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(Stroke. 2004;35:443.)
© 2004 American Heart Association, Inc.
Original Contributions |
2 (COL1A2) Is the Susceptible Gene for Intracranial Aneurysms
From the Department of Neurosurgery, Neurological Institute (T.Y., H.K., H.O., H.A., T.H.) and Maternal and Perinatal Center (K.H.), Tokyo Womens Medical University; and Division of Genetic Diagnosis, Institute of Medical Science, University of Tokyo (T.Y., H.A., T.N., I.I.), Tokyo, Japan.
Correspondence to Ituro Inoue, MD, Division of Genetic Diagnosis, Institute of Medical Science, University of Tokyo, Shirokanedai 4-6-1, Minato-ku, Tokyo 108-8639, Japan. E-mail ituro{at}ims.u-tokyo.ac.jp
| Abstract |
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2(I) gene (COL1A2) on chromosome 7q22.1, a positional and functional candidate for intracranial aneurysm (IA), was extensively screened for susceptibility in Japanese IA patients.
Methods Twenty-one single nucleotide polymorphisms (SNPs) of COL1A2 were genotyped in genomic DNA from 260 IA patients (including 115 familial cases) (mean age, 59.9 years) and 293 controls (mean age, 61.6 years). Differences in allelic and genotypic frequencies between the patients and controls were evaluated with the
2 test. Circular dichroism spectrometry was monitored with collagen-related peptides that mimic triple-helical models of type I collagen with Ala-459 and Pro-459 to estimate the conformation and stability of alterations.
Results Significant genotypic association in the dominant model was observed between an exonic SNP of COL1A2 and familial IA patients (
2=11.08; df=1; P=0.00087; odds ratio=3.19; 95% CI, 2.22 to 6.50). This SNP induces Ala to Pro substitution at amino acid 459, located on a triple-helical domain. Circular dichroism spectra showed that the Pro-459 peptide had a higher thermal stability than the Ala-459 peptide.
Conclusions The variant of COL1A2 could be a genetic risk factor for IA patients with family history.
Key Words: aneurysm collagen genetics subarachnoid hemorrhage
| Introduction |
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Recently, gene expressions in various tissues have been extensively investigated with the use of cDNA microarray and other methods to elucidate the molecular pathology and pathogenesis. The differential gene expression between IA and the superficial temporal artery was investigated by Peters et al11 using SAGE analysis. In this report the genes of extracellular matrix proteins such as fibronectin, collagen families, elastin, and others were overexpressed in IA. Because the collagen
2(I) gene (COL1A2) is overexpressed in IA and is located on 7q22.1 where the best evidence of linkage was detected, we considered that COL1A2 could be a positional and functional candidate for IA.12 We genotyped 21 SNPs of COL1A2 in 553 individuals, including IA patients and controls, for an allelic association study. We identified a variant leading to an amino acid substitution that is genetically associated with IA and may have functional significance for IA formation.
| Subjects and Methods |
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SNP Selection of COL1A2 and Genotyping for Association Study
SNPs of COL1A2 were identified in the National Center for Biotechnology Information (NCBI) database (http://www.ncbi.nlm.nih.gov/SNP/) (Figure 1), and the polymorphisms were confirmed by sequencing 8 patients with IA. In addition, all the coding regions and exon-intron junctions of COL1A2 were sequenced to detect SNPs in the 8 subjects. After the identification of SNPs in COL1A2, those without complete linkage disequilibrium were selected for further genotyping. Genomic DNA was subjected to polymerase chain reaction (PCR) amplification followed by sequencing with BigDye Terminator cycle sequencing with the use of an ABI PRISM 3700 DNA analyzer (Applied Biosystems). The primer sets to amplify each COL1A2 segment were designed with the use of genomic sequences obtained from the GenBank database, accession No. GI:22050628. Ten nanograms of genomic DNA was amplified in 10 µL of reaction mixture containing 1x AmpliTaq Gold Buffer (Applied Biosystems), 300 nmol/L of each primer, 200 nmol/L of dNTP, 1.5 mmol/L of MgCl2, and 0.2 U of AmpliTaq Gold DNA Polymerase (Applied Biosystems). The PCR conditions were as follows: initial denaturation at 95°C for 12 minutes was followed by 35 cycles of denaturation at 94°C for 15 seconds, annealing at 55°C for 15 seconds, and extension at 72°C for 30 seconds and was completed by a final extension at 72°C for 10 minutes with the use of a thermal cycler (GeneAmp PCR System model 9700, Applied Biosystems).
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Statistical Analysis
Allelic association with IA was evaluated with the use of the
2 test for each SNP in the cases and controls. Linkage disequilibrium was estimated as D=xij-pipj, where xij is the frequency of haplotype A1B1, and p1 and p2 are the frequencies of alleles A and B, respectively. A standardized linkage disequilibrium coefficient, r, is given by D/(p1p2q1q2)1/2, where p2 and q2 are the frequencies of the other alleles at loci A and B, respectively.14 Lewontins coefficient, D', is given by D'=D/Dmax, where Dmax=min(p1q2,p2q1) when D<0 or Dmax=min(p1q1,p2q2) when D>0.15 These statistics were generated with the use of SNPAlyze (Dynacom).
Collagen-Related Peptides and Circular Dichroism Spectra
Standard fluorenylmethoxycarbonyl-based solid-phase peptide synthesis methods were used to prepare the collagen-related peptides and polypeptides GPP (GlyProPro)10 and GPA (GlyProPro)4GlyProAla(GlyProPro)5 (Sigma). The peptides were purified by high-performance liquid chromatography (C18 8-µm column with a linear gradient of 80% acetonitrile/H2O plus 0.1% trifluoroacetic acid) with >90% purity and verified by electrospray mass spectrometry.
The circular dichroism (CD) spectra were recorded in 50 mmol/L acetic acid at 10°C after incubation for 24 hours at 4°C on a JASCO J-820 equipped with a thermostatically controlled cell holder.16 The peptides (0.23 mmol/L) were incubated in 50 mmol/L acetic acid for 24 hours at 4°C. Ellipticity at 225 nm was monitored by CD spectroscopy, while the sample temperature was raised from 5°C to 95°C in increments of 0.5°C at a rate of 50°C per hour.
| Results |
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Allelic Association Study With SNPs of COL1A2
A total of 57 SNPs of COL1A2 were identified, and 21 SNPs without complete linkage disequilibrium (r2=1) were subjected to further genotyping. The results of the allelic association study are summarized in Table 2. The genotype frequencies for all the SNPs met Hardy-Weinberg expectations in both cases and controls (data not shown). Three SNPs28, 36, and 50showed significant differences in allelic frequency between the controls and familial cases and between the controls and total case samples. These differences in allelic frequency were more evident in IA patients with family history than in overall IA patients. The most significant allelic association was observed with SNP28 (
2=10.59; P=0.001; odds ratio=3.00; 95% CI, 2.11 to 6.00). The genotype frequency of SNP28 also showed significant association with familial IA in the dominant model (
2=11.08; P=0.00087; odds ratio=3.19; 95% CI, 2.22 to 6.50) (Table 3).
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SNP28 is located on exon 28, resulting in an amino acid substitution, Ala to Pro at 459.18 SNP36 is located on exon 32, resulting in a synonymous change in amino acid. SNP50 is an intron.
Characteristics of Collagen-Related Triple-Helical Models
Collagen model peptides mimicking our target variants were prepared and designated as GPP and GPA to investigate the effect of the variant at amino acid position 459 on the structure of the triple helix (Figure 2a). The conformational properties of the peptide homotrimer were assessed with the use of the CD spectrum. GPP is known to form a homotrimer showing a characteristic triple-helical CD of 225 nm maximum in the native state, and GPA showed identical spectra, suggesting that GPA also forms a triple helix (Figure 2b). The thermal stability of the model peptides was measured on the basis of their thermal denaturation with CD spectroscopy (Figure 2c). For equilibrium melting temperature (Tm) transitions (or the transition midpoint), the ellipticity at 225 nm was monitored (Figure 2d). The value of the Tm of 2 triple helices differed, ie, GPP had a higher Tm than GPA.
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| Discussion |
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Type I collagen is by far the most abundant collagen of the vertebrate connective tissue and is found in a large number of tissues, including vessels.21 It is well known that type I collagen is a key structural component of broad tissues; however, it also has many physiological functions, such as specific interactions with various molecules and cells. Among the interaction sites, Ala-459 in the triple helix domain of type I collagen is a location for keratan sulfate proteoglycan binding.22 Conformational alteration due to amino acid substitution was investigated with the use of collagen-related peptides on a CD spectrum. We synthesized 2 collagen-related peptides, GPP and GPA, that mimic Pro-459 and Ala-459 of the collagen type I
2-chain, respectively (Figure 2a). Both peptides form a triple-helical structure in solution (Figure 2b).23 The differences in stability between GPP and GPA were assessed by Tm transition in a thermal stability test. The Tm value for GPP was higher than that for GPA (Figure 2c and 2d), indicating that the triple helix of GPP is more stable than that of GPA, which may lead to overall structural stability of the Pro-459 variant.
Connective tissue alterations were observed in skin specimens from IA patients, and the active expression of matrix metalloproteinases may be associated with IA.2426 The deposition of interstitial type I collagen was documented at the surfaces of smooth muscle cells or along elastin filaments in the cerebral vascular wall, and type I collagen may thus play a role in the rigidity and elasticity of the vascular wall.21 It is therefore speculated that the causative Pro-459 variant of type I collagen
2-chain may affect the rigidity or elasticity of the vascular wall because altered thermal stability was demonstrated in the model peptide (Figure 2). It is also possible that local conformational change due to amino acid substitution could affect interaction with other molecules, eventually altering the vascular wall strength.27
In conclusion, SNP28 of COL1A2 is possibly susceptible to IA, and the functional SNP results in stability changes that may lead to IA formation. However, because the allelic frequency of SNP28 is low, the variant could account for only a portion of IA susceptibilities.
| Acknowledgments |
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Received June 26, 2003; revision received September 18, 2003; accepted October 14, 2003.
| References |
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