
The expression of the two Crya genes is not uniform, even if
both are expressed at very high levels in the lens. The
A-crystallin
can be considered a lens-specific protein, because only trace amounts
are found outside the lens in the spleen.6
7
During
embryogenesis, Cryaa expression is observed in the lens cup
of the rat8
and the mouse at embryonic day
(E)10.9
Then,
A-crystallin is present in the posterior
half of the lens vesicle,10
and later on, it becomes very
abundant in lens fiber cells.9
In contrast, Cryab is expressed ubiquitously. In rat lenses,
B-crystallin can first be detected at the end of embryonic
development11
; however, in the mouse, it is already
present at E9.5. After birth, it is found preferentially in the
epithelial cells.9
Transcripts can also be found in marked
amounts in brain, heart, skeletal muscle, lung, thymus, and kidney and
in a variety of cell lines.7
During embryogenesis,
expression of Cryab mRNA was detected by in situ
hybridization in the primitive heart of the mouse at E8.5 and in the
myotome of the somites at E10.5, supporting the hypothesis that
functions of
B-crystallin may be coupled to the activation of
genetic programs responsible for myogenic differentiation and cardiac
morphogenesis.12
Because of the expression pattern of the Crya genes, it is
thought that several diseases may be caused or accompanied by
alterations in the
-crystallins. In the mouse, Brady et
al.13
reported the consequence of the loss of the
Cryaa gene in homozygous knockout mutants. The only defects
produced by the targeted disruption of the Cryaa gene are
lens opacities resulting from inclusion bodies containing
B-crystallin. Furthermore, a recessive missense mutation in the
Cryaa gene was reported recently as a cause for the lens
opacity in the lop18 mouse.14
The homozygous
mutants exhibit a degeneration of the lens cortex, posterior migration
of lens epithelial nuclei, and formation of abnormal lens fibers at the
posterior pole resulting in a large white cataract. In a human family
with dominant zonular central cataract, a missense mutation in the
CRYAA gene leading to replacement of Arg at position 116
with Cys (R116C) was also identified.15
A further human
cataract, but with recessive inheritance was reported to be caused by
the nonsense mutation W9X, resulting in a premature stop codon of
CRYAA.16
A mutation in the human
B-crystallinencoding gene
(CRYAB), an A-to-G transition at nucleotide 3787 in the
genomic sequence, leads to a replacement of Arg 120 with Gly and to
dominant, desmin-related myopathy with cataract
formation.17
In contrast, the knockout of the
Cryab gene in the mouse does not lead to the formation of
cataracts, even in the homozygous stage.18
In the course of the analysis of mouse mutants obtained by a large-scale ethylnitrosourea (ENU) mutagenesis program,19 20 we identified several dominant cataract mutations. We report one of them, which was previously referred to as Aey7. It has been mapped to chromosome 17 and finally characterized as the first dominant Cryaa mutation in the mouse.
| Materials and Methods |
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Male C3HeB/FeJ mice were treated with ENU (160 mg/kg) at the age of 10 weeks according to Ehling et al.21 and mated to untreated female C3HeB/FeJ mice. The offspring of the ENU-treated mice were screened at the ages of 4 to 6 months for the presence of cataracts with the aid of a slit lamp (SLM30; Carl Zeiss, Oberkochen, Germany).22 Mice with lens opacities were tested for a dominant mode of inheritance. Homozygotes were obtained by brother-x-sister mating.
Phenotypic Characterization
The eyes of the mutants were examined continuously during the
postnatal life with the slit lamp. For documentation, lenses were
enucleated under a dissecting microscope (MZ APO; Leica, Heidelberg,
Germany) and photographed.
Histologic analysis of the eyes was performed at birth and at the ages of 3 and 7 weeks. The eye globes were fixed for 3 hours in Carnoys solution and embedded in JB-4 plastic medium (Polysciences, Inc., Eppelheim, Germany), according to the manufacturers protocol. Sectioning was performed with an ultramicrotome (OMU3; Reichert-Jung, Walldorf, Germany). Serial transverse 3-µm sections were cut with a glass knife and stained with methylene blue and basic fuchsin. The sections were evaluated with a light microscope (Axioplan; Zeiss). Images were acquired by means of a scanning camera (Progress 3008; Jenoptik, Jena, Germany) and imported into an image-processing program (Photoshop, ver. 5.5 and Adobe Illustrator, ver. 8.0; Adobe, Unterschleissheim, Germany).
Mapping
Homozygous carriers (first generation) were mated to wild-type
C57BL/6J mice, and the offspring (second generation) were backcrossed
to the wild-type C57BL/6J mice. DNA was prepared from tail tips of
cataractous offspring of the third generation (G3) according to
standard procedures. DNA was adjusted to a concentration of 50 ng/µl.
For a genome-wide linkage analysis, several microsatellite markers were
used for each autosome.23
PCR and Sequencing
For the molecular analysis, RNA was isolated from lenses of
C3H/El, T-stock, 129, JF-1, and C57BL/6J wild-type mice and from
homozygous mutant mice at the age of 4 weeks. RNA samples from lenses
were reverse transcribed to cDNA using a kit (Ready-to-Go; Pharmacia
Biotech, Freiburg, Germany); genomic DNA was isolated from tail tips or
spleen of wild-type C3HeB/FeJ and C57BL/6J mice or from homozygous
mutants, according to standard procedures. For amplification of cDNA
from the Cryaa gene, primers were selected from the EMBL and
GenBank databases (Table 1
; EMBL is provided in the public domain by the European Molecular
Biology Laboratory, Heidelberg, Germany, and is available at
http://www.embl-heidelberg.de; GenBank is provided in the public domain
by the National Center for Biotechnology Information and is available
at http://www.ncbi.nlm.nih.gov/genbank/).
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Computer-assisted predictions of the biochemical properties of proteins were performed using the Proteomics tools of the ExPASy molecular biology server (provided by the Swiss Institute of Bioinformatics, Geneva, and available at no charge to academic users at http://www.expasy.ch).
Materials
Chemicals were from Merck (Darmstadt, Germany) or Sigma Chemical
Co. (Deisenhofen, Germany). The enzymes used for cloning and reverse
transcription were from Roche (Mannheim, Germany).
| Results |
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Within the coding sequence, several polymorphic sites were observed in our C3H/El colony compared with the database (accession no. J00375 for the 5' part and J00376 for the 3' end); however, all predicted amino acids do not alter the primary sequence of the protein. The entire cDNA from wild-type C3H/El mice (including the insert exon) has been added to the EMBL database (accession no. AJ310308).
The only sequence alteration cosegregating with the mutant phenotype was a replacement of T with A at position 371 (exon 3) of the Cryaa gene, counting the first base in the ATG start codon as base pair 1. The insert exon, which is spliced into the mature mRNA of only 10% of the rodent Cryaa transcripts, was not considered in this calculation (Fig. 4) . The mutation leads to the appearance of a new MnlI restriction site in the mutant mice, but is not present in the wild-type sequence of C3H/El and several other mouse strains. The cosegregation of the mutation was validated by the presence of the MnlI restriction site in five homozygous mutants (Fig. 5) . Therefore, the new allele should be referred to as CryaaAey7.
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Glu exchange at codon 124,
counting the Met start codon as 1. To allow comparison across species
borders, the amino acids encoded in rodents by the insert exon are not
considered. At this position (124), Glu occurs in none of the known
-crystallins from 28 mammalian species, from chicken and frog (which
has Leu instead of Val at this position), or from 13
primates.24
25
The ProSite scan of ExPASy
(http://www.expasy.ch) does not reveal changes at this particular site;
particularly, the recognition sequence for the phosphorylation site at
Ser122 is not affected. The ProtScale analysis of ExPASy, looking for
hydrophobic or hydrophilic regions within the protein, revealed also
only a very slight difference in the region surrounding position 124. | Discussion |
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A exchange within the third exon of the
Cryaa gene, and the mutant allele was therefore designated
CryaaAey7. The predicted amino acid in the
mutants is Glu instead of Val at position 124, close to the C terminus.
The lens opacity is already visible at eye opening. The cataractous
changes coincide with the expression of the Cryaa gene,
which is observed first in the lens cup. Later on,
A-crystallin
becomes very abundant in lens fiber cells8
9
10
11
and is
considered as a major structural protein of the ocular
lens.2
-Crystallin participates in the intracellular architecture through
cellular filaments. Together with CP49, a lens-specific cytoskeletal
protein,
-crystallins form the beaded filaments.26
A-Crystallin also interacts with tubulin27
and
actin.28
Moreover, there are several lines of biochemical
evidence that
-crystallin may become associated with the plasma
membrane. Because the molecular sites of these interactions are not yet
known in detail,29
whether the V124E mutation described
herein is involved in these interactions is open to speculation.
Besides its structural properties,
A-crystallin is the target of
posttranslational alterations, and one of its most important
modifications is phosphorylation. Past studies have demonstrated that
the major site of
A-crystallin in vivo phosphorylation is Ser122,
which is phosphorylated in a cAMP-dependent manner.30
This
phosphorylated form of
A-crystallin could be detected in human
lenses only from adolescent, adult, and senile donors, but not in
infants, suggesting a developmental regulation of this particular kind
of modification.31
Computer-assisted prediction programs,
such as ProSite (ExPASy), did not support the hypothesis that the V124E
mutation influences the use of the very close phosphorylation site at
Ser122.
However, the most exciting finding concerns the function of
A-crystallin as a molecular chaperone.32
A-Crystallin prevents thermal aggregation of several enzymes and
even of ß- and
-crystallins. Chaperone activity is essential for
the lens, because degradation and extrusion of defective proteins is
not possible as it is in other tissues. Moreover, the lens is exposed
to a variety of damaging agents, particularly in light of various
wavelengths leading to oxidative effects on quite a number of lens
proteins.
-Crystallin has a substrate specificity different from
other chaperones and recognizes specific nonnative intermediates formed
during denaturation only.33
Based on the gene structure, Wistow34
thought the overall
structure of
-crystallins to consist of a globular N-terminal domain
of two symmetry-related motifs and a somewhat longer C-terminal domain
also consisting of two motifs. The two globular domains, which are
built up by two exons, are fused by a short connecting peptide, which
is extended in the rodent
Ains-crystallin by
23 amino acids5
and has a significant reduced chaperone
activity compared with the normal
A-crystallin.35
36
The C terminus of the
A-crystallin consists of two rather
hydrophilic domains2
that are exposed to the surface and
tend to form tetrameric assemblies.37
Moreover, there is
considerable evidence for the involvement of the flexible C-terminal
extension in chaperone activity. Its truncation or immobilization
greatly reduces this capacity, suggesting that the hydrophilic tail is
likely to be important in keeping complexes of chaperones and bound
proteins in solution.4
The chaperone activity of
-crystallin was first localized to amino acid residues 158-173 of
the C-terminal region of
A-crystallin,38
but recently,
reduced chaperone-like activity was demonstrated also in an
A-crystallin mutation (Asp69Ser)39
and by the
introduction of a hydrophobic tryptophan at position
172.40
The mutation described in this article replaces a neutral, hydrophobic
amino acid (Val) with an acidic amino acid (Glu) in the C-terminal part
(position 124) of the
A-crystallin. The same region of the
A-crystallin is affected by a mutation in human CRYAA
causing a dominant zonular central cataract phenotypically very similar
to Aey7. It was characterized by a missense mutation gene
leading to replacement of Arg with Cys at position 116
(R116C).15
Detailed biochemical analysis of this mutant
protein demonstrated a fourfold reduction in chaperone-like activity,
but it tends to bind to membranes 10 times more than the wild-type
form.41
Similar explanations may be suggested for
cataractogenesis in the CryaaAey7 mutants.
In mouse and human, a few other mutations in the Cryaa/CRYAA genes have already been described. The loss of the entire Cryaa gene in mouse knockout mutants leads to an opacification of the lens only in homozygous conditions.13 In the recessive lop18 mouse mutants, a missense mutation in the first exon of the Cryaa gene converting codon 54 Arg to His was demonstrated recently as cause for the phenotype.14 A recessive cataract has been also reported in human, which is caused by the nonsense mutation W9X, resulting in a premature stop codon of CRYAA.16
Of interest and in contrast to the other abundant lens proteins, the
ß- and
-crystallins, mutations in the Cryaa and
CRYAA genes lead to both recessive and dominant phenotypes.
Mutations affecting the N-terminal part or even the loss of the entire
protein result in recessive phenotypes, whereas mutations in the
C-terminal part leads to dominant phenotypes. Most likely, the dominant
mutation inhibits interactions with other proteins affecting the
chaperone activity, affinity with the membranes, or interactions with
the lens intermediate filaments.
| Acknowledgements |
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| Footnotes |
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Supported by Grant 01KW9610/1 from the German Human Genome Project (RB, EW, MHdA).
Submitted for publication April 19, 2001; revised July 23, 2001; accepted August 1, 2001.
Commercial relationships policy: C, I (RB, MHdA); N (all others).
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Jochen Graw, GSF - National Research Center for Environment and Health, Institute of Mammalian Genetics, Ingolstädter Landstr. 1, D-85764 Neuherberg, Germany. graw{at}gsf.de
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