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1 From the Department of Ophthalmology, Keio University School of Medicine, Tokyo, Japan; the 2 Department of Ophthalmology, University of Tokyo Graduate School of Medicine, Japan; the 3 Ophthalmic Genetics and Visual Function Branch, National Eye Institute, National Institutes of Health, Bethesda, Maryland; the 4 Department of Ophthalmology, Tenri Yorozu Hospital, Nara, Japan; the 5 Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Japan; the 6 Department of Ophthalmology, National Childrens Hospital, Tokyo, Japan; and the 7 National Institute of Sensory Organs, National Tokyo Medical Center, Japan.
| Abstract |
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METHODS. Sixty-five unrelated Japanese patients with PCG were screened by PCR-single-strand conformational polymorphism (SSCP) analysis followed by direct sequencing. No patients were offspring of consanguineous marriages, a common occurrence among patients in previous reports. PCG haplotypes were constructed with intragenic polymorphisms in affected individuals. Three-dimensional atomic structures of human CYP1B1 and four mutant CYP1B1 sequences representing missense mutations were assembled using homology modeling and were regularized by an energy-minimization procedure.
RESULTS. Eleven novel mutations, including seven definite and four probable mutations, were detected in 13 (20%) of the 65 unrelated patients. Of the seven definite mutations, three were predicted to truncate the CYP1B1 open reading frame. The other four were missense mutations (Asp192Val, Ala330Phe, Val364Met, and Arg444Gln), all located in conserved core structures determining proper folding and heme-binding ability of cytochrome P450 molecules. Molecular modeling demonstrated that two of four mutations in positions 330 and 364 were structurally neutral, but Arg444Gln caused significant structural change. Of the four probable mutations, three were missense (Val198Ile, Val320Leu, and Glu499Gly); the other was a base substitution in the noncoding region of exon 1.
CONCLUSIONS. The 11 varied CYP1B1 mutations found in 13 unrelated Japanese patients with sporadic occurrence of PCG represent an allelic heterogeneity and may be unique to a specific population.
| Introduction |
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The incidence of PCG varies geographically, reported to be 1 in 5,000 to 1 in 22,000 newborns in Western countries, 1 in 2,500 in the Middle East, and 1 in 1,250 in the Rom (gypsy) population of Slovakia, in whom PCG is the major cause of blindness.2 3 4 5 6 The incidence of this disease is unclear in Japan. Most cases of PCG are sporadic in occurrence. In approximately 10% of cases in which a hereditary pattern is evident, inheritance is usually believed to be autosomal recessive.7 8 However, boys are affected more often than girls,9 and cases of incomplete penetrance have been documented.10 In some PCG-affected families, the inheritance pattern may be multifactorial or polygenic.9
Recently, a putative PCG locus, GLC3A, was linked to markers on the short arm of chromosome 2p21.11 An additional PCG locus, GLC3B, has been localized to chromosome 1p36.12 Thus, PCG is a genetically heterogeneous disease with at least two loci. Furthermore, Stoilov et al.13 identified three different mutations in the cytochrome P4501B1 gene, CYP1B1, in five Turkish families. Later, three mutations in the same gene were identified in 24 of 25 families with PCG in Saudi Arabia,14 and 16 mutations were found in 22 families with PCG in Turkey, the United States, Canada, and the United Kingdom.15 These data strongly suggest that mutations affecting CYP1B1 are responsible for the PCG phenotype associated with the GLC3A locus. CYP1B1 gene mutations were associated with over 85% of families with PCG in Saudi Arabia, Turkey, and Slovakia.10 15 16 In the present study, we screened the CYP1B1 gene in 65 unrelated Japanese probands with PCG and identified 11 novel mutations in 13 probands (20%).
| Methods |
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Mutation Screening
Genomic DNAs were prepared from leukocytes by the methods of
proteinase K-phenol-chloroform extraction. First, we screened the
CYP1B1 gene (GenBank accession number U56438) by PCR-single
strand conformation polymorphism (SSCP) analysis in a standard
protocol, using nine specific sets of primers in three exonic regions.
The putative open reading frame begins in the second exon. Samples
showing abnormal mobility in this analysis were further analyzed by
direct sequencing of the PCR products with an automated DNA sequencer
(model 373A; Applied Biosystems, Foster City, CA) using a sequencing
kit (Thermo Sequenase II Dye Terminator Cycle Sequencing kit; Amersham
Pharmacia Biotech, Uppsala, Sweden). Sixty unaffected Japanese subjects
(20 males and 40 females) were examined as control subjects after their
informed consent was obtained.
DNA sequencing of the PCR products of the exon 2 region of
CYP1B1 revealed substitution of two nucleotides at codon
330 (GCC
TTC). To identify the change of two nucleotides in
one mutant allele, the PCR products of this region were subcloned into
a vector (pT7 Blue T-Vector; Takara, Tokyo, Japan). This was followed
by DNA sequencing, using the kit and DNA sequencer. Primers M13 and M13
RV (Takara) were used for sequencing.
Sequence variations detected in control subjects and affected individuals were recorded for analysis. PCG haplotypes were constructed with intragenic polymorphisms in affected individuals with mutations and also in their parents.
Analysis of the Primary Structure of CYP1B1 by
Comparative Sequence Alignment
Amino acid sequences of CYP1B1, CYP1A1,
and CYP1A2 obtained from SwissProt (Swiss Institute of
Bioinformatics, Geneva, Switzerland) were compared between human,
mouse, and rat using a computer sequence alignment program (Omiga; ver.
2.0; Oxford Molecular Ltd., Oxford, UK). Analysis was performed
according to the manufacturers default setting.
Molecular Modeling of the Conserved Terminal Half of the
CYP1B1 Protein
Structural coordinates of cytochromes P450 (2hpd and 2c17) were
obtained from the Brookhaven Protein Data Bank (Brookhaven,
NY).17
The structures of both cytochromes were used as
templates to construct two separate models of human CYP1B1.
Multiple sequence alignment was performed by the method of Needleman
and Wunsch,18
as incorporated in a software
program (Look, ver. 3.5.2; Molecular Applications Group, Palo Alto,
CA). The human CYP1B1 structure was assembled by using the
automatic segment-matching method available in the computer
program.19
Superposition of two predicted models of human
CYP1B1 demonstrated high similarity for the conserved
portion of both structures. However, the model based on the structure
of 2c17 was more complete and therefore was used for analysis of
mutations. In each mutant structure, the conformation of the replaced
and neighboring residues was refined by self-consistent ensemble
optimization,20
which uses a statistical mean force field
approximation to obtain the structure with minimum global energy. Four
models of mutant CYP1B1 corresponding to each of four
missense mutations (Asp192Val, Ala330Phe, Val364Met, and Arg444Gln)
were prepared by this method. Finally, the predicted structures were
regularized by an energy-minimization procedure (Insight II, ver. 2000;
Molecular Simulations, Inc., San Diego, CA). Predicted structures were
tested by computer (Procheck; Roman Laskovsky, University College,
London, United Kingdom; information on availability is found at
http://www.biochem.ucl.ac.uk).21
| Results |
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In the present study, two patients with PCG had a heterozygous mutation of Val320Leu (G7463T) and a C3130T substitution in the noncoding region of exon 1. Two novel amino acid changes, Val198Ile (G4397A) and Glu499Gly (A8333G), were detected in only one allele of the coding regions in two patients with PCG. These four substitutions were not present in the 120 chromosomes, which were analyzed by SSCP mobility, from 60 unaffected Japanese control subjects.
Polymorphisms in CYP1B1 Gene and Haplotype
Analysis
PCR-SSCP or sequence analysis of the three exons in the
CYP1B1 gene in the 65 affected individuals resulted in
identification of five single-nucleotide polymorphisms. Four of these,
Arg48Gly (C3947), Ala119Ser (G4160T), Val432Leu (G8131C), and Asp449Asp
(T8184C), have been reported previously in patients with
PCG.10
15
Arg48Gly and Ala119Ser are always found together
in normal Japanese subjects. This linked amino acid substitution has
been reported not to alter CYP1B1 function.22
One polymorphism, Val243Val(G4534C), was novel and was not detected in
the 60 unaffected Japanese individuals. The frequency of the five
polymorphisms in the 60 unaffected Japanese individuals analyzed is
shown in Table 2 . This suggests that the most common Japanese haplotype was C-G-G-C-C.
The Asn453Ser polymorphism (A8195G)10
15
previously
identified in British, Turkish, and Saudi Arabian populations was not
found in the present study.
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Evaluation of Mutated Positions by Multiple Sequence Alignment
Comparison of amino acid sequence alignment in nine different
cytochrome P450 proteins revealed that all missense mutations had
occurred at highly conserved positions (Fig. 1)
. Ala330Phe and
Arg444Gln affected positions that were conserved among the nine
different CYP1 gene families analyzed and were located in
highly conserved regions of the I helix and of the meander,
respectively. The meander, which was just C terminal of the K' helix,
was so named because at first glance this region appears to make a
random walk from the K' helix to the heme-binding region; actually,
this was not the case. Asp192Val affected a position that was conserved
in six of the nine different CYP1 gene families analyzed and
immediately preceded the invariant residue Pro193. The Asp192 position
was occupied by a hydrophilic residue, specifically Asp (D), Glu (E),
or Asn (N), in the nine CYP1 gene families analyzed. The
Asp192Val mutation would result in substitution of the hydrophilic Asp
residue for the hydrophobic Val amino acid. The Val364 position was
occupied by a hydrophobic residue with Val (V) or Ile (I) in the nine
CYP1 gene families analyzed. This position was located in
the conserved region of the K helix.
Analysis of Missense Mutations in the CYP1B1Structure
Four missense mutations (Asp192Val, Ala330Phe, Val364Met, and
Arg444Gln) were analyzed based on the atomic structure of
CYP1B1 predicted by homology modeling as described in the
Methods section. Mutations in positions 330 and 364 did not produce
significant structural changes. Only a small recognizable
conformational change was observed in Ala330Phe, because of the
replacement of the small Ala side chain by the large hydrophobic
Phe. Another missense mutation (Asp192Val) was predicted to be located
ahead of the conserved residue Pro193 in the fragment of polypeptide
chain containing helix H and ß-strand 6 of ß-sheet 3. Modeling
showed that replacement of the negatively charged Asp192 residue by a
hydrophobic Val affects conformations of 11 neighbor residues. As a
result, the Leu343 residue changed conformation and lost the hydrogen
bond between the side chain of Arg194 and the main chain of Asn228
(Fig. 2A)
.
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| Discussion |
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CYP1B1 gene mutations were identified in more than 85% of
PCG-affected families with a minimum of two affected subjects in Saudi
Arabia, Turkey, and Slovakia.10
15
In contrast, only 20%
of our 65 unrelated Japanese patients with sporadic PCG harbored a
CYP1B1 gene mutation. Similarly, Kakiuchi et
al.24
reported a truncation mutation (1620insG) in the
CYP1B1 gene in only one (17%) of six Japanese families with
PCG. In the present study, 11 novel mutations were detected in 13
unrelated families. An insertion of two extra base pairs (AT) after the
second nucleotide of codon 324 (1318insAT) created a stop codon 104
amino acids downstream from the original amino acid Ile324. This
insertion in codon 324 did not alter the amino acid at codon 324,
Ile324Ile (ATC
ATA). Deletion of one of the three base pairs of C at
the third nucleotide (3964delC) of codon 53, at the first nucleotide
(3965delC) of codon 54, or at the second nucleotide (3966delC) of codon
54 created a stop codon 59 amino acids downstream from the original
amino acid Gly53. This deletion in codon 53 or 54 did not alter the
amino acid in codon 53, Gly324Gly (GGC
GGC).
A hydrophilic Asp residue occupied position 192 in the nine CYP1 gene families analyzed. Replacement of the hydrophilic Asp192 by nonpolar Val (Asp192Val) increases the hydrophobic potential of this residue and may affect proper protein folding of native structure.
Ala330Phe is located in the conserved helix I and represents the first missense mutation to be detected in this domain. This position was conserved in the nine different CYP1 gene families analyzed. The mutation is unique, because the two nucleotides at codon 330 were changed in only one allele. The Val364 residue is located at the end of helix J,15 a highly conserved core structural element. Here, Val is present in the CYP1B1 gene of human, mouse, and rat. However, the predicted three-dimensional structure models of mutant CYP1B1 corresponding to these two missense mutations did not demonstrate significant structural changes, probably because these mutations did not alter hydrophobic properties at their sites.
The Arg residue at position 444 was conserved among the nine different CYP1 gene families analyzed. Residue Arg444 is involved in nonpolar interactions, forms two hydrogen bonds in the predicted CYP1B1 structure, and stabilizes the structure of meander, as indicated in the Results section. In addition, Glu387 lost two hydrogen bonds with the side chain of Arg390 and the carbonyl oxygen of Asp441. Residues Glu387, Arg390, and Arg444 are involved in the so-called ERR triad, invariant in most cytochrome P450 structures.25 Because the ERR triad may maintain heme-binding, mutations in this region could destabilize this important protein function. Indeed, in cytochrome P450, a mutation of arginine involving the meander that was located in a position similar to that of Arg444 in the human CYP1B1 protein, resulted in a completely inactive protein.26 Further, the Phe445 residue, located next to Arg444, stabilizes the flap between the heme-binding domain and meander and may help to form the redox-partner binding site.27 Therefore, destabilization of the meander structure by the Arg444Gln mutation may influence the heme-binding and redox-partner functions of CYP1B1.
In the present study, four probable mutations, Val198Ile, Val320Leu, and Glu499Gly, and a C3130T alteration in the noncoding region of exon 1, were detected. These four alterations were not present in 120 chromosomes isolated from unaffected Japanese control individuals. Mutations in the 5'-untranslated regions have been shown to affect mRNA stability,28 29 although we could not confirm that this occurred with C3130T. Two unrelated patients with PCG were compound heterozygotes with both C3130T and Val320Leu. However, two amino acid changes, Val198Ile and Glu499Gly, were detected on only one allele in two patients with PCG. No second mutation was identified in these individuals, possibly because of a change in the promoter region or because of limited sensitivity of our PCR-SSCP screening analysis.
In conclusion, 11 novel mutations, including 7 definite and 4 probable mutations, were detected in the CYP1B1 gene in 13 unrelated sporadic Japanese subjects with PCG. Considerable molecular heterogeneity in this gene was seen in nonconsanguineous Japanese families with sporadic disease occurrence. The mutations appear to be unique to Japanese patients.
| Footnotes |
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Commercial relationships policy: N.
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: Yukihiko Mashima, Department of Ophthalmology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-8582, Japan. mashima{at}med.keio.ac.jp
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ík, A. (1989) Epidemiology and genetics of primary congenital glaucoma in Slovakia: description of a form of primary congenital glaucoma in gypsies with autosomal-recessive inheritance and complete penetrance Dev Ophthalmol 16,75-115
ík, A, Gen
íková, A, Gerinec, A. (1980) Genetic heterogeneity of congenital glaucoma Clin Genet 17,241-248[Medline][Order article via Infotrieve]
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