Structure of the human beta2-glycoprotein I (apolipoprotein H) gene

Valine/Leucine247 polymorphism of β2-glycoprotein I in patients with antiphospholipid syndrome: lack of association with anti-β2-glycoprotein I antibodies

In antiphospholipid syndrome (APS) the presence of anti-β2-glycoprotein I (β2GPI) antibodies is strongly associated with thromboembolic complications. It has been suggested that the common β2GPI Valine/Leucine247 (Val/Leu247)polymorphism could be found more commonly in APS and might influence the generation of anti-β2GPI antibodies. Therefore we studied β2GPI Val/Leu247single-nucleotide polymorphism (SNP) by PCR in 338 patients with various autoimmune diseases (46 with secondary and 84 with primary APS) and 147 sex and age-matched healthy controls. In all patients lupus anticoagulant, anticardiolipin and anti-β2GPI antibodies (both IgG and IgM) were also determined. All patients and controls were Caucasians. Frequencies of the SNP genotypes in patients did not depart from genetic equilibrum and did not differ from those found in controls. There was also no association between the presence of β2GPI Val/Leu247genotypes and the presence or absence of lupus anticoagulant, anticardiolipin antibodies, anti-β2GPI antibodies or clinical APS symptoms in all patients studied. In conclusion, among the exclusively Caucasian, Polish population of autoimmune patients β2GPI Val/Leu247SNP has the same distribution as in healthy subjects and does not influence the production of anti-β2GPI antibodies.

Genome-wide significant results identified for plasma apolipoprotein H levels in middle-aged and older adults

Apolipoprotein H (ApoH) is a multi-functional plasma glycoprotein that has been associated with negative health outcomes. ApoH levels have high heritability. We undertook a genome-wide association study of ApoH levels using the largest sample to date and replicated the results in an independent cohort (total N = 1,255). In the discovery phase, a meta-analysis of two cohorts, the Sydney Memory and Ageing Study (Sydney MAS) and the Older Australian Twins Study (OATS) (n = 942) revealed genome-wide significant results in or near the APOH gene on chromosome 17 (top SNP, rs7211380, p = 1 × 10⁻¹¹). The results were replicated in an independent cohort, the Hunter Community Study (p < 0.002) (n = 313). Conditional and joint analysis (COJO) confirmed the association of the chromosomal 17 region with ApoH levels. The set of independent SNPs identified by COJO explained 23% of the variance. The relationships between the top SNPs and cardiovascular/lipid/cognition measures and diabetes were assessed in Sydney MAS, with suggestive results observed for diabetes and cognitive performance. However, replication of these results in the smaller OATS cohort was not found. This work provides impetus for future research to better understand the contribution of genetics to ApoH levels and its possible impacts on health.

Genetic determinants of plasma β₂-glycoprotein I levels: a genome-wide association study in extended pedigrees from Spain

BACKGROUND

β2 -Glycoprotein I (β2 -GPI), also designated apolipoprotein H, is a 50-kDa protein that circulates in blood at high concentrations, playing important roles in autoimmune diseases, hemostasis, atherogenesis, and angiogenesis, as well as in host defense against bacteria and in protein/cellular waste removal. Plasma β2 -GPI levels have a significant genetic component (heritability of ~ 80%).

OBJECTIVES

To present the results of a genome-wide association study for plasma β2 -GPI levels in a set of extended pedigrees from the Genetic Analysis of Idiopathic Thrombophilia (GAIT) Project.

PATIENTS/METHODS

A total of 306 individuals for whom β2 -GPI plasma measurements were available were typed for 307,984 single-nucleotide polymorphisms (SNPs) with the Infinium 317k Beadchip (Illumina). Association with the β2 -GPI phenotype was investigated through variance component analysis, and the most significant results were followed up for association with coronary artery disease (CAD) in an independent in silico analysis involving 5765 CAD cases from the PROCARDIS Project and 7264 controls from the PROCARDIS Project and the Wellcome Trust Case Control Consortium (WTCCC) collection.

RESULTS

After correction for multiple testing, three SNPs located in/around two genes (ELF5 and SCUBE2) reached genome-wide significance. Moreover, an SNP in the APOH gene showed suggestive association with the β2 -GPI phenotype. Some of the identified genes are plausible biological candidates, as they are actually or potentially involved in inflammatory processes.

CONCLUSIONS

Our results represent a first step towards identifying common variants reflecting the genetic architecture influencing plasma β2 -GPI levels, and warrant further validation by functional experiments, as the functions of some of the discovered loci are still unknown.

Beta2-glycoprotein I gene polymorphisms Val247Leu and Trp316Ser in Spanish patients with primary antiphospholipid syndrome

The significance of beta2-glycoprotein I (β2GPI) polymorphisms in the production of anti-β2GPI and other antiphospholipid autoantibodies (aPL) and in the pathogenesis of primary antiphospholipid syndrome (PAPS) is not well understood. We performed a study comparing the distribution of polymorphisms at codons 247 (Val247Leu) and 316 (Trp316Ser) of the β2GPI gene in a Caucasian Spanish population of PAPS patients and healthy controls, and then making correlations with the development of anti-β2GPI antibodies and other aPL and associated clinical manifestations. A total of 57 PAPS patients and 100 control subjects were included. In the analysis of Val247Leu polymorphism, alleles (V and L) and genotypes (V/V, V/L, L/L) were similarly distributed in PAPS patients and controls (P = 0.66 and P = 0.22, respectively). Regarding Trp316Ser polymorphism, we found a higher percentage of patients with respect to controls expressing S allele (11.4 vs. 5%, P = 0.02) and T/S genotype (22.8 vs. 10%, P = 0.02). However, when we compared T/T and T/S genotypes in PAPS patients, we found no differences regarding generation of anti-β2GPI, other aPL and clinical manifestations favoring any genotype. Our findings suggest that among Spanish Caucasians, polymorphisms at codon 247 (Val247Leu) do not seem to influence PAPS pathogenesis. On the contrary, polymorphisms at codon 316 (Trp316Ser), by means of an increased S allele and T/S genotype presence in Spanish Caucasian patients, might play a role in the pathogenic development of PAPS, although mechanism would not involve an increased production of anti-β2GPI and other aPL.

Functional and genetic characterization of the promoter region of apolipoprotein H (beta2-glycoprotein I)

This study characterized the human apolipoprotein H [APOH; beta(2)-glycoprotein I (beta(2)GPI)] promoter and its variants by in vitro functional experiments and investigated their relationship with human plasma beta(2)GPI levels. We examined the individual effects of 12 APOH promoter single nucleotide polymorphisms in the 5' flanking region of APOH (approximately 1.4 kb) on luciferase activity in COS-1 cells and HepG2 cells and their impact on plasma beta(2)GPI levels in 799 American White people, the DNA binding properties of the APOH promoter using an electrophoretic mobility shift assay in HepG2 cells, the effects of serial deletion analysis of the APOH 5' flanking region in COS-1 and HepG2 cells and cross-species conservation of the APOH promoter sequence. The variant alleles of three single nucleotide polymorphisms (-1219G>A, -643T>C and -32C>A) showed significantly lower luciferase expression (51, 40 and 37%, respectively) as compared with the wild-type allele. The electrophoretic mobility shift assay demonstrated that these three variants specifically bind with protein(s) from HepG2 cell nuclear extracts. Three-site haplotype analysis (-1219G>A, -643T>C and -32C>A) revealed one haplotype carrying -32A (allele frequency = 0.075) to be significantly associated with decreased plasma beta(2)GPI levels (P < 0.001). Deletion analysis localized the core APOH promoter to approximately 160 bp upstream of ATG codon with the presence of critical cis-acting elements between -166 and -65. Cross-species conservation analysis of the APOH promoters of seven species indicated that basic promoter elements are highly conserved across species. In conclusion, we have characterized the functional promoter of APOH and identified functional variants that affect the transcriptional activity of the APOH promoter.

Val/Leu247 polymorphism of beta2-glycoprotein I in Brazilian patients with antiphospholipid syndrome--a genetic risk factor?

A genetic polymorphism of the beta2-glycoprotein I (beta2-GPI) is recognized by antiphospholipid antibodies (aPL) and may even play a role in the development of antiphospholipid syndrome (APS). The objectives of this study were to determine a Val/Leu SNP at position 247 of the beta2-GPI gene in Brazilian patients with APS and to compare these data with clinical and laboratory manifestations. Polymorphism assignment was performed by PCR followed by Rsa I restriction endonuclease. The titration of anti-beta2-GPI antibodies was detected by ELISA. The results showed significantly higher frequencies of the V-encoding allele and the homozygous VV genotype in patients with APS than in control subjects (OR = 1.781, P = 0.0068; and OR = 6.413, P < 0.0001, respectively). The frequency of this genotype was also significantly higher in patients with arterial and venous thrombosis than in the control group (52% and 44%, respectively, versus 13%). Anti-beta2-GPI-positive patients had significantly higher frequencies of the VV genotype than the controls subjects (OR = 8.179, P < 0.0001). These results suggest that the V-encoding allele and the homozygous VV genotype at position 247 of the beta2-GPI gene may play a role in the generation of anomalous beta2-GPI, with consequent auto-antibody production, and in phenotype expression of arterial and venous thrombosis in APS patients.

Mapping of the serum proteome of hepatocellular carcinoma induced by targeted overexpression of epidermal growth factor to liver cells of transgenic mice

Epidermal growth factor (EGF) is an important mitogen for hepatocytes. Its targeted overexpression induced hepatocellular carcinomas (HCC), as recently reported by us (Borlak et al. 2005). Early detection of disease is essential for successful therapy and overall survival. Here we describe our efforts in identifying serum biomarkers of liver cancer in a transgenic disease model that mimics effectively the consequence of exaggerated EGF signaling. We report a reference 2-DE map of mouse serum proteins. About 180 proteins were detected per gel and 130 proteins were identified by 2-DE-MALDI-MS analysis. We compared serum proteins of healthy nontransgenic and HCC tumor-bearing mice and identified 25 regulated proteins, of which n = 7 reached statistical significance ( p < 0.05). Furthermore, we identified several fragments of fibrinogens and of the alpha-2-macroglobulin to be disease-associated. We also found immunoglobulins to be repressed or absent in serum samples of tumor-bearing mice, and this included, among others, the Ig K and L class. In contrast, amyloid component P and apolipoprotein M were highly significantly increased by 10- and 8-fold, respectively, in serum samples of HCC mice. Overall, our findings provide a rationale for further clinical evaluation of the herein identified biomarker candidates.

Beta2-glycoprotein I and its clinical significance: from gene sequence to protein levels

In order to elucidate beta2-GPI at the DNA level and characterize its polymorphisms, mRNA expression, protein levels and clinical significance at each of these steps, a molecular review of beta2-GPI literature was performed. The human beta2-GPI complete nucleotide sequence has been reported and it consists of 8 exons separated by large introns. The beta2-GPI gene is polymorphic with four alleles. The distribution of point mutations can be significantly different between various racial populations. DNA variation studies of the beta2-GPI gene identified a total of 151 single-nucleotide polymorphisms, 26 of which are within regions with potential clinical significance. Southern blot analysis indicated the presence of one gene product only. An atypical TATA box and a hepatic nuclear factor-1 element are both essential for beta2-GPI promoter activity. Transcription factor binding sites for STAT, CREB, C/EBPbeta, NF-1, AP-1, NFAT, HNF-3beta and HNF-1 have been identified in the promoter region of the beta2-GPI gene by computer analysis. The beta2-GPI transcriptional signal of 1.5 kb was detected in Northern blot analysis and its 326-amino-acid sequence was found to be one of the most proline-rich eukaryotic proteins. Amino acid substitutions have been shown to be associated with loss of phospholipid binding, development and recognition of antiphospholipid antibodies.

Val/Leu247 and Trp/Ser316 polymorphisms in β2 glycoprotein I and their association with thrombosis in unselected Chilean patients

It is known that polymorphisms of beta (2)-glycoprotein I (beta (2)GPI) in exon 7 affect interaction between the phospholipid binding site and the antibodies, and that other polymorphisms in exon 8 increase the generation of antibodies. In this study, we analyzed genetic polymorphisms of beta (2)GPI in unselected Chilean patients to determine the prevalence of beta (2)GPI polymorphisms in the phospholipid domain in patients with venous and arterial thrombosis and the clinical correlation with thromboembolic complications. This study comprised 149 patients with venous and arterial thrombosis (62 with venous thrombosis and 87 with arterial thrombosis) and 160 healthy controls with no previous history of thrombosis. Polymorphisms of exons 7 and 8 of beta (2)GPI, which encode for its fifth domain, were determined by PCR-RFLP. The presence of aPL or anti-beta (2)GPI in the patients was detected by ELISA. Anti-beta (2)GPI were present in 8/149 patients (5.4%); of these, five had aCL antibodies of low titer. The allele containing Val/Leu(247) and Trp/Ser(316) was significantly more frequent in patients with thrombosis than in the control group (OR=3.1, CI 1.6-6.0, p=0.0003; OR=2.9, CI 1.1-8.6, p=0.027, respectively). These polymorphisms did not correlate with aPL or anti-beta (2)GPI but significant differences were observed with venous thrombosis (p=<0.0001) and arterial thrombosis (p=0.026). In conclusion, the beta (2)GPI polymorphisms Val/Leu(247) and Trp/Ser(316) are not related to the presence of anti-beta (2)GPI antibodies in unselected Chilean patients with venous and arterial thrombosis, but they are significantly associated with venous and arterial thrombosis.

Detection of a major gene effect for LDL peak particle diameter and association with apolipoprotein H gene haplotype

Low-density lipoprotein (LDL) size, a coronary heart disease risk factor, is influenced by both genetic and environmental factors. Results from the Quebec Family Study (QFS) revealed that the LDL peak particle diameter (LDL-PPD) aggregates in families with a heritability coefficient above 50% and is affected by a major quantitative trait locus on chromosome 17q (LOD=6.8). Complex segregation analyses have consistently demonstrated a major gene effect influencing LDL size. In the present study, we report a similar analysis in the QFS cohort, which suggests that a major gene explains 23% of the variance in age-body mass index and triglyceride-adjusted LDL-PPD. The most intuitive positional candidate gene on chromosome 17q is the apolipoprotein H gene. Direct sequencing of the promoter, coding regions, and exon-intron splicing boundaries of this gene revealed the presence of three missense mutations and two polymorphisms in the untranslated regions. Using family-based association tests, none of these variants was individually associated with LDL-PPD. However, analysis of the haplotypes constructed from the three missense mutations, suggested that one particular haplotype (frequency=20.9%) was associated with a significant increase in LDL-PPD trait values (p=0.046). Taken together, these results suggest the presence of a major gene effect influencing LDL-PPD and a positive association with a positional candidate gene located on chromosome 17q. Replication of the association between apolipoprotein H gene haplotype and LDL-PPD is required before reaching firm conclusion.

Complete DNA Sequence Variation in the Apolipoprotein H (beta2-glycoprotein I) Gene and Identification of Informative SNPs

Apolipoprotein H (APOH), also known as beta2-glycoprotein I, is a major antigen for the production of antiphospholipid antibodies in autoimmune diseases. Previously we have examined DNA variation in the coding region of the APOH gene and determined the molecular basis of the common protein polymorphism. Here we report the results of DNA sequence variation in the entire APOH gene encompassing a 20.3 kb region in 46 Caucasian Americans and 48 African American chromosomes. A total of 150 single nucleotide polymorphisms (SNPs) and one tri-allelic polymorphism were identified, including 8 in the coding region, 14 in the 5'-region and 2 in the 3'- region; the remainder were observed in introns. The observed number of SNPs was higher in the African American sample than in the Caucasian sample (130 vs. 84). We examined the race-specific linkage disequilibrium pattern among SNPs and identified maximally informative SNPs for future association studies. Altogether, we have identified 17 informative SNPs among Caucasians and 35 in blacks. The discovery of a full range of sequence variation and identification of race-specific informative SNPs in the APOH gene may facilitate the rapid evaluation of this variation in relation to autoimmune diseases.

Cloning and characterization of the human beta2-glycoprotein I (beta2-GPI) gene promoter: roles of the atypical TATA box and hepatic nuclear factor-1alpha in regulating beta2-GPI promoter activity

Beta2-glycoprotein I (beta2-GPI) is a plasma glycoprotein primarily synthesized in the liver. The interindividual variability of beta2-GPI expression in subjects with various metabolic syndromes and disease states suggests that it may have clinical importance. However, the regulation of beta2-GPI gene expression has not been clarified. To gain more insight into the control of beta2-GPI gene expression, we cloned the 4.1-kb 5'-flanking region and characterized the proximal promoter of the beta2- GPI gene in this study. Cis -acting elements required for beta2-GPI promoter activity were identified with transient transfection assays in the hepatoma cell lines HepG2 and Huh7 and in non-hepatic HeLa cells. Serial deletion analyses of the beta2-GPI 5'-flanking sequence revealed that the region from -197 to +7 had strong promoter activity in hepatoma cells but not in HeLa cells. Truncation and site-directed mutagenesis of putative cis -elements within this region showing an atypical TATA box and a HNF-1 (hepatic nuclear factor-1) element were both essential for the beta2-GPI promoter activity. Subsequent gel mobility shift assays confirmed the interaction of HNF-1alpha with the HNF-1 site residing downstream of the TATA box. Co-transfection of beta2-GPI promoter-luciferase vector with HNF-1alpha expression vector in Huh7 and HNF-1-deficient HeLa cells demonstrated the transactivation effect of HNF-1alpha on beta2-GPI promoter activity. In addition, overexpression of HNF-1alpha enhanced the endogenous beta2-GPI expression. These results suggest that the atypical TATA box and HNF-1 cis-element are critical for beta2-GPI transcription and HNF-1alpha may play an important role in cell-specific regulation of beta2-GPI gene expression.

Apolipoprotein H gene polymorphisms and risk of primary cerebral hemorrhage in a Chinese population

BACKGROUND AND PURPOSE

Apolipoprotein H (ApoH) has been implicated in several physiologic pathways including lipid metabolism, coagulation and the production of hypertension, which are related to the pathogenesis of primary cerebral hemorrhage (PICH). The gene coding for ApoH is polymorphic, with the occurrence of several common alleles in the general population. This genetically determined variation can effect lipid metabolism and the production of hypertension. We determined the distribution of ApoH gene polymorphisms in Chinese people and investigated whether these polymorphisms were associated with increased risk of PICH in a Chinese population.

METHODS

We studied polymorphisms of the ApoH gene by the polymerase chain reaction-single strand conformation polymorphism technique and DNA sequencing in 140 PICH patients and 100 healthy control subjects. Serum antiphospholipid antibodies and lipid levels were also examined in all subjects.

RESULTS

Four polymorphisms of the ApoH gene have been identified in Chinese people. No difference in genotype frequencies of G817T (Leu247Val) polymorphism, G1025C (Try316Ser) polymorphism and C1080T polymorphism was observed between PICH patients and control subjects (p > 0.05). The G341A (Ser88Asn) polymorphism correlated significantly with PICH. The frequencies of the A allele were significantly higher in PICH patients than in controls, especially in PICH patients with hypertension and a family history of stroke.

CONCLUSIONS

Our results suggest that the G341A (Ser88Asn) polymorphism might be associated with increased risk of PICH in a Chinese population. The association appeared to be mediated by the generation of hypertension.

Studies on specific interaction of beta-2-glycoprotein I with HBsAg

AIM: To observe the binding activity of beta-2-glycoprotein I (β₂GPI) to hepatitis B surface antigen (HBsAg) and the possible roles of β₂GPI in hepatitis B virus (HBV) infection.

METHODS: The rationale of ELISA methods and ELISA-based research method and ligand-blotting technique were used to detect the specific interaction of β₂GPI with HBsAg.

RESULTS: With the increase of rHBsAg, the binding of β₂GPI to rHBsAg elevated, and these changes had statistic significance. When we added non-biotinlyated β₂GPI, the OD value significantly decreased though they still were positively relevant to rHBsAg, suggesting non-biotinlyated β₂GPI competed with biotinlyated β₂GPI to saturate the binding sites on rHBsAg. Meanwhile BSA was used as negative control to substitute for rHBsAg coating the plates. The results indicated no interaction between β₂GPI and BSA, suggesting the affinity of β₂GPI to rHBsAg was specific. The ligand blotling indicated that β₂GPI might bind to rHBsAg no matter whether it was under reduced condition or not.

CONCLUSION: The binding of β₂GPI to HBsAg suggests that β₂GPI may be a carrier of HBV and that β₂GPI may play important roles in HBV infection.

A functional polymorphism at the transcriptional initiation site in beta2-glycoprotein I (apolipoprotein H) associated with reduced gene expression and lower plasma levels of beta2-glycoprotein I

Human beta2-glycoprotein I (beta2GPI), also known as apolipoprotein H, has been implicated in haemostasis and the production of anti-phospholipid antibodies. There is a wide range of interindividual variation in beta2GPI plasma levels that is thought to be under genetic control, but its molecular basis remains unknown. To understand the genetic basis of beta2GPI variation, we analyzed the 5' flanking region of the beta2GPI gene for mutation detection by DHPLC and identified a point mutation at the transcriptional initiation site (-1C-->A) with a carrier frequency of 12.1%. The mutation was associated with significantly lower beta2GPI plasma levels (P < 0.0001) and low occurrence of anti-phospholipid antibodies in lupus patients (4.8% antibody-positive group vs. 16.6% in the antibody-negative group; P = 0.019). Northern blot analysis confirmed that the -1C-->A mutation was associated with lower mRNA levels and it reduced the reporter (luciferase) gene expression by twofold. Electrophoretic gel mobility shift assay (EMSA) revealed that the -1C-->A mutation disrupts the binding for crude hepatic nuclear extracts and purified TFIID. These results suggest that the substitution of C with A at the beta2GPI transcriptional initiation site is a causative mutation that affects its gene expression at the transcriptional level and ultimately beta2GPI plasma levels and the occurrence of anti-phospholipid antibodies.